Halo is the circular region around a bright light that appears “brighter” – It’s caused by elastic collisions of electrons with the MCP surface which subsequently then bounce off and down another hole. Halo’s are the same size all over the screen and the size is dictated by the distance between the photocathode and the MCP. Basically, it’s the round circle around lights when you look at them with Night Vision and it’s generally used as an indication that you’re looking at something that’s too bright.
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