Introduction Contrast ratiois defined as the ratio of the luminance of an excited area of the CRT screen to that of an unexcited or dark area of the screen. Contrast is normally measured under various ambi ent lighting conditions in order to simulate the environment in which the CRT will be used. Some of the factors that affect th e measured Contrast ratio of the CRT are: 1) Internal reflections of light from excited areas falling on unexcited areas af the screen. 2) Unexcited areas being excited by scattered electrons and/or photons. 3) Ambiont Light falling on the CRT screen.