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Interest in improving backlight performance is increasing rapidly, driven by the large world-wide investment in high-perfonnance LCD display screens. At the simplest level, afl backlights consist of one or more light sources (usually tubular fluorescent lamps) and an optical system. The optical system collects the fluorescent light and causes it to pass through the LCD display screen towards the viewer. NiOptics' proprietary optical designs have been used to increase flat-panel backlight luminance by a factor of 2 to 2.5 over comparable conventional systems. This dramatic improvement results from matching illumination to preferred viewing angles. The maximum theoretical improvement is quantified by deriving the fundamental thermodynamic limits which restrict two key backlight sub-systems, the light source and the distribution panel. These sub-system analyses are then combined to show thesurprisingly complex set oftrade-offs intrinsic to all backlight designs, completely independent ofthe specific device architecture. Specifically, we show that backlight luminance has an upper bound given by Po ILot< x— D1D2Sin90t cxlcclL hsinOi 1—cx In this expression L0t is the output luminance, øout is the vertical output half-angle, P is the lamp luminous output, Di and D2 are the linear dimensions of the backlight emission surface, h is the backlight thickness, dL is the lamp I.D., and a is the lamp's effective self-absorptivity coefficient. Two limiting special cases of this expression are developed to illustrate the design trade-offs. The comparisondemonstrates that superior performance requires that angular reduction be achieved within the distribution panel. Finally, we compare these two theoretical limits to NiOptics' experimental results. The results show that NiOptics proprietary backlights perform at 66% of the theoretical limit, with shortfall due primarily to material absorption and reflection losses.
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