Backside Illuminated Sensors: How They Improve Low Light Quality

What is a backside illuminated sensor?

A conventional CMOS sensor, called front side illuminated (FSI), places the photodiodes, the light-sensitive elements, under a layer of metal wiring that carries signals across the chip. Light has to pass through that maze of traces and insulating layers before reaching the photodiodes. Some photons get reflected or absorbed before they ever register.

A backside illuminated sensor flips that arrangement. The silicon wafer is thinned and turned over so the photodiodes face the incoming light directly. The wiring layer moves behind the photodiodes, out of the light path. This is not a change to pixel count or sensor size; it is a change to how efficiently each pixel captures the photons that reach it.

• FSI: light passes through wiring before hitting the photodiode
• BSI: light hits the photodiode first, wiring sits behind
• Same pixel size, same resolution, but more light captured per pixel

Front side illuminated vs backside illuminated: what changes

The practical difference is photon collection efficiency. In an FSI sensor, the metal traces that route electrical signals occupy part of the pixel area. They block light. The smaller the pixel, the larger the fraction of the pixel area lost to wiring, and the stronger the effect. On large pixels, the wiring occupies a smaller share of the area, so the penalty is modest. On small pixels, as used in smartphones, action cameras, and compact cameras, the penalty is more significant.

BSI removes that tradeoff. Because the photodiodes face the light source directly, the fill factor, the proportion of the pixel area that is actually light sensitive, rises. That is why BSI is especially valuable on small sensors where pixels are already tiny. For a given pixel size, a BSI sensor collects more photons than an FSI sensor with the same design.

Why BSI improves low light quality

Low light performance is governed by how many photons each pixel captures and how much noise the camera electronics add. More captured photons means a stronger signal relative to the fixed noise floor. BSI increases the signal without increasing the noise, so the signal to noise ratio improves. In practice this shows up as less visible noise at high ISO settings and cleaner shadows.

The improvement matters most when pixels are small. A 1-inch sensor with 20 megapixels has tiny pixels. An action camera with a smaller sensor has even tinier pixels. BSI helps those cameras behave as though their pixels were larger than they physically are. For full-frame cameras with large pixels, the benefit is smaller but still present, which is why most full-frame sensors introduced in the last several years use BSI construction.

BSI also helps with other image quality factors. Some BSI designs include a deeper photodiode that captures light across a wider range of angles, which can improve edge performance with wide aperture lenses. And because the wiring is no longer in the light path, there is less optical crosstalk between neighboring pixels, which can improve color accuracy at the pixel level.

• Higher quantum efficiency: more photons become electrons
• Better signal to noise ratio at high ISO
• Cleaner shadows and smoother tonal transitions
• Reduced crosstalk between adjacent pixels

Where you will find BSI sensors

BSI is not exclusive to any one brand or camera class. Starting in the early 2010s it appeared in smartphone sensors and compact cameras, where the benefit is largest. Today BSI is the default architecture for most newly designed sensors, from 1-inch compacts to APS-C and full-frame mirrorless cameras.

The Sony Alpha line and Nikon Z cameras use BSI or stacked BSI sensors in many models. Canon has adopted BSI in several of its sensors. Action cameras from GoPro, DJI, and others use BSI sensors in most current models. If you buy a camera released in the last few years, there is a good chance its sensor is backside illuminated.

The key point for buyers: BSI is a sensor construction technique, not a feature you select. You do not choose BSI on its own; you choose a camera that uses it. The best way to benefit from BSI is to pick a camera within your preferred sensor size and resolution range that uses a recent sensor design.

Stacked BSI: the next step

Many recent sensors combine BSI with a stacked design. In a stacked CMOS sensor, the photodiode layer and the signal processing layer are manufactured separately and then bonded together. This allows the sensor to read out data much faster than a single layer design, enabling higher burst rates, faster electronic shutter, and reduced rolling shutter artifacts.

BSI is the enabling technology for stacking. Because the photodiode layer is thinned and flipped, there is room to bond a second chip underneath. Stacked BSI sensors appear in the fastest cameras on the market, including the Sony Alpha 1 and Nikon Z8, and in many premium action cameras where fast readout is needed for high frame rate video.

For low light quality, stacking does not add a direct benefit beyond BSI. The benefit is speed, which improves autofocus, burst shooting, and video. If you shoot fast action or need silent shooting, a stacked BSI sensor is a strong reason to consider a camera. If you mostly shoot still subjects in low light, a standard BSI sensor may be sufficient.

Do you need BSI?

For most buyers, the answer is not whether you need BSI but whether the camera you are considering has a recent sensor. BSI is so widely adopted that it is difficult to find a new camera without it, especially in the compact and action camera categories where it matters most.

If you buy a compact camera, a 1-inch sensor camera, or an action camera, BSI is worth checking for because it directly affects the low light quality you will get from a small sensor. In APS-C and full-frame cameras, BSI is the norm in current models, so the sensor architecture matters less than the overall camera system, lens selection, and your shooting needs.

The practical recommendation is to focus on the camera that fits how you shoot. If low light matters, pair a BSI sensor camera with a fast lens and learn how to use ISO effectively. The sensor is only one part of the image quality equation, but it is the part that makes the other parts work better in the dark.

What to pick for your use

Questions

What is the difference between backside illuminated and front side illuminated sensors?

In a front side illuminated sensor, the metal wiring layer sits between the lens and the photodiodes. Light must pass through the wiring to reach the light-sensitive area. A backside illuminated sensor flips the chip so the photodiodes face the light directly, and the wiring sits behind them. More light reaches each pixel.

Does BSI really improve low light performance?

Yes, but the size of the improvement depends on pixel size. On small sensors with tiny pixels, BSI can make a noticeable difference in the amount of light captured per pixel, which improves the signal to noise ratio at high ISO. On large sensors with big pixels, the benefit is smaller but still present in the form of slightly better efficiency.

Are all new cameras using BSI sensors?

Most new sensor designs use BSI, especially in compact, action, and mirrorless cameras. Some budget cameras and older designs still use FSI sensors. The best way to confirm is to check the camera specifications and look for the term backside illuminated or BSI in the sensor description.

What is a stacked BSI sensor?

A stacked BSI sensor combines a backside illuminated photodiode layer with a separate processing chip bonded underneath. This allows much faster data readout, which enables high burst rates and reduced rolling shutter. The low light benefits are the same as standard BSI; the stacking adds speed.

Do I need BSI for good low light photos?

BSI helps, but it is not the only factor. A camera with an older FSI sensor and a larger sensor area may still outperform a BSI camera with a much smaller sensor. For compact and action cameras, BSI matters most. For full-frame cameras, the advantage is small enough that you should choose based on the whole camera system.