Colour vision

Photoreceptor responses to the scene

Cone photoreceptor sensitivities

The spectral profiles that we have been investigating describe the information that is available at the eye. The next step is for the eye to have a mechanism for capturing this information. For the purposes of colour vision, the cone photoreceptors are the key elements.

The typical human retina contains three classes of cone photoreceptor, named 'S', 'M', and 'L' after the location of peak sensitivity ('S'hort, 'M'edium, 'L'ong). Each of these cones has varying sensitivities to different wavelengths, and their response is the sum of the product of such sensitivities with the spectrum of light energy.

The graph below shows the photoreceptor sensitivity profiles for a human with typical colour vision.

Photoreceptor responses

As described above, we can simulate how a given cone photoreceptor will respond to a given spectrum of light. One way to think of such responses is as a way of summarising the light spectrum; rather than representing the light spectrum by a number for each wavelength, we can now represent it by three numbers that are given by the responses of the three cone types.

Since we have summarised the spectrum at each pixel by a single number for a given cone type, we can visualise this number to see how the cone type responds to different parts of the image.

Next we will consider the transformation of these cone photoreceptor responses into the perceptual colour space of the scene.