If I understand you correctly, you're saying that it's not a relative effect -- that if you left the whole screen blank white for an extended period you'd be causing damage, even if you didn't notice it relative to the other pixels? In that case, you're shortening the life of the monitor by using the system at all, aren't you?

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This is exactly right. You wouldn't notice the burn as much because you're in effect burning the entire screen at the same rate.

Phosphor has an overall life limit that will eventually be reached by all CRT displays, but by displaying one image for extended periods of time, you're essentially 'wearing out' that one area faster than all the others.