I recently had the pleasure of interviewing Scott Wegner and Kevin Broughton, principal optical engineers for Eaton. The topic: LED optics. I’m happy to share their responses with you here. The interview informed an article I wrote for the June 2016 issue of tED.
DiLouie: What is the purpose of an optical system, as applied to a light source in any luminaire or directional lamp?
Wegner and Broughton: The optical system redirects the uncontrolled light emitted from a light source into a controlled beam or distribution specifically formulated for a given task.
DiLouie: What optical approaches are common for traditional fluorescent, HID and incandescent/halogen luminaires and directional lamps?
Wegner and Broughton: Traditional fluorescent, HID and incandescent/halogen sources emit light in all directions. 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.
DiLouie: What optical approaches are common for LED luminaires and lamps? Why were these approaches developed?
Wegner and Broughton: LEDs are unique because they are Lambertian light sources (i.e. hemispherical output). Reflectors are less effective since only a small fraction of energy can be captured compared to traditional sources. Refractors are more commonly used with LEDs. Due to their low temperature, polymers may be used to form small, precise features impossible for molded glass.
DiLouie: What are the benefits of these optical approaches compared to optics for traditional products?
Wegner and Broughton: Because LEDs emit light only into a hemisphere, a refractor or lens can completely encompass the LED, gathering all the energy for redistribution. The optical efficiency is very high, commonly 90-95 percent, compared to 75-87 percent with optics of traditional sources. The geometry of an LED is tightly controlled and as such, optical distributions can be considerably more precise than luminaires with traditional sources.
DiLouie: Based on relative performance and cost, what applications are targeted for each LED optical approach?
Wegner and Broughton: Due to significant drop in costs as well as the extremely long operating life, LEDs are slated to replace all traditional sources in all applications.
Exit and Emergency: LEDs have been offered in exit signs for years due to their high efficacy in green and red. Emergency luminaires benefit from the high efficacy of LEDs, long life and DC operation, which simplifies driver designs.
Recessed: For indoor architectural lighting applications, recessed luminaires were the logical choice to serve as an alternative to incandescent and fluorescent sources. In addition to the obvious energy savings, LED sources are easier to control than fluorescent lamps and make dimming luminaires much easier.
Area and Roadway: Area and roadway lighting are the most demanding applications for optical control. In addition to their high efficacy, LEDs are small, allowing for precise optical control and have an advantage compared to traditional sources.
Troffers (formerly linear fluorescent): This product type is the most recent adoption of LEDs due to the long life and cost of linear fluorescent lamps as well as a lesser need for precise optical control. As costs have continued to drop, and the desire for building controls has increased, LEDs have become the logical choice for ambient lighting.
DiLouie: What are the top three trends in LED optical design?
Wegner and Broughton:
1. Refractors for wide distributions such as those for area and roadway luminaires, and low bay or high bay applications
2. TIR (total internal reflecting) refractors for narrow beam distributions, floodlighting, and sports/stadium lighting
3. Edge-lit panels distribute light over a broad surface to minimize glare. High-end indoor and outdoor architectural applications benefit from the uniform, low glare appearance.
DiLouie: Some manufacturers are offering 3D printed optical systems allowing optics to be made to individual specification. What are the pros and cons and target markets for this offering?
Wegner and Broughton: 3D printed optics are produced directly from CAD solid models, without the need for molds. The setup can be done in several hours and optics delivered to the customer in less than a week. A timely speed to market without a penalty for low volumes is a major benefit for any fledgling industry. This is in stark contrast to the traditional method of injection molded plastic, which requires expensive tooling taking weeks to machine. Injection molding requires large product volumes for reasonable part costs, which poses a risk if rapid adoption of a new product is uncertain. 3D printed optics are deposited on a flat transparent substrate. Hollow voids in the optic are not possible at this point. Typically, both the inside and outside surfaces of a refractor are used to control light. In this case, only one surface can be curved to modify the light distribution thus limiting control. However, it is possible to flip the substrate over, printing on both sides of the substrate for more complicated geometry. Today, there are limits to the maximum optic height and minimum feature size. Sharp details cannot be reproduced. The current materials are somewhat temperature sensitive and can degrade over time if used in close proximity to high power LEDs and are not UV stabilized.
DiLouie: Some manufacturers are now offering TIR lenses for chip-on-board arrays. What are the pros and cons and target markets for this offering?
Wegner and Broughton: For discriminating applications, such as museums, it is desirable for lighted objects to produce sharp, crisp shadows. A plurality of individual optics produces multiple shadows instead of a single defined edge. A single optic for chip-on-board arrays (COB) creates a single shadow. The optic does need to increase in size, however, proportionally to the source size and eventually reaches the limit of manufacturability. A larger optic is inherently more expensive.
DiLouie: What is the overall trend in LED optical design? What will LED optics look like in 3-5 years?
Wegner and Broughton: The practicality of LED optics has already been identified. In general, the optics will not be significantly different from what is produced today. We may see more versatility in materials. Glass is extremely resilient to extreme environments and UV exposure. Optical silicone, since it is flexible, can be molded with slight undercuts that would be expensive to mold with rigid materials. It is possible some optical distributions might be further refined and perfected.
DiLouie: What is the main thing distributors should understand about optics when selling LED products to their customers?
Wegner and Broughton: LED optical systems are much more consistent than traditional sources, especially HID sources. When purchased from reputable manufacturers, the customer can expect to get high quality and consistency without the variability seen by using lamps from different manufacturers.
DiLouie: If you could tell the entire electrical industry just one thing about LED optics, what would it be?
Wegner and Broughton: When compared to traditional light sources, LED optics are highly efficient, precise and consistent. 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.
DiLouie: Is there anything else you’d like to add about this topic?
Wegner and Broughton: LEDs are here to stay. There is no other technology waiting in the wings threatening to make LEDs obsolete.