In this article, we look at the digital camera imaging sensor, providing a guide to what it is for, and how it works
Much is made of the ‘digital’ in ‘digital camera’, but the digital camera imaging sensor is the heart of a digital camera; a semiconductor integrated circuit or chip, but it is not digital – it is analogue! All digital cameras contain a version of the digital imaging sensor and they are all analogue. So is a digital camera really digital? The simple answer is yes, of course. This is how it works.
As the camera’s sensor is photosensitive. When light photons are collected in the photosites (one photosite for each pixel), a tiny electrical charge is produced. The brighter the light, the more photons are collected, and a higher electrical charge is generated. Different pixel photosites will register different electrical charges and, once the exposure is complete, each individual pixel photosite’s electrical charge must be measured and then turned into a digital value by an analogue-to-digital converter. From then on, the process is entirely digital.
Being an up-to-date camera image sensor, it is a CMOS type, or Complementary Metal Oxide Semiconductor. Older cameras and even some current ones use a different type of sensor chip called a CCD, or Charge-Coupled Device. CMOS sensors are cheaper to make, but used to be inferior in sensitivity and noise performance. Thanks to steady technical refinement, CMOS sensors are now the image sensor of choice.
To make colour images, pixel photosites that make up a sensor need to register the brightness of red, green and blue light. In a conventional sensor, a quarter of the pixels record red light, another quarter blue light and half the pixels record green light. This is achieved by placing a coloured filter above each pixel photosite, also known as a Bayer filter array. The camera’s image processor then interpolates the colour data in neighbouring photosites to assign a full colour value to each pixel. However, this means that the colour resolution of an image is much less than the luminance (brightness) resolution – unless you’re using a Foveon sensor, which registers red, green and blue for every image pixel.