Manufacturing of linear variable filters with straight iso-thickness lines

Linear variables filters are band-pass multilayer coatings manufactured with a thickness gradient that allows a significant wavelength shift of the centring wavelength of the filter according to the point that is illuminated. In case such a filter is associated with a 2D matrix detector in order to form a compact spectrometer, iso-thickness lines must be as straight as possible, perpendicularly to the thickness gradient. To answer this problem, we developed a masking mechanism that combine the classical rotation movement of substrates and a translation movement for the mask, this last movement being induced during the rotation by the mean of a cam. Thickness gradient can be freely adjusted according to specifications, while transverse uniformity is 99.9%.