In many ways, the testing of plastic optical elements and systems is similar to the testing of glass optical elements and systems. However, because plastic optical elements may have features not normally seen in glass optics, additional test equipment and/or techniques may need to be utilized. This chapter on testing is intentionally placed ahead of the chapter on prototyping because we feel it is important to consider test issues while the system is being designed, not while it is being built.
In this chapter, we discuss the parameters that are typically tested on plastic optical elements and systems as well as the equipment and techniques that are commonly used to test them. In some cases, we discuss multiple methods of measuring a parameter. We also discuss decisions and methods that can make the testing of plastic optics easier.
6.1 Parameters, Equipment, and Techniques
In theory, every parameter that is called out on a drawing (whether in the notes or on the figures) should be tested to prove that a part is conforming. With a new mold (or changes to a mold), this is often done as part of the first article inspection (FAI) process or as a condition of purchase of the tool. Once an FAI has been performed and the manufacturing process is under control, the number of parameters measured is normally reduced, which saves cost due to the labor involved in performing and reporting the measurements.
Earlier, we discussed the notes and callouts on a sample lens drawing, which was shown in Figs. 4.28 and 4.29. We begin our discussion of testing by referring to this drawing once more. The first parameter we discuss, though it is out of order with the notes on the drawing, is the form of the optical surfaces. Surface form can be a critical parameter in the performance of a system. An incorrectly figured aspheric surface can wreak havoc on a system's image quality; the Hubble telescope, before its repair, is a prime example of this. Surface form can also be one of the more difficult specifications to initially meet, sometimes requiring several mold iterations to achieve. Accurate measurement of the surface form, besides determining conformance to the drawing, provides valuable feedback to both the mold processor and the mold maker. Correct mold compensation can only be achieved by knowing what surface form is being produced, both before and after any adjustment to the form of the mold inserts. As noted before, both surfaces of the lens shown in the drawing are aspheric, each described by its base radius and conic constant.
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