Photon detectors wavelength response

The output of a thermal detector depends on the power absorbed by the detector. To make it absorb as much energy as possible, we blacken the surface of the detector. If the detector absorbs all of the incident energy, then the output of the bolometer is proportional to the arriving power, and the responsivity (in volts/watts) is independent of wavelength. This is very different from photon detectors, whose responsivity (in volts/watts) increases linearly with wavelength. 

These instruments use photon detectors which are semiconductor devices that are responsive only to a narrow band of wavelengths. Many narrow-bond thermometers further restrict the wavelength bandwidth by means of a narrow pass filter. In this case the total power detected is(4> ... 

Another unit, lux, measured using a photometer, is the amount of VIS photonic energy received by stability samples. As used in the ICH document, this measurement does not refer to the entire visible spectrum but rather to that amount measured using a detector with a photonic (eye-like) response. Hence, not all of the radiation present is measured, and that which is measured is not accurate. Chapter 6 of this book discusses this problem in much more detail. A direct conversion of the irradiance into the illuminance is not possible as only the irradiance considers the amount of the energy of a photon corresponding to its wavelength. 

Photon detectors consist of a thin film of semiconductor material, such as lead sulfide, lead telluride, indium antimonide, or germanium doped with copper or mercury, deposited on a nonconducting glass and sealed into an evacuated envelope. Photon flux impinging on the semiconductor increases its conductivity. Lead-sulfide detectors are sensitive to radiation below about 3 fj.m in wavelength and have a response time of about 10 /nsec. Doped germanium detectors cooled to liquid-helium temperatures are sensitive to radiation up to about 120 jitm in wavelength, and have a response time of approximately 1 nsec. 

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. 

Detection devices for optical spectroscopy fall into two distinct groups photon detectors and thermal detectors. The fundamental difference is illustrated in an idealized way in Figure 1. For a photon detector, the spectral responsivity is a function of wavelength... 

Note that the sample here includes both cell and solvent. The important feature of eqn [4] is that it clearly indicates that the apparent transmittance may be greater or less than the true transmittance depending on the magnitudes of and Tx- One important point to be made here is that the only ISL considered is the one that causes a detector signal. This may appear pedantic but as photon detectors show a marked upper cutoff in response, so stray light of wavelengths longer than that will not matter. 

Fig. 2.14. Ratio of Df at wavelength of peak response (assumed to be long wavelength limit) to D (500 K) as a function of long wavelength limit for an ideal photon detector...

Spectral range co The wavelength above which the photonic detector response quickly starts to drop, corresponds to the photons with an energy equal to the material bandgap, the unit is micrometer Ko = 1.239/ j Eg in [eV], A, in [pm]... 

Spectral response is shorthand for the responsivity variation with wavelength. For an ideal photon detector, the spectral response per photon is independent of wavelength - up to the cutoff wavelength - and zero beyond that wavelength. For an ideal thermal detector, the spectral response per watt is independent of wavelength. For real detectors, we will see deviations from those ideal. 

For background-limited " photon detectors, at wavelengths less than the cutoff, substitution of the responsivity and photon-generated noise equations derived earlier yields the BLIP D (Equations 4.31 a and b). 

There are three main detector categories photon, thermal, and multichannel. Photon detectors respond to the arrival rates of the photons and have a spectral response that changes with wavelength rather than responding to photon energies such as thermal detectors, which exhibit a near-uniform wavelength response. The performance criteria for aU... 

Delta function response - Over most of the wavelengths of interest, optical and infrared detectors produce one photoelectron for every detected photon, which provides a one-to-one correspondence between detected photons and photoelectrons. This means that the detector response is exactly linear to the intensity incident on the detector - an attribute that allows astronomers to precisely remove sky background and electronic bias to accurately measure the intensity of the astronomical object. 

The photomultiplier tube a very sensitive device that has a linear response over seven decades - has for a long time been the most widely used detector in spectrophotometers. Its efficiency depends on the yield of the photocathode, which varies with wavelength (e.g. 0.1 e /photon at 750 nm). and with the signal gain provided by the dynode cascade (e.g. gain of 6 x 105). With such values, the impact of 10000 photons per second produces a current of 0.1 nA. 

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