What is "overflow" in photography?

"Overflow" refers to highlight overflow.

Halo phenomenon refers to the phenomenon that white stripes (bands) are formed from top to bottom in the whole picture when a high-brightness point light source is injected into the picture.

High light overflow phenomenon

No highlight overflow phenomenon

The pixels on the photosensitive element of a digital camera are responsible for collecting photons, converting them into charges through photodiodes, and then forming a digital image through a series of processing. As we have said in the ranking explanation of what is dynamic range, once the bucket of receiving photons is full, the charge converted by excess photons will overflow, and this overflow has no effect on the pixel value, so it will lead to insufficient or excessive sensitivity of the pixel value. When the charge overflows to the next pixel, the next pixel is overexposed in the process of processing photons. As can be seen from the following figure, the bright pixels representing the sun have charge overflow, which makes the darker pixels on the edge of the building overexposed and forms a white belt. Overflow of highlights will not only make the picture lose details, but also increase the chances of purple edges. There is a serious purple edge phenomenon on the right side of the picture below.