Freeform design and fabrication: where the proof of the pudding is in verification

A freeform optical surface is typically defined as any surface that does not have an axis of rotational symmetry. These surfaces provide additional degrees of freedom that can lead to improved performance compared to systems that make use solely of conventional optics. The benefits of using freeforms are: • Less optics can be used in the opto-mechanical system and therefore a decrease in the amount of optical surfaces occurs. Since every surface is a reduction of light intensity (e.g. by scattering), a higher throughput of the optical system is the result. • Less optics also means a reduction in mass and size • An improvement in optical quality (e.g. spherical aberration, coma, distortion) • A more favourable position of the optical components is possible. Generally, these freeform surfaces are more difficult to manufacture, and therefore more expensive both in cost and in development time. Furthermore, there is a danger with the application of exotic freeforms that have a large number of parameters, and consequently, degrees of freedom. For the latter reason they may naively seem attractive from the point of view of an optical designer, providing many knobs to optimize the performance; however, the overwhelming number of parameters may also cause a hotchpotch of local optima in merit function landscape. Many of those will not lead to an improved design because e.g. the particular freeform representation does not provide the correct handles, or the location in the optical train is unsuitably chosen. Another difficulty in the application of freeforms is the complexity of describing and evaluating the surface form tolerances of non-symmetric surfaces. Yet another difficulty is the following. An appropriate conventional design can often be obtained as an optimization from a paraxial system that can be defined analytically. For instance, both the telecentric beam expander and the push broom telescope that will be described in more detail below, can be derived from standard optical rules. Then, the paraxial design is a good starting point for an optimization algorithm as is used in optical design software in the hunt for an optimum design. In contrast, for a design that includes nonsymmetric optics, the parameters that provide the departure from symmetry are not so easily obtained with an analytically determined first guess. This makes it difficult to coax the optimizer into the right direction. Thus, with the benefits of using freeforms come disadvantages: • Difficult to determine the optimal freeform representation and location in the optical train • Optical tolerance analyses are not yet common practice in optical design packages • Difficult to manufacture with classical production technologies • Difficult to validate the surface shape • More difficult to align, because of the increased degrees of freedom • For all of the reasons above: more expensive Consequently, optical designers sometimes are hesitant to apply free form surfaces in an optical design difficult as they are to handle both from design and manufacturability of point of view.