Stretched seven-segment LED-displays with increased contrast at high ambient light levels

The optical principles for stretching the segments of displays using specular reflector/diffuse reflector/light guide are compared. A new technique to form light guides by injection molding has been developed. The polycarbonate material used in this process has an index of refraction n = 1.6 and hence a better angle of total reflection compared to the commonly used epoxy material. This allows a higher light transmission from the LED pellet to the bar and hence a good brightness and brightness distribution of the bar in the "on status". In this design the light guides are formed upon a specular reflecting lead frame with the attached seven LED-pellets and finally masked by a properly shaped cover. The reflectivity of the cover and of the bar in the "off status" has been measured under various ambient light levels and optimized for best readability. Whereas most displays have a "black" cover the new design rule calls for a "gray value" of the cover equal to the "gray value" of the bars. The improved contrast and readability at high ambient light levels for this design and others will be shown. This design is fully compatible with the commonly used color filters for contrast enhancement and allows further design flexibility not only for red but also for yellow and green displays.