White Paper: Performance Improvements In Back Panel Display Lighting Using Near-Lambertian Diffuse High-Reflectance Materials

LCD backlighting applications require diffuse illumination over an extended area of a display unit while maintaining high luminance levels. Since such applications involve multiple reflections within a reflective cavity, the efficiency of the cavity can be affected significantly by relatively small changes in the reflectance of the cavity material. Materials with diffuse rather than specular (or mirror-like) reflectance scatter light, averaging out hot spots and providing a uniform field of illumination. Reflectors with specular components tend to propagate non-uniformities in the illuminator system. The result is a spatial variation in brightness visible to the viewer of the display. While the undesirability of specular materials for such applications has been widely recognized, some diffuse materials in common use exhibit a significant specular component. This paper describes a method for measuring the specular component of such materials, and presents a simple approach to evaluating the effect of such secondary specular behavior on the performance of a backlight cavity. It is demonstrated that significant differences exist among available diffuse reflectance materials, and that these differences can lead to significant differences in the performance of the displays in which these materials are used.