Vision

• The cornea and the lens help to concentrate and focus light onto the retina – the photosensitive layer of cells at the back of the eyeball.
• The amount of light that reaches the retina is regulated by the iris, which sits between the cornea and the lens, gives us eye colour, and controls the size of the pupil.
• The retina is responsible for translating the differences in the wavelength and brightness of the light that strikes the retina into electrical signals.
• These signals are transmitted through the optic nerve that exits at the back of the eye to the visual processing areas of the brain.

Visual perception

• Vision, as experienced by human beings, provides the basis for visual perception.
• Visual perception is associated with eyesight but also refers to the brain’s ability to make sense of what our eyes see.
• Visual perception augments the physiological sensitivity of our eyes to light with all the inferences from which our understanding of the world is derived.

Light, colour and vision

• The human eye and so human vision are tuned and respond to the visible spectrum and so to colours between red and violet.
• Light is rarely of a single wavelength, so an observer will usually be exposed to a contiguous spread of different wavelengths of light or a mixture of wavelengths from different areas of the spectrum.
• There are no properties of electromagnetic radiation that distinguish visible light from other parts of the electromagnetic spectrum other than wavelength and frequency.
• Colour is not a property of electromagnetic radiation, but a feature of vision and the visual perception of an observer.
• Colour is what human beings see in the presence of light.
• Objects appear to be different colours to an observer depending on the wavelengths and intensity of light at the moment it strikes the retina at the back of the eye.

Trichromatic colour vision (Trichromacy)

Trichromatic colour theory explains the system the human eye uses to see colour.

• Trichromatic colour theory is based on the presence of three types of light-sensitive cone cells in the retina at the back of our eyes, each sensitive to a different spread of colour.
• All the colours we observe result from the simultaneous response of all three types of cones.
• The sensitivity of cone cells is the physiological basis for trichromatic colour vision in humans.
• The fact that we see colour is, in the first instance, the result of interactions among the three types of cones, each of which responds with a bias towards its favoured wavelength within the visible spectrum.
• The result is that the L, M and S cone types respond best to light with long wavelengths (biased towards 560 nm), medium wavelengths (biased towards 530 nm), and short wavelengths (biased towards 420 nm) respectively.