Detectors may be categorized as photon detectors or as thermal detectors. They may also be cast as elemental detectors or as arrays. Photon detectors sense the arrival of individual photons, which cause some change in the state of the detector. The photons can cause an electron to become freed from its local site so that it can participate in current flow and thereby increase the conductivity of the detector. Photoemissive detectors, in which electrons are freed of the surface, are not used in imaging spectrometers. Thermal detectors sense a change in their own temperature, and that change may be a result of the absorption of the power in the input beam. Elemental detectors provide a single output no matter what the areal distribution of the flux on the detector. An array is a collection of individual detector elements.
Single-element (elemental) detectors are described by their responsivity, noise, quantum efficiency, and in many cases their specific detectivity. The quantum efficiency of a photon detector is usually written as Î· and is simply the number of electrons generated for each photon. The number is never greater than one. The responsivity is the output electrical signal, often specified as a voltage, divided by the input power. So this is specified as volts per watt or volts per watt per micrometer or some equivalent unit. The quantum efficiency for most intrinsic photon detectors is almost constant over the band in which it has any sensitivity, although there is a sharp rolloff at the long-wavelength end of the response. The responsivity, on the other hand, because it is defined in terms of the power input, is almost a linear function of wavelength, since the detector responds to photons and photons have decreasing energy with increasing wavelength; the responsivity increases linearly with wavelength until it reaches its cutoff, where it drops abruptly.
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