LIGA technology has been used to fabricate linear gratings having free-standing nickel walls a few micrometers wide and as much as 50 µm high, with period on the order of 10 µm. With additional MEMS processing steps, such devices are intended for use in a tunable infrared filter. Prediction of optical performance is a particularly challenging problem for gratings with these parameters and materials and requires a robust Maxwell solver. We have applied our own code, described elsewhere, in the form of a finite-element implementation of the equivalent variational problem, to examine the optical properties of this class of gratings. Here, we describe our predicted results for the transmittance as a function of wavelength and polarization for various grating parameters and incident conditions. Measurements of fabricated gratings were also carried out, and the predictions are shown to agree well with the measured data. The filter cutoff is shown to be sensitive to the cone angle of the incident radiation, and the implications of this effect for system performance are discussed.