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Goniometer
From Wikipedia, the free encyclopedia For other uses, see Goniometer (disambiguation).

Goniometer made by Develey le Jeune in Lausanne, late 18th–early 19th century

Manual (1), and Mitscherlich's optical (2) goniometers for use in crystallography, c. 1900 A goniometer is an instrument that either measures an angle or allows an object to be rotated to a precise angular position. The term goniometry is derived from two Greek words, gōnia, meaning angle, and metron, meaning measure.

Contents
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1 Applications
○ ○ ○ ○ ○ ○

1.1 Communications 1.2 Crystallography 1.3 Light measurement 1.4 In Medicine 1.5 Physical therapy 1.6 Surface science
  

1.6.1 Static contact angle 1.6.2 Advancing and receding contact angles 1.6.3 Surface tension

○ ○ • • •

1.7 Positioning 1.8 Knife and blade cutting edge angle measurement

2 See also 3 References 4 External links

[edit] Applications
[edit] Communications
Goniometers are used for direction finding in signals intelligence applications for military and civil purposes,[1] e.g. interception of satellite and naval communications as performed on the French warship Dupuy de Lôme uses multiple goniometers.

[edit] Crystallography

A goniometer for crystallography

In crystallography, goniometers are used for measuring angles between crystal faces. They are also used in X-ray diffraction to rotate the samples. The groundbreaking investigations of physicist Max von Laue and cohorts into the atomic structure of crystals in 1912 involved a goniometer.

[edit] Light measurement
Goniophotometers measure the spatial distribution of light visible to the human eye at a specific angular position.

[edit] In Medicine
Medical practitioners, (Physicians, Physician Assistants, Physical Therapists, Athletic Trainers, Chiropractors and Nurse Practitioners) use a goniometer to document initial and subsequent range of motion, at the visits for Occupational injuries, and by disability evaluators to determine a permanent disability. This is to evaluate progress, and also for medico-legal purposes. It is a tool to evaluate Waddell's signs (findings that may indicate symptom magnification.)

[edit] Physical therapy
In physical therapy and occupational therapy, a goniometer is an instrument which measures an axis and range of motion. If a patient or client experiences decreased range of motion in a joint (e.g. a knee or elbow), the therapist can use a goniometer to assess what the range of motion is prior to intervention, and then make sure the intervention is working by using the goniometer in subsequent interventions.

[edit] Surface science

Surface scientists use a contact angle goniometer to measure contact angle, surface energy and surface tension. In surface science, an instrument generally called a contact angle goniometer is used to measure the static contact angle, advancing & receding contact angles, and surface tension. The first contact angle goniometer was designed by Dr. William Zisman of the United States Naval Research Laboratory in Washington, DC and manufactured by ramé-hart (now ramé-hart instrument company), New Jersey, USA. The original manual contact angle goniometer used an eyepiece with microscope. The current generation of contact angle instruments uses cameras and software to capture and analyze the drop shape and are better suited for dynamic and advanced

studies. A Gonioreflectometer is used to measure the reflectivity of a surface at a variety of angles. [edit] Static contact angle

A liquid droplet rests on a solid surface and is surrounded by gas. The contact angle, θC, is the angle formed by a liquid at the three phase boundary where the liquid, gas, and solid intersect. The contact angle, θ, is the angle formed by a liquid at the three phase boundary where the liquid, gas, and solid intersect. The contact angle depends on the interfacial tensions between the gas & liquid, liquid & solid, and gas & solid. Young’s Relation expresses the contact angle analytically.[2]

where = Interfacial tension between the solid and gas = Interfacial tension between the solid and liquid = Interfacial tension between the liquid and gas Contact angle goniometers measure a droplet’s contact angle by assuming the droplet fits the geometry of a sphere, ellipsoid, or the Young-Laplace equation. Another perspective that describes contact angles uses cohesion vs. adhesion. Cohesion is the force between the liquid molecules which hold the liquid together. Adhesion is the force between the liquid molecules and the solid molecules. The contact angle is a quantitative measure that tells the user the ratio of cohesion vs. adhesion. If the contact angle is near zero, meaning the liquid droplet spreads completely on the solid surface, adhesive forces are dominating. If the contact angle is very high, meaning the liquid droplet beads up on the solid surface as water does on a freshly waxed car, cohesive forces are dominating. [edit] Advancing and receding contact angles While static contact angles give static information about the interfacial tensions between the solid, liquid, and gas, advancing and receding contact angles give some information about the dynamic interaction of the liquid, solid, and gas. An advancing contact angle is determined by pushing a droplet out of a pipette onto a solid. When the liquid initially meets the solid it will form some contact angle. As the pipette injects more liquid through the pipette, the droplet will increase in volume, the contact angle will increase, but its three phase boundary will remain stationary until it suddenly jumps outward. The contact angle the droplet had immediately before jumping outward is termed the advancing contact angle. The receding contact angle is now measured by sucking the liquid back out of the droplet. The droplet will decrease in volume, the

contact angle will decrease, but its three phase boundary will remain stationary until it suddenly jumps inward. The contact angle the droplet had immediately before jumping inward is termed the receding contact angle. The difference between advancing and receding contact angles is termed contact angle hysteresis which can be used to characterize surface heterogeneity, roughness, and mobility.[3] Surfaces that are not homogeneous will have domains which impede motion of the contact line. [edit] Surface tension

Surface tension exists because the molecules inside a liquid experience roughly equal cohesive forces in all directions, but molecules at the surface experience larger attractive forces toward the liquid than toward gas Contact angle goniometers can also determine the surface tension for any liquid in gas or the interfacial tension between any two liquids. If the difference in densities between the two fluids is known, the surface tension or interfacial tension can be calculated by using the pendant drop method. Surface tension quantifies the cohesive force present at the surface of a droplet. Contact angle goniometers measure surface tension by examining a drop hanging from a syringe tip in air. The balance of forces acting on the drop include the surface tension. The surface tension can be calculated using the below equation

where:
• • • • •

is the liquid-air surface tension (J/m² or N/m) β is the shape factor Δρ is the difference in density between fluids at the interface (kg/m3) g is acceleration due to gravity (m/s²) R is the radius of the drop curvature at the drop’s apex (m) dx/ds = cosθ dz/ds = sinφ dφ/ds = 2 + βz – sinφ/x

3 dimensionless first order equations can define the shape factor • • •

Iterative approximations can solve for β.

[edit] Positioning
Main article: Positioning goniometer

A miniature electro-mechanical goniometer stage. This type of stage is used primarily in the field of lasers and optics. A positioning goniometer or goniometric stage is a device used to rotate an object precisely about a fixed axis in space. It is similar to a linear stage, however, rather than moving linearly with respect to its base, the stage platform rotates partially about a fixed axis above the mounting surface of the platform. Positioning goniometers typically use a worm drive with a partial worm wheel fixed to the underside of the stage platform meshing with a worm in the base. The worm may be rotated manually or by a motor as in automated positioning systems.

[edit] Knife and blade cutting edge angle measurement
The included cutting angles of all kinds of sharp edge blades and knives is measured using a laser reflecting goniometer. Developed by CATRA in the UK in 1979, a range of devices can accurately determine the cutting edge profile including a rounding of the tip to ½°. The included angle of a blade is important in controlling its cutting ability and edge strength, i.e. a low angle makes the edge thin and optimized for cutting while a large angle makes it thick, which cuts poorly, but is very strong. One of CATRA's products is the strangely named hobbigoni, which is a low cost version of the industrial angle measurement lasers and is very useful for knife enthusiast.

Laser
From Wikipedia, the free encyclopedia For other uses, see Laser (disambiguation).

United States Air Force laser experiment

Laser beams in fog, reflected on a car windshield A laser is a device that emits light (electromagnetic radiation) through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation.[1][2] The emitted laser light is notable for its high degree of spatial and temporal coherence, unattainable using other technologies. Spatial coherence typically is expressed through the output being a narrow beam which is diffraction-limited, often a so-called "pencil beam." Laser beams can be focused to very tiny spots, achieving a very high irradiance. Or they can be launched into a beam of very low divergence in order to concentrate their power at a large distance. Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance (the coherence length) along the beam.[3] A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase which vary randomly with respect to time and position, and thus a very short coherence length. Most so-called "single wavelength" lasers actually produce radiation in several modes having slightly different frequencies (wavelengths), often not in a single polarization. And although temporal coherence implies monochromaticity, there are even lasers that emit a broad spectrum of light, or emit different wavelengths of light simultaneously. There are some lasers which are not single spatial mode and consequently their light beams diverge more than required by the diffraction limit. However all such devices are classified as "lasers" based on their method of producing that light: stimulated emission. Lasers are employed in applications where light of the required spatial or temporal coherence could not be produced using simpler technologies.

Contents
[hide]
• • • •

1 Terminology 2 Design 3 Laser physics 4 Continuous and pulsed modes of operation
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4.1 Continuous wave operation 4.2 Pulsed operation
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4.2.1 Q-switching 4.2.2 Mode-locking 4.2.3 Pulsed pumping



5 History
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5.1 Foundations 5.2 Maser 5.3 Laser 5.4 Recent innovations 6.1 Gas lasers
 



6 Types and operating principles


6.1.1 Chemical lasers 6.1.2 Excimer lasers

○ ○ ○ ○ ○ ○ ○ ○ •

6.2 Solid-state lasers 6.3 Fiber lasers 6.4 Photonic crystal lasers 6.5 Semiconductor lasers 6.6 Dye lasers 6.7 Free electron lasers 6.8 Bio laser 6.9 Exotic laser media 7.1 Examples by power 7.2 Hobby uses

7 Uses
○ ○

• • •

8 Safety 9 As weapons 10 Fictional predictions

• • •

11 See also 12 References 13 External links

Terminology
The word laser started as an acronym for "light amplification by stimulated emission of radiation"; in modern usage "light" broadly denotes electromagnetic radiation of any frequency, not only visible light, hence infrared laser, ultraviolet laser, X-ray laser, and so on. Because the microwave predecessor of the laser, the maser, was developed first, devices of this sort operating at microwave and radio frequencies are referred to as "masers" rather than "microwave lasers" or "radio lasers". In the early technical literature, especially at Bell Telephone Laboratories, the laser was called an optical maser; this term is now obsolete.[4] A laser which produces light by itself is technically an optical oscillator rather than an optical amplifier as suggested by the acronym. It has been humorously noted that the acronym LOSER, for "light oscillation by stimulated emission of radiation," would have been more correct.[5] With the widespread use of the original acronym as a common noun, actual optical amplifiers have come to be referred to as "laser amplifiers", notwithstanding the apparent redundancy in that designation. The back-formed verb to lase is frequently used in the field, meaning "to produce laser light,"[6] especially in reference to the gain medium of a laser; when a laser is operating it is said to be "lasing." Further use of the words laser and maser in an extended sense, not referring to laser technology or devices, can be seen in usages such as astrophysical maser and atom laser.

Design

Principal components: 1. Gain medium 2. Laser pumping energy 3. High reflector 4. Output coupler 5. Laser beam Main article: Laser construction A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material with properties that allow it to amplify light by stimulated emission. In its simplest form, a cavity consists of two mirrors

arranged such that light bounces back and forth, each time passing through the gain medium. Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror. Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light. The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

Laser physics

A helium-neon laser demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6. The pinkorange glow running through the center of the tube is from the electric discharge which inadvertently produces incoherent light, just as in a neon tube. That glowing plasma however also acts as the gain medium through which the internal beam passes as it is reflected in between the two mirrors. Laser radiation output from the front mirror can be seen to produce a tiny (about 1mm in diameter) intense spot on the screen to the right. Although it is a deep and pure red color, spots of laser light are so intense that cameras are typically overexposed and distort their color, often appearing more white.

Spectrum of a helium neon laser illustrating its very high spectral purity (limited by the measuring apparatus). The .002 nm bandwidth of the lasing medium is well over 10,000 times narrower than the spectral width of a light-emitting diode (whose spectrum is shown here for comparison), with the bandwidth of a single longitudinal mode being much narrower still. See also: Laser science The gain medium of a laser is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission. It can be of any state: gas, liquid, solid, or plasma. The gain medium absorbs pump energy, which raises some electrons into higher-energy ("excited") quantum states. Particles can interact with light by either absorbing or emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved and the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. By itself, this makes an optical amplifier. When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser. The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and often monochromaticity established by the optical cavity design. The optical resonator is sometimes referred to as an "optical cavity", but this is a misnomer: lasers use open resonators as opposed to the literal cavity that would be employed at microwave frequencies in a maser. The resonator typically consists of two mirrors between which a coherent beam of light travels in both directions, reflecting back on itself so that an average photon will pass through the gain medium repeatedly before it is emitted from the output aperture or lost to diffraction or absorption. If the gain (amplification) in the medium is larger than the resonator losses, then the power of the recirculating light can rise exponentially. But each stimulated emission event returns an atom from its excited state to the ground state, reducing the gain of the medium. With increasing beam power the net gain (gain times loss) reduces to unity and the gain medium is said to be saturated. In a continuous wave (CW) laser, the balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser. If the applied pump power is too small, the gain will never be sufficient to overcome the resonator losses, and laser light will not be produced. The minimum pump power needed to begin laser action is called the lasing threshold. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons in a spatial mode supported by the resonator will pass more than once through the medium and receive substantial amplification. The beam in the cavity and the output beam of the laser, when travelling in free space (or a homogenous medium) rather than waveguides (as in an optical fiber laser), can be approximated as a Gaussian beam in most lasers; such beams exhibit the minimum divergence for a given diameter. However some high power lasers may be multimode, with the transverse modes often approximated using Hermite-Gaussian or Laguerre-Gaussian functions. It has been shown that unstable laser resonators (not used in most lasers) produce fractal shaped beams.[7] Near the beam "waist" (or focal region) it is highly collimated: the wavefronts are planar, normal to the direction of propagation, with no beam divergence at that point. However due to diffraction, that can only remain true well within the Rayleigh range. The beam of a single transverse mode (gaussian beam) laser eventually diverges at an angle which varies inversely with the beam

diameter, as required by diffraction theory. Thus, the "pencil beam" directly generated by a common helium-neon laser would spread out to a size of perhaps 500 kilometers when shone on the Moon (from the distance of the earth). On the other hand the light from a semiconductor laser typically exits the tiny crystal with a large divergence: up to 50°. However even such a divergent beam can be transformed into a similarly collimated beam by means of a lens system, as is always included, for instance, in a laser pointer whose light originates from a laser diode. That is possible due to the light being of a single spatial mode. This unique property of laser light, spatial coherence, cannot be replicated using standard light sources (except by discarding most of the light) as can be appreciated by comparing the beam from a flashlight (torch) or spotlight to that of almost any laser. The mechanism of producing radiation in a laser relies on stimulated emission, where energy is extracted from a transition in an atom or molecule. This is a quantum phenomenon discovered by Einstein who derived the relationship between the A coefficient describing spontaneous emission and the B coefficient which applies to absorption and stimulated emission. However in the case of the free electron laser, atomic energy levels are not involved; it appears that the operation of this rather exotic device can be explained without reference to quantum mechanics.

Continuous and pulsed modes of operation
A laser can be classified as operating in either continuous or pulsed mode, depending on whether the power output is essentially continuous over time or whether its output takes the form of pulses of light on one or another time scale. Of course even a laser whose output is normally continuous can be intentionally turned on and off at some rate in order to create pulses of light. When the modulation rate is on time scales much slower than the cavity lifetime and the time period over which energy can be stored in the lasing medium or pumping mechanism, then it is still classified as a "modulated" or "pulsed" continuous wave laser. Most laser diodes used in communication systems fall in that category.

Continuous wave operation
Some applications of lasers depend on a beam whose output power is constant over time. Such a laser is known as continuous wave (CW). Many types of lasers can be made to operate in continuous wave mode to satisfy such an application. Many of these lasers actually lase in several longitudinal modes at the same time, and beats between the slightly different optical frequencies of those oscillations will in fact produce amplitude variations on time scales shorter than the round-trip time (the reciprocal of the frequency spacing between modes), typically a few nanoseconds or less. In most cases these lasers are still termed "continuous wave" as their output power is steady when averaged over any longer time periods, with the very high frequency power variations having little or no impact in the intended application. (However the term is not applied to mode-locked lasers, where the intention is to create very short pulses at the rate of the round-trip time). For continuous wave operation it is required for the population inversion of the gain medium to be continually replenished by a steady pump source. In some lasing media this is impossible. In some other lasers it would require pumping the laser at a very high continuous power level which would be impractical or destroy the laser by producing excessive heat. Such lasers cannot be run in CW mode.

Pulsed operation

Pulsed operation of lasers refers to any laser not classified as continuous wave, so that the optical power appears in pulses of some duration at some repetition rate. This encompasses a wide range of technologies addressing a number of different motivations. Some lasers are pulsed simply because they cannot be run in continuous mode. In other cases the application requires the production of pulses having as large an energy as possible. Since the pulse energy is equal to the average power divided by the repetition rate, this goal can sometimes be satisfied by lowering the rate of pulses so that more energy can be built up in between pulses. In laser ablation for example, a small volume of material at the surface of a work piece can be evaporated if it is heated in a very short time, whereas supplying the energy gradually would allow for the heat to be absorbed into the bulk of the piece, never attaining a sufficiently high temperature at a particular point. Other applications rely on the peak pulse power (rather than the energy in the pulse), especially in order to obtain nonlinear optical effects. For a given pulse energy, this requires creating pulses of the shortest possible duration utilizing techniques such as Q-switching. The optical bandwidth of a pulse cannot be narrower than the reciprocal of the pulse width. In the case of extremely short pulses, that implies lasing over a considerable bandwidth, quite contrary to the very narrow bandwidths typical of CW lasers. The lasing medium in some dye lasers and vibronic solid-state lasers produces optical gain over a wide bandwidth, making a laser possible which can thus generate pulses of light as short as a few femtoseconds (10−15 s). Q-switching Main article: Q-switching In a Q-switched laser, the population inversion is allowed to build up by introducing loss inside the resonator which exceeds the gain of the medium; this can also be described as a reduction of the quality factor or 'Q' of the cavity. Then, after the pump energy stored in the laser medium has approached the maximum possible level, the introduced loss mechanism (often an electro- or acousto-optical element) is rapidly removed (or that occurs by itself in a passive device), allowing lasing to begin which rapidly obtains the stored energy in the gain medium. This results in a short pulse incorporating that energy, and thus a high peak power. Mode-locking Main article: Mode-locking A mode-locked laser is capable of emitting extremely short pulses on the order of tens of picoseconds down to less than 10 femtoseconds. These pulses will repeat at the round trip time, that is, the time that it takes light to complete one round trip between the mirrors comprising the resonator. Due to the Fourier limit (also known as energy-time uncertainty), a pulse of such short temporal length has a spectrum spread over a considerable bandwidth. Thus such a gain medium must have a gain bandwidth sufficiently broad to amplify those frequencies. An example of a suitable material is titanium-doped, artificially grown sapphire (Ti:sapphire) which has a very wide gain bandwidth and can thus produce pulses of only a few femtoseconds duration. Such mode-locked lasers are a most versatile tool for researching processes occurring on extremely short time scales (known as femtosecond physics, femtosecond chemistry and ultrafast science), for maximizing the effect of nonlinearity in optical materials (e.g. in second-harmonic generation, parametric down-conversion, optical parametric oscillators and the like) due to the large peak power, and in ablation applications.[citation needed] Again, because of the extremely short pulse duration, such a laser will produce pulses which achieve an extremely high peak power.

Pulsed pumping Another method of achieving pulsed laser operation is to pump the laser material with a source that is itself pulsed, either through electronic charging in the case of flash lamps, or another laser which is already pulsed. Pulsed pumping was historically used with dye lasers where the inverted population lifetime of a dye molecule was so short that a high energy, fast pump was needed. The way to overcome this problem was to charge up large capacitors which are then switched to discharge through flashlamps, producing an intense flash. Pulsed pumping is also required for three-level lasers in which the lower energy level rapidly becomes highly populated preventing further lasing until those atoms relax to the ground state. These lasers, such as the excimer laser and the copper vapor laser, can never be operated in CW mode.

History
Foundations
In 1917, Albert Einstein established the theoretic foundations for the laser and the maser in the paper Zur Quantentheorie der Strahlung (On the Quantum Theory of Radiation); via a rederivation of Max Planck’s law of radiation, conceptually based upon probability coefficients (Einstein coefficients) for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation; in 1928, Rudolf W. Ladenburg confirmed the existences of the phenomena of stimulated emission and negative absorption;[8] in 1939, Valentin A. Fabrikant predicted the use of stimulated emission to amplify “short” waves;[9] in 1947, Willis E. Lamb and R. C. Retherford found apparent stimulated emission in hydrogen spectra and effected the first demonstration of stimulated emission;[8] in 1950, Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of optical pumping, experimentally confirmed, two years later, by Brossel, Kastler, and Winter.[10]

Maser
Main article: Maser

Aleksandr Prokhorov In 1953, Charles Hard Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first microwave amplifier, a device operating on similar principles to the laser, but amplifying microwave radiation rather than infrared or visible radiation. Townes's maser was

incapable of continuous output.[citation needed] Meanwhile, in the Soviet Union, Nikolay Basov and Aleksandr Prokhorov were independently working on the quantum oscillator and solved the problem of continuous-output systems by using more than two energy levels. These gain media could release stimulated emissions between an excited state and a lower excited state, not the ground state, facilitating the maintenance of a population inversion. In 1955, Prokhorov and Basov suggested optical pumping of a multi-level system as a method for obtaining the population inversion, later a main method of laser pumping. Townes reports that several eminent physicists — among them Niels Bohr, John von Neumann, Isidor Rabi, Polykarp Kusch, and Llewellyn Thomas — argued the maser violated Heisenberg's uncertainty principle and hence could not work.[1] In 1964 Charles H. Townes, Nikolay Basov, and Aleksandr Prokhorov shared the Nobel Prize in Physics, “for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser–laser principle”.

Laser
In 1957, Charles Hard Townes and Arthur Leonard Schawlow, then at Bell Labs, began a serious study of the infrared laser. As ideas developed, they abandoned infrared radiation to instead concentrate upon visible light. The concept originally was called an "optical maser". In 1958, Bell Labs filed a patent application for their proposed optical maser; and Schawlow and Townes submitted a manuscript of their theoretical calculations to the Physical Review, published that year in Volume 112, Issue No. 6.

LASER notebook: First page of the notebook wherein Gordon Gould coined the LASER acronym, and described the technologic elements for constructing the device. Simultaneously, at Columbia University, graduate student Gordon Gould was working on a doctoral thesis about the energy levels of excited thallium. When Gould and Townes met, they spoke of radiation emission, as a general subject; afterwards, in November 1957, Gould noted his ideas for a “laser”, including using an open resonator (later an essential laser-device component). Moreover, in 1958, Prokhorov independently proposed using an open resonator, the first published appearance (the USSR) of this idea. Elsewhere, in the U.S., Schawlow and Townes

had agreed to an open-resonator laser design — apparently unaware of Prokhorov’s publications and Gould’s unpublished laser work. At a conference in 1959, Gordon Gould published the term LASER in the paper The LASER, Light Amplification by Stimulated Emission of Radiation.[1][5] Gould’s linguistic intention was using the “-aser” word particle as a suffix — to accurately denote the spectrum of the light emitted by the LASER device; thus x-rays: xaser, ultraviolet: uvaser, et cetera; none established itself as a discrete term, although “raser” was briefly popular for denoting radio-frequencyemitting devices. Gould’s notes included possible applications for a laser, such as spectrometry, interferometry, radar, and nuclear fusion. He continued developing the idea, and filed a patent application in April 1959. The U.S. Patent Office denied his application, and awarded a patent to Bell Labs, in 1960. That provoked a twenty-eight-year lawsuit, featuring scientific prestige and money as the stakes. Gould won his first minor patent in 1977, yet it was not until 1987 that he won the first significant patent lawsuit victory, when a Federal judge ordered the U.S. Patent Office to issue patents to Gould for the optically pumped and the gas discharge laser devices. The question of just how to assign credit for inventing the laser remains unresolved by historians.[11] On May 16, 1960, Theodore H. Maiman operated the first functioning laser,[12][13] at Hughes Research Laboratories, Malibu, California, ahead of several research teams, including those of Townes, at Columbia University, Arthur Schawlow, at Bell Labs,[14] and Gould, at the TRG (Technical Research Group) company. Maiman’s functional laser used a solid-state flashlamppumped synthetic ruby crystal to produce red laser light, at 694 nanometres wavelength; however, the device only was capable of pulsed operation, because of its three-level pumping design scheme. Later in 1960, the Iranian physicist Ali Javan, and William R. Bennett, and Donald Herriott, constructed the first gas laser, using helium and neon that was capable of continuous operation in the infrared (U.S. Patent 3,149,290); later, Javan received the Albert Einstein Award in 1993. Basov and Javan proposed the semiconductor laser diode concept. In 1962, Robert N. Hall demonstrated the first laser diode device, made of gallium arsenide and emitted at 850 nm the near-infrared band of the spectrum. Later, in 1962, Nick Holonyak, Jr. demonstrated the first semiconductor laser with a visible emission. This first semiconductor laser could only be used in pulsed-beam operation, and when cooled to liquid nitrogen temperatures (77 K). In 1970, Zhores Alferov, in the USSR, and Izuo Hayashi and Morton Panish of Bell Telephone Laboratories also independently developed room-temperature, continual-operation diode lasers, using the heterojunction structure.

Recent innovations

Graph showing the history of maximum laser pulse intensity throughout the past 40 years. Since the early period of laser history, laser research has produced a variety of improved and specialized laser types, optimized for different performance goals, including: • • • • • • • new wavelength bands maximum average output power maximum peak pulse energy maximum peak pulse power minimum output pulse duration maximum power efficiency minimum cost

and this research continues to this day. Lasing without maintaining the medium excited into a population inversion[dubious – discuss] was discovered in 1992 in sodium gas and again in 1995 in rubidium gas by various international teams.[citation needed] This was accomplished by using an external maser to induce "optical transparency" in the medium by introducing and destructively interfering the ground electron transitions between two paths, so that the likelihood for the ground electrons to absorb any energy has been cancelled.

Types and operating principles
For a more complete list of laser types see this list of laser types.

Wavelengths of commercially available lasers. Laser types with distinct laser lines are shown above the wavelength bar, while below are shown lasers that can emit in a wavelength range. The color codifies the type of laser material (see the figure description for more details).

Gas lasers
Main article: Gas laser Following the invention of the HeNe gas laser, many other gas discharges have been found to amplify light coherently. Gas lasers using many different gases have been built and used for many purposes. The helium-neon laser (HeNe) is able to operate at a number of different wavelengths, however the vast majority are engineered to lase at 633 nm; these relatively low

cost but highly coherent lasers are extremely common in optical research and educational laboratories. Commercial carbon dioxide (CO2) lasers can emit many hundreds of watts in a single spatial mode which can be concentrated into a tiny spot. This emission is in the thermal infrared at 10.6 µm; such lasers are regularly used in industry for cutting and welding. The efficiency of a CO2 laser is unusually high: over 10%. Argon-ion lasers can operate at a number of lasing transitions between 351 and 528.7 nm. Depending on the optical design one or more of these transitions can be lasing simultaneously; the most commonly used lines are 458 nm, 488 nm and 514.5 nm. A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an inexpensive gas laser, often home-built by hobbyists, which produces rather incoherent UV light at 337.1 nm.[15] Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two examples. Like all low-pressure gas lasers, the gain media of these lasers have quite narrow oscillation linewidths, less than 3 GHz (0.5 picometers),[16] making them candidates for use in fluorescence suppressed Raman spectroscopy. Chemical lasers Chemical lasers are powered by a chemical reaction permitting a large amount of energy to be released quickly. Such very high power lasers are especially of interest to the military, however continuous wave chemical lasers at very high power levels, fed by streams of gasses, have been developed and have some industrial applications. As examples, in the Hydrogen fluoride laser (2700-2900 nm) and the Deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride. Excimer lasers Excimer lasers are a special sort of gas laser powered by an electric discharge in which the lasing medium is an excimer, or more precisely an exciplex in existing designs. These are molecules which can only exist with one atom in an excited electronic state. Once the molecule transfers its excitation energy to a photon, therefore, its atoms are no longer bound to each other and the molecule disintegrates. This drastically reduces the population of the lower energy state thus greatly facilitating a population inversion. Excimers currently used are all noble gas compounds; noble gasses are chemically inert and can only form compounds while in an excited state. Excimer lasers typically operate at ultraviolet wavelengths with major applicatons including semiconductor photolithography and LASIK eye surgery. Commonly used excimer molecules include ArF (emission at 193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), and XeF (351 nm).[17] The molecular fluorine laser, emitting at 157 nm in the vacuum ultraviolet is sometimes referred to as an excimer laser, however this appears to be a misnomer inasmuch as F2 is a stable compound.

Solid-state lasers

A frequency-doubled green laser pointer, showing internal construction. Two AAA cells and electronics power the laser module (lower diagram) This contains a powerful 808 nm IR diode laser that optically pumps a Nd:YVO4 crystal inside a laser cavity. That laser produces 1064 nm (infrared) light which is mainly confined inside the resonator. Also inside the laser cavity, however, is a non-linear KTP crystal which causes frequency doubling, resulting in green light at 532 nm. The front mirror is transparent to this visible wavelength which is then expanded and collimated using two lenses (in this particular design). Solid-state lasers use a crystalline or glass rod which is "doped" with ions that provide the required energy states. For example, the first working laser was a ruby laser, made from ruby (chromium-doped corundum). The population inversion is actually maintained in the "dopant", such as chromium or neodymium. These materials are pumped optically using a shorter wavelength than the lasing wavelength, often from a flashtube or from another laser. It should be noted that "solid-state" in this sense refers to a crystal or glass, but this usage is distinct from the designation of "solid-state electronics" in referring to semiconductors. Semiconductor lasers (laser diodes) are pumped electrically and are thus not referred to as solidstate lasers. The class of solid-state lasers would, however, properly include fiber lasers in which dopants in the glass lase under optical pumping. But in practice these are simply referred to as "fiber lasers" with "solid-state" reserved for lasers using a solid rod of such a material.

Laser spots (650, 532, 405 nm) Neodymium is a common "dopant" in various solid-state laser crystals, including yttrium orthovanadate (Nd:YVO4), yttrium lithium fluoride (Nd:YLF) and yttrium aluminium garnet (Nd:YAG). All these lasers can produce high powers in the infrared spectrum at 1064 nm. They are used for cutting, welding and marking of metals and other materials, and also in spectroscopy and for pumping dye lasers. These lasers are also commonly frequency doubled, tripled or quadrupled, in so-called "diode pumped solid state" or DPSS lasers. Under second, third, or fourth harmonic generation these produce 532 nm (green, visible), 355 nm and 266 nm (UV) beams. This is the technology behind the bright laser pointers particularly at green (532 nm) and other short visible wavelengths. Ytterbium, holmium, thulium, and erbium are other common "dopants" in solid-state lasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around 1020-1050 nm. They are potentially very efficient and high powered due to a small quantum defect. Extremely high powers in ultrashort pulses can be achieved with Yb:YAG. Holmium-doped YAG crystals emit at 2097 nm and form an efficient

laser operating at infrared wavelengths strongly absorbed by water-bearing tissues. The Ho-YAG is usually operated in a pulsed mode, and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones. Titanium-doped sapphire (Ti:sapphire) produces a highly tunable infrared laser, commonly used for spectroscopy. It is also notable for use as a mode-locked laser producing ultrashort pulses of extremely high peak power. Thermal limitations in solid-state lasers arise from unconverted pump power that manifests itself as heat. This heat, when coupled with a high thermo-optic coefficient (dn/dT) can give rise to thermal lensing as well as reduced quantum efficiency. These types of issues can be overcome by another novel diode-pumped solid-state laser, the diode-pumped thin disk laser. The thermal limitations in this laser type are mitigated by using a laser medium geometry in which the thickness is much smaller than the diameter of the pump beam. This allows for a more even thermal gradient in the material. Thin disk lasers have been shown to produce up to kilowatt levels of power.[18]

Fiber lasers
Main article: Fiber laser Solid-state lasers or laser amplifiers where the light is guided due to the total internal reflection in a single mode optical fiber are instead called fiber lasers. Guiding of light allows extremely long gain regions providing good cooling conditions; fibers have high surface area to volume ratio which allows efficient cooling. In addition, the fiber's waveguiding properties tend to reduce thermal distortion of the beam. Erbium and ytterbium ions are common active species in such lasers. Quite often, the fiber laser is designed as a double-clad fiber. This type of fiber consists of a fiber core, an inner cladding and an outer cladding. The index of the three concentric layers is chosen so that the fiber core acts as a single-mode fiber for the laser emission while the outer cladding acts as a highly multimode core for the pump laser. This lets the pump propagate a large amount of power into and through the active inner core region, while still having a high numerical aperture (NA) to have easy launching conditions. Pump light can be used more efficiently by creating a fiber disk laser, or a stack of such lasers. Fiber lasers have a fundamental limit in that the intensity of the light in the fiber cannot be so high that optical nonlinearities induced by the local electric field strength can become dominant and prevent laser operation and/or lead to the material destruction of the fiber. This effect is called photodarkening. In bulk laser materials, the cooling is not so efficient, and it is difficult to separate the effects of photodarkening from the thermal effects, but the experiments in fibers show that the photodarkening can be attributed to the formation of long-living color centers.[citation
needed]

Photonic crystal lasers
Photonic crystal lasers are lasers based on nano-structures that provide the mode confinement and the density of optical states (DOS) structure required for the feedback to take place.[clarification needed] They are typical micrometre-sized[dubious – discuss] and tunable on the bands of the photonic crystals.[19][clarification needed]

Semiconductor lasers

A 5.6 mm 'closed can' commercial laser diode, probably from a CD or DVD player

Laser beams (red, green, violet) Semiconductor lasers are diodes which are electrically pumped. Recombination of electrons and holes created by the applied current introduces optical gain. Reflection from the ends of the crystal form an optical resonator, although the resonator can be external to the semiconductor in some designs. Commercial laser diodes emit at wavelengths from 375 nm to 3500 nm. Low to medium power laser diodes are used in laser printers and CD/DVD players. Laser diodes are also frequently used to optically pump other lasers with high efficiency. The highest power industrial laser diodes, with power up to 10 kW (70dBm)[citation needed], are used in industry for cutting and welding. External-cavity semiconductor lasers have a semiconductor active medium in a larger cavity. These devices can generate high power outputs with good beam quality, wavelength-tunable narrow-linewidth radiation, or ultrashort laser pulses. Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers whose emission direction is perpendicular to the surface of the wafer. VCSEL devices typically have a more circular output beam than conventional laser diodes, and potentially could be much cheaper to manufacture. As of 2005, only 850 nm VCSELs are widely available, with 1300 nm VCSELs beginning to be commercialized,[20] and 1550 nm devices an area of research. VECSELs are external-cavity VCSELs. Quantum cascade lasers are semiconductor lasers that have an active transition between energy sub-bands of an electron in a structure containing several quantum wells. The development of a silicon laser is important in the field of optical computing. Silicon is the material of choice for integrated circuits, and so electronic and silicon photonic components (such as optical interconnects) could be fabricated on the same chip. Unfortunately, silicon is a difficult lasing material to deal with, since it has certain properties which block lasing. However,

recently teams have produced silicon lasers through methods such as fabricating the lasing material from silicon and other semiconductor materials, such as indium(III) phosphide or gallium(III) arsenide, materials which allow coherent light to be produced from silicon. These are called hybrid silicon laser. Another type is a Raman laser, which takes advantage of Raman scattering to produce a laser from materials such as silicon.

Dye lasers
Dye lasers use an organic dye as the gain medium. The wide gain spectrum of available dyes, or mixtures of dyes, allows these lasers to be highly tunable, or to produce very short-duration pulses (on the order of a few femtoseconds). Although these tunable lasers are mainly known in their liquid form, researchers have also demonstrated narrow-linewidth tunable emission in dispersive oscillator configurations incorporating solid-state dye gain media.[21] In their most prevalent form these solid state dye lasers use dye-doped polymers as laser media.

Free electron lasers
Free electron lasers, or FELs, generate coherent, high power radiation, that is widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to soft X-rays. They have the widest frequency range of any laser type. While FEL beams share the same optical traits as other lasers, such as coherent radiation, FEL operation is quite different. Unlike gas, liquid, or solid-state lasers, which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium, hence the term free electron.

Bio laser
Living cells can be genetically engineered to produce Green fluorescent protein (GFP). The GFP is used as the laser's "gain medium", where light amplification takes place. The cells are then placed between two tiny mirrors, just 20 millionths of a metre across, which acted as the "laser cavity" in which light could bounce many times through the cell. Upon bathing the cell with blue light, it could be seen to emit directed and intense green laser light.[22][23]

Exotic laser media
In September 2007, the BBC News reported that there was speculation about the possibility of using positronium annihilation to drive a very powerful gamma ray laser.[24] Dr. David Cassidy of the University of California, Riverside proposed that a single such laser could be used to ignite a nuclear fusion reaction, replacing the banks of hundreds of lasers currently employed in inertial confinement fusion experiments.[24] Space-based X-ray lasers pumped by a nuclear explosion have also been proposed as antimissile weapons.[25][26] Such devices would be one-shot weapons.

Uses

Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments. Main article: List of applications for lasers When lasers were invented in 1960, they were called "a solution looking for a problem".[27] Since then, they have become ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, and the military. The first use of lasers in the daily lives of the general population was the supermarket barcode scanner, introduced in 1974. The laserdisc player, introduced in 1978, was the first successful consumer product to include a laser but the compact disc player was the first laser-equipped device to become common, beginning in 1982 followed shortly by laser printers. Some other uses are:
• • • • • • •

Medicine: Bloodless surgery, laser healing, surgical treatment, kidney stone treatment, eye treatment, dentistry Industry: Cutting, welding, material heat treatment, marking parts, non-contact measurement of parts Military: Marking targets, guiding munitions, missile defence, electro-optical countermeasures (EOCM), alternative to radar, blinding troops. Law enforcement: used for latent fingerprint detection in the forensic identification field[28][29] Research: Spectroscopy, laser ablation, laser annealing, laser scattering, laser interferometry, LIDAR, laser capture microdissection, fluorescence microscopy Product development/commercial: laser printers, optical discs (e.g. CDs and the like), barcode scanners, thermometers, laser pointers, holograms, bubblegrams. Laser lighting displays: Laser light shows



Cosmetic skin treatments: acne treatment, cellulite and striae reduction, and hair removal.

In 2004, excluding diode lasers, approximately 131,000 lasers were sold with a value of US$2.19 billion.[30] In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold.[31]

Examples by power

Laser application in astronomical adaptive optics imaging Different applications need lasers with different output powers. Lasers that produce a continuous beam or a series of short pulses can be compared on the basis of their average power. Lasers that produce pulses can also be characterized based on the peak power of each pulse. The peak power of a pulsed laser is many orders of magnitude greater than its average power. The average output power is always less than the power consumed. The continuous or average power required for some uses: Power Use 1-5 mW Laser pointers 5 mW CD-ROM drive 5–10 mW DVD player or DVD-ROM drive 100 mW High-speed CD-RW burner 250 mW Consumer 16x DVD-R burner Burning through a jewel case including disk within 4 seconds[32] 400 mW DVD 24x dual-layer recording.[33] 1 W Green laser in current Holographic Versatile Disc prototype development Output of the majority of commercially available solid-state lasers used for micro 1–20 W machining 30–100 W Typical sealed CO2 surgical lasers[34] 100–3000 Typical sealed CO2 lasers used in industrial laser cutting W 5 kW Output power achieved by a 1 cm diode laser bar[35] 100 kW Claimed output of a CO2 laser being developed by Northrop Grumman for military

(weapon) applications Examples of pulsed systems with high peak power:
• •

700 TW (700×1012 W) – National Ignition Facility, a 192-beam, 1.8-megajoule laser system adjoining a 10-meter-diameter target chamber.[36] 1.3 PW (1.3×1015 W) – world's most powerful laser as of 1998, located at the Lawrence Livermore Laboratory[37]

Hobby uses
In recent years, some hobbyists have taken interests in lasers. Lasers used by hobbyists are generally of class IIIa or IIIb, although some have made their own class IV types.[38] However, compared to other hobbyists, laser hobbyists are far less common, due to the cost and potential dangers involved. Due to the cost of lasers, some hobbyists use inexpensive means to obtain lasers, such as salvaging laser diodes from broken DVD players (red), Blu-ray players (violet), or even higher power laser diodes from CD or DVD burners.[39] Hobbyists also have been taking surplus pulsed lasers from retired military applications and modifying them for pulsed holography. Pulsed Ruby and pulsed YAG lasers have been used.

Safety

Warning symbol for lasers Main article: Laser safety Even the first laser was recognized as being potentially dangerous. Theodore Maiman characterized the first laser as having a power of one "Gillette" as it could burn through one Gillette razor blade. Today, it is accepted that even low-power lasers with only a few milliwatts of output power can be hazardous to human eyesight, when the beam from such a laser hits the eye directly or after reflection from a shiny surface. At wavelengths which the cornea and the lens can focus well, the coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localized burning and permanent damage in seconds or even less time. Lasers are usually labeled with a safety class number, which identifies how dangerous the laser is: • Class I/1 is inherently safe, usually because the light is contained in an enclosure, for example in CD players.



Class II/2 is safe during normal use; the blink reflex of the eye will prevent damage. Usually up to 1 mW power, for example laser pointers. Class IIIa/3R lasers are usually up to 5 mW and involve a small risk of eye damage within the time of the blink reflex. Staring into such a beam for several seconds is likely to cause damage to a spot on the retina. Class IIIb/3B can cause immediate eye damage upon exposure. Class IV/4 lasers can burn skin, and in some cases, even scattered light can cause eye and/or skin damage. Many industrial and scientific lasers are in this class.

• • •

The indicated powers are for visible-light, continuous-wave lasers. For pulsed lasers and invisible wavelengths, other power limits apply. People working with class 3B and class 4 lasers can protect their eyes with safety goggles which are designed to absorb light of a particular wavelength. Certain infrared lasers with wavelengths beyond about 1.4 micrometres are often referred to as being "eye-safe". This is because the intrinsic molecular vibrations of water molecules very strongly absorb light in this part of the spectrum, and thus a laser beam at these wavelengths is attenuated so completely as it passes through the eye's cornea that no light remains to be focused by the lens onto the retina. The label "eye-safe" can be misleading, however, as it only applies to relatively low power continuous wave beams; any high power or Q-switched laser at these wavelengths can burn the cornea, causing severe eye damage.

As weapons
Laser beams are famously employed as weapon systems in science fiction, but actual laser weapons are still in the experimental stage. The general idea of laser-beam weaponry is to hit a target with a train of brief pulses of light. The rapid evaporation and expansion of the surface causes shockwaves[citation needed] that damage the target. The power needed to project a highpowered laser beam of this kind is beyond the limit of current mobile power technology thus favoring chemically powered gas dynamic lasers. Lasers of all but the lowest powers can potentially be used as incapacitating weapons, through their ability to produce temporary or permanent vision loss in varying degrees when aimed at the eyes. The degree, character, and duration of vision impairment caused by eye exposure to laser light varies with the power of the laser, the wavelength(s), the collimation of the beam, the exact orientation of the beam, and the duration of exposure. Lasers of even a fraction of a watt in power can produce immediate, permanent vision loss under certain conditions, making such lasers potential non-lethal but incapacitating weapons. The extreme handicap that laser-induced blindness represents makes the use of lasers even as non-lethal weapons morally controversial, and weapons designed to cause blindness have been banned by the Protocol on Blinding Laser Weapons. The U.S. Air Force is currently working on the Boeing YAL-1 airborne laser, mounted in a Boeing 747, to shoot down enemy ballistic missiles over enemy territory. In the field of aviation, the hazards of exposure to ground-based lasers deliberately aimed at pilots have grown to the extent that aviation authorities have special procedures to deal with such hazards.[40] On March 18, 2009 Northrop Grumman claimed that its engineers in Redondo Beach had successfully built and tested an electrically powered solid state laser capable of producing a 100kilowatt beam, powerful enough to destroy an airplane or a tank. According to Brian Strickland, manager for the United States Army's Joint High Power Solid State Laser program, an

electrically powered laser is capable of being mounted in an aircraft, ship, or other vehicle because it requires much less space for its supporting equipment than a chemical laser.[41] However the source of such a large electrical power in a mobile application remains unclear.

Fictional predictions
See also: Raygun Several novelists described devices similar to lasers, prior to the discovery of stimulated emission:
• •

A laser-like device was described in Alexey Tolstoy's science fiction novel The Hyperboloid of Engineer Garin in 1927. Mikhail Bulgakov exaggerated the biological effect (laser bio stimulation) of intensive red light in his science fiction novel Fatal Eggs (1925), without any reasonable description of the source of this red light. (In that novel, the red light first appears occasionally from the illuminating system of an advanced microscope; then the protagonist Prof. Persikov arranges the special set-up for generation of the red light.)

Stethoscope
From Wikipedia, the free encyclopedia

Modern stethoscope. The stethoscope (from Greek στηθοσκόπιο, from στήθος, stéthos - chest and σκοπή, skopé examination) is an acoustic medical device for auscultation, or listening to the internal sounds of an animal body. It is often used to listen to lung and heart sounds. It is also used to listen to intestines and blood flow in arteries and veins. In combination with a sphygmomanometer, it is commonly used for measurements of blood pressure. Less commonly, "mechanic's stethoscopes" are used to listen to internal sounds made by machines, such as diagnosing a malfunctioning automobile engine by listening to the sounds of its internal parts. Stethoscopes can also be used to check scientific vacuum chambers for leaks, and for various other small-scale acoustic monitoring tasks. A stethoscope that intensifies auscultatory sounds is called phonendoscope. •

[edit] History

Early stethoscopes. The stethoscope was invented in France in 1816 by René Laennec at the Necker-Enfants Malades Hospital in Paris.[1] It consisted of a wooden tube and was monaural. His device was similar to the common ear trumpet, a historical form of hearing aid; indeed, his invention was almost indistinguishable in structure and function from the trumpet, which was commonly called a "microphone". The first flexible stethoscope of any sort may have been a binaural instrument with articulated joints not very clearly described in 1829.[2] In 1840, Golding Bird described a stethoscope he had been using with a flexible tube. Bird was the first to publish a description of such a stethoscope but he noted in his paper the prior existence of an earlier design (which he thought was of little utility) which he described as the snake ear trumpet. Bird's stethoscope had a single earpiece.[3] In 1851, Irish physician Arthur Leared invented a binaural stethoscope, and in 1852 George Cammann perfected the design of the instrument for commercial production, which has become the standard ever since. Cammann also wrote a major treatise on diagnosis by auscultation, which the refined binaural stethoscope made possible. By 1873, there were descriptions of a differential stethoscope that could connect to slightly different locations to create a slight stereo effect, though this did not become a standard tool in clinical practice. Rappaport and Sprague designed a new stethoscope in the 1940s, which became the standard by which other stethoscopes are measured, consisting of two sides, one of which is used for the respiratory system, the other for the cardiovascular system. The Rappaport-Sprague was later made by Hewlett-Packard. HP's medical products division was spun off as part of Agilent Technologies, Inc., where it became Agilent Healthcare. Agilent Healthcare was purchased by Philips which became Philips Medical Systems, before the walnut-boxed, $300, original Rappaport-Sprague stethoscope was finally abandoned ca. 2004, along with Philips' brand (manufactured by Andromed, of Montreal, Canada) electronic stethoscope model. The Rappaport-Sprague model stethoscope was heavy and short (18–24 in (46–61 cm)) with an

antiquated appearance recognizable by their two large independent latex rubber tubes connecting an exposed-leaf-spring-joined-pair of opposing "f"-shaped chrome-plated brass binaural ear tubes with a dual-head chest piece.

Early flexible tube stethoscopes. Golding Bird's instrument is on the left. The instrument on the right was current around 1855 when this image was first published. Several other minor refinements were made to stethoscopes, until in the early 1960s Dr. David Littmann, a Harvard Medical School professor, created a new stethoscope that was lighter than previous models and had improved acoustics.[4] In the late 1970s, 3M-Littmann introduced the tunable diaphragm: a very hard (G-10) glass-epoxy resin diaphragm member with an overmolded silicone flexible acoustic surround which permitted increased excursion of the diaphragm member in a "z"-axis with respect to the plane of the sound collecting area. The left shift to a lower resonant frequency increases the volume of some low frequency sounds due to the longer waves propagated by the increased excursion of the hard diaphragm member suspended in the concentric accountic surround. Conversely, restricting excursion of the diaphragm by pressing the stethoscope diaphragm surface firmly against the anatomical area overlying the physiological sounds of interest, the acoustic surround could also be used to dampen excursion of the diaphragm in response to "z"-axis pressure against a concentric fret. This raises the frequency bias by shortening the wavelength to auscultate a higher range of physiological sounds. 3-M Littmann is also credited with a collapsible mold frame for sludge molding a single column bifurcating stethoscope tube[5][dead link] with an internal septum dividing the single column stethoscope tube into discrete left and right binaural channels (AKA "cardiology tubing"; including a covered, or internal leaf spring-binaural ear tube connector). In 1999, Richard Deslauriers patented the first external noise reducing stethoscope, the DRG Puretone. It featured two parallel lumens containing two steel coils which dissipated infiltrating noise as inaudible heat energy. The steel coil "insulation" added .30 lb to each stethoscope. In 2005, DRG's diagnostics division was acquired by TRIMLINE Medical Products.[6]

[edit] Current practice

A German physician listens to a patient's lower lung regions with a stethoscope applied to his back. Stethoscopes are often considered as a symbol of the doctor's profession, as doctors are often seen or depicted with stethoscopes hanging around their necks.

[edit] Types of stethoscopes
[edit] Acoustic

Acoustic stethoscope, with the bell upwards. Acoustic stethoscopes are familiar to most people, and operate on the transmission of sound from the chest piece, via air-filled hollow tubes, to the listener's ears. The chestpiece usually consists of two sides that can be placed against the patient for sensing sound; a diaphragm (plastic disc) or bell (hollow cup). If the diaphragm is placed on the patient, body sounds vibrate the diaphragm, creating acoustic pressure waves which travel up the tubing to the listener's ears. If the bell is placed on the patient, the vibrations of the skin directly produce acoustic pressure waves traveling up to the listener's ears. The bell transmits low frequency sounds, while the diaphragm transmits higher frequency sounds. This two-sided stethoscope was invented by Rappaport and Sprague in the early part of the 20th century. One problem with acoustic stethoscopes was that the sound level is extremely low. This problem was surmounted in 1999 with the invention of the stratified continuous (inner) lumen, and the kinetic acoustic mechanism in 2002. Acoustic stethoscopes are the most commonly used. A recent independent review

evaluated twelve common acoustic stethoscopes on the basis of loudness, clarity, and ergonomics. They did acoustic laboratory testing and recorded heart sounds on volunteers. The results are listed by brand and model.[7]

[edit] Electronic
An electronic stethoscope (or stethophone) overcomes the low sound levels by electronically amplifying body sounds. However, amplification of stethoscope contact artifacts, and component cutoffs (frequency response thresholds of electronic stethoscope microphones, pre-amps, amps, and speakers) limit electronically amplified stethoscopes' overall utility by amplifying mid-range sounds, while simultaneously attenuating high- and low- frequency range sounds. Currently, a number of companies offer electronic stethoscopes. Electronic stethoscopes require conversion of acoustic sound waves to electrical signals which can then be amplified and processed for optimal listening. Unlike acoustic stethoscopes, which are all based on the same physics, transducers in electronic stethoscopes vary widely. The simplest and least effective method of sound detection is achieved by placing a microphone in the chestpiece. This method suffers from ambient noise interference and has fallen out of favor. Another method, used in Welch-Allyn's Meditron stethoscope, comprises placement of a piezoelectric crystal at the head of a metal shaft, the bottom of the shaft making contact with a diaphragm. 3M also uses a piezo-electric crystal placed within foam behind a thick rubber-like diaphragm. Thinklabs' Rhythm 32 inventor, Clive Smith uses an Electromagnetic Diaphragm with a conductive inner surface to form a capacitive sensor. This diaphragm responds to sound waves identically to a conventional acoustic stethoscope, with changes in an electric field replacing changes in air pressure. This preserves the sound of an acoustic stethoscope with the benefits of amplification. Because the sounds are transmitted electronically, an electronic stethoscope can be a wireless device, can be a recording device, and can provide noise reduction, signal enhancement, and both visual and audio output. Around 2001, Stethographics introduced PC-based software which enabled a phonocardiograph, graphic representation of cardiologic and pulmonologic sounds to be generated, and interpreted according to related algorithms. All of these features are helpful for purposes of telemedicine (remote diagnosis) and teaching. Electronic stethoscopes are also used with Computer-aided Auscultation programs to analyze the recorded heart sounds pathological or innocent heart murmurs. [edit] Recording stethoscopes Some electronic stethoscopes feature direct audio output that can be used with an external recording device, such as a laptop or MP3 recorder. The same connection can be used to listen to the previously-recorded auscultation through the stethoscope headphones, allowing for more detailed study for general research as well as evaluation and consultation regarding a particular patient's condition and telemedicine, or remote diagnosis.

[edit] Fetal stethoscope
A fetal stethoscope or fetoscope is an acoustic stethoscope shaped like a listening trumpet. It is placed against the abdomen of a pregnant woman to listen to the heart sounds of the fetus. The fetal stethoscope is also known as a Pinard's stethoscope or a pinard, after French obstetrician Adolphe Pinard (1844–1934).

[edit] Stethoscope earpieces
Stethoscopes usually have rubber earpieces which aid comfort and create a seal with the ear improving the acoustic function of the device. Stethoscopes can be modified by replacing the

standard earpieces with moulded versions which improve comfort and transmission of sound. Moulded earpieces can be cast by an audiologist or made by the stethoscope user from a kit.

[edit] Maintenance
The flexible vinyl, rubber, and plastic parts of stethoscopes should be kept away from solvents, including alcohol and soap. Solvents can have detrimental effects, including accelerating the natural aging process by dissolving the plasticizers that keep these parts flexible and looking new. In addition, when they are manufactured stethoscopes with two-sided chestpieces are lubricated where the chestpiece rotates around the stem and need to be re-lubricated periodically, just like any other machine. If these moving parts are not lubricated, they grind together and ruin the fine tolerances required for the proper acoustic performance of the stethoscope. Cleaning the stethoscope will also remove lubricants, making periodic lubrication essential. Most lubricants must be kept away from rubber, vinyl, and plastic parts. Only products that have been tested to be safe and effective for cleaning stethoscopes and similar medical instruments should be used.

Balance board
From Wikipedia, the free encyclopedia For the Wii video game accessory, see Wii Balance Board. •

A balance board is a device used for recreation, balance training, athletic training, brain development, therapy, musical training and other kinds of personal development. It is a lever similar to a see-saw that the user usually stands on, usually with the left and right foot at opposite ends of the board. The user's body must stay balanced enough to keep the board's edges from touching the ground and to keep from falling off the board. A different challenge is presented by each of the four basic types of balance boards and their subtypes. Some of them can be attempted successfully by three-year-olds and elderly people, and some, because of their steepness and speed, are difficult and dangerous for professional athletes. In their design, what differentiates the four types (and their subtypes) is how unstable each of them is, i.e., in how many and in which of the three dimensions of space each board turns and/or sways and how freely its fulcrum contacts the board and the ground.



[edit] Uses and users

Originally produced for skiers and then surfers to practice their skills in the off season and at night, a balance board is a device that has come to be used for training in all sports and martial arts, for physical fitness and for non-athletic purposes that are listed here. It is used to develop balance, motor coordination skills, weight distribution and core strength; to prepare people, before and after [1] they reach old age, to avoid injurious falls; to prevent sports injuries,[2] especially to the ankle [3] and knee;[4][5] and for rehabilitation after injuries to several parts [6] of the body. Uses of a balance board that are distant from the athletic purpose of its origin have gradually become more common: to expand neural networks that enable the left and right hemispheres of the brain [7] to communicate with each other, thereby increasing its efficiency; to develop sensory integration and cognitive skills in children with developmental disorders; to make dancers lighter on their feet; to teach singers optimal posture for the control of air-flow;[8][9] to teach musicians how to hold their instrument;[10] to shake off writer's block and other inhibitors of creativity; as an accessory to yoga and as a form of yoga, cultivating holistic health, self-awareness and calm.

Users who may not be interested in any of those practical purposes use a balance board to entertain themselves; it is a game of thrills that is somewhat frightening because of the almost constant sensation of being at risk of falling in the next moment if one does not adjust carefully enough before then. Skillful and dramatic balancing acts are presented by circus performers who stack multiple boards on top of one another to increase the challenge, also combining this with other circus arts to create a more exciting spectacle.

[edit] Structure

The user/rider/player stands on a board or other platform which is on top of an unstable groundcontacting member, the fulcrum. The height of the fulcrum of most models is between 3 and 6 inches (i.e., the top of the fulcrum is that distance above the ground). Due to the fulcrum's instability, all of a user's balance skills must be activated and coordinated in order to prevent the board from touching the ground.

The roller and underside of a rocker-roller board Thus, the rider stimulates, exercises and teaches the parts of the body that implement the act of balancing (toes, soles, ankles, knees, hips, shoulders, arms and neck) and the parts of the body and brain that create the sense of balance and that engineer the implementation of the act of balancing (inner ears, cerebellum, proprioceptors [nerve receptors in muscles, tendons and ligaments around joints that let a person visualize a bone's changing location] and eyes).

Video of a wobble board's instability The degrees of movement through which the board can move – sliding, pivoting, rotating, tilting, rolling or some combination of those – and the speed of the board differ in different types and subtypes of models, depending on the shape and size of the fulcrum, whether it is attached to the board and, if it isn't attached, the method(s) by which it is constrained by the board, if any. With an increase of speed and with each additional degree of movement through which one model or another can move, the need to avoid losing control of the board (to avoid landing or falling) forces the rider to exercise considerably more skill.

The underside of a sphere-and-ring board In rocker boards and wobble boards, the fulcrum is attached to the board. In rocker-roller boards and sphere-and-ring boards, the fulcrum is a separate piece. In sphere-and-ring boards, the fulcrum (an inflatable or solid ball) is constrained by a ring on the board's underside. In some rocker-roller boards, the fulcrum (a cylinder or mainly a cylinder) isn't constrained by the board (except by their friction), and in most rocker-rollers the cylinder is constrained by the board in any of five ways (a different number and combination of those ways in each type of rockerroller) that are described below, in this article's "Rocker-Roller Boards" section. Positions other than standing are also used, in order to work on particular muscles and skills. See below, under the heading "Fitness Exercises." For better foot traction, the stood-on surface of most models is manufactured with an unsmooth texture: for plastic models, in the molding; for wooden models, with grip tape or rubber. A smooth surface under the feet or shoes can cause a user to slip off a balance board and fall. Wobble boards are the only type of balance board that is commonly made of plastic. Being no longer than their width, they don't need to be as strong and warp-resistant as other balance boards.

[edit] Types
[edit] Classifying balance boards into four basic types
There are more than a hundred models of balance boards on the market in the United States. Each of them is a version of one of about fifteen types of balance board. Each of these models and types can be classified as one of four basic types of balance board according to two binary parameters: whether its fulcrum is attached to the board and whether the board tilts in only two opposite directions (left and right or forward and back) or in every direction (360 degrees). More specifically: bipolar 360 degrees the task attached rocker wobble static balance not attached rocker-roller sphere-and-ring dynamic balance In other words: • • • • A rocker-roller board is a rocker board whose fulcrum is a separate piece. A sphere-and-ring board is a wobble board whose fulcrum is a separate piece. A wobble board is a rocker board that tilts toward 360 degrees. A sphere-and-ring board is a rocker-roller board that tilts toward 360 degrees.

Those four analogies are not precise definitions. They ignore some details of models' structure. [edit] Rocker boards A rocker board is the most basic and least challenging type of balance board. It is a flat board with a fulcrum attached to the board's underside. In some models the fulcrum is perpendicular to the board's length and in the other models the fulcrum is two rockers that are parallel to each other and parallel to the board's length, one in front of the person who is standing on the board and one behind. The ground-contacting edge of the fulcrum is curved in most models and is flat in some models.

With one foot placed at each end of the board, the user can tilt it from side to side until the balance point is found and can then either try to keep the board stationary or continue rocking. Rocker boards offer only one degree of movement: part rotation about the longitudinal axis, i.e., banking (left-right tilting). Most rocker boards are made by manufacturers of toys or of gym equipment. [edit] Rocker-roller boards

The Bongo Board in a newsreel from the 1950's Rocker-roller boards add a degree of instability to the rocker board that makes them much more challenging for the rider than a rocker board is. Rather than on a fixed pivot, a rocker-roller's board is placed on a cylindrical roller; this fulcrum is a wheel that moves in relation to the ground and in relation to the board. The board's pivot point shifts back and forth as the cylinder rolls beneath it. In almost all models the two flat ends of the wheel/roller are about as far from each other as the width of the board. In most models the axis of the roller is perpendicular to the board's length. Thus, as the rider's weight moves over the roller, the board both tilts from side to side and also slides sideways. In models whose roller can be placed with its axis parallel to the board's length, the board slides and tilts toward the front and back (if the rider's feet are oriented accordingly).

The rollers of seven rocker-roller boards The roller has a different form in different models. Some are a cylinder and some are a cylinder in their midsection that tapers toward the two ends. That tapering enables tricky moves by the rider. Four of the models produced by Vew-Do Balance Boards each have a roller whose shape has a different taper, designed for doing different tricks and simulating different boardsports. (One of those four is the roller at the top of the photograph to the side of this paragraph.) VewDo's patent gives information about the effect that its roller's shape has on the board's movement. How the roller and rocker interface can vary. Rollers may have grooves to fit a guide on the board to keep the roller aligned with the rocker and prevent the rocker from sliding along the roller. Rockers may have guard rails at the ends to prevent the rocker from rolling off the roller. The diameter of the roller of almost all rocker-rollers is between 3.5 and 6 inches at its widest section. The rollers of the rola bolas that circus performers use are usually between 7 and 9 inches in diameter. [edit] Wobble boards

Video of an oversized wobble board whose fulcrum, unlike the fulcrum of a standard wobble board, is connected to a stationary base Two videos of a standard wobble board are in this article, one in the "Uses and Users" section and one in the "Structure" section. The fulcrum of almost all wobble boards is a semi-sphere or smaller spherical cap (or a shape that is approximately such) whose flat side is attached to the center of the board's underside. This allows the board to pivot in all directions during the same ride: forward-backward, left-right and anywhere in between, i.e., toward 360 degrees. Standing on a wobble board exercises muscles that are not exercised by standing on boards that tilt in only two (opposite) directions. In almost all models, the board's length and width are about the same size; a circle is the usual shape. Wobble boards are widely used in child development, gymnasiums, sport training, prevention of injuries to the ankle and knee, rehabilitation after ankle, knee and hip injuries and physiotherapy. The basic exercise is standing on the wobble board with both feet and tilting in any direction without letting the board tilt so far that its edge touches the ground. Some of the many other common exercises are squats; standing on the board with one foot while keeping the other foot off the ground; push-ups (pressing down on the board with the hands while lying face-down with only the knees or toes contacting the ground); and sit-ups (with the board under one's rear end).

Any exercise is much more work when a person's weight is on a wobble board than when s/he is supported by a stable and level base such as a floor. For additional muscle exercise while wobbling, some models can be attached to an elastic stretch band. Each hand pulls up one of the band's two ends. Each end of the band fits through one of two opposed holes near the rim of the board, for quick attachment and detachment. A wobble board offers full rotation about the vertical axis (i.e., yawing, i.e., twisting), part rotation about the transverse/lateral axis (i.e., pitching, i.e., backward-forward tilting) and part rotation about the longitudinal axis (i.e., banking, i.e., left-right tilting). A fourth and fifth degree of movement, translation (i.e., sliding) in the dimensions of the two horizontal axes, are possible for almost all wobble boards, i.e., all models except ones that have a stationary base. This sliding occurs much less often and usually across a shorter distance than in the case of a rocker-roller board and sphere-and-ring board. It is usually unintended and unwanted by a wobble board user and, presumably, also by the manufacturer and might more accurately be called skidding than sliding. Plastic is the material that most wobble boards are made of. Wooden models are better able than plastic ones to withstand long use, such as in a gym. Among plastic models, how heavy-duty the plastic is varies. Wobble boards are made by manufacturers of gym, sports and physical therapy equipment. [edit] Sphere-and-ring boards

The fulcrum is an inflatable rubber ball. A sphere, either an inflatable rubber ball (such as a basketball) or a solid polyurethane ball, is the fulcrum on which the board is balanced, and the fulcrum is kept contained under the board by a guard rail or ring on the underside. By redistributing his/her weight across the board, the rider can move the board in any direction– side to side, forward and backward, twisting, diagonal, and full rotations or any combination of these movements. A rider can move the board vertically by doing an advanced maneuver called an ollie. It can also be tilted in any direction and fully rotated. Sphere-and-ring boards provide the greatest freedom of movement of any type of balance board, allowing rotation about all axes (yawing, pitching and banking) and translation

(i.e., sliding) in both transverse (i.e., lateral) and longitudinal directions. They, like wobble boards, simultaneously exercise muscles that are not exercised by use of boards that tilt about only one axis (in two opposite directions).

The fulcrum is a solid urethane ball. When balancing or riding on a sphere-and-ring board, the difficulty and ride speed, which is how fast the rider can move the board on the ball, are determined by the following: Ring shape and size Larger rings allow more movement of the fulcrum. Different shapes change how the fulcrum might move. Sphere's size, weight, and rigidity These affect how fast the fulcrum moves and how much strength is required to move it. Board's shape and size Changing these can affect the weight and weight distribution of the board, changing how much strength is required to move it.

[edit] Aquatic balance boards
These are underwater balance boards. They were developed for physical therapy and are used also for recreation. Besides the general advantages of aquatic therapy over non-aquatic therapy (the use of the smooth resistance of water instead of the jerky resistance of weights and the avoidance of burdening an injured joint with excessive weight– in this case, the weight of the patient's own body), aquatic balance boards have the specific advantage over non-aquatic balance boards of saving a patient who slips off of a board from the impact of falling and crashing into a floor. Slipping off of an aquatic balance board is safe as long as the user knows to avoid inhaling while underwater and knows how to tread water. Three models are produced by Theraquatics Australia: The Theraquatics Balance Board is a Vshaped rocker board that a user stands, kneels or sits on. The Wonder Board is a V-shaped rocker board that a user kneels or sits on. The Aquatic Balance Board (a.k.a. the Aquafit Balance Board) is a wobble board. The holes in it allow water to fill it, making it neutrally buoyant (i.e., neither sinking on its own nor floating up to the water's surface) so that it is easier to control and safer than it would be if this wobble board were more buoyant. Two products that Theraquatics Australia calls balance boards, its Star Balance Board and its Theraquatics Balance Board with Straps, are not balance boards in the familiar sense of the term, though they can be used for practicing balance skills.

Psychological aspects
Proper use of a balance board is a test of both physical skill and the user's sense of balance. Another sensation often experienced by a user of a balance board is the sensation of falling. Apart from actually falling, this often occurs during sharp accelerations caused by leaning too far or too quickly. Feeling like falling can raise fears and provoke reflexes that while useful on a stable surface can be counter-productive on a balance board, such as throwing the arms forward to catch the fall or over-correcting and sharply shifting weight onto the opposite leg, causing a fall in the opposite direction.

Injury risk and prevention
Balance boards can break bones, sprain joints, and tear tendons, ligaments and cartilage. These risks can be diminished by preparing the space, wearing protective gear and following manufacturers' other safety recommendations. Risk can be lowered by anticipating falls and clearing the surrounding area of objects that the rider might fall onto, and making sure that the surface is soft. Important protective gear is gear that protects the joints, the head and face, and otherwise protects from bumps and scrapes during falls. Care should be taken in selecting a helmet, as the weight could make falls worse or the shape might be unsuited for protecting from falls and might be pressed into the neck during impact. Standing on a balance board is extremely dangerous for a person who is prone to dizziness or whose balance is impaired, such as by being tired or wikt:under the influence of alcohol or other drug.

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