Light Management Films In Led Luminaire Design
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1) The document discusses the role of light management films in LED luminaire design. It describes how light management films can help obscure individual LED points and create a more uniform glow, while efficiently extracting light. 2) Two types of films are covered: diffusion films, which scatter and soften light, and collimation films, which concentrate and shape light. Proper film and cavity design are needed to balance light redistribution and extraction. 3) Measurements of films alone can be misleading, as film performance depends on interacting with the optical cavity. Integrating sphere measurements capturing total system output are most informative.
Light Management Films In Led Luminaire Design
- 1. Light Management Films in LED‐based Luminaire Design Donald Hirsh & Robert Wood Bright View Technologies, Inc. Morrisville, NC USA Introduction LED-based designs for every form of illumination are expanding at a rapid pace. With LED manufacturers promoting exceptional product lifetimes, environmentally friendly materials, and high efficacy, lighting designers are following suit with new generations of product appearing on the market from both startups and the mainline players in the industry. Efficiency figures posted by Department of Energy show solid state lighting (SSL) far ahead of incandescent lighting, and rapidly overtaking compact fluorescent (CFL) lighting efficacy in 2007 1 . Regarding lifetimes, Cree rates its XR-E power LED at 50,000 hours with lumen maintenance of 70% of original output as long as LED junction temperatures do not exceed 80° C 2 . The winning advantages of LEDs must be translated into winning systems designs for widespread adoption to take place. In Bright View’s work, supplying light management films to the display and illumination marketplaces, we have observed many luminaire designs that are effective in mechanical, thermal and electrical efficiency, but relatively fewer that are optically efficient as well. Ultimately, if a light fixture does not exhibit optical efficacy, it will not fulfill the potential of LED technology. There are several approaches to achieving a commercially acceptable combination of aesthetic quality and optical efficacy. Light management films, used in conjunction with well-designed optical cavities, can be a critical and enabling component in good systems design. Two recent reviews provide overviews of efficient LED lighting design principles 3,4 . Solid state lighting metrology is well documented 5 , but until recently little has been written about efficient optical design 6 . 1 DOE Solid‐State Lighting CALiPER Program, detailed at http://www.netl.doe.gov/ssl/comm_testing.htm 2 Negley, Jerry and van de Ven, Anthony, “Essentials of designing efficient luminaires with LEDs,” LED Magazine, February, 2008 3 Negley, Op Cit. 4 Riesebosch, S., Essentials for Designing LED Luminaires. LEDs Webcast, LEDs Magazine, Pennwell Publishing, (2008). 5 Approved Method: Electrical and Photometric Measurements of Solid‐State Lighting Products, IES LM‐79‐08, IESNA, 2008. 1
- 2. In this white paper we will present a review of the critical role of light management films in creating attractive solid-state lighting fixtures. A Hundred Points of Light In the not-too-distant future, we may be able to design high brightness luminaires with a very small number of very bright LEDs. The newest LED packages such as PhlatLight LED packages from Luminus or the recently announced XLamps-MC from Cree point to this. But for the time being, virtually every SSL developer finds the most cost effective way to deliver high flux fixtures is to develop luminaires constructed from arrays of LEDs, often-times large arrays with dozens or even hundreds of elements. For example, Cree recently introduced the LR6, an LED based downlight compatible with conventional ceiling fixtures. At 600 lumens, the LR6 has output comparable to a 65 Watt BR30, 50W PAR30, or a 100W A-lamp when installed in typical 6" downlights. The LR 6 also provides an impressive CRI of 95. This is achieved through the use of forty-two mixed color LEDs in the LR6 light engine. Per‐LED or Per‐System Optics There are two basic schools of thought in the development of LED-based fixtures. In the first approach the system and optics are designed around the LED itself. The point-source of light is integral to the aesthetic quality of the LED and the LED fixture does nothing to hide this. This type of fixture “celebrates” the pointy light source and makes it an integral part of the design. Diffusion and mixing are performed in the environment rather than in the luminaire. Fixtures from Beta-Kramer-LED are exemplary of this school of design and employ discrete optics, coupled to individual LEDs to deliver efficient light extraction from the luminaire. Figure 1: Emitting Surface Drawing, & Picture of a Kramer‐LED Linear Pendant This class of design is naturally aligned with contemporary or modern design and relies on the diffusive character of surfaces in the illuminated space to soften and diffuse the light. However, the corollary is that direct, multi-point LED illumination does not emulate traditional lighting designs compatible with 6 Lewin, Ian, “Absolute photometry measurement has relative benefits for LED and SSL performance evaluation,” LED Magazine, August, 2008. 2
- 3. traditional architectural and interior design. Under some circumstances, the point-source nature of the individual LEDs can pose glare problems. The other school of thought in LED-based luminaire design is to obscure the individual LEDs and seek to have the fixture “glow” like a conventional luminaire. One approach is to obscure by the use of indirect reflection in the fixture itself. This is the design principle at work in the downlights developed by Renaissance Lighting. Figure 2: Cross Sectional Illustration of Renaissance Lighting ED04/6 Downlight Another approach is to obscure a multi-point grid of LEDs by the use of diffusion films in the path of the LEDs. These two photographs of a representative multi-LED downlight (with and without its diffuser) illustrate the approach. Figure 3: A multi‐LED downlight with and without a task‐optimized diffuser The challenge for both these designs is the conflicting imperatives to redistribute the flux from the multiple LEDs and to extract as much of the redistributed light out of the luminaire and into the illuminated space. In both approaches, the design of the optical cavity and its interaction with the components within the fixture are critical in determining the overall light extraction efficiency. 3
- 4. Light Management Films There are two major classes of light management films used in illumination applications, diffusion and collimation films. The former scatter the illumination source, the latter concentrate or shape the source. From stretched animal skins, to patterned and etched glass, people have attempted to soften the glare of harsh illumination sources - well, ever since there have been harsh illumination sources to diffuse. For example, the emergence of frosted glass diffusers in the 1920s was a significant advancement in electric lighting because the glare of a single filament could be could be converted to a much larger glowing orb with less glare and softer shadows. The most common light management film in illumination applications, diffusion - no matter what the technology - redistributes the light through scattering and/or refraction. The ability to achieve an acceptable balance between light extraction efficiency, light redistribution, and aesthetic quality relies heavily on the design of the light management film. Modern micro-optic production techniques have allowed for the creation of a new generation of diffusion structures that offer considerable flexibility in distribution pattern and level of diffusion. Modern diffusers can exhibit deterministic patterns through refraction, rather than scattering – thereby delivering far-field geometric patterns with greater precision than previous technologies. Management of the interaction between the diffusion film and the optical cavity permits optimization of the quality versus quantity proposition necessary in multipoint luminaire design. Unlike previous generations of diffusion materials, newer microstructure-based diffusers can be “tuned” to the properties of the optical subsystem to achieve the desired result. Bright View’s development of ACEL LED light-management films was driven by 4 major criteria, common to all LED-based luminaires. The film must; Depixellate (hide) the individual LEDs Create a homogeneous, pleasing “glow” Extract light efficiently from the luminaire Provide optimum shaping of the far-field light pattern Bright View’s ACEL diffusion films are designed to work in combination with efficient back reflectors lining the optical cavity. The diffuser reflects a portion of incident light back into the light cavity in order to create the required mixing. Light that is reflected back into the cavity must be efficiently recycled by the reflective material lining the cavity to achieve high efficiency. We found the use of reflective materials having white diffuse reflectance exceeding 96% could give light extraction efficiency as high as 92% based on integrating sphere measurement. The optical effect of the Bright View film in conjunction with the reflective lining material creates a new light emission pattern, a cone that may be much wider than the emission profile of the LED itself. The measurement of the total light output from the luminaire can only be done using a large format integrating sphere or goniophotometer capable of capturing light output into a full hemisphere (+/-90° in all directions). 4
- 5. c d a b LED backplane Diffuser Figure 4: Side view of luminaire emission/reflection: the light paths within an LED luminaire may be classified by (a) primary emission from LEDs within a defined cone; (b) reflection from the diffuser; (c) reflection from the light cavity; and (d) transmission through the diffuser within a cone that is broader then the LED primary cone. Complex interaction among the components of the luminaire determines the final resulting light cone and aesthetic “look” of the light fixture. Because of the strong interactions between optical cavity and light management film, measurement of either component in isolation can produce misleading results. For example, measuring diffuser film total transmission via ASTM D1003 7 may show a particular diffuser has less than 80% transmission, while an integrating sphere measurement of the total light output from a luminaire equipped with the same diffuser may show more than 90% extraction. How can this be? One of the subtleties of diffusers used in luminaires is that a significant portion of the light incident on the diffuser surface is reflected, rather than transmitted on the first pass. Reflected light (path (b) in the figure above) may still be extracted from the luminaire if it can be “recycled” through efficient reflection from the cavity (path (c) in the figure). Thus, it is important to consider the efficiencies of both the diffuser and the cavity. The geometric design of the LED luminaire has profound and often unpredictable effects on efficiency, depixellation, far-field light pattern, and the aesthetic “look” of the light fixture. Spacing between LEDs and distance of LEDs from the diffuser plane are the most important parameters, but the exact shape of the cavity plays a critical role as well. As such we have developed a variety diffuser models to work with specific luminaire designs, such as the generic example shown below 8 . 7 ASTM D1003 ‐ Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, (2000). 8 This design is not modeled or optimized in any way. It is a simple reference design that allows us to describe a series of luminaire production techniques. 5
- 6. BVT-LED-ACEL Diffuser Light mixing cavity LED Backplane (heat sink not shown) White LED 0.5W 110 70 White reflector R > 96% Dimension in mm 45 Side View End View Figure 5: Generic LED‐Luminaire Layout In Figure 5, an illustration of a generic LED luminaire design, the LED backplane consists of twelve white LEDs (~0.5W each) mounted on a white reflective board and heat sink. The light mixing cavity is conically shaped and lined with a white reflector having diffuse reflectance >96%. The Bright View film is mounted behind a rigid clear acrylic disk, with the film clamped along its edges, providing an optical air gap between the film and the disk 9 . The design of optical cavities in luminaires is incredibly varied and modes of film/cavity/source interactions are complex. Task-optimized modeling software such as Photopia, Optis or LightTools may be employed to create useful and predictive performance models, which can offer illuminating evidence of these complex interactions. But an excellent example of the strong interaction between certain kinds of films and optical cavities comes from the world of backlight design, where Bright View has a substantial product development and systems modeling program creating collimation films. Designed for the LCD-backlight market, Bright View’s LCD-ACE-1F film is a single film solution for LCD backlights – the illumination components of LCD TVs and monitors, replacing multiple diffusers and 9 Application notes: 1. For best results, the ACEL diffuser must be mounted with its backside facing the LED sources and its front side facing the viewer space. When placed on a flat surface with the clipped notch in the upper right‐hand corner, the front side will be facing up and the backside will be facing down. 2. Both surfaces of many ACEL Light Management Films contain optically active structures and therefore must not be mounted in a manner that causes optical coupling or planarization of either surface. Use edge clamping or edge taping to mount the film in the luminaire. 6
- 7. gain films with a single film. The backlights illuminate the liquid crystal displays and are designed to stringent tolerances for uniformity of illumination field (flux, colorimetry and homogeneity) and on-axis gain. LCD-ACE-1F performs homogenization and mixing as well as delivering on axis gain. Figure 6: Cross Section of a Liquid Crystal Display (LCD) The gain profile of the film in the floor-to-ceiling axis, compared to the roughly lambertian profile of a backlight without a film, looks like this. Figure 7: Normalized Profile of 32” LCD TV Backlight With and Without BVT Collimation Film Experimental data corroborates the graph below from one of our system models, which shows the gain of our 1F film as function of the efficiency of reflector used in the optical cavity which our film covers. 7
- 8. Figure 8: Gain versus Cavity Efficiency ‐ Using BVT‐LCD‐ACE‐1F Basic measurements of light management films do not tell the whole story Integrating sphere measurements of Bright View’s ACEL Light Management Films show efficiencies above 92% when coupled with an efficient optical cavity. This same film, when measured in a less controlled setting with a conventional hemispheric-headed photometer and a single LED might demonstrate 55-70% transmission. And the same film/configuration when measured with a spot photometer yields stranger results still, as the table below illustrates. Generic Square Spot Hemispheric LED‐based Photometer Photometer Luminaire (cd/m²) @ 8” (cd/m²) Uncovered 6800 1005 LEDs BVT Diffuser 3240 914 BVT Diffuser + 1/8” Clear 940 Acrylic Generic Acrylic 812 752 Diffuser 715 Sabic 3805GZ Table 1: Measurements of a Luminaire with Photometers 8
- 9. Clearly, simple measurements of transmission, reflectivity and haze do not tell the whole story about optical films when used in illumination systems. This is one of several cases where system efficiency, particularly luminous efficiency, is not simply the linear multiplication of components multiplied together. In good design, system performance can be greater than sum of the parts. Conclusion Common wisdom holds the acceptance of LED lighting rests on three features: green technology, long life, and energy efficiency. But LED luminaires will not be accepted until they are also beautiful, aesthetic and functional. Diffusion and collimation are already prevalent in almost every form illumination and makes our living and working environments feel warm, comfortable and inviting. A working environment with properly diffused illumination makes for good visibility in task-driven work and lack of glare. Good illumination – uniform, smooth and glare-free – is what makes for space that “feels good” to be in. Bright View’s ACEL Light Management Films give the lighting designer advanced new elements to include in modern, efficient and attractive luminaire design. 9