Contrast enhancement of light emitting diode displays

This paper describes a technique for calculating the resultant contrast of displays incorporating various filters or filter combinations. In particular, contrast of such displays employing red or green light emitting diodes (LED's) as the active elements is tabulated. The results indicate that adequate contrast enhancement of red LED's in high light level environments is usually achieved using neutral density or bandpass filters with antireflective coatings. Green LED'S must be designed to have a high degree of specular reflectance so that a circular polarizer may be employed in order to achieve similar results. A discussion of required filter bandwidth to accommodate shifts in peak spectral output of LED'S due to manufacturer's tolerances and temperature effects is also included.