FAQ: What’s a Back Side-Illuminated Sensor?

Low-light salvation

Digital photography took a digital step forward in reducing digital noise and improving image quality started in 2009 with the first Back Side-Illuminated Sensor. What is it and how is it different from other sensors?

A reader recently wrote: “I’ve been hearing the term ‘back side-illuminated sensor’ when referring to the imaging sensors in some digital cameras. What is it, and what’s the advantage of a camera with a back side illuminated sensor?”

Photo courtesy Sony

A back side-illuminated sensor (BSI) is an imaging sensor for digital cameras with a different layout from other sensors that allows more light to be collected at the pixel level. The resulting images have less digital noise, and low-light performance can be improved by as much as a stop or more.

The tech background

Each sensor is made up of individual picture elements, or pixels, which are laid out in a matrix design. For each pixel, there’s a lens in front, sensors in the back, and wires somewhere in the middle. In a traditional sensor, a matrix with transistors is placed in front of the lenses. So, the light has to travel through a maze of wires and transistors before it hits the lens, and some of that light is blocked. In a back side-illuminated sensor, the transistor matrix is moved behind the lenses. This allows more light to hit each pixel. In terms of signal-to-noise ratio, they are claimed to offer an 8dB improvement, which is significant.


Illustration courtesy Sony

Sounds like a no-brainer, right? Shouldn’t all cameras have these things? Until recently, manufacturing back side-illuminated sensors was prohibitively expensive, and these sensors were more likely to be found in security cameras, microscope and astronomy cameras and other specialized applications where low light was common. However, Sony saw the widespread commercial potential of the sensors and developed a process that let them mass-produce the chips. The first backside-illuminated CMOS Exmor R sensor was announced in 2009, and numerous  compact cameras and camera phones are now using that sensor, or similar designs made by other camera makers.

As new compact cameras are announced, you can expect backside-illuminated sensors to become much more widespread. Canon, Samsung and others have developed their own backside-illuminated CMOS sensors, many of which can be found in recently-introduced compacts such as the Canon G12. Low-end compact cameras probably won’t get BSI sensors for a while, because they add to the cost.

Photos courtesy Sony

The results? Better low-light photography!

Cameras with BSI sensors that have been tested by DxOMark labs and others are showing a dramatic improvement in digital noise levels. In some high-end compact cameras, when combined with improved JPEG noise reduction, it is now possible to get images with decent image quality at ISO 400 or higher.

Bottom line: A positive step

BSI CMOS sensors have become more affordable to make and are being found on a growing number of cameras. If you are concerned about image quality, they make a real difference especially when shooting in low light at higher ISO settings. If you are in the market for a new compact camera, backside-illuminated sensor is a feature to look for!

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