This report identifies and characterizes those segments of the population for whom the visibility of electroluminescent diodes is significantly abnormal. Red- and green-emitting GaP and red-emitting GaAsP diodes are considered explicitly. It is concluded tht the most important classes of anomalous and dichromatic observers are the protanopes and protanomalous, for whom the brightness of red electroluminescent diodes is substantially reduced from that perceived by normal observers. No classes of anomalous or dichromatic observers have been found to experience significantly decreased visibility for green GaP diodes.