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Chapter 7:
Every radiometric measurement uses a detector in some way. This chapter discusses how they are described and delineates their most important properties with respect to radiometric measurements. 7.1 Detector Descriptions The responsivity of a detector is the electrical output divided by the optical input. The inputs and outputs can take different forms. The output is often a voltage or a current, but sometimes it is a count of pulses or even a temporal frequency (which may have been generated by a voltage-to-frequency converter). In this text, for most purposes, the electrical output will be assumed to be either voltage or current. The input is a power or photon rate on the detector. It may be monochromatic, bandlimited, or cover the entire optical spectrum (say 95% to 99% of the radiation). The responsivity is almost always written as a script R, i.e., ℜ. Thus the signal voltage can be written as V s =ℜΦ=∫ℜ(λ)Φ(λ)dλ. The total noise arises from several sources, but, no matter how many and what kind, the total noise can be written V n =∫V 2 n (f)df − − − − − − − − − √ , where the spot noise, the noise at a given frequency, V n (f) , is the sum of all the noise components at that frequency. Therefore the signal-to-noise ratio, SNR, is given by SNR=V s V n =ℜΦ V n =∫ℜ(λ)Φ(λ)dλ ∫V 2 n (f)df √ .
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