In the camera, CMOS is a kind of photosensitive sensor (the other is CCD), which is responsible for sensing light and converting it into electrical signals, and acting as the film in the film machine.

BSI CMOS is called back-illuminated CMOS, which is a kind of CMOS. The structure of traditional CMOS is lens, filter, circuit and photodiode from top to bottom. The light passes through from top to bottom, finally reaches the photodiode, and is illuminated to generate an electrical signal. This traditional structure is relatively easy to manufacture, low cost and high transmittance, but the circuit layer has a certain influence on the transmittance, which makes the photodiode at the bottom receive insufficient light. In order to solve this problem, back-illuminated CMOS came into being. The principle is to move the circuit layer below the photodiode to reduce the occlusion, so as to maximize the sensitivity of the photodiode.

The principle is simple, but it is not easy to implement. This design was put forward by 1990. However, due to the manufacturing technology, even if the sample can be developed, it has not reached the commercial level. It was not until 2009 that Sony first introduced commercial mass-produced back-illuminated CMOS and registered the Exmor R trademark.

The disadvantage of back-illuminated CMOS is that the bottom circuit layer is prone to crosstalk, which affects the image quality at low sensitivity (contrary to the shortcomings of traditional CMOS) and is gradually improving.

At present, there are only a few companies that can manufacture back-illuminated CMOS, including Panasonic.