Abstract
Dispersive hyperspectral VNIR (visible and near-infrared) imagers using back-illuminated CCDs will suffer from interference fringes in near-infrared bands, which can cause a sensitivity modulation as high as 40% or more when the spectral resolution gets higher than 5 nm. In addition to the interference fringes that will change with time, there is fixed-pattern non-uniformity between pixels in the spatial dimension due to the small-scale roughness of the imager’s entrance slit, creating a much more complicated problem. A two-step method to remove fringes for dispersive hyperspectral VNIR imagers is proposed and evaluated. It first uses a ridge regression model to suppress the spectral fringes, and then computes spatial correction coefficients from the object data to correct the spatial fringes. In order to evaluate its effectiveness, the method was used to remove fringes for both the calibration data and object data collected from two VNIR grating-based hyperspectral imagers. Results show that the proposed method can preserve the original spectral shape, improve the image quality, and reduce the fringe amplitude in the 700–1000 nm region from about ±23% (10.7% RMSE) to about ±4% (1.9% RMSE). This method is particularly useful for spectra taken through a slit with a grating and shows flexible adaptability to object data, which suffer from time-varying interference fringes.
Highlights
IntroductionBack-illuminated CCDs are frequently used for astronomical applications due to their superior performance in terms of quantum efficiency, good linearity, and high spatial resolution [1]
Back-illuminated CCDs are frequently used for astronomical applications due to their superior performance in terms of quantum efficiency, good linearity, and high spatial resolution [1].a property of back-illuminated CCDs that is often overlooked is their propensity to generate constructive and destructive interference fringes when illuminated with coherent photons in NIR bands [2]
We have presented a two-step method that allows us to remove fringes for hyperspectral VNIR
Summary
Back-illuminated CCDs are frequently used for astronomical applications due to their superior performance in terms of quantum efficiency, good linearity, and high spatial resolution [1]. A property of back-illuminated CCDs that is often overlooked is their propensity to generate constructive and destructive interference fringes when illuminated with coherent photons in NIR (near infrared, at wavelength within ~700–1100 nm) bands [2]. The typical thickness of the depletion region in back-illuminated thinned CCDs is about 10–50 μm [3]. The absorption length of NIR photons in silicon can be several times or even ten times this thickness, which indicates that the depletion region is semi-transparent in NIR. This can improve the quantum efficiency in NIR bands; on the other hand, interference fringes occur due to the multi-beam interference
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