Managing Optical Performance - AN117

The viewability of standard EL displays with reflective aluminum rear electrodes typically require a contrast enhancing filter for improved visual performance. The inherent contrast of Planar's integral contrast and brightness enhancement (ICETM and ICEBriteTM) technology is so great that any additions to the front of the display can only detract from the basic performance. Now that high contrast is built into the display, a secondary effect of surface reflections become a relatively more important part of the contrast the user sees. The surface of every element in the display optical system can reflect light and reduce contrast.

Wherever mismatched indices of refraction meet (like air meeting glass) light will be reflected, and contrast will be lost. Lens designers increase contrast in their optical systems by coating the surfaces of each element. The same technique can be used in display optical systems. Display contrast is typically expressed as contrast ratio. Contrast ratio is the measured brightness of an on area / measured brightness of an off area. The measured brightness of an on area is the sum of the light emitted by the display and the light reflected by the display. The measured brightness of an off area is the suLm of the off brightness of the display and the light reflected by the display.