Gratings are excellent low-pass filters, provided they have the appropriate mounting. We have measured the spectral reflectivity of filters composed of four gratings, from the 0.2 mm to 2 mm wavelength region. In order to contain the size of the gratings, they were placed near a lens focus, where the beam section is smaller. The effect of non-parallel light is to smooth the cut-off edge of the filters. The measurements are in good agreement with the diffraction theory of gratings. © 1992.
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