An optical chopper is a device which periodically interrupts a light beam. Three types are available: variable frequency rotating disc choppers, fixed frequency tuning fork 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.
Optical choppers, usually rotating disc mechanical shutters, are widely used in science labs in combination with lock-in amplifiers. [1] 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.
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.
Choppers were widely used in early missile guidance systems, and in this role are sometimes known as "reticle seekers". [2] The earliest uses were on air-to-air missiles. A photocell sensitive to infrared light is positioned behind a chopper driven by a synchronous motor. 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. [2]
The same basic system has been used in a number of other roles. Early ICBMs used a similar chopper system connected to a small visible-light telescope to produce a star tracker that was used to improve accuracy by measuring the angles to one or more stars once they climbed above the atmosphere. Anti-tank missiles used an infrared photocell on the launcher that tracked a flare on the missile, using the X and Y error signals to drive the missile into the line of sight of the operator's aiming telescope. [2]
These systems are no longer in use in modern weapons, as they have generally been replaced by imaging systems that provide much more information.
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Optical incremental rotary encoders are a form of choppers. These are used in many industrial machines. Some early anti-lock braking systems used rotary encoders for wheel speed sensors. Late 20th century opto-mechanical computer mice used two encoders for X-Y position measurement. Optical linear encoders also exist.
LCD televisions use millions of LCD shutters paired with red, green or blue filters to control the color of the pixels 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 with a hand-operated shutter.
Optical communication, also known as optical telecommunication, is communication at a distance using light to carry information. It can be performed visually or by using electronic devices. The earliest basic forms of optical communication date back several millennia, while the earliest electrical device created to do so was the photophone, invented in 1880.
A tachometer is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. The device usually displays the revolutions per minute (RPM) on a calibrated analogue dial, but digital displays are increasingly common.
In computing, an optical disc drive is a disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can only read from certain discs, but recent drives can both read and record, also called burners or writers. Compact discs, DVDs, and Blu-ray discs are common types of optical media which can be read and recorded by such drives.
An air-to-air missile (AAM) is a missile fired from an aircraft for the purpose of destroying another aircraft. AAMs are typically powered by one or more rocket motors, usually solid fueled but sometimes liquid fueled. Ramjet engines, as used on the Meteor, are emerging as propulsion that will enable future medium- to long-range missiles to maintain higher average speed across their engagement envelope.
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.
A Nipkow disk, 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 fundamental component in mechanical television, and thus the first televisions, through the 1920s and 1930s.
An opto-isolator is an electronic component that transfers electrical signals between two isolated circuits by using light. Opto-isolators prevent high voltages from affecting the system receiving the signal. Commercially available opto-isolators withstand input-to-output voltages up to 10 kV and voltage transients with speeds up to 25 kV/μs.
A movie projector is an opto-mechanical device for displaying motion picture film by projecting it onto a screen. Most of the optical and mechanical elements, except for the illumination and sound devices, are present in movie cameras. Modern movie projectors are specially built video projectors.
Beam-riding, also known as Line-Of-Sight Beam Riding (LOSBR), beam guidance or radar beam riding is a technique of directing a missile to its target by means of radar or a laser beam. The name refers to the way the missile flies down the guidance beam, which is aimed at the target. It is one of the simplest guidance systems and was widely used on early missile systems, however it had a number of disadvantages for long-range targeting and is now found typically only in short-range roles.
Mechanical television or mechanical scan television is an obsolete television system that relies on a mechanical scanning device, such as a rotating disk with holes in it or a rotating mirror drum, to scan the scene and generate the video signal, and a similar mechanical device at the receiver to display the picture. This contrasts with vacuum tube electronic television technology, using electron beam scanning methods, for example in cathode-ray tube (CRT) televisions. Subsequently, modern solid-state liquid-crystal displays (LCD) and LED displays are now used to create and display television pictures.
Infrared homing is a passive weapon guidance system which uses the infrared (IR) light emission from a target to track and follow it seamlessly. Missiles which use infrared seeking are often referred to as "heat-seekers" since infrared is radiated strongly by hot bodies. Many objects such as people, vehicle engines and aircraft generate and emit heat and so are especially visible in the infrared wavelengths of light compared to objects in the background.
A chart recorder is an electromechanical device that records an electrical or mechanical input trend onto a piece of paper. Chart recorders may record several inputs using different color pens and may record onto strip charts or circular charts. Chart recorders may be entirely mechanical with clockwork mechanisms, electro-mechanical with an electrical clockwork mechanism for driving the chart, or entirely electronic with no mechanical components at all.
In photography, a shutter is a device that allows light to pass for a determined period, exposing photographic film or a photosensitive digital sensor to light in order to capture a permanent image of a scene. A shutter can also be used to allow pulses of light to pass outwards, as seen in a movie projector or a signal lamp. A shutter of variable speed is used to control exposure time of the film. The shutter is constructed so that it automatically closes after a certain required time interval. The speed of the shutter is controlled either automatically by the camera based on the overall settings of the camera, manually through digital settings, or manually by a ring outside the camera on which various timings are marked.
The ANS synthesizer is a photoelectronic musical instrument created by Russian engineer Evgeny Murzin from 1937 to 1957. The technological basis of his invention was the method of graphical sound recording used in cinematography, which made it possible to obtain a visible image of a sound wave, as well as to realize the opposite goal—synthesizing a sound from an artificially drawn sound spectrogram.
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 be used to determine the orientation of the spacecraft with respect to the stars. In order to do this, the star tracker must obtain an image of the stars, measure their apparent position in the reference frame of the spacecraft, and identify the stars so their position can be compared with their known absolute position from a star catalog. A star tracker may include a processor to identify stars by comparing the pattern of observed stars with the known pattern of stars in the sky.
An infrared countermeasure (IRCM) is a device designed to protect aircraft from infrared homing missiles by confusing the missiles' infrared guidance system so that they miss their target. Heat-seeking missiles were responsible for about 80% of air losses in Operation Desert Storm. The most common method of infrared countermeasure is deploying flares, as the heat produced by the flares creates hundreds of targets for the missile.
A CD-ROM is a type of read-only memory consisting of a pre-pressed optical compact disc that contains data. Computers can read—but not write or erase—CD-ROMs. Some CDs, called enhanced CDs, hold both computer data and audio with the latter capable of being played on a CD player, while data is only usable on a computer.
This is a list of infrared topics.
This glossary of electrical and electronics engineering is a list of definitions of terms and concepts related specifically to electrical engineering and electronics engineering. For terms related to engineering in general, see Glossary of engineering.