Noise thermal detectors

Readout resolution thermal detector noise dark current and amplifier noise... 

At a difference with thermal detectors, the background noise of photoconducting detectors is frequency-dependent. If it is assumed that the photoconductor is used to detect radiation at a frequency just above its cut-off frequency z/c, the detectors with a cut-off in the near IR display a much smaller background noise than those with a cut-off at lower energies. This is because in the near IR, the black body emissivity contribution at room temperature and below is very small. 

In thermal detectors and especially in bolometers, the energy exchange between the sensing element and the heat sink through a thermal link of conductance G results in a thermal noise known as phonon noise. The NEP associated with this phonon noise, which is a white (frequency-independent) noise, is given by ... 

Figure 16.27 in practice approximates the error only for instruments with Johnson or thermal noise-limited detectors, such as photoconductive detectors like CdS or PbS detectors (400 to 3500 nm) or thermocouples, bolometers, and Golay detectors in the infrared region. Johnson noise is produced by random thermal motion in resistance circuit elements. 

Infrared and near-infrared spectroph( tometcrs also exhibit Case 1 behavior. With these, the limiting random error usually arises from Johnson noise in the thermal detector. Recall (Section. 8-2) that this type of noise is independent of the magnitude of the pho-locurrcnt indeed, iluctuations arc observed even in the absertce of radiation when there is essentially zero net current. 

The detection of molecules in a molecular beam by a bolometer is based on the bolometer s response to the total beam energy, including the center of mass translational energy (Zen, 1988). The bolometer consists of a liquid-helium-cooled thermocouple whose electrical response varies rapidly with the energy of the bolometer. The low temperature is necessary in order to reduce the heat capacity of the thermocouple, thereby increasing its sensitivity, as well as to minimize the thermal detector noise. 

We can evaluate the impact of indeterminate error due to instrumental noise on the information obtained from transmittance measurements. The following discussion apphes to UV/ VIS spectrometers operated in regions where the hght source intensity is low or the detector sensitivity is low and to IR spectrometers where noise in the thermal detector is signihcant. 

What range of % transmittance results in the smallest relative error for an instrument limited by (a) noise in the thermal detector of an IR spectrometer ... 

The topics included here are limited to the usual types of noise in the common types of infrared photon detectors. Noise in thermal detectors, such as temperature noise in bolometers, is not included. Noise associated with the avalanche process is omitted. The detailed noise theory of phototransistors, an extension of shot noise in photodiodes, is not included. Modulation noise, an example of which arises from conductivity modulation by means of carrier trapping in slow surface states, is not included. Pattern noise, due to the... 

Because the performance of infrared detectors is limited by noise, it is important to be able to specify a signal-to-noise ratio in response to incident radiant power. An area-independent figure of merit is D ( dee-star ) defined as the rms signal-to-noise ratio in a 1 Hz bandwidth per unit rms incident radiant power per square root of detector area. D can be defined in response to a monochromatic radiation source or in response to a black body source. In the former case it is known as the spectral D, symbolized by Df X, f, 1) where A is the source wavelength,/is the modulation frequency, and 1 represents the 1 Hz bandwidth. Similarly, the black body D is symbolized by Z> (T,/1), where T is the temperature of the reference black body, usually 500 K. Unless otherwise stated, it is assumed that the detector Held of view is hemispherical 2n ster). The units of D are cm Hz Vwatt. The relationship between )J measured at the wavelength of peak response and D" (500 K) for an ideal photon detector is illustrated in Fig. 2.14. For an ideal thermal detector, Df = D (T) at all wavelengths and temperatures. 

The simplest representation of the thermal circuit of an infrared detector is shown in Fig. 3.1. The detector is represented by a thermal mass H coupled via a conductance Gto a heat sink at a constant temperature T. In the absence of a radiation input the average temperature of the detector will also be T, although it will exhibit a fluctuation about this value. This fluctuation gives rise to a source of detector noise ( temperature noise ) which sets the ultimate limit to the minimum signal detected by a perfect thermal detector. When a radiation input is received by the detector, the rise in temperature is found by solving the equation ... 

The definitions of Wj and of Pj assume the amplified noise bandwidth Af is reduced to 1 Hz. For wider bandwidth Af the rms noise increases as (A Equation (3.10) represents the best performance attainable from a thermal detector. If all the incident radiation is absorbed by the detector, 17 = 1. 

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