A pixel on a digital camera sensor collects photons which are converted into an electrical charge by its photodiode. As explained in the dynamic range topic, once the "bucket" is full, the charge caused by additional photons will overflow and have no effect on the pixel value, resulting in a clipped or overexposed pixel value. Blooming occurs when this charge flows over to surrounding pixels, brightening or overexposing them in the process. In the example below, the charge overflow of the overexposed pixels in the sky causes the dark pixels at the edges of the leaves and branches to be brightened and overexposed as well. As a result detail is lost. Blooming can also increase the visibility of purple fringing.
Some sensors come with "anti-blooming gates" which drain away the overflowing charge so it does not affect the surrounding pixels, except for extreme exposures (very bright edge against a virtually black edge).
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
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