I recently had the pleasure of interviewing Chris Bailey LC, LEED AP BD+C, DDI, MIES, Director, Business Development and Product Innovation, Hubbell Lighting, Inc. The topic: trends in LED optics. I’m happy to share his 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?
Bailey: The purpose of an optical system is to redirect the light emitted from the source or sources as a means to achieve a desired photometric effect. This may vary from the overtly creative to the highly technical. Whether the goal is to enhance productivity, reduce errors, prevent crime, increase occupant safety, satisfaction and comfort or to simply create drama, light is capable of doing so much, given the appropriate optical system.
DiLouie: What optical approaches are common for traditional fluorescent, HID and incandescent/halogen luminaires and directional lamps?
Bailey: Light emitted from most any lamp is controlled in a similar way, through reflection and refraction. Lamp characteristics such as surface temperature, lamp profile/shape, lamp source distribution pattern, lamp envelope (size), surface brightness, etc. play a role in how optics are physically carried out. To the extent that these characteristics are somewhat similar, the optical systems deployed are similar as well.
DiLouie: What optical approaches are common for LED luminaires and lamps? Why were these approaches developed?
Bailey: Just as the use and application of LED technology broadly covers general ambient and specialized categories, the optical systems in use today for LED luminaires and lamps ranges from the simple, like frosted diffusers, to the complex and elaborate, for example nested TIR (total internal reflection).
In residential applications, as well as some recessed linear commercial applications, it is relatively common to use an interior reflective white surface or frosted (at times prismatic) diffuser, prismatic film, or a combination thereof, to collect, shape, diffuse, control and transmit the light emitted from a luminaire. Typical materials for both recessed and suspended linear products include matte and mirrored linear reflectors, molded prismatic lenses (for control and diffusion), louvers and light guides.
Recessed downlights intended for architectural and commercial applications more frequently utilize specular reflectors in an effort to maximize efficacy, enhance photometric precision, manage striations, minimize the apparent brightness of the reflector surface and generally blend into a commercial or architectural ceiling. Typical materials and approaches include Alzak reflectors, molded baffles, Fresnel lenses and regressed optics. More recently, the use of TIR optics and/or prismatic films have been deployed in this category of products. As such, the role of the specular reflector, in some cases, is has become simply aesthetic or used for lamp source shielding. The use of diffusion film of “frosted” internal optical elements are also used today where different (color or color temperature) LEDs are being utilized (ex: multi-die LED array) within a single luminaire; as would be in the case in a “color tunable” and some “dim-to-warm” products.
Perhaps the most common optical system in use today for industrial, outdoor and some indoor products is the TIR optic, which are available in a range of materials such as acrylic, polycarbonate, silicone and glass. While some reflector-based LED optical systems are in used today, this approach has 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 and encompass some of the necessary mechanical provisions.
DiLouie: What are the benefits of these optical approaches compared to optics for traditional products?
Bailey: While some of these optical approaches should seem quite familiar, some are distinctly new (ex: light guides and TIR optics). Given the relatively small format of LED sources and inherent directionality of light emission, these approaches uniquely enable LED luminaire designers to imagine new luminaire physical profiles and maximize luminaire efficacy.
DiLouie: What are the top three trends in LED optical design? Light guides, TIR and diffusion
Bailey: Light guides, TIR and combination optics (reflector + TIR overlay).
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?
Bailey: 3D printing holds much promise for the solid-state lighting industry. Given the relatively high cost of injection molding reflectors or TIR optics, 3D printing enables optical designers and manufacturers to either quickly evaluate several optical approaches or confirm design assumptions prior to committing to the cost and lead-time for optical tooling and first article parts. While the precision of some 3D printers might be high enough for production optics, these printers are still rare, and the piece part pricing is still much higher than traditional injection molding. Also, the UV stability, transmission (clarity) and surface finish available from the resins and 3D printers in use today may limit the applications to those indoors, at least for the time being. However, the recent developments with 3D printing technology certainly holds promise for cost-optimized and production-grade 3D printed optics in the future.
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?
Bailey: Generally speaking, the sheer size of chip-on-board LED array require larger discrete optics. This can be somewhat limiting when molding precision plastic parts. Also, efficiently coupling to a plurality of LEDs within a single LED package can be challenging as well. If a generally symmetrical optical pattern is desired, then a COB with a TIR or combination (reflector + TIR) optic may hold promise. However, it may not be ideal for COBs – at least today – in asymmetric outdoor distributions or those applications requiring high center beam candlepower (CBCP). Companies, such as Fraen, have developed nested two-piece TIR optics for COBs which utilize a transition layer to aid in the collimation of light emitted by larger COB arrays. Most recently, many manufactures have resorted to “oversizing” the COB and lowering the current density to the internal LED array which helps overcome lower optical efficiencies by increasing source efficacy. While this may lead to relatively high delivered system efficacies, this approach does not necessarily improve lumen utilization or light on target.
DiLouie: What is the overall trend in LED optical design? What will LED optics look like in 3-5 years?
Bailey: As luminaires are purposely designed around LED sources, versus the common practice of “LEDifying” luminaires designed for legacy technologies, optical systems will continue to grow in significance. It is the “business end” of the luminaire. It represents a significant opportunity for specifiable differentiation and will remain such for years to come. As the overall physical footprint of LED luminaires scales, presumably down, to LED sources, optical systems may also play an increasing role in the overall fixture design and aesthetics.
DiLouie: What is the minimum electrical distributors should understand about optics when selling LED products to their customers?
Bailey: It’s not about lumens and efficacy. Some of the better performing luminaires may require less light (and corresponding fixture power) to meet the required light levels. Also, some products may sacrifice system efficiency to provide significant improvements in uniformity, reductions in glare and increased target efficacy. Suppliers should demonstrate a solid understanding of how to make LEDs work for you and provide compelling application performance for your customer though advanced optical design and technology.