Stray light represents a major performance limitation for optical instruments. Analyses are done with ray-tracing software to evaluate the stray light performances of a design and, if necessary, improve it before manufacturing. Accurate simulations, however, require sending a sufficient number of rays. Hence, the process can be very time-consuming. We introduce the concept of stray light entrance pupil (SLEP) and demonstrate how it can be an efficient tool for simulating stray light for point sources. The SLEP defines a pupil over which light entering the optical system generates stray light reaching the detector. When that pupil is smaller than the first lens of the system, rays can be sent only through that pupil instead of the full lens aperture. Therefore, the time required to perform the simulation is reduced. Moreover, the efficiency can be further improved by defining a source with nonuniform ray density. The SLEP method is demonstrated on a wide-angle Earth observation camera and a time reduction up to about 20 is obtained. The SLEP concept can also be used to facilitate experimental characterization.
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