Essential Guide to Sensors – Glossary

Glossary of terms…

Analogue-to-digital converter (ADC)

A device which converts a continuous signal into discrete digital data. The higher the bit depth of an ADC, the greater the number of possible values that may be assigned to the signal upon conversion, thus the smaller the chance of an error occurring. Many recent DSLRs have 14bit ADCs on board, which allow 16,384 tones per colour channel; since JPEGs are stored as 8bit files as standard, they cannot contain the full information from such converters. This level of information may be maintained in Raw images, or alternatively in uncompressed TIFF files with a high enough bit depth.

APS-C

A sensor whose dimensions are roughly the same as the “Classic” format negatives from the APS film system (25.1×16.7mm). APS-C sensors are used in many DSLRs and Compact System Camera models, as well as in enthusiast compact cameras such as Fujifilm’s X100s. Being physically smaller than full frame sensors, these apply a crop factor to mounted lenses, typically 1.5-1.6x.

Back-side illumination (BSI)

A back-side illuminated sensor features a different construction from standard types, where the wiring and other physical obstructions usually on the top of the sensor sit behind the substrate. As these are no longer in the path of incoming light, it helps a sensor to gather light more effectively, thus reducing image noise.

Bit depth

The number of possible tones that can be recorded in an image, with a higher bit depth signifying a greater range of tones available. With sensors, the bit depth refers to the analogue-to-digital converter.

CCD

Charged Coupled Device. A type of sensor which has long featured in compact cameras, DSLRs and scanners, as well as in high-end cameras developed for scientific purposes. These work by transferring charge via a bucket-brigade system down to the end of each column on the sensor, before they are read out and converted to a discrete digital value. In many consumer applications these have largely been replaced by CMOS alternatives.

CMOS

Complementary metal-oxide-semiconductor, the main type of sensors used in cameraphones, compacts, Compact System Cameras and DSLRs. These work in a similar manner to CCDs, although their construction sees more functionality integrated onto the sensor itself.

Colour Filter Array (CFA)

Sensors do not see colour as standard, so some way of determining colour information at each photosite is necessary. A CFA is the usual way this is achieved: this is an array of coloured filters which sit over the sensor, through which light passes before it enters each photosite. As only one colour sits over each pixel, other colours for that pixel are determined through demosaicing (right). Although a red-green-blue-green mosaic pattern is usually used, some sensors use a different colour combination, or work on a different principle entirely.

Crop Factor

For any lens, the smaller the sensor with which it is used, the more peripheral areas are cut away. As this restriction leads to a smaller angle of view, it replicates the effect of using a longer lens. APS-C DSLRs typically apply a 1.5-1.6x crop factor; Micro Four Thirds sensors – being roughly a quarter the size of full frame – apply 2x.

Demosaicing

The process of interpolating colour information into an image so that each pixel contains complete colour data. Values are calculated by looking at the values of surrounding pixels of that colour, and this is required for sensors that use a standard colour filter array, as each pixel only receives colour information for one colour as standard. This is a process that can lead to false colour patterning and softness.

EXR CMOS

Fujifilm’s proprietary sensor technology. This uses rotated photodiodes together with a non-standard colour filter array and back-side illumination, and adjusts its behaviour to best capture the scene.

Foveon X3

A sensor technology that is found in Sigma’s DSLRs and compact cameras, where the standard colour filter array is replaced by layers of silicon. These are penetrated by different wavelengths of light to varying depths, which allows each pixel to receive full red, green and blue colour information without the need for demosaicing. Sigma claims that this system creates sharper images with more accurate colour.

Full Frame

A sensor whose dimensions roughly match those of a frame of 35mm (36x24mm). Because of this they apply no “crop factor” to mounted lenses. These are the largest sensors used in DSLRs and feature in the most expensive models, such as Nikon’s D4 and Canon’s EOS-1D X.

Microlenses

An array of very small lenses which sit over a camera’s sensor. These help to funnel in as much light as possible into each photosite, although they can cause purple fringing too.

Pixel pitch

The distance between the centre of two pixels on a sensor, stated in micrometres (μm). As an example, a 20MP compact camera with a small sensor may feature a pixel pitch of around 1.2µm, while the same pixel count on a full frame sensor may be around 5.5µm.

Signal-to-noise ratio

The ratio of the light to unwanted noise. The higher this ratio is in favour of the signal, the less noise can be seen and thus the better the image quality.