Theory and application of gain ranging to Fourier transform infrared spectroscopic imaging

Gain ranging is incorporated into the data acquisition and processing protocol for a Fourier transform infrared (FT-IR) imaging spectrometer employing a focal plane array (FPA) detector. A model for predicting the signal, noise, and signal-to-noise ratio (SNR) for an FPA in terms of the dynamic range of the analog-to-digital converter (ADC) is presented. Conventional gain ranging theory, modified to account for variation of noise with gain by incorporating a linear model for noise prediction, is shown to provide excellent agreement with observed values for the SNR advantage afforded by gain ranging. SNR improvement, as affected by collection parameters, was shown to be limited by the noise characteristics of the FPA detector. The advantages of gain ranging were demonstrated by spectroscopic imaging of a thin section of human prostate tissue.