You might be surprised to know that every photograph you take – and every digital photograph ever taken – starts in black-and-white (and in shades of grey).
Digital images are recorded on a sensor situated at the back of the camera. Each pixel (short for picture element) contains a tiny photocell. This responds to light by generating a small electrical charge – just like a solar cell. The size of the charge is proportional to the brightness of the light falling on the cell – the brighter the light, the greater the charge.
Each charge is given a number based on its strength. Taking the 8 bit JPEG file format as an example; if no light falls on the cell, no charge is generated and the number is ‘0’ (black). If the cell generates its maximum power it is given the number ‘255’ (white). Values in between are given numbers from ‘1’ to ‘254’ (shades of grey). These numbers are saved in binary format on your media card as an image file.
Your computer reads the image file and recreates the image by reproducing ‘0’ pixels as black, ‘255’ pixels as white, and all the other pixels in their appropriate shade of grey. However RAW files offer a far greater range of shades; a 14 bit RAW image spanning ‘0’ to ‘16384’.
Adding colour to the mix
Where does the colour in your digital photographs come from? All the photocell can do is record the brightness of the light. It is not sensitive to colour.
This is where it gets clever. Covering the sensor is a mosaic of colour filters – one filter for each photocell. The filters are grouped into squares of four, one red (R), two green (G) (as the human eye is twice as sensitive to green light as red and blue) and one blue (B). The cells respond to the red, green and blue components of light. Each group of four cells now has sufficient colour data to recreate the image in colour.
So how do just 3 colours create realistic colour images?