BSI stands for backside illuminated (also called back-illuminated). It describes a type of image sensor in which the wiring and circuits are placed behind the light-sensitive photodiodes instead of in front of them.
Because the wiring no longer blocks part of the light, more light reaches each pixel. This gives better sensitivity, less noise and better image quality, especially in low light. BSI sensors are now common in smartphones and in many modern cameras.
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