Optical Receivers (Ground Based Astronomy)
At optical wavelengths it is convenient to think of the radiation as consisting of a stream of photons. All optical detectors function by transferring some or all of the energy of an incident photon to one or more electrons. The excited electrons arc then detected in a wide variety of ways.
A number of basic parameters characterize all detectors. The first is the sensitivity of the device as a function of wavelength (termed the SPECTRAL RESPONSE of the device). Associated with this is a measurement of the QUANTUM EFFICIENCY of the device. No device is totally efficient. Some photons may be reflected away from and others may pass through the device without absorption. In some devices the excited electrons may lose their energy before they are detected, while in others the detection procedure may itself be inefficient. Effects such as these reduce the detector efficiency; the quantum efficiency specifies the relative effectiveness of the detector.
All detectors suffer, to some extent, from what is variously known as FOG LEVEL (in photographs) or DARK CURRENT (with photo-cathode). This appears as a source of signal even when the detector is completely shielded from all external radiation. To understand this we must remember that the detector is relatively warm (it will normally be at room temperature) and this means that electrons in the detector will have an average energy appropriate to that temperature. When a photon is absorbed, an electron acquires extra energy from the photon. Sometimes, however, two electrons in the material collide so as to give much of their combined energy to one electron which will also be detected in the same way as an electron excited by only one photon. If a detector has appreciable infrared sensitivity, relatively small amounts of photon energy are needed to excite an electron. Therefore it is much more likely that excited electrons will be produced by the random motions within the sub-stance of the detector. Consequently, infrared sensitivity and dark current go hand in hand. The dark current may be reduced by cooling the detector. This happens because the random energy of electrons in the detector material is reduced when the temperature is reduced. This in turn reduces the probability of an excited electron being generated within the detector itself, so reducing the dark current.