Implication of Illumination Beam Geometry on Stray Light and Bandpass Characteristics of Diode Array Spectrometer

This paper describes spectral stray light measurements of diode array spectrometers and, in particular, the dependence of stray light and bandpass properties on the illumination beam geometry. The effects of underfilling and overfilling the nominal field of view of the instrument are presented. Our measurements showed that the line spread function of a commercially available miniature spectrometer module having a fiber optic entrance with 0.22 nominal numerical aperture can depend on fiber illumination cone apex angle up to $40^\circ$ , which is well above the $25^\circ$ nominal acceptance cone apex angle of the fiber. The bandpass width and amount of stray light inside the spectrometer increased proportionally with the illumination cone angle. Spectral stray light measurements with different setups gave consistent and repeatable results for similar illumination geometries. Care should be taken to ensure that illumination geometry during spectral stray light characterization of a spectrometer matches the conditions during target measurements for which the spectral stray light corrections are applied. The impact of illumination geometry during stray light characterization depends on the target signal and particular application. Photochemical reflectance index (PRI) and normalized difference vegetation index (NDVI) of a hemiboreal birch forest are used as contrasting examples in this paper. Compared to natural variability, the relative impact on PRI exceeds that of NDVI by 10 times.