Cones come in 3 types:
red, green and blue. Each responds differently to various frequencies of
light. The following figure shows the spectral-response functions of the
cones and the luminous-efficiency function of the human eye (Fig. 6.16.
Cones and Luminous-efficiency Function of the Human Eye
The profile above is called a spectra.
The colour signal to the brain comes from the response of the
3 cones to the spectra being observed (Fig 6.17). That is, the signal
consists of 3 numbers:
where E is the light and S are the sensitivity
A colour can be specified as the sum of three colours.
So colours form a 3 dimensional vector space.
The following figure shows the amounts of three primaries
needed to match all the wavelengths of the visible spectrum (Fig. refspectrum).
Wavelengths of the Visible Spectrum
The negative value indicates that some colours cannot be
exactly produced by adding up the primaries.