Like the bricks and mortar that hold up a building, optical engineering is the foundation of any system based on photonics. In a photonic system, the key to accomplishing a task is getting the photons where you want them to go, when you want them to be there, and in the proper form, whether that is a concentrated beam, a diffraction pattern, or a diffuse reflectance. That goal is accomplished by optical engineering, which involves design issues beyond just components. “It’s a lot more than lenses and mirrors,” says Robert Fischer, president and CEO of Optics1 (Westlake Village, CA), who prefers to use the term photon management. “You have to think about mechanical and optomechanical elements, packaging issues, configuration, cost, manufacturability—all the things they don’t teach you in school.”
An important part of optical engineering is design tradeoffs. Warren Smith, chief scientist at Kaiser Electro-Optics (Carlsbad, CA) points out the challenges in the hot area of LCD-based head-mounted displays. “The big problems amount to trying to get a big eyebox and a wide field of view simultaneously,” he says. Indeed, optical design has always been about making tradeoffs and balancing requirements. “Wide field and high resolution for a given LCD are kind of an invariant—you can’t raise one without cutting the other. The way people are doing it is to use more than one LCD.”
Fischer agrees. His group recently completed a multispectral imaging system for the Navy that can simultaneously image a scene in the visible, medium-wave infrared (IR), and long-wave IR spectral regions. “There were a total of five different images that could in theory all be presented simultaneously. Completing it took all the aspects of optical engineering, including packaging, mechanical and optomechanical design, lens design, coating design, and issues regarding the IR sensors,” Fischer says. “Then of course once you assemble it you have to align all of these things. It was a pretty massive project.”
Image isn’t everything
In recent years, optical engineering has broadened out to include sophisticated nonimaging system design, according to Fischer. “Ten years ago almost everything was imaging optics. Now with medical devices, sensors, and fluorescence measuring equipment where they’re detecting photon flux, [nonimaging optical design] is becoming really important, much more than it ever was in the past,” he says.
Nonimaging optics can range from illumination optics to concentrators and collectors for applications such as solar power, radiant heating, and signal detection. “You’re basically building a light funnel,” says Ben Jacobson, president of Illumitech (Chicago, IL). “The materials are invariably the same [as for lens designs]; the innovation is in the shape.” Engineers are now designing collectors for nonplanar absorbers with tube shapes, for example. “You need quite different solutions,” Jacobson says.
It also involves a different mind set. “Classical nonimaging optics is for applications where it’s very important to get maximum efficiency and also radiance concentration within a specific aperture,” Jacobson says. “Traditional optics allows you to collect all the energy and conserve radiance only for a single point or a small area. Once you get off-axis, you have aberrations, which force you to choose between conserving radiance and collecting all the energy. Nonimaging optics allows me to collect all of those off-axis points as well.”
Doug Goodman of Polaroid (Waltham, MA) agrees. “An imaging system has constraints that a nonimaging system does not,” he says.
What this basically means is that for nonimaging optical design, you need to throw away aberration theory and start from scratch—completely the opposite of most optical-design education. “Most of the practitioners don’t come from an optics background,” says Jacobson. “The analogies to thermodynamics and mechanical engineering are better than the analogies to imaging optics in some ways.”
Nonimaging optics also involves tailoring, which is used to create a uniform illumination field or a specified irradiance distribution for applications such as transportation lighting, automotive lighting, traffic signals, and solid-state lighting. “LEDs are now more efficient than incandescent lamps, so people are willing to pay the cost penalty. The new generation of LEDs has such compelling benefits that lighting designers have to use them,” says Jacobson. The change will open up opportunities for nonimaging optics designers. “The arrival of LED lighting is probably going to leverage another wave of changes in lighting optical design, and nonimaging optics is in the right place at the right time to ride on top of that.”
(By Kristin Lewotsky, OEmagazine, October, 2001)