Rather than replacing or challenging the physical theory of colour, psychology extends the model by incorporating the neurological behaviour of the eye and the brain in the process. As defined by the emission and absorption spectra of the source and object our eye will be exposed to a spectrum of light with varying intensity per wavelength. In the eye the light stimulates the retinal cones, receptor cells used to turn incoming photons (packages of light energy) into a neural signal for transmission to the brain. In the human eye we find three distinct kinds of cone receptor cells labelled beta, gamma and rho cells as well as so-called rod receptors which primarily deal with intensity information rather than colour.