An optical wavemeter includes a slit, a diffraction grating, a mask, a complementary grating, and a detector. A monochromatic source is incident on the slit. The diffraction grating produces an image of the slit in an image plane at a horizontal position that is wavelength dependent. The mask has a two-dimensional pattern of transmission variations and produces different vertical intensity channels for different spectral channels. The complementary grating produces a stationary image of the slit independent of wavelength. The detector measures vertical variations in intensity of the stationary image, and the mask is created so that the number of measurements made by the detector is less than the number of spectral channels sampled.