Wavefront information for an optical system is calculated based on the
intensity of an image of a plurality of gratings having different periods
and orientations taken from at least two different planes a predetermined
distance apart. The image of a plurality of gratings having different
spatial frequencies or periods and orientations, the location of which are
precisely known, are imaged in a nominal focal plane of the optical
system, and, preferably, in two additional planes displaced a
predetermined distance from the nominal focal plane. The phase shift, if
any, from a fundamental frequency of the image intensity, is determined
based on the known location of the grating and the grating image
intensity. The grating image intensity is detected and measured in a first
detection plane in a nominal focal plane and in a second detection plane a
predetermined distance from the nominal focal plane. From these
measurements wavefront information is calculated. By utilizing a plurality
of different gratings, the wavefront at different spatial locations in the
optical system may be mapped, providing information relating to image
quality. The present invention provides wavefront information, which is
directly related to image quality, relatively easily and without having to
remove the optical system from the device, such as a photolithographic
device used in semiconductor manufacturing. The present invention is
particularly applicable to optical systems designed for UV illumination,
which tends to degrade or modify optics over time. More consistent, better
quality imaging is obtained with reduced down time.