WAVEFRONT SENSING

摘要

The wavefront shape of radiation arriving at a pupil plane (4) is defined by a set of predetermined orthonormal functions such a Zernike functions with a weighting coefficients as a function of the shape. Apparatus for determining at least one such weighting coefficient comprises rate means responsive to radiation as it traverses the input pupil (4) to determine a pixelwise distribution indicative of rate of radiation intensity change thereat, and converting means for converting the intensity distribution to one or more weighting coefficients. The converting means comprises a store holding one or more matrices of predetermined values, each said matrix corresponding to one said orthonormal function, and the size of each said matrix corresponding to the number of pixels in the pixelwise distribution, and calculating means for multiplying said pixelwise distribution by a said matrix and adding the results to provide the weighting coefficient for its orthonormal function. The rate of radiation intensity change at pupil (4) is conveniently approximated by measurements of intensity in closely adjacent planes (22, 23) either side of pupil (4).