- The detector should match the application, even if the camera doesn't match the clothes.Richard Ellis/Getty Images News/Getty Images
There are two basic purposes for a detector: either to provide an image of a scene or to make a measurement of total input. Imaging systems must have multielement detectors, or have one element that "moves around" to record a scene, while single-element detectors can measure total input power. For example, a detector at the end of the fiber optic cable that carries a telephone signal only needs to measure the total power, while a television camera that only measures one average signal for a whole scene would not be very valuable. Multielement detectors can range from 4-pixel by 4-pixel photodiode arrays to such sizes as 16,384-pixel by 16,384-pixel charge-coupled devices for scientific imaging. - The spectral range is the wavelength region over which a given detector will provide good response. Some detectors, such as bolometers --- which measure the change in temperature caused by absorbed light --- can have a very wide and flat spectral response. Others, like photodiodes, are limited to a certain range of energies, which means they are limited to a certain range of wavelengths.
- Responsivity is a measure of how much output a detector will provide, which is important to match to the illumination conditions.Ethan Miller/Getty Images News/Getty Images
Responsivity is a measure of how much output is generated for a given input. For most electronic detectors this will be given in amperes per watt, or the current generated by one watt of incident radiation. Usually, responsivity will vary over wavelength. Considering the human eye as a detector, for example, the brightness sensed for a given power of yellow light is much greater than the brightness sensed for the same power of blue light. - Dynamic range is an indication of over what range of intensity a detector provides a reasonable response. A cellphone camera, for example, will typically respond well to daylight conditions but its response for a nighttime outdoor shot will be pretty poor. Many overall sensor systems will extend the detector dynamic range by including adjustment mechanisms. For example, the dynamic range of the human retina is extended by opening or closing the iris in response to light levels.
- Every detection mechanism is subject to noise --- signals the detector puts out that have no relation to light into the system. Some significant components can be shot noise, due to randomly generated electrons in a semiconductor detector, and Johnson noise from electrons that absorb enough energy from the temperature of the detector to free themselves to be sensed. Going back to that cellphone camera, noise is why a cellphone picture taken at night will be grainy: Pixels give signals as if they detected low levels of light, but it's just noise.
- For the consumer application, nonuniformity isn't much of an issue, but it's critical for scientific applications.NASA/Getty Images News/Getty Images
Every detector varies across its active area --- some areas have higher responsivity than others. For most consumer applications, this variation is too small to worry about. For scientific applications it's a different story. Astronomical applications, for example, are often finding the most interesting information just at the edge of the detection capability. If one pixel provides a larger signal than its neighbor, the interpretation of the image can vary greatly.
next post