The relationship between video input to the monitor and the luminance of the CRT. Monitors are required to conform to a narrower range of performance specifications than is expected of commercial receivers. Confirming these tighter tolerances requires attention to measurement details since, for example, the luminance may vary if different areas of the tube face are selected. Light output is routinely measured in the centre of large, uniform “patches” or windows.

Since there is significant “bleeding” of light within a CRT face, the monitor transfer function also decreases with decreasing size of the windows (it is thus reduced for fine detail) and with increasing video level of the raster surrounding the windows.