BSI Sensor

A BSI, or back-side illuminated sensor, rearranges the layers of a camera sensor so that the wiring and circuitry sit behind the light-gathering photodiodes rather than in front of them. In a conventional front-side illuminated sensor, the metal wiring layer partially blocks incoming light from reaching the photodiodes. Moving that wiring to the back of the sensor exposes more of each pixel's surface area to incoming light, increasing sensitivity and improving performance in low-light conditions.

The practical effect of BSI on image quality is most visible at higher ISO settings. BSI sensors produce less noise at elevated ISOs compared to front-side illuminated sensors of the same generation and resolution, because each pixel captures more light per unit of exposure. Dynamic range, particularly in shadow areas, also benefits from the improved light-gathering efficiency. In M43, BSI sensors are used in OM System bodies including the OM-1, OM-1 Mark II, OM-3, and OM-5 II, and in Panasonic bodies including the G9 II and GH7.

BSI should not be confused with stacked sensor design, which is a further refinement. A stacked sensor adds a separate processing layer bonded beneath the BSI sensor, allowing the sensor to read out data much faster. All current stacked sensors are BSI, but not all BSI sensors are stacked. The OM-1 Mark II uses a stacked BSI sensor; the OM-5 uses a BSI sensor without stacking. The stacked design enables features like 120fps burst and faster electronic shutter with less rolling shutter distortion.