Effect of changing mirror velocity

The effect of changing the scan speed of a rapid-scanning interferometer may also be inferred from Eq. 7.8. If D does not change with modulation frequency, doubling the mirror velocity will simply halve the measurement and reduce the SNR by a factor of V5. Doubling the number of scans will recover the SNR of the original measurement. 

Unfortunately, this simple treatment is rarely valid in practice, since the D value of most detectors varies with the modulation frequency, /, of the radiation being measured (see Chapter 6). With pyroelectric detectors, D varies approximately thus if the speed of the moving mirror is halved while keeping the measurement time constant, the SNR will increase. Conversely, D of most quantum detectors usually increases as / increases up to some maximum value ( 1 kHz for photoconductive MCT detectors). It then remains approximately constant as the modulation frequency is increased, and Anally drops off at frequencies above 1 MHz. Since it is rare that interferometers are operated at scan speeds that modulate the incident radiation at frequencies greater than 1 MHz, operating at high scan speeds when an MCT detector is employed is usually beneficial. 

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