Engineering & Laboratory Notes

Folk Wisdom in Optical Design

By Anthony E. Smart, Titan Spectron, Santa Ana, California 92705

Abstract Building optical systems that actually meet specifications requires more than a knowledge of optics and the use of contemporary design codes. Excellence also comes with a kind of "street" knowledge, learned not from textbooks but from experience, mostly from failures. These hard-won les­sons are colloquially described here as "folk wisdom." If considered during system conceptualization and design, these notes may improve efficiency, effectiveness, and ui-

8130 Sυpplement to Applied Optics I 1 December 1994

timate performance. Costs, mechanical constraints, parts choice, specification and procurement, available adjust­ments, alignment sequence and retention, deterioration of performance with aging, and other topics are considered. Introduction These notes and tables may be useful in the creation of op­tical systems and their necessary mechanical support struc­tures. While minimum function may often be achieved by competent application of textbook optical and mechanical design, excellence also depends on the intelligent applica­tion of experience. The lessons here have been learned slowly and painfully, over almost three decades, by design-

TABLE 1. An Approach to Making a Successful Optical System

Engineering & Laboratory Notes

TABLE 3. Environmental Considerations

ing and fielding many laser-based instruments of various types and operating principles. While many designers maintain checklists such as are included here, none have been archived, and I thought it useful to formalize my own set as a basis that others may exploit and extend.

The notes may be applied to coherent, incoherent and non-imaging systems, radiation sources and detectors, ac­tive and passive devices, and other system elements. The intent here is to stress the goals to reach, and the tasks to accomplish them. Once these objectives are clearly identi­fied, the methods are available and accessible in the litera­ture. Whether the application be at the esoteric edges of research or simple, reliable, and cheap enough to be prof­itable, careful analysis and optimization reap benefits. Early emphasis on systems analysis is advantageous with veri­fication of principle, specification of performance, cost, in­terfaces, testing, and anticipated reliability. Also beneficial is a sensitivity to partly remembered experiences, fre­quently accessible only as "intuition." Concepts and caveats

TABLE 2. Optical System Properties

here are reported elsewhere scattered in publications and over years. They are summarized here as mnemonics to prevent some problems and offer possible solutions to others.

Creating an Optical System Table 1 suggests a possible sequence of actions that may yield a working optical system. "Optical system" is defined as optical components, mechanical support, alignments, and documentation that describes the system and allows testing. Today it also includes electronics and computing.

Thought-experiments can simulate your proposed system and its application. These experiences may suggest additional adjustments, identify necessary design changes, recommend special tools, improve calibration methods, or just make it easier to use. One may need to loop back in the sequence of Table 1 for verification, forced changes, or cor­rection of errors. Certain activities, such as documentation and acceptance tests, permeate the entire process.

Schedule times to involve peers and colleagues in de­sign reviews. Find the most critical detractor, and listen carefully to the objections raised. That person will help to identify obstacles early enough to surmount them. Leave

Supplement to Applied Optics I 1 December 1994 8131

Engineering & Laboratory Notes

TABLE 4. Proverbs and Aphorisms TABLE 6. Obvious Homilies

TABLE 5. Purposes of a Model

ego outside the design review. Think of participants as dev­il's advocates rather than adversaries. Handle any matters of doubt or confusion and differences of opinion in supple­mentary reviews. Any contention must be pursued until agreement is reached: It will take less time and cost less overall. A plausible project plan is required for organiza­tional and technical purposes. This can be simple and still expose potential problem areas needing attention in time for solutions to be found.

Optical systems can be active or passive, and may also be adaptive. Active implies that the system embodies a ra­diation source for interrogating or stimulating the phenom­enon in question. A passive system behaves as a sensor only, observing a natural properly of the phenomenon. Either system may be adaptive, meaning some capability for au­tonomous self-reconfiguration. Whether a system is active,

adaptive, or passive, the response is what counts to the user. Table 2 (previous page) shows some practical consider­ations for general systems.

Environment There are two aspects of environment, the local conditions of the optical system and the conditions under which the phenomenon occurs. These may interact and generate ques­tions such as, "Is the window transparent at the wavelength of interest?", "Does the facility have a floor upon which to place the optical system?", "Are the necessary services available?", "Will the lenses melt?" or "How far away must I be to avoid the impact crater?" These are humorous only until they happen without having been anticipated.

Table 3 (previous page) considers environmental fac­tors necessary for successful operation of the optical system. The equipment must perform to specification as long as required. State-of-the-art optical systems are often needed for environments less benign than the laboratory, including detrimental factors that may be unexpected until final in­stallation.

Table 4 is a group of observations that I have been careless enough to disregard on one or more occasions.

Modeling Numerical models and simulations of a system are always desirable, for the reasons listed in Table 5.

Conclusions It is impossible to be certain of success although it may be possible to anticipate many of the obvious problems. Find­ing solutions before serious consequences occur makes even difficult programs proceed acceptably. Thinking about all the facets of a complex system, at the planning stage when enthusiasm outweighs realism, takes courage, dedication, and mental discipline: It also avoids later pain. Typical of folk wisdom, it is not always the most obvious nor appar­ently logical approach that leads to the best solution. Ex­perience and apprenticeship teach more than written texts. Table 6, a list of homilies, may help make the difference between an adequate and excellent system.

To make an optical system requires detailed analysis, followed by synthesis of the best ideas into a design to achieve acceptable performance. Success requires not only standard scientific and engineering texts, but also experi­ence available personally and from colleagues, anecdotal observations, and historical preferences. Making mistakes may not be the only way to learn, but it is certainly effective.

8132 Supplement to Applied Optics I 1 December 1994