The Strehl ratio (SR) is a measure of image quality that is especially relevant to recording media of finite contrast, such as photoresist, film, and the human retina. The SR is accessible experimentally from the total power and maximum irradiance in the point spread function (PSF). Measurement error is introduced by the finite dimension of the pinhole aperture that is used to generate a detectable image. This systematic error can be related parametrically to the object geometry and illumination system, and the SR can be corrected to within a few percent. Systematic corrections also compensate for the portion of the PSF irradiance that falls outside the data integration window in the image plane. An optical test bench is constructed to measure the SR over the lens field and azimuth to an accuracy of 5% and a repeatability ofl%. © 1992 Optical Society of America.