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This article may require cleanup to meet Wikipedia's quality standards.
Please improve this article if you can. (September 2006)
Not to be confused with censure, censer, or censor.
"Detector" redirects here. For the radio electronics component, see Detector (radio).
"Detector" redirects here. For detector in particle physics, see Particle detector.
A sensor is a device which measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, all sensors need to be calibrated against known standards.
Sensors are used in everyday objects such as touch-sensitive elevator buttons and lamps which dim or brighten by touching the base. There are also innumerable applications for sensors of which most people are never aware. Applications include automobiles, machines, aerospace, medicine, industry, and robotics.
A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1cm when the temperature changes by 1°, the sensitivity is 1cm/1°. Sensors that measure very small changes must have very high sensitivities.
Technological progress allows more and more sensors to be manufactured on a microscopic scale as microsensors using MEMS technology. In most cases, a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches. See also MEMS sensor generations.
Contents [hide]
1 Types
1.1 Thermal
1.2 Electromagnetic
1.3 Mechanical
1.4 Chemical
1.5 Optical radiation
1.6 Ionising radiation
1.7 Acoustic
1.8 Other types
1.8.1 Non Initialized systems
1.8.2 Initialized systems
2 Classification of measurement errors
2.1 Resolution
3 Biological sensors
4 Geodetic sensors
5 See also
6 External links

Because sensors are a type of transducer, they change one form of energy into another. For this reason, sensors can be classified according to the type of energy transfer that they detect.
temperature sensors: thermometers, thermocouples, temperature sensitive resistors (thermistors and resistance temperature detectors), bi-metal thermometers and thermostats
heat sensors: bolometer, calorimeter, heat flux sensor
electrical resistance sensors: ohmmeter, multimeter
electrical current sensors: galvanometer, ammeter
electrical voltage sensors: leaf electroscope, voltmeter
electrical power sensors: watt-hour meters
magnetism sensors: magnetic compass, fluxgate compass, magnetometer, Hall effect device
metal detectors
pressure sensors: altimeter, barometer, barograph, pressure gauge, air speed indicator, rate-of-climb indicator, variometer
gas and liquid flow sensors: flow sensor, anemometer, flow meter, gas meter, water meter, mass flow sensor
gas and liquid viscosity and density: viscometer, hydrometer, oscillating U-tube
mechanical sensors: acceleration sensor, position sensor, selsyn, switch, strain gauge
humidity sensors: hygrometer
Chemical proportion sensors: oxygen sensors, ion-selective electrodes, pH glass electrodes, redox electrodes, and carbon monoxide detectors.
[edit]Optical radiation
light time-of-flight. Used in modern surveying equipment, a short pulse of light is emitted and returned by a retroreflector. The return time of the pulse is proportional to the distance and is related to atmospheric density in a predictable way - see LIDAR.
light sensors, or photodetectors, including semiconductor devices such as photocells, photodiodes, phototransistors, CCDs, and Image sensors; vacuum tube devices like photo-electric tubes, photomultiplier tubes; and mechanical instruments such as the Nichols radiometer.
infra-red sensor, especially used as occupancy sensor for lighting and environmental controls.
proximity sensor- A type of distance sensor but less sophisticated. Only detects a specific proximity. May be optical - combination of a photocell and LED or laser. Applications in cell phones, paper detector in photocopiers, auto power standby/shutdown mode in notebooks and other devices. May employ a magnet and a Hall effect device.
scanning laser- A narrow beam of laser light is scanned over the scene by a mirror. A photocell sensor located at an offset responds when the beam is reflected from an object to the sensor, whence the distance is calculated by triangulation.
focus. A large aperture lens may be focused by a servo system. The distance to an in-focus scene element may be determined by the lens setting.
binocular. Two images gathered on a known baseline are brought into coincidence by a system of mirrors and prisms. The adjustment is used to determine distance. Used in some cameras (called range-finder cameras) and on a larger scale in early battleship range-finders
interferometry. Interference fringes between transmitted and reflected lightwaves produced by a coherent source such as a laser are counted and the distance is calculated. Capable of extremely high precision.
scintillometers measure atmospheric optical disturbances.
fiber optic sensors.
short path optical interception - detection device consists of a light-emitting diode illuminating a phototransistor, with the end position of a mechanical device detected by a moving flag intercepting the optical path, useful for determining an initial position for mechanisms driven by stepper motors.
[edit]Ionising radiation
radiation sensors: Geiger counter, dosimeter, Scintillation counter, Neutron detection
subatomic particle sensors: Particle detector, scintillator, Wire chamber, cloud chamber, bubble chamber. See Category:Particle detectors
acoustic : uses ultrasound time-of-flight echo return. Used in mid 20th century polaroid cameras and applied also to robotics. Even older systems like Fathometers (and fish finders) and other 'Tactical Active' Sonar (Sound Navigation And Ranging) systems in naval applications which mostly use audible sound frequencies.
sound sensors : microphones, hydrophones, seismometers.
[edit]Other types
motion sensors: radar gun, speedometer, tachometer, odometer, occupancy sensor, turn coordinator
orientation sensors: gyroscope, artificial horizon, ring laser gyroscope
distance sensor (noncontacting) Several technologies can be applied to sense distance: magnetostriction
[edit]Non Initialized systems
Gray code strip or wheel- a number of photodetectors can sense a pattern, creating a binary number. The gray code is a mutated pattern that ensures that only one bit of information changes with each measured step, thus avoiding ambiguities.
[edit]Initialized systems
These require starting from a known distance and accumulate incremental changes in measurements.
Quadrature wheel- A disk-shaped optical mask is driven by a gear train. Two photocells detecting light passing through the mask can determine a partial revolution of the mask and the direction of that rotation.
whisker sensor- A type of touch sensor and proximity sensor.
[edit]Classification of measurement errors

A good sensor obeys the following rules:
the sensor should be sensitive to the measured property
the sensor should be insensitive to any other property
the sensor should not influence the measured property
Ideal sensors are designed to be linear. The output signal of such a sensor is linearly proportional to the value of the measured property. The sensitivity is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement.
If the sensor is not ideal, several types of deviations can be observed:
The sensitivity may in practice differ from the value specified. This is called a sensitivity error, but the sensor is still linear.
Since the range of the output signal is always limited, the output signal will eventually reach a minimum or maximum when the measured property exceeds the limits. The full scale range defines the maximum and minimum values of the measured property.
If the output signal is not zero when the measured property is zero, the sensor has an offset or bias. This is defined as the output of the sensor at zero input.
If the sensitivity is not constant over the range of the sensor, this is called nonlinearity. Usually this is defined by the amount the output differs from ideal behavior over the full range of the sensor, often noted as a percentage of the full range.
If the deviation is caused by a rapid change of the measured property over time, there is a dynamic error. Often, this behaviour is described with a bode plot showing sensitivity error and phase shift as function of the frequency of a periodic input signal.
If the output signal slowly changes independent of the measured property, this is defined as drift.
Long term drift usually indicates a slow degradation of sensor properties over a long period of time.
Noise is a random deviation of the signal that varies in time.
Hysteresis is an error caused by when the measured property reverses direction, but there is some finite lag in time for the sensor to respond, creating a different offset error in one direction than in the other.
If the sensor has a digital output, the output is essentially an approximation of the measured property. The approximation error is also called digitization error.
If the signal is monitored digitally, limitation of the sampling frequency also can cause a dynamic error.
The sensor may to some extent be sensitive to properties other than the property being measured. For example, most sensors are influenced by the temperature of their environment.
All these deviations can be classified as systematic errors or random errors. Systematic errors can sometimes be compensated for by means of some kind of calibration strategy. Noise is a random error that can be reduced by signal processing, such as filtering, usually at the expense of the dynamic behaviour of the sensor.
The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a digital display, the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made. For example, a scanning probe (a fine tip near a surface collects an electron tunnelling current) can resolve atoms and molecules.
[edit]Biological sensors

All living organisms contain biological sensors with functions similar to those of the mechanical devices described. Most of these are specialized cells that are sensitive to:
light, motion, temperature, magnetic fields, gravity, humidity, vibration, pressure, electrical fields, sound, and other physical aspects of the external environment;
physical aspects of the internal environment, such as stretch, motion of the organism, and position of appendages (proprioception);
an enormous array of environmental molecules, including toxins, nutrients, and pheromones;
many aspects of the internal metabolic milieu, such as glucose level, oxygen level, or osmolality;
an equally varied range of internal signal molecules, such as hormones, neurotransmitters, and cytokines;
and even the differences between proteins of the organism itself and of the environment or alien creatures.
Artificial sensors that mimic biological sensors by using a biological sensitive component, are called biosensors.
The human senses are examples of specialized neuronal sensors. See Sense.
[edit]Geodetic sensors

Geodetic measuring devices measure georeferenced displacements or movements in one, two or three dimensions. It includes the use of instruments such as total stations, levels and global navigation satellite system receivers.
[edit]See also

Car sensor: reversing sensor and rain sensor.
Data acquisition
Data acquisition system
Data logger
Detection theory
Fully Automatic Time
Hydrogen microsensor
Lateral line
List of sensors
Machine olfaction
Receiver operating characteristic
Sensor network
Sensor Web
[edit]External links

Look up Sensor in
Wiktionary, the free dictionary.
Capacitive Position/Displacement Sensor Theory/Tutorial
Capacitive Position/Displacement Overview
M. Kretschmar and S. Welsby (2005), Capacitive and Inductive Displacement Sensors, in Sensor Technology Handbook, J. Wilson editor, Newnes: Burlington, MA.
C. A. Grimes, E. C. Dickey, and M. V. Pishko (2006), Encyclopedia of Sensors (10-Volume Set), American Scientific Publishers. ISBN 1-58883-056-X
SensEdu; how sensors work
Clifford K. Ho, Alex Robinson, David R. Miller and Mary J. Davis. Overview of Sensors and Needs for Environmental Monitoring. Sensors 2005, 5, 4-37
Wireless hydrogen sensor
Sensor circuits
Categories: Measuring instruments | Sensors | Transducers
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This page was last modified on 8 April 2008, at 00:33. All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)
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Image sensor
From Wikipedia, the free encyclopedia

A CCD-sensor on a flexible circut board
An image sensor is a device that converts an optical image to an electric signal. It is used mostly in digital cameras and other imaging devices. It is a set of charge-coupled devices (CCD) or CMOS sensors such as active-pixel sensors.
There are several main types of color image sensors, differing by the means of the color separation mechanism:
Bayer sensor, low-cost and most common, using a Bayer filter that passes red, green, or blue light to selected sensels, or pixels, forming interlaced grids sensitive to red, green, and blue. The image is then interpolated using a demosaicing algorithm.
Foveon X3 sensor, using an array of layered sensors where every pixel contains three stacked sensors sensitive to the individual colors.
3CCD, using three discrete image sensors, with the color separation done by a dichroic prism. Considered the best quality, and generally more expensive than single-CCD sensors.
Contents [hide]
2 Performance
3 Specialty sensors
4 See also
5 References
[edit]CCD Vs CMOS

Today, most digital still cameras use either a CCD images sensor or a CMOS sensor. Both types of sensor accomplish the same task of capturing light and converting it into electrical signals.
A CCD is an analog device. When light strikes the chip it is held as a small electrical charge in each photo sensor. The charges are converted to voltage one pixel at a time as they are read from the chip. Additional circuitry in the camera converts the voltage into digital information.
A CMOS chip is a type of active pixel sensor made using the CMOS semiconductor process. Extra circuitry next to each photo sensor converts the light energy to a voltage. Additional circuitry on the chip converts the voltage to digital data.
Neither technology has a clear advantage in image quality. CMOS can potentially be implemented with fewer components, use less power and provide data faster than CCDs. CCD is a more mature technology and is in most respects the equal of CMOS.[1] [2]

There are many parameters that can be used to evaluate the performance of an image sensor, including its dynamic range, its signal-to-noise ratio, its low-light sensitivity, etc. For a detailed guide to digital sensor performance, see Roger Clark's article.
[edit]Specialty sensors

Special sensors are used for various applications. The most important are the sensors for thermal imaging, creation of multi-spectral images, gamma cameras, sensor arrays for x-rays, IR Rays Infrared Rays and other highly sensitive arrays for astronomy.
[edit]See also

Video camera tube
Semiconductor detector
Contact Image Sensor (CIS)
Charge-coupled device (CCD)
Active pixel sensor (MOS, CMOS)
Image sensor format: discusses the sizes and shapes of common image sensors
This photography-related article is a stub. You can help Wikipedia by expanding it.

^ [1] CCD Vs CMOS from Photonics Spectra 2001
^ [2] Sensors By Vincent Bockaert
Categories: Photography stubs | Digital photography | Image sensors
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This page was last modified on 27 March 2008, at 19:15. All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)
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