Abstract: The light power not occluded during a repetitive, transverse knife-edge scanning of a focused spot is measured by a photodetector while an interferometer measures the displacement of the knife-edge during the scanning operation. The knife-edge is supported on a glass plate carried by the photodetector with a liquid medium being located between the glass plate and the light sensitive surface of the photodetector to enhance light reception by the photodetector. The differentiated output of the photodetector when displayed (e.g., by means of an oscilloscope) provides a graphic representation of the intensity profile of the spot which is dependent on the composite refractive index of the glass plate and the liquid medium.