The entire surface of an LCD panel is backlit from behind by a light source (CCFL or LED) and the LCD blocks out the light that is not needed when displaying a particular image. Backlight bleeding occurs when this light is not 100% blocked, allowing excess light to "bleed" through the LCD.
This backlight bleeding issue leaves spots of lighter areas on a dark or black background. Unfortunately, almost all LCD panels suffer from at least a small amount of backlight bleed because the opacity of LCD panels is not enough to block all light, though it only causes problems if it can be easily detected by the human eye.
Backlight Bleeding sample
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