  Combining image sensor simulation tools (e.g., ISETCam) with physically based
ray tracing (e.g., PBRT) offers possibilities for designing and evaluating
novel imaging systems as well as for synthesizing physically accurate, labeled
images for machine learning. One practical limitation has been simulating the
optics precisely: Lens manufacturers generally prefer to keep lens design
confidential. We present a pragmatic solution to this problem using a black box
lens model in Zemax; such models provide necessary optical information while
preserving the lens designer's intellectual property. First, we describe and
provide software to construct a polynomial ray transfer function that
characterizes how rays entering the lens at any position and angle subsequently
exit the lens. We implement the ray-transfer calculation as a camera model in
PBRT and confirm that the PBRT ray-transfer calculations match the Zemax lens
calculations for edge spread functions and relative illumination.
