Optical chopper
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An optical chopper is a device which periodically interrupts a
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
beam. Three types are available: variable frequency rotating disc choppers, fixed frequency
tuning fork A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs ( ''tines'') formed from a U-shaped bar of elastic metal (usually steel). It resonates at a specific constant pitch when set vibrating by striking it ag ...
choppers, and optical shutters. A rotating disc chopper was famously used in 1849 by Hippolyte Fizeau in the first non-astronomical measurement of the speed of light.


Use in science laboratories

Optical choppers, usually rotating disc mechanical shutters, are widely used in science labs in combination with
lock-in amplifier A lock-in amplifier is a type of amplifier that can extract a signal with a known carrier wave from an extremely noisy environment. Depending on the dynamic reserve of the instrument, signals up to a million times smaller than noise components, ...
s. The chopper is used to modulate the intensity of a light beam, and a lock-in amplifier is used to improve the
signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...
. To be effective, an optical chopper should have a stable rotating speed. In cases where the 1/f noise is the main problem, one would like to select the maximum chopping frequency possible. This is limited by the motor speed and the number of slots in the rotating disc, which is, in turn, limited by the disc radius and the beam diameter.


Use in guidance systems

Choppers were widely used in early
missile guidance Missile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness. Guidance systems improve missile accuracy by improving its P ...
systems, and in this role are sometimes known as "reticle seekers". The earliest uses were on
air-to-air missile An air-to-air missile (AAM) is a missile fired from an aircraft for the purpose of destroying another aircraft (including unmanned aircraft such as cruise missiles). AAMs are typically powered by one or more rocket motors, usually solid-fuel roc ...
s. A photocell sensitive to
infrared light Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those o ...
is positioned behind a chopper driven by a
synchronous motor A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integer number of AC cycles. Sync ...
. As the chopper rotates, it periodically blocks the photocell's view of the target aircraft, creating a series of pulses of output. This signal is then smoothed to make a sinusoidal output which is then compared to the signal driving the motor. The difference in phase between these two signals reveals the angle of the target compared to a given point in the motor signal. Sampling the signal at two different points directly produces X and Y ''error signals'' that can drive the missile's flight controls. The same basic system has been used in a number of other roles. Early
ICBM An intercontinental ballistic missile (ICBM) is a ballistic missile with a range (aeronautics), range greater than , primarily designed for nuclear weapons delivery (delivering one or more Thermonuclear weapon, thermonuclear warheads). Conven ...
s used a similar chopper system connected to a small visible-light telescope to produce a
star tracker A star tracker is an optical device that measures the positions of stars using photocells or a camera. As the positions of many stars have been measured by astronomers to a high degree of accuracy, a star tracker on a satellite or spacecraft may ...
that was used to improve accuracy by measuring the angles to one or more stars once they climbed above the atmosphere.
Anti-tank missile An anti-tank guided missile (ATGM), anti-tank missile, anti-tank guided weapon (ATGW) or anti-armor guided weapon is a missile guidance, guided missile primarily designed to hit and destroy armoured fighting vehicle, heavily armored military v ...
s used an infrared photocell on the launcher that tracked a
flare A flare, also sometimes called a fusée, fusee, or bengala, bengalo in several European countries, is a type of pyrotechnic that produces a bright light or intense heat without an explosion. Flares are used for distress signaling, illuminatio ...
on the missile, using the X and Y error signals to drive the missile into the
line of sight The line of sight, also known as visual axis or sightline (also sight line), is an imaginary line between a viewer/ observer/ spectator's eye(s) and a subject of interest, or their relative direction. The subject may be any definable object taken ...
of the operator's aiming telescope. These systems are no longer in use in modern weapons, as they have generally been replaced by imaging systems that provide much more information.


Examples

Optical incremental rotary encoders are a form of choppers. These are used in many industrial machines. Some early
anti-lock braking system An anti-lock braking system (ABS) is a Automotive safety, safety anti-Skid (automobile), skid Brake, braking system used on aircraft and on land motor vehicle, vehicles, such as cars, motorcycles, trucks, and buses. ABS operates by preventing t ...
s used rotary encoders for wheel speed sensors. Late 20th century opto-mechanical computer mice used two encoders for X-Y position measurement. Optical
linear encoder A linear encoder is a sensor, transducer or readhead paired with a scale that encodes position. The sensor reads the scale in order to convert the encoded position into an analog or digital signal, which can then be decoded into position by a ...
s also exist.
LCD television A liquid-crystal-display television (LCD TV) is a television set that uses a liquid-crystal display to produce images. It is by far the most widely produced and sold type of television display. LCD TVs are thin and light, but have some disadvanta ...
s use millions of LCD shutters paired with red, green or blue filters to control the color of the
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...
s on the screen. Movie cameras use an optical shutter to record individual frames of the movie. Movie projectors use an optical shutter synchronized with the movie frames to produce the effect of apparent motion on the movie screen. Liquid crystal shutter glasses are used in conjunction with a synchronized display screen to create the illusion of a three-dimensional image. Light signals are sent at sea and at airports using a
signal lamp Signal lamp training during World War II A signal lamp (sometimes called an Aldis lamp or a Morse lamp) is a visual signaling device for optical communication by flashes of a lamp, typically using Morse code. The idea of flashing dots and dashes ...
with a hand-operated shutter.


See also

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Stroboscope A stroboscope, also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. It consists of either a rotating disk with slots or holes or a lamp such as a flashtube which produces br ...
*
Nipkow disk A Nipkow disk (sometimes Anglicized as Nipkov disk; patented in 1884), also known as scanning disk, is a mechanical, rotating, geometrically operating image scanning device, patented by Paul Gottlieb Nipkow in Berlin. This scanning disk was a f ...


References

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External links


Frequently asked questions
on optical choppers. Optical metrology