Accurate modeling of optical system aberrations applied to the design of a stationary Fourier transform spectroradiometer
Abstract
Stationary Fourier transform spectrometry is a well-known concept to build reliable field or embedded spectroradiometers, especially for the mid- and far- infrared. However, the quality of the interference pattern imaged on the focal plane array is crucial to obtain a good spectrum by Fourier transform. We describe here an accurate modeling of the interferometer behavior that takes into account the instrument aberrations and field of view in order to quantitatively predict, at each wavelength and for a spatially extended uniform incoherent source, the real interferogram defects, namely, fringe distortion, fringe blurring and illumination non-uniformity. To investigate these effects, we first derive the properties of the elementary interferograms built by each source point. For this purpose, we use ray-tracing to extract optical path and vignetting information with the help of a commercial optical design software package, and we reconstruct from them the two wavefronts that hit the detector using general numerical methods with the help of standard computing tools. The whole interferogram being formed by the incoherent superposition of all elementary interferograms, we next, compute the relevant quantities by appropriate numerical quadratures. We illustrate this method with two potential layouts of a Fourier transform spectrometer that we are currently designing for accurate radiometric measurements in the 2.9μm-9.6μm range with a spectral resolution better than 8cm-1 on a 4.5°x0.6° field of view.