The cones in a typical human eye have the ability to separately sense three different portions of the spectrum.
We identify these peak sensitivities as red, green and blue - the primary colors.
All rod light sensors have the same broadband sensitivity and therefore only provide luminance information.
Rods cannot create color images.
The brain and nervous system perform extensive processing of the rod and cone outputs in order to generate an image.
Our eyes have three sets of sensors with peak sensitivities at light frequencies that we call red (580 nm), green (540 nm) and blue (450 nm).
Light at any wavelength in the visual spectrum range from 400 to 700 nanometres will excite one or more of these three types of sensors.
Our perception of which color we are seeing is determined by which combination of sensors are excited and by how much.
The following illustration shows the spectral sensitivity of the typical human visual system.
It is customary to denote the RGB sensors with the Greek letters Rho (red), Gamma (green) and Beta (blue).