Choosing software: Different design paths lead to different solutions, Part I
The abundance of sophisticated optical-design software products on the market can present a bewildering maze to engineers seeking solutions to specific design problems.
Hassaun A. Jones-Bey
The abundance of sophisticated optical-design software products on the market can present a bewildering maze to engineers seeking solutions to specific design problems. To provide some clarity, we interviewed about 15 high-profile vendors of optical-design software for the types of applications covered in Laser Focus World. A brief summary of their responses for classical (as opposed to micro- or nanoscale) optical-design problems is included here. Part II, to appear in a subsequent issue, will cover optical-design software for photonics applications such as telecommunications and integrated optics, in which the structural dimensions of the optical components are on the same order of magnitude as the electromagnetic energy propagating through them.
If your project involves lens design, as in objective lenses for cameras, telescopes or microscopes, or imaging of one form or another, you probably want to investigate sequential ray-tracing software, which essentially projects an image from a point source of light onto a target area (see Laser Focus World, September 1999, p. 147). The electromagnetic propagation is modeled as rays of light traveling predictably and sequentially through optical system elements from source to image (as in a traditional camera lens system for instance). The relative simplicity of the basic propagation model in such systems enables designers to obtain a lot of information by tracing relatively few rays, as well as a sophisticated level of design optimization (see Laser Focus World, April 1998, p. 71).
The primary strengths of sequential ray-tracing software lie in rendering the best images possible through very precise control of tolerances, and in their well-developed optimization capabilities. Among popular sequential ray-tracing programs originally developed primarily for lens-design type problems (listed in alphabetical order) are names such as CODE V from Optical Research Associates (ORA; Pasadena, CA), OSLO from Lambda Research (Littleton, MA), Solstis-Odyssey from Optis (La Farléde, France), OPTEC from Sciopt (San Jose, CA), SYNOPSYS from Optical Systems Design (East Boothbay, ME), and ZEMAX from Zemax (Bellevue, WA).
If your project involves nonimaging illumination, such as the design of automotive headlamps, solar collectors or toasters, or other spatial light distribution problems, you will probably want to investigate nonsequential ray-tracing software (see Laser Focus World, May 2005, p. 71; December 2004, p. 61; May 2004, p. 85; October 2003, p. 58; and October 2002, p. 63). In the real world, light interacts with a wide variety of optical components and the behavior of each ray of light is not as predictable as the sequential model assumes. Just because one ray or photon follows a particular path, the others don’t have to. So not all ray-tracing problems can be modeled by sequential propagation assumptions with a relative handful of rays.
Even the relatively straightforward lens systems in cameras and telescopes are subject to stray light “contamination” from a variety of factors including reflection from lenses (about 4% on noncoated optics), or from direct sunlight entering an imaging pathway. With improvements in computing power over the years, sequential ray-tracing programs have incorporated sophisticated nonsequential ray-tracing capabilities for dealing with light contamination problems and other real world issue. But the same Moore’s Law growth of computer speeds has also spawned full-fledged nonsequential ray-tracing software with a primary application in illumination design in particular and geometrical optics in general.
The emphasis in nonsequential ray-tracing software, rather than on precise tolerances, is on distributing light from multiple real-world sources-which in the typical office space might include overhead fluorescent lights, daylight through a window, and incandescent or LED desk lamps, as well as numerous reflections from light fixtures, and other surfaces with varying reflective and absorptive properties. Rather than producing an image on a selected plane, the desired outcome may be more qualitative or even subjective, such as brightness distribution over an illuminated area or ambience in an illuminated interior (see figure). The added complexity requires tracing many (several orders of magnitude) more rays than in lens design, and of course along nonsequential and often unpredictable pathways.
Two major areas of current development in illumination systems are optimization, which is much more challenging in complex and data-intense illumination environments, and compatibility or integration with CAD software, which is needed to place the optical aspects of the design in proper context amidst the numerous nonoptical structures and surfaces that also affect system performance (see Laser Focus World, November 2004, p. 70, and December 2003, p. 59). For both sequential and nonsequential ray-tracing software, vendor offerings can vary widely in terms of how their software approaches these and other aspects of lens and illumination design problems. And the variation can occur not only between vendors, but also in different offerings from the same vendor. Ray-tracing software prices also vary based on a range of factors including the availability of technical support. Prices quoted in interviews for this article ranged from as low as $1000 for software priced to meet the budget of a one-person design shop to as high as $150,000 for software intended for use in the industrial design operations of the world’s largest corporations.
This exterior illumination model of the front of a church was produced using SPEOS software from Optis.
Among nonsequential ray-tracing software products originally and exclusively designed for illumination design problems, one finds names (listed alphabetically) such as ASAP from Breault Research Organization (Tucson, AZ), FRED from Photon Engineering (Tucson, AZ), LightTools from ORA, SPEOS from Optis, and TracePro from Lambda Research. As an example of how some of traditional category lines have begun to blur, Zemax added an illumination-design module to its traditional lens-design product in the late 1990s. For design needs that cross over categories, either in the classical optics discussed here or into the micro and nano-optics to be discussed in Part II, an important point to discuss with prospective vendors is compatibility or interoperability with relevant software from other vendors.
Several ray-tracing programs also include physical optics and diffraction capabilities for handling problems such as polarization and the propagation of coherent light. But vendors of dedicated laser-design software, such as GLAD from Applied Optics Research (AOR; Woodland, WA), LASCAD from Micro Systems Design (MSD; Munich, Germany) and PARAXIA-Plus from Sciopt maintain that unlike the very precise tolerances and optimization of dedicated sequential ray-tracing, or the complex spatial design strengths of dedicated nonsequential ray-tracing software, dedicated physical optics software offers an exceptional level of accuracy required to predict the near and far-field diffractive effects caused by beam interactions with hard-edged apertures, for instance. Typical problems for this type of software might include analysis and design of resonators or Q-switched lasers (see Laser Focus World July 2000, p. 137; May 2000 p. 293; May 1998, p. 297; and January 1998, p. 207).
That said, even as these words are being written, Moore’s Law improvements in computer capabilities are blurring these few categories even further. So there is no substitute for actually talking with vendors in the appropriate categories about solutions for your specific design problems, rather than just looking for a specific type of software (see Laser Focus World, April 2004, p. 73). You should also ask for examples. Different software offerings are likely to take different algorithmic approaches, with differing strengths and weakness for different types of designs. Despite rapid growth in recent years, the field remains small and scientific enough that you are still likely to get a very experienced and technically capable person on the other end of the phone who would probably prefer to send you to a competitor for an appropriate solution than to sell you something in-house that might turn you into a dissatisfied customer.❏
Companies mentioned in this article can be found on-line at:
Applied Optics Research, www.aor.com
Breault Research Organization, www.breault.com
Lambda Research, www.lambdares.com
Micro Systems Design, www.las-cad.com
Optical Research Associates, www.opticalres.com
Optical Systems Design.home.gwi.net/OSD
Photon Engineering, www.photonengr.com