In the case of LCDs an all white screen happens to be the maximum brightness. Colors and gray scales are obtained by reducing the brightness of one or more colors. LCDs backlight the screen to 120-235 cd/m2, then subtract color(s) to produce features.
For CRTs an all white screen represents the maximum average current that can be handled. However, when displaying material like white text on a black screen, bursts of white much brighter than the average can be interspersed with areas of black to achieve the same average current load as the all white screen. CRTs can typically deliver current peaks that allow areas of the screen to approach 500-600cd/m2.
Illumination measurements made on uniform white screens do not reflect the actual brightness of the screens in normal use very well. In practice, a CRT with a brightness of 100 cd/m2 is likely to be more readable in bright lighting than an LCD with a brightness of 200cd/m2 -- especially when viewing small amounts of light colored material on a dark background as when reading white text on a black screen.
Cold cathode fluorescent backlighting can bring practical LCD brightness to levels comparable with or better than CRTs. But it needs more power than conventional LCD backlighting. The increase battery drain may not be acceptable if the equipment is to be used in portable applications.
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