Here is a simple example of a veiling glare calculation.  Veiling glare is defined (in the Photonics Directory) as “diffuse stray light at the image plane of an optical system that results in reduced contrast and resolution.” 

Veiling glare typically results when unwanted light from an out-of-field source enters an optical system. For example, consider the case of a telescope looking up from the earth.  No matter where in the sky the telescope is aimed, some light from the sun (or moon, or other bright "out-of-field" object) may strike the front of the telescope and then scatter around inside, eventually ending up at the telescope’s focal plane.

A telescope’s designer must take this extraneous light – the veiling glare – into account.  A good way to measure and compare the amount of veiling glare in different optical systems is to calculate the Normalized Detector Irradiance (NDI).  From “Stray Light Performance Optimization through System Design,” by Francis Reininger (SPIE Vol.2260, p.21; 1994):

The Normalized Detector Irradiance (NDI) is the standard figure of merit for evaluating a system’s stray radiation performance as a function of off-axis point source angles.  The NDI is the ratio of the stray light irradiance at the detector to the stray light source irradiance at the entrance port when evaluated for a plane normal to the line of sight to the source point.  Because it is a ratio of irradiance rather than power, the NDI can be used to compare the stray light suppression capability of instruments of various sizes without the need for scaling.

The images below show a telescope system before and after interior stray light baffles are introduced.  For each of the two systems (one with, and the other without, interior baffles) we have set up ZEMAX to perform dozens of stray light calculations for various stray light source angles and then automatically calculate the NDI as a function of angle.  Using the ZEMAX Programming Language (ZPL) we have created customized NDI plots, the comparison of which clearly shows the value added by including the interior baffles.