LEDs and doped polymer light guides for efficient illumination and colour engineering
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This project involves the study of optical properties of polymers doped with TRIMM (transparent refractive index matched micro-particles), and their uses in light guides. The refractive index difference between dopant and host material is small (0.02), so forward transmittance is high, and losses due to backscattering are negligible. Flexible polymer optical fibre (POF) and polymethylmethacrylate (PMMA) rods are being incorporated into an increasing range of lighting and light mixing applications. For energy efficient mixing of red, green and blue (RGB) light-emitting diodes (LEDs) to produce white light and a range of other colours, light is transmitted from the end of a light guide ('endlight'). A major problem here is solved, namely the achievement of uniform illumination, simultaneously with low losses from scattering. Light output from RGB LEDs is shown to be completely mixed by short TRIMM-doped light guides. Alternatively, long lengths of TRIMM-doped POF can be used for 'side-light'. The concentration of TRIMM for these is chosen such that light is emitted from the side walls of the guide to give even illumination along its length. A geometrical method of ray tracing in particle-doped rectangular and cylindrical light guides is derived, and Monte Carlo ray tracing simulations performed for undoped and TRIMM-doped light guides. The evolution of the distribution of ray angles, internal and external to a light guide, with propagation distance are studied. Computer simulations of angular distribution of light emitted from the wall of POF agree with measurements performed using a photogoniometer. Simulations and measurements of light output intensity and colour from RGB LED arrays when projected from the end of a mixing rod, are also presented. Colour calculations agree with photometric measurements of RGB LED output from clear and TRIMM-doped PMMA mixing rods. Results of transmittance measurements and computer simulations show that light losses are almost entirely due to Fresnel reflectance from the entrance and exit surfaces of the rods. Photogoniometer measurements of the angular distribution of light from LEDs are used as a basis for LED source models used in ray tracing simulations. Results of an investigation comparing the effect of using a smoothed LED source model instead of measurement-based models on simulated light output distributions are presented. The light output from LEDs can have sudden peaks in intensity at certain angles, resulting in distinctive patterns with clear colour separation, after mixing in clear polymer mixing rods. These caustic patterns are eliminated by using TRIMM-doped mixing rods, with a transmittance of 90% after Fresnel losses, which can be readily reduced.
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