BLOOMING

 

CCD camera detector arrays consist of a collection of individual components called picture elements (pixels). Each pixel is designed to collect and contain a certain number of electrons generated by the impact of photons. Each pixel is separated from its neighbors by the physical structure etched onto the chip. If the object being imaged is very bright, the pixel may fill to its capacity and further electrons can leak over into adjacent pixels corrupting the charge in that pixel and so on. Cameras designed for daytime use such as DSLRs or cellphone cameras usually have circuitry called anti-blooming gates (ABG) added to bleed off electrons when the pixel gets close to full, in order to prevent the pixels overflowing.

From a scientific point of view this is not desirable. Ideally, one photon would generate one electron. By reading the electron charge you can then determine the number of photons and hence the brightness of the object. The closer to capacity the ABG fitted CCD gets to capacity, the less the trapped electrons represent the photons imaged. My science camera does not have anti-blooming gates (NABG). The relationship of photons received vs electron charge is very linear, however I have to be careful to prevent blooming in the images that I take for brightness measurements. For pretty pictures where the brightness is not critical, there is software that cosmetically corrects the blooming. If the blooms are too great the software will leave visible artifacts.

 Posted by at 16:43

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