Study of accessible performances of a spectro imager using a wedge filter

Linearly variable filters, also called wedge filters are special optical interference filters with a shift of the spectral characteristics along one axis of the filter. These optical components have the huge advantage of allowing simple and small volume spectroimager designs. But they also have drawbacks such as their sensitivity to the instrument design (F number, distance between filter and detector), and to the stray light, especially stray light between the detector and the filter. And performances of such spectro-imagers are difficult to model because of the angle dependence of the filter response (as every interference coating show) and because of the spatial dependence of wedge filters spectral response. These variations make purely geometric stray light calculation unusable because the stray light spectrum can be very different from the nominal beam spectrum, and because of the mixing between imagery and spectrum aspects. A MatLab code has thus been developed to be able to take into account a non uniform filter with both spectral and imaging aspects. The purpose of this study was, using this code.. to evaluate the achievable performances of a spectro imager using a wedge filter, and to quantify the sensitivity of these performances with respect to instrument design.