chapter 4, Design Guidelines

Excerpt

In this chapter, we discuss guidelines that apply to the design of plastic optical systems. We begin by reviewing some basics of optical design, consider tolerances and their effects, and cover various element parameters, such as thickness, shape, and different surface types. We also discuss guidelines for optomechanical design, stray light prevention and analysis, issues associated with drawings, and interacting with vendors.

We noted in the last chapter that injection molding is the most popular method of producing plastic optical elements. As such, most of the guidelines discussed are associated with the injection-molding production process. In some cases, the guidelines for injection-molded optics may differ from optics made from other processes, such as diamond turning. In these situations, I will attempt to point out the differences.

It should be kept in mind that what we discuss in this chapter are guidelines, not hard and fast rules. The guidelines presented are based mostly on practical experience (often bad experiences) and an understanding of the manufacturing process. Various designers and vendors may have different opinions regarding any given guideline, and trades can often be made between cost, performance, and producibility.

4.1 Design Basics

Optical design is a skill that previously was learned under the guidance of an experienced designer in somewhat of an apprentice relationship. The optical design community was relatively small; there were few courses of instruction and limited opportunities to enter the field. With the advent of personal computers and a number of commercially available optical design programs, access to tools to easily perform optical design has greatly increased. Currently, almost any company or individual with a computer and enough money to buy or lease an optical design program can get into the field. In this section, we present a brief discussion of the basics of optical design, beginning with a short, general review of geometric optics and aberrations. There are entire books and classes devoted to geometric optics and optical design, and given the length of this text, we will not cover the subject in great depth. We refer the reader to any (or all) of several well-known works for more detailed study.

A basic assumption in our discussion is that light can be represented by rays, and that light rays travel in straight lines (at least in homogeneous media). In Chapter 2, we discussed the refraction of a ray at the boundary between two media of different refractive indices. The rule that governs the refraction of the ray is known as Snell's law.