Lithographic tool performance depends in part on the partial coherence (how the lens pupil is illuminated) during photoresist exposure. The partial coherence dial is set prior to exposure and it is usually assumed it correctly indicates actual pupil illumination. The validity of this assumption is tested in situ by utilizing a 'negative pinhole,' an occluding spot on the back side of a clear reticule, that forms a negative image of the pupil illumination. A quantitative sequence of dose contours is obtained from the resist boundaries of images formed by exposing positive photoresist through a clearing dose sequence. These contours are pieced together and generate a source file describing the illumination distribution that is used as input to a lithography simulator to determine tool performance under actual operating conditions.