Our discernment of the world is via our five senses of sight, hearing, taste, touch and smell. Of these sight is the most important. Over 80 per cent of our experience of the world comes via our sight. But how do we see?

2.1 What is light?
To see we need light, and light is an emission of electromagnetic radiation. The electromagnetic spectrum varies from radio waves through infrared, ultra-violet, X-rays and finally to gamma rays, and light is a very small part of this spectrum with wavelengths from 380 to 760 nanometres (1nm=10-9m). This is the part of the spectrum whose rays are visible to the human eye and lies between infrared and ultra-violet. Light may be further divided as the wavelength of the light relates to the colour we see. As the wavelength changes so does the colour of the light, from blue at 400nm to red at 700nm.

2.2 The eye and vision
Rays of light entering the eye are directed onto the retina, which is a layer of light sensitive cells within the eye. The retina is composed of two basic types of light sensitive cells, the rods and the cones. These cells have different properties. Cones operate during the day and enable us to see in detailed colour (photopic vision). As the light level drops, say to that of a well-lit street, the cones become less effective and are assisted by the more sensitive rods (mesopic vision). However, the rods only give black and white vision. Therefore we see a less brightly coloured view as we are using a mixture of the rod and cone cells, the relative mixture varying depending upon the actual light level. At much lower light levels, say that of dim moonlight, the cones cease to function at all, and our vision becomes totally monochromatic using just the rods (scotopic vision). The unit for this measure of light is the lumen.

These concepts are important as we consider the appearance of a space under different lighting conditions with respect to the amount of light and the colour spectrum of the light.