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Here's what Donals Livingston has to say in Fink's Televisin Engineering Handbook (1957).
He claims the effect is minimized by using a light background, but it looks to me like it is more prominent, although maybe less objectionable becuase you can see detail in the background away from the halo.
The effect is caused by redistribution of electrons on the IO target. It is very similar to the early Xerox machines, which could not reproduce a large black area, but only made full black near the edge of the black area, where it was next to a lighter area.
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