Computer modeling and analysis of veiling glare and stray light in Fresnel lens optical systems

Veiling glare (or flare) occurs in an imaging system when non-image forming flux strikes the focal surface. This light fends to reduce image contrast and can be caused by surface defects, dust, or anything else that might cause light to deviate from its intended imaging path. Fresnel lenses contain many such non-image forming structures as a result of their faceted and discontinuous nature. Sample optical systems for an extensive air shower observatory have been designed that consist of multiple Fresnel lenses. Veiling glare is expected to be significant in these systems, and it is desired to quantify its extent and ramifications. To this end, a series of tests has been performed on the Fresnel lens systems using non-imaging optical analysis software. A method for obtaining veiling glare information efficiently has been developed based upon the manner in which it is measured in real optical systems. By using computer software, the effects that several different parameters have on the veiling glare are analyzed independently without the use of expensive prototypes. It is shown that the amount of veiling glare is highly dependent upon the system configuration and that generalized conclusions regarding veiling glare and the number of Fresnel surfaces in a system are not possible. The analysis methods developed here, however, can be used to efficiently analyze the stray light in any system.