A modified Twyman-Green interferometer is in use that makes possible wavefront testing of optical filters at any wavelength from 200 to 1100 nm. The use of mirrors for collimation and pupil imaging makes the instrument achromatic, and therefore the focus is fixed over the entire bandwidth. The beamsplitter and compensator plates are made of
fused silica, and the detector is a UV-enhanced CCD TV camera. A tunable monochrometer with a broadband light source permits selection of any wavelength. Fringe distortion, even when the collimating mirror is spherical, is small enough to keep measurement errors within 0.1 wave peak-to-valley over the 2- x 2-in, aperture.