This Small Business Innovation Research (SBIR) Phase I project aims to design and fabricate a novel Substrate-Guided Holographic Diffuser (SGHD) for liquid crystal display (LCD) backlights and for light-emitting diode (LED) lighting. There is a substantial need to reduce the power consumption of backlit LCDs, while making them more affordable to consumers. Finding new techniques and materials both to reduce substantially the number of optical components in LCD backlights and make them more efficient and bright requires application of new scientific approaches. Holographic diffusers give a substantial brightness enhancement in a given direction provided the incident light input is in normal direction with respect to its surface. To date, the necessity to have incident light at a normal angle has prevented the widespread use of holographic diffusers in the LCD backlighting industry, because one or two additional optical films are needed to direct the light from LEDs towards the holographic diffuser. This Phase I effort will result in eliminating the need for one-two additional collimating optical films, and will provide a thin one-component bright LCD backlight with a controlled output angular distribution. The increased efficiency will reduce the power consumption.
The broader impact/commercial potential of this project includes development of highly efficient and thin LED lighting panels for architectural, street, and similar lighting designs. Building a model of polychromatic light propagation and diffraction on SGHD will provide a basis for modeling and understanding of light propagation in a number of confined geometries: e.g., in thin waveguides for integrated optics and, in biological tissues. The commercial impact of the SGHD project includes development of more efficient and more cost effective LCD backlights aimed for mobile phones, and potentially for larger size displays, such as LCDs in notebook computers and in desktop monitors. The SGHD provides more compact illumination light patches for LED lighting. At the same time, SGHD should prove to be more power efficient than current solutions. Other applications of SGHD will include lighting components for medical, scientific, and industrial instrumentation. The success of this project will benefit society by reducing the cost and power consumption of LCD backlighting. The market sectors impacted will include LED lighting, color displays.
Luminit developed a Substrate-Guided Holographic diffuser (SGHD) as an Efficient Backlighting Solution for LCDs and LED Lighting Applications (Figure 1). Current LCD backlights usually consist of 6-7 components: LEDs, light guide, one or two brightness enhancement films (BEFs), and two or three diffusers. The proposed SGHD will advance the backlight technology by providing a single-component backlight diffuser that reduces the number of components to two (SGHD and LEDs) and provides a substantially greater power efficiency. The proposed SGHD has the following advantages: • Replaces a number of components in state-of-the-art LCD backlights (6 to 7) with just two components (SGHD and LEDs), • Provides ~up to 30% energy savings, • Can be applied to LED lighting applications as a luminous pad as well. Liquid crystal displays (LCDs) have become a very important, part of information display in today’s information society. As the demand grows, there is a substantial need to reduce the power consumption of backlit LCDs while making them more affordable for consumers. A current trend is to design and manufacture thinner, less costly, and more power efficient LCDs. A major power-consuming part of an LCD is its backlight module. Backlight units for backlit LCDs account for ~up to 80% of the power consumption and 40% of the material costs, five or even six diffusers/other optical elements may be needed to build an LCD backlight for notebooks, computer screens and for other applications. SGHD is an innovative single component optical element comprising a light guide plate, a brightness enhancement film, and a light-homogenizing diffuser with a controlled angular diffusion angle paired with an LED module. Collimated light from LED module enters the transparent light guide with a holographic diffusion element laminated to it. This light bounces inside the light guide due to total internal reflection. On each bounce, the hologram diffracts light out of the plate with a pre-determined diffusion cone angle and brightness, towards the LCD. The increased brightness is based on the brightness gain property of the holographic diffuser. This SGHD technology will enable LCD manufacturers to meet growing display demands with a lightweight, cost efficient backlighting system. RESULTS OF PHASE I PROJECT With support from the National Science Foundation, Luminit realized the following significant SGHD Phase I achievements. • Demonstrated 50.8 mm × 50.8 mm × 5 mm RGB SGHD diffusers, capable of providing white light for LCD backlighting and for LED lighting applications. • Identified scientific and technical challenges for implementing a SGHD prototype for commercialization. • Began developing a comprehensive scientific ray-tracing computer simulation model with which to design the SGHD prototype for commercialization. • Fabricated SGHD feasibility demonstration samples with an RGB color holographic setup developed at Luminit. • Demonstrated both non-bounce and single-bounce SGHDs. • Worked with two LCD backlight manufacturers to understand current and future trends in the LCD backlighting industry and to align the SGHD project with their commercialization and manufacturing goals. Both companies have expressed interest in the SGHD. These interactions have given us a foundation for implementing our commercialization goals. • Applied for and received Phase IB Supplement funding, the purpose of which was to re-align the Phase I tasks with the commercialization/ manufacturing goals of a major manufacturer and distributor of optical films for LCDs. • Advanced computer ray tracing of light propagation through substrate-guided diffusers • Designed and fabricated SGHD with non-collimated light input. • Implemented comparative evaluation of field-sequential RGB LEDs vs. RGB white LEDs for backlight implementation, the RGB white LED backlighting was identified as a better suited approach to commercializing SGHD. As a result, during Phase I, Luminit successfully completed all major tasks for demonstrating the feasibility of the SGHD concept. Tasks of Phase IB were successfully implemented as well. Identified Scientific Challenges. In the course of the Phase I effort, identified the following major scientific challenges: (i) building a fully-functional computer ray-trace simulation software model for SGHD that accurately simulate both the angular and wavelength selectivity of holographic optical elements, includes multi-bounce geometry for light rays inside the SGHD substrate, provides the flexibility to design non-homogeneous (‘apodized’) diffraction efficiency over the surface of the SGHD, and simulates RGB color retrieval for arbitrary ratios of R:G:B luminous intensities of RGB color primaries in a retrieval light source (white LED); (ii) providing a means to reduce cross-talk in the retrieval of white LED light with a RGB white SGHD diffuser. Identified Technical Challenges. Our experimental work during Phase I effort identified the following major technical/engineering challenges: (i) providing a maximum achievable diffraction efficiency for small-area (50.8 mm × 50.8 mm) SGHDs in ‘one bounce’ retrieval geometry for separate R, G, B colors and for a composed RGB white light; (ii) providing experimentally correct R:G:B ratios of diffracted light from a white LED in order to generate correct white color for LCD backlights.
