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Optics

Richard T. Williams

Computer Ray Tracing used in Optics labs and lecture:

In Physics 352, Physical Optics and Optical Design , one of the professional-level ray trace optical design programs, in this case ZEMAX by Focus Software, Inc., is used hands-on by the students in some of the laboratories accompanying Physics 352, and in lecture demonstration using a computer display projection system. Physics 352 is a junior-senior course. When the phased-in issue of student thinkpads reaches the senior class in 1999, hands-on ray-tracing in class can be used.

In its basic form, ray tracing propagates a number of rays through a mathematically constructed optical system, using the exact geometrical results of Snell's law and the law of reflection at each interface. In this sense there are no approximations, and the ray trajectories are "exact", showing the user the true system performance, aberrations and all. Ray tracing is widely used in optics courses now, partly because it is a near-universal tool of people proceeding professionally in optics, and also because it replaces a certain amount of expensive and fragile "real optics" inventory with a virtually unlimited inventory of mathematically generated optics that the student can design and set up.

There are two categories of ray tracers that fill the latter function: instructional packages without optimization, and professional packages with optimization. "Optimization" is the process whereby the program changes specified system parameters while continually tracing rays, in order to minimize the ray deviation from some specified target. We have chosen to use a program with optimization, in our case ZEMAX, not only because the student receives training in using optimization, which would be the main use in later professional applications, but also because of what we have found to be a very useful pedagogical feature of optimization ray tracing performed in a Windows environment. As a specific example, students set up an optical system whose aberrations are to be minimized. Three windows are opened simultaneously:

The students set up the system, define the target and the test of the aberration of interest, decide on starting parameters and what they will allow to change to achieve optimization, and then kick it off and follow the progress in the three windows. One cannot help but notice the correlation between lens bending or element placement and the (usually) steady shrinking of a spot diagram or flattening of a ray fan plot. Then when we let the students define a Cartesian (conic section) optical element instead of the usual sphere, and they see the spot diagram shrink dramatically to the geometric point focus as optimization proceeds, it is visibly effective.

We do not entirely replace bench optics experiments, even within the subunit on imaging optics. It is important for the student to see it both ways. Ray tracing is used in approximately 3 labs and in parts of 3 weeks of lecture. [an error occurred while processing this directive]