The analytical illuminance monitoring technique provides an exact expression within the geometrical optics limit for the illuminance over an image surface for light that has passed through a multiinterface optical system. The light source may be collimated rays, a point source, or an extended source. The geometrical energy distributions can be graphically displayed as a line or point spread function over selected image planes. The analytical illuminance technique gives a more accurate and efficient computer technique for evaluating the energy distribution over an image surface than the traditional scanning of the spot diagram mathematically with a narrow slit. The analytical illuminance monitoring technique also provides a closed form expression for the caustic surface of the optical system. It is shown by examining the caustic surface for a number of lens systems from the literature that the caustic is a valuable merit function for evaluating the aberrations and the size of the focal region. © 1976 Optical Society of America.