Glossary

BSI sensor

What is a back-illuminated sensor?

Back-illuminated sensors, also known as backside illumination (BSI or BI), hit the digital photography market ten years ago, offering significant image quality improvement in low-light situations. In this particular type of image sensor, imaging components are arranged in a manner that increases the amount of light captured: while traditional sensors have wires and transistors placed in front of the lens, BSI technology allowed for the placement of the transistor matrix behind the lens, which prevents illumination loss.

BSI manufacturing was initially highly expensive, which restricted its application to specialized cameras for low-light conditions (such as security cameras). However, the considerable improvement in image quality – especially the reduction in digital noise levels – provided by backside sensors motivated the photography industry to invest in mass BSI production, resulting in lower costs.

Sony introduced the first back-illuminated sensor developed for the consumer market in 2009 (the Exmor R CMOS sensor), and competitors soon followed suit by exploring similar sensor designs. Although Sony is to be thanked for introducing BSI to consumers, this technology is now widespread in the digital photography industry, having been embraced by every major brand. Nowadays, back-illuminated sensors can be found not only in high-end cameras but also in mobile devices such as smartphones and tablets.

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