Sensor Technology Advances: Gapless Microlenses | Sensor Phase Detection AF | Copper Wiring
Sensor Types: CCD (Bayer Filter) | CCD Vs CMOS | CMOS | Backlit CMOS (BSI) | BSI CMOS vs CMOS | Fujifilm X-Trans | Foveon | Black and white

Here's our complete guide to camera image sensor technology, the different types, and what it means to image quality. Image sensors are used in smartphones, cameras, video cameras, security cameras, action cameras, and in the automotive industry. 

 

Sensor Technology

The imaging sensor sits in the middle of your digital camera, right behind the lens, and turns the optical image into a digital version, converting light into electronic signals. The quality, size and make up of the image sensor, combined with the lens and image processor, play a massive part in the image quality produced by the camera.

Fujifilm X-E2s

 

The image sensor is made up of millions of pixels, and is made out of a silicon wafer like computer chips, which you will have seen if you remember the Intel bunnymen adverts, a picture is shown below:

 

A sensor typically features an infrared filter, following by an anti-aliasing (or optical low-pass) filter, on top of the sensor.

Each pixel point on a sensor is made up of a layer starting with:

• On-chip microlens (microlenses)
• Colour filter (Bayer or other)
• Sensor aperture
• Sensor area

From Sony PDF: Super HAD CCD II Diagram. Side note: Sony HAD sensors use a variation of the Bayer sensor pattern, and instead of having a square of red, green, blue, and green, they use red, green, blue and turquoise / cyan / emerald (RGBE). More recently, other filters alter the colour filter and some of the latest sensors collect Red, Green, Blue and White for increased light sensitivity. More information here, and here.

 

Sensor Technology Improvements:

Gapless MicroLenses:

Canon introduced gapless microlenses with the introduction of the Canon EOS 50D (2008), which removed the gap between the microlenses on the sensor, this means that more light is directed to the photodiode or pixel well, and means less amplification is needed, which in turn means an improvement in noise performance. This was further developed in the EOS 5D Mark II (2008) and EOS 7D (2009) with a reduction in the distance between the sensor and the gapless microlenses, further improving noise performance thanks to higher pixel sensitivity to light. The Canon EOS-1D X (2012) and EOS 5D Mark III (2012) also benefit from this technology.

Gapless microlenses can be found on a wide range of other cameras as well, including the Sony Alpha A77-II, A6000, and A7R to name a few.

 

Sensor Based Phase-Detection Auto Focus (AF):

Olympus OM-D E-M1 Sensor, with central phase-detection focus.

First introduced in the Fujifilm FinePix F300EXR (2010), phase-detection pixels are found on the sensor to improve focus speeds. It's also found on the Canon EOS 650D (2012), Olympus OM-D E-M1 (2013), and Canon EOS 70D (2013), and gives Digital SLRs quicker focus during live view and video, which is particularly useful, as DSLR cameras often have slower focus speeds when using live view. 

 

Image courtesy Canon (PDF)

Canon's Phase-Detection sensor features a dual-pixel system that covers 80% of the sensor. Canon call this system Dual Pixel AF, and each pixel features two photo diodes that can be read independently during autofocus or together to capture the image, as shown above. We go into more detail on the system in our full Canon EOS 70D review.

 

Copper Wiring:

Introduced with the Fujifilm X-Pro2 (2016), and Sony Alpha A6300 (2016), is the introducing of a new copper wiring process that enables a thinner wiring layer, which means that more light can get to the photodiode, which is also larger than previous sensors. The copper wiring also enables a faster readout from the sensor, which also enables 4K video recording (on the A6300).

 

Sensor Types Explained:

CCD - Charge-Coupled Device

Each pixel is responsible for collecting light, but as the pixel is simply detecting light, a colour filter is needed in order for the sensor to pick up the different colours in the scene. The CCD sensor is often used in lower price cameras, as is often limited to slower continuous shooting and lower resolution video recording (720p).

The Bayer Sensor pattern is the most popular and common arrangement of the filter, with a Blue, Green, Green, and Red arrangement:

 

However, the traditional RGBG (Red, Green, Blue, Green) colour arrangement isn't the only colour arrangement possible, with a number of companies using different colour filter arrangements on the sensor, to try to improve colour reproduction, as well as improve light sensitivity. The first, or more common alternative has been the RGBE (Red, Green, Blue, Emerald/Cyan) colour filter used in Sony's Super HAD CCD sensor.

More recently smartphones have been using a variety of different colour filters, with Huawei using an RYBY (Red, Yellow, Blue, Yellow) colour filter, in the Huawei P30 Pro, which they say is more sensitive to light compared to green. There are also sensors designed use an RGBW filter, with the fourth colour being white or clear, so as to capture the maximum possible light, without any colour filter reducing the amount of light getting to the individual pixel, however, it doesn't look like these sensors have taken off, with only a limited number of cameras using this sensor technology.

 

CCD vs CMOS

We thought we'd show the difference between a 12 megapixel CCD sensor, compared to a newer 12 megapixel BSI CMOS sensor, both 1/2.3inch in size. On the left is the result of shooting at ISO1600 with a Nikon Coolpix S2500, and on the right is the result of shooting at ISO1600 with a Panasonic Lumix TZ70. Click the image to view at full size.

 

CMOS

CMOS (Complementary Metal–Oxide–Semiconductor) sensors use less power than CCD sensors and often allow quicker read speeds than CCD sensors, allowing high speed continuous shooting and high speed FullHD video, as well as 4K video recording in some cameras. More information on CMOS technology can be found here: Exmor CMOS sensor (Sony's CMOS sensors). The majority of CMOS sensors use the Bayer filter pattern.

Back-lit CMOS

Also known as B.S.I / Back Side Illuminated (Sony call it Exmor R / RS Backlit CMOS) and first introduced in 2009.

BSI CMOS sensors are of particularly benefit to small sensors such as those found in camera phones, and compact cameras where the small size of the sensor means the sensor is mostly dominated by the wiring which blocks light from reaching the sensor. More recent developments has seen the use of Back-lit CMOS sensors used in APS-C CMOS sensors, as well as full-frame CMOS sensors for improved noise performance. The majority of BSI CMOS sensors use the Bayer filter pattern.

From Sony's website, this diagram shows how moving the wiring structure improves light collection:

And if you've ever wondered why it's called "Back-illuminated" when by the looks of things, they've actually moved the light detecting surface to the front / top of the sensor, it's because the light receiving surface is on the back side of the silicon layer. The benefits of back-lit CMOS sensors compared to front illuminated is shown below in this low light, f/2.0 30lux, image from Sony:

Recent developments in CMOS technology, by Sony, has seen additional image processing and memory being built into the back of the CMOS sensor, something not possible with CCD sensors. Sony has introduced a BSI Full-Frame sensor in the 42 megapixel Sony Alpha A7R Mark II. 

Using a "stacked" sensor has enabled even quicker continuous shooting as well as high speed video recording, as found in the Sony Cyber-shot RX10 II and RX100 IV. 

 

Backlit (BSI) CMOS vs CMOS Sensor

We thought we'd show an example of the difference between a standard CMOS sensor (Front-illuminated), compared to the BSI (Back-side illuminated) CMOS sensor, and have shown ISO6400 taken with the Sony Cyber-shot RX100, which features a 20 megapixel 1inch-type CMOS sensor, and the same subject and lighting conditions, taken with the Sony Cyber-shot RX100 Mark II, which features a 20 megapixel 1inch-type BSI CMOS sensor. Click the image to view at full size.

Sony 20mp RX100 CMOS Sensor (Left) Vs RX100 II BSI CMOS Sensor (Right) @ ISO6400 

 

Fujifilm X-Trans CMOS Sensor

The latest sensor design from Fujifilm is the Fujifilm X-Trans CMOS sensor first introduced in the Fujifilm X-Pro1, and used in other X-Mount cameras. This uses a new arrangement of the colour filter on the sensor, said to mimic film, it is designed to improve detail capture, as well as reduce noise levels. Fujifilm also say that false colour and moire is reduced by not using the standard Bayer filter array. Detail captured is improved by not using an Optical Low-Pass Filter (OLPF), also known as an Anti-Aliasing (AA) filter.

X-Trans CMOS II, III and IV

X-Trans CMOS II, III and IV - These incorporate phase-detection auto-focus at the sensor level. The X-Trans CMOS III is a new 24 megapixel version of the sensor, with copper wiring. The X-Trans IV CMOS sensor is a 26mp BSI (Back-side illuminated) sensor, giving higher resolution, improved noise performance, and the sensor does not feature an anti-aliasing filter. It can be found in the Fujifilm X-T3, X-T30, X-T4, X100V, and X-Pro3.

 

Fujifilm EXR Sensor - CCD/CMOS (No longer used)

First introduced in "Super CCD" form, Fujifilm used a diagonal "honeycomb" arrangement of pixels in the typical Bayer pattern, they then re-arranged the pixels and put blocks of colour pixels next to each other, creating the Super CCD EXR sensor:

The EXR arrangement gives:
HR - High resolution - uses all pixels
DR - Dynamic range - combines neighbouring pixels reading different exposures
SN - Signal/noise sensitivity - combines neighbouring pixels

The EXR sensor is different from the Bayer sensor pattern, and Fujifilm has re-arranged the Red, Green, and Blue pixels to enable pixel combining: Super CCD EXR explained (PDF).

Fujifilm has also introduced a backlit CMOS version of the sensor, moving the wiring to increase sensitivity to light:

 

Live MOS Sensor 

Olympus and Panasonic use this name for their sensors, but there is little information on-line about the sensor technology. When used in compact cameras, it enables high speed shooting, much like backlit CMOS sensors. The sensors use the Bayer filter pattern. It is also used in Panasonic and Olympus Micro Four Thirds cameras such as the Panasonic Lumix G7.

Foveon X3 CMOS Sensor

Departing from the standard Bayer sensor, the Foveon sensor is unique in that it has the Red, Green, and Blue pixels on top of each other, so that at each point the camera is reading the full colour. The benefits of this seem clear, however Foveon sensors are only used in Sigma cameras, which means that development of the sensor and technology seems to be at a slower pace compared to the speed of development of other sensors, for example sensors in mobile phones are sold in their billions every year.

Black and White Sensors

Image courtesy Leica Camera AG.

A black and white sensor does not feature the Bayer colour filter, and is therefore able to record detail at every pixel. The black and white sensor can be found in specialist cameras, as well as the Leica M Monochrom with 18 megapixel CCD sensor, and Leica M Monochrom (Typ 246) with 24 megapixel CMOS sensor. Some smartphones have featured a secondary black and white camera, for example the Huawei Leica P9.
 

 

These are the different types of sensors available, CCD vs CMOS, as well as various technologies involved including Bayer and other colour filter arrays, and whether one is better than the other is regularly debated, with the lens and image processing playing a big part in the final image quality. For the most informed view on how a sensor performs, you need to have a look at the latest camera reviews of the model you're interested in, as the results from one sensor could be very different when used by one manufacturer over another. 

We have seen the switch to backlit CMOS sensors improve image quality, with lower noise at higher ISO settings, higher continuous shooting speeds, high speed video and other benefits such as quicker AF. Further developments have added on-sensor phase detection focus, copper wiring, as well as new and interesting arrangements of the colour filter array.

Further reading: Sensor Size Explained | Sensor Pixel Size in Detail

Side note: Some camcorders use "3CCD" sensors - this is literally three different sensors, and in front is a prism that splits the light to red, green, and blue and sends each colour to an individual sensor.