Even the most efficient light sources would be rendered useless without high-quality optics. A compact fluorescent lamp, for example, can lose up to 70 percent of its light if paired with an inappropriate optic, says Nadarajah Narendran, director of research at the Lighting Research Center (LRC), in Troy, N.Y. Likewise, the touted efficacy of LEDs wouldn’t exist without the right optics.
Designing lenses and reflectors for solid-state lighting requires more than scaling them down from legacy sources. Yes, LEDs have smaller form factors than their conventional counterparts, but they differ in how they emit light. Incandescents illuminate in 360 degrees but LEDs are directional, illuminating only 180 degrees. This stems from the design of an LED package, typically comprising: one or more semiconductor chips, or die, mounted atop heat-conducting material; a primary optic—a lens or encapsulant—that encloses the die; and components to regulate heat and power. When current is applied, the chips produce light through electroluminescence.
Conventional lamps emit light via radiance or fluorescence. The source is surrounded by glass, metal, and acrylic reflectors that capture the omnidirectional light, guide it into the specified distribution, and work with lenses and optical accessories, such as louvers and baffles, to further shape the beam.
Although the output from LEDs is more concentrated, the distribution is too broad for most applications, and the light lacks intensity over distance. Therefore, LED lamps and fixtures typically incorporate one or more secondary optics, which can consist of lenses, reflectors, total internal reflection (TIR) optics (a lens and a reflector), and diffusers that collect the light, magnify its intensity, direct it to a target surface, and then blur it to enhance beam and color uniformity.
Choosing the appropriate optic depends on the application. Reflectors and TIR optics, which are common in LED MR16s and directional lighting, both have their advantages and disadvantages, says Frank Shum, vice president of LED products for Soraa.
Reflectors are simpler to implement and less expensive to manufacture than TIR optics. How well they collimate light—or propagate light into parallel rays—depends, in part, on their shape. Faceting or segmenting the surface can improve beam uniformity, as can applying different textures or finishes. If needed, lenses can further diffuse the light.
But reflectors don’t solve everything. Light from an LED can evade parabolic reflectors, for example, and become spill light or, worse, glare. Moreover, many reflectors are vapor-coated with aluminum, a conductive material that can cause electrical shorting. Manufacturers can separate the reflector and LED circuit board with an insulating material, but the farther an LED package is to the reflector’s input aperture, the less a reflector can “capture as much of the light as possible and re-direct it,” says Catherine Leatherdale, product development specialist at 3M.
New specular films, such as 3M’s recently introduced D50 series, are closing that gap. Made from polymeric material, these films can be applied onto a plastic substrate and are non-conductive, highly reflective, and, in some cases, optically superior to aluminum. Alternatively, a specular polymer can be custom molded into reflectors to control the light precisely, enhance surface reflectance, and sit close to the LED.
Designed around the phenomenon where light traveling from one medium to another of lesser optical density hits the interface at an angle and reflects with 100 percent of the beam energy, TIR optics, or TIR lenses, consist of a refractive lens nestled inside a reflector and are typically cone-shaped with optical efficiencies as high as 92 percent. The lens directs light from the source’s center to the reflector, which sends it out in a controlled beam. An additional surface over the assembly provides another opportunity to modify the light.
Generally injection-molded from polymers, TIR optics are sculpted to a precise beam pattern with a variety of surface treatments—such as rippling, pillowing, or polishing—to diffuse the light, widen the beam spread, or shape distribution. Injection molding, however, limits lens size and wall thickness, typically to 0.5 inch. The larger the optics, the greater the risk of shrinkage and distortion. Maintaining a higher temperature and pressure on the machines for a longer time period can reduce the risk, Shum says, but at a cost.
TIR optics capitalize on characteristics unique to LEDs. Unlike incandescents, which radiate heat outward, LEDs send heat out their base, allowing TIR optics to fit snugly over their domed top. As a result, says Chris Bailey, director of the Lighting Solutions Center at Hubbell Lighting, “LEDs afford an opportunity for the designer to extract light directly from the source and precisely direct it through key vertical and horizontal planes.”
Though prevalent in outdoor and industrial lighting, TIR optics are still gaining in indoor applications. While ideal for beam control, they don’t work for all applications, Bailey says. For example, coupling is not necessary in architectural recessed lighting, where the emphasis is on diffused illumination, low glare, and a gradient distribution.
Shum says that the size ratio of an LED or LED package to an optic determines the beam angle. That is, narrower beams require smaller light sources or larger optics. Choosing the former affects output while choosing the latter can stress the limits of injection molding or system design if the lamp is small, as in the case of an MR16.
However, source sizes are increasing, driven by a need for higher luminous flux and convenience. To attract fixture designers to their products, LED manufacturers have introduced modular, high-output, chip-on-board (COB) LED arrays, which are becoming more common and can output 600 to 20,000 lumens at 6W to 200W, Bailey says. COB LEDs consist of multiple die that are wired to operate as one electrical device and assembled onto a ceramic package with a single emitting surface. Designed to produce a specific color temperature and lumen output, they take the guesswork out of creating a well-integrated LED.
But controlling the output from COB LEDs is more difficult. Their size increases the cost of injection molding the appropriate optics, and their flat surface does not lend well to coupling, which is key to the efficiency of TIR optics. Consequently, COB arrays fare better in applications where beam control is less critical, such as wide-distribution floodlighting or downlighting, Bailey says. But their optical and electrical needs are also simpler. Typically, one optic is sufficient and “electrical connections can be made using simple plug-and-play,” he says.
High-flux density (HFD) LEDs are also growing in popularity. They also consist of multiple die, but each is smaller than those in a COB array and can handle more current, Bailey says. The result is more light output from a smaller lighting-emitting surface. The die are in a domed surface, which easily accommodates a TIR optic and therefore offers more output control. These high-powered LEDs come in 4W to 60W and output anywhere from 400 to 6,000 lumens, Bailey says.
Although the laws of physics have limited the availability of small LED lamps with tight beam angles, progress is being made. Soraa’s Point Source Optics, which features a folded prismatic optic designed by Shum, led to the company’s LED MR16 lamp with a 10-degree beam spread. The folded optic reflects the light emitted from a single LED package multiple times before propagating it out, and blurs the light near the beam’s central axis—as opposed to its outer edges—creating a smooth, well-defined beam. Shum says this larger diameter optic can be injection molded because its diameter to height ratio is 6:1, whereas typical TIR optics have a ratio of 1.25:1. The Soraa Snap System allows users to magnetically snap a variety of lenses and optical accessories to the MR16 and shape, widen, or narrow the beam and eliminate glare.
Also tackling the issue of source size, Reading, Mass.–based Fraen Corp. has developed a multi-TIR nested lens for use with any COB LED product to create a narrow beam spread. Though the larger footprint of the COB LED would typically require a proportionally larger optic to fully control its illumination, the compact nested lens design produces a collimated beam with little spill light. And, of course, a smaller optic means a smaller luminaire.
Taking the Heat
The increasing lumen output of COB and HFD LEDs also means more heat. As these sources become more common, proper thermal management will become more critical in ensuring the performance and life of a diode, as well as the fixture’s circuitry. In fact, because of the increased heat, LEDs today are encapsulated with silicone instead of epoxy, which was used in the early days of LEDs when outputs were measured in milliwatts. Epoxy degrades above 80 C, whereas silicone can withstand temperatures of up to 200 C, Narendran says.
While coupling can improve optical efficiency, exposure to heat and light—particularly from the high-energy blue portion of the spectrum—can cause the materials to degrade over time. Lenses and reflectors can yellow, leading to color shifting and performance discrepancies between fixtures, Narendran says. A uniform lighting design on day one may “start producing different colors, which, in turn, will affect the aesthetics of the space,” he says. Hazing can also occur, reducing the optics’s ability to direct the lumen output.
Not surprisingly, heat-resistant materials are garnering interest from LED optics and fixture makers. One material of choice for TIR optics is PMMA (polymethyl methacrylate) acrylic, favored for its clarity, UV stability, and high transmissivity, Hubbell Lighting’s Bailey says. However, long-term heat exposure may cause deformations.
Manufacturers have also turned to glass, which, Narendran says, “is a good candidate because it’s much more robust than polymers.” It also offers high transmission, but can be heavy, fragile, and expensive to manufacture.
Polycarbonates that address the specific needs of LEDs are another strong contender. Bayer MaterialScience’s Makrolon LED-grade materials are designed to tolerate long-term heat exposure, transmit light effectively, and have good clarity. A diffusion additive can also be added to polycarbonates to mitigate glare.
As manufacturers continue to explore the opportunities unique to LEDs, the push for higher efficacies will introduce new heat-resistant materials and more customized optical solutions. According to Marco de Visser, manager of marketing communications for Dutch 3D printed optics maker Luxexcel, the latter has already begun; 3D printing technology is speeding up prototyping and printing optics to order. Such developments will further emphasize solid-state lighting’s importance as a source of not only efficiency but also beautiful and controlled light.
Resources A list of introductory articles and white papers that discuss optics for LEDs.
TIR Lens Guide, by LEDiL, 2012. Available at: bit.ly/LEDiLTIR.
“Quality of Light: Perfect Spectrum, Perfect Beam,” by Soraa, 2013. Available at: bit.ly/1tpsmDb.
CALiPER Report 22: LED MR16 Lamps, by the DOE, Office of Energy Efficiency & Renewable Energy, Building Technologies Program, Sept. 14, 2014. Available at: 1.usa.gov/YYwvTm.
Secondary Optics Design Considerations for SuperFlux LEDs, by Lumileds Lighting, 2002. Available at: bit.ly/1mgWgZB.