Aspheres Stand Up to the Test

H. Philip Stahl

As optical designers tackle the challenge of improving system performance while at the same time reducing overall package size, aspheric optics undoubtedly will grow in significance. Unfortunately, just because aspheres are easy to design does not mean they are easy to manufacture. In routinizing the manufacturing process, optical testing represents an important first step.
Aspheric surfaces can be troublesome to test for several reasons. Among them, the need to increase the aspheric departure of an asphere with fewer residual errors is a hindrance to designers. As any engineer knows, it is difficult to measure large numbers with such precision.
With these considerations in mind, there are three basic approaches for testing aspheres: measuring the actual shape, the aspheric departure from a base sphere or the null departure from the desired aspheric shape.