MODULE: Colour perception

Step 07 of 09: After images

Earlier we mentioned that, in the human eye, the cone photoreceptors are responsible for colour vision. This is because cones come in three types that are each sensitive to a different range of wavelengths of light. It was previously easy to attribute these to being sensitive to red, green and blue on the spectrum of long through short wavelengths. It is now understood that a more complicated system characterised by the "Opponent process" first discussed by Edward HERING in 1878, but not understood until more recently, may be more accurate. It proposes that the three types of conical photoreceptive cells exist as opposites in a range and can acknowledge the reception of only one of their triggers at any one time, they exist as red-green receptors, blue-yellow receptors and white-black receptors. This area of colour theory is still evolving and requires more research. But the opponent process explains a few peculiarities of human vision such as the retention of negative after-images in the retina, sometimes referred to as PURKINJE's image. When a red shape is stared at for long periods of time a "ghosted" image of that shape remains perceptible when the eye is moved to a white surface, but the ghosted image is not red, but a blue/green because the "inhibited" colours are no longer suppressed;