Blooming
As Blooming ( engl. Bloom "bloom", such as "blooming") is known in the digital photography of the formation of a bright spot is a local overexposure. This effect occurs mainly in older CCD - cameras on, if they do not have an anti-blooming circuit. However, it can also occur with digital cameras with CMOS sensors.
The reason for this is that the individual light-sensitive elements (pixels) of a CCD sensor can only absorb a limited amount of charge (caused by photons ) . If a picture element is exposed too strongly, this amount of charge is exceeded and the cell transfers the excess charges to the neighboring cells. Since these can of course only hold a limited amount of charge, the blooming effect can expand significantly depending on the illuminance. The charges mainly flow between the cells that are coupled to each other for charge transport when the CCD sensor is read out. For all video cameras and most other CCD cameras this is the vertical direction (parallel to the short side of the picture). Most of the cells affected by blooming therefore have the maximum amount of charge and appear overexposed in the image (maximum brightness; with color cameras at least one color is saturated or white if all colors are saturated).
As a measure to counter the blooming effect, anti-blooming gates (ABG) can be installed between the cells so that excess charges can be released. However, since these ABGs reduce the pixel size and thus the sensitivity, this solution is not always practical. In addition, an anti-blooming gate can also ensure that, with long exposure times, charges flow away before a storage cell is full. This is why highly sensitive CCD sensors without ABG are still produced today.
Blooming should not be confused with the “ smear ” effect, in which the charge carriers do not continue to flow undesirably, but rather the exposure of the CCD sensor while the charges are shifting leads to a streak. In contrast to the mostly sharply defined blooming stripe, the stripe of the smear effect always extends to the edge of the picture and does not appear completely overexposed.