Abstract
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.
Highlights
I N FIELD spectroscopy and remote sensing applications, diode array spectrometers have become rather common.Manuscript received December 15, 2017; revised April 6, 2018 and May 17, 2018; accepted May 23, 2018
We have shown in our study that the stray light matrix (SLM) of the Zeiss MMS 1 spectrometer depends on the illumination geometry
Our results indicate that spectral stray light measurements performed with different setups give similar and repeatable results if the illumination beam geometry is similar
Summary
I N FIELD spectroscopy and remote sensing applications, diode array spectrometers have become rather common. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org They present several advantages over scanning double monochromator systems such as compact size, lack of moving elements, relatively low cost, and simultaneous acquisition of all the spectral bands, which significantly improves the measurement speed. LSFs measured with monochromatic sources corresponding to central wavelengths of every pixel of the spectrometer form a spectral stray light matrix (SLM). We present a case study based on a class of commercial spectrometer modules with fiber optic input, and show how illumination geometry can affect the measured LSFs. In Section II, we present measurements of LSFs using both a tunable laser and a monochromator system and report the results of LSFs of a Zeiss MMS 1 (Monolithic Miniature Spectrometer 1). The LSFs were measured with a spectrally tunable absolute irradiance and radiance source (STAIRS) [18] and verified with a double monochromator setup
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