Digital Cameras and Colour

RGB Colour

According to high school science, every colour in the world can be made by combining the appropriate amounts of the primary colours: red, green, and blue light. Conversely, every colour can be resolved into a combination of red, green, and blue light.

A system whereby colours are characterised by the contributions of the red, green, and blue primary colours is called an RGB colour model.

How digital cameras capture a scene

Digital camera sensors work by breaking down a scene into millions of ‘pixels’ each representing the colour and intensity of the light at a spot in the scene.

By using tiny coloured filters over light sensitive electronics in the sensor, the digital camera is able to record a set of three electronic signals representing the amounts of red, green and blue light at each pixel in a scene. The electronic signals are turned into numbers by our digital imaging software.

The range of tonality that can be recorded, stored, and manipulated in a digital imaging system, is dependent on the 'bit depth' of the electronics and image files. Although real digital cameras are at least 12-bit devices, for simplicity we will consider 8-bit cameras. Smoother gradations of colour and tone are possible the greater the bit-depth, but the principles are the same.

In an 8-bit camera system, 256 different levels of light intensity could be measured by the sensor under each filter. The camera's built-in computer would assign a value from 0 to 255: 0 = no light detected; 255 = the most intense the sensor can record. The intensity of red light at a point can be assigned any number from 0 to 255. The amount of green light at the point can be assigned any value from 0 to 255 and the blue component can also be assigned a value from 0 to 255.

• The intensity and colour of the light at the point can, therefore, be represented as a set of three numbers corresponding to the amount of red, green, and blue light that makes it up.
• A fully saturated red dot captured by a digital camera, for example, might be recorded as 255,0,0 because it has maximum red and nothing else.
• A fully saturated green dot might be recorded by 0,255,0
• A black dot — meaning no light — would be recorded as 0,0,0
• A dot representing a pale skin colour might be recorded as 240,210,180 because such a colour is made up of a mixture of red, green, and blue. A dot in a pair of 'blue' jeans, for example might be recorded as 32, 35, 156.
• A whole image is recorded in a digital file that consists of many millions of these number triples.

A system whereby colour and tonality recorded numerically by means of number-triples representing the contributions of red, green, and blue is called an RGB colour space.

The complete set of number-triples (and associated colours) that a particular digital camera can record is a device-dependent colour space. The actual colours associated with any number triple in a given camera are found by measurement in a process called 'profiling' the camera.

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