Trends in LED Optics
Below is my lighting column published in the June issue of tED Magazine. Reprinted with permission.
The large majority of commercial building luminaires feature some type of optical system that controls the light emission. The type of optics employed ultimately determines where light is directed, how much light reaches that point, and with how much visual comfort. So when evaluating a luminaire or lamp, we are concerned with the pattern the light emission projects onto the space, percentage of produced lumens exiting the luminaire (luminaire efficiency) and degree to which the luminaire controls direct glare.
Distributors selling LED products may benefit by understanding the general principles of lighting optics and, specifically, new developments in LED optical design. This allows them to provide better guidance to their customers and identify products best suited for an application. It also allows them to better appreciate the role of application efficiency and visual comfort in addition to luminaire efficacy (lm/W) in product selection. Some high-performance products sacrifice a degree of efficacy in order to optimize light distribution and visual comfort, concerns that may take precedence in a given project. While energy efficiency is important, the right product choice isn’t always the most energy-efficient.
Optical design is primarily concerned with reflection and refraction:
Reflection: The optical element reflects light from its surface, with the angle of incidence (the angle at which the ray of light impacts the reflector) being equal to the angle of reflectance (where the ray of light is directed).
The contour of the reflector—whether it’s flat, angled, curved, etc.—can change the angle of reflectance. The beam of reflected light tighter with a specular reflector and more diffused with a diffuse reflector.
Refraction: Light travels through the optic, which bends the light of light in a desired direction. A common example is a prismatic lens. Some refractors allow the light to travel along the optic and be reflected back to a reflector for directed emission, which is total internal reflection (TIR).
“Traditional fluorescent, HID and incandescent/halogen sources emit light in all directions,” notes Reed Bradford, Director, Optical Design and Engineering Services, Eaton’s Lighting Division. “Reflectors are very common for these sources, as a large amount of energy can be captured and redirected. Refractors are less common for directing light into a specific light pattern, as a reflector is also generally required. Lenses are often used to disperse or diffuse light and minimize direct view of the light source. High heat from HID lamps typically necessitates using glass.”
New LED optics
Chris Bailey, LC, LEED-AP BD+C, DDI, MIES, Director, Business Development and Product Innovation, Hubbell Lighting, says a range of familiar optical approaches are used for LED, depending on the market and application. Integrated, purpose-built LED products, however, increasingly feature new optical approaches that emphasize refraction over reflection.
LED sources have three characteristics that make them suited to new optical approaches. First, the light source is very small. A reflector is typically designed to be five times the size of the light source, so with a very compact source like LED, the reflector can also be very small. Second, LEDs are one-sided light sources with hemispherical (directional) light output. For that reason, the amount of energy that can be captured with a reflector is smaller than with traditional sources. For this reason, refraction is often used in LED optical systems. And third, LEDs produce little radiated heat, so polymers can be molded into compact and precise lenses, which would be impossible for glass, and mounted directly to the light source as transparent lenses.
“Perhaps the most common optical system in use today for industrial, outdoor and some indoor products is the TIR optic, which is available in a range of materials such as acrylic, polycarbonate, silicone and glass,” Bailey says. “While some reflector-based LED optical systems are in use today, TIR optics have proven to be one of the most efficient means of maximizing both optical coupling—light extraction—and optical efficiency—light transmission. These lenses can be designed as single elements or molded in an array to control the light from multiple LEDs.”
As a result, he adds, in some cases the role of the specular reflector has become aesthetic or used for lamp shielding. Another approach is prismatic films, which may be used instead of or to supplement TIR optics, and another is light guides. Larger TIR optics are now being applied to chip-on-board arrays in addition to individual sources. For tunable-white and some dim-to-warm products, a diffusion film of frosted internal optical elements may be used.
Dr. Stephen Malkmus, Head of Technology Field Optics, OSRAM GmbH, emphasizes that because new LED optics like TIR are manufactured in an injection-molded process, their shape can be arbitrary. This allows for free-form surface shapes that in turn enable greater optical efficiency and more precise emission. “As for lenses for LEDs, they are smaller, can be used as covers and can be more precise if necessary,” he says. “Thus their freeform character gives more freedom to shape the light distribution. In addition, the system can be much smaller with an extremely flat form factor.”
“The optical efficiency is very high, commonly 90-95 compared to 75-87 percent with optics of traditional sources,” Bradford adds. “The geometry of an LED is tightly controlled, and, as such, optical distributions can be considerably more precise than luminaires with traditional sources.”
He points to three major trends in LED optical design:
• Refractors for wide distributions such as area and roadway luminaires and high- and low-bay applications
• TIR refractors for narrow-beam distributions, floodlighting and sports/stadium lighting
• Edge-lit panels that distribute light over a broad surface to minimize glare, well suited to high-end indoor and outdoor architectural applications
An interesting potential emerging trend is 3D printed optical systems.
“This technology has gone through an interesting development to deliver better surface quality, larger optics and higher transmittance,” Malkmus says. “This highly dynamic development is still ongoing and will enable customized optics for low- and middle-volume applications.”
Bailey says the technology presents great opportunities for LED lighting, but needs further development due to high cost, rarity of printers capable of the kind of precision demanded for production optics, and performance characteristics that largely limit application to indoor luminaires. “However,” he adds, “recent developments with 3D printing technology certainly holds promise for cost-optimized and production-grade 3D printed optics in the future.”
“LED optical systems are much more consistent than traditional sources, especially HID sources,” Bradford says. “They have the ability to control and distribute light where it is needed with better uniformity, and can be tailored to meet a broader number of applications.”
“Selling LED products means understanding the final application in detail,” Malkmus notes. “Therein, the optical part is a very important aspect. By making the choice for a certain light source/LED, you can make the optical task much more complicated, and the optical parts much more expensive, so be careful. Electrical distributors should collaborate with a proven company with a solid history of lighting application engineering.”