Luminance

Brightness
Perceiving brightness forms another aspect of visual perception. Brightness is a subjective reaction to levels of light and is affected by the amount of light emitted by an object. This is known as luminance. Visual acuity increases with increased luminance. Eye is exponential in sensitivity (so something that looks half as bright is in fact 10 times less bright - this is why photos in low light are not as good as what we see).
The 3 colours in the trichromatic system are identified by 3 cells in the retina of the eyes (receptors). Theories have been developed that receptors take signals which take the form of red-green, yellow-blue and black-white. These types of signals are known as opponent colour channels because they transmit opposite colours. To display the best detail luminance contrasts should always be used.