Light is the electromagnetic radiation that also includes for example X-rays, ultraviolet and infrared radiation. All these types of radiation are described either by the frequency (cycles per second) or the wavelength of one cycle. The wavelength of light is usually expressed in nanometres, nm; (1nm = 10 ). The human eye can respond to electromagnetic radiation between 380 nm and 780 nm as light. Light with a short wave length appears blue - as the wavelength increases the colour appears to change through green, yellow and orange to red. Radiation combining all the wavelengths of the visible spectrum in almost equal proportions is perceived as white light.
Radiation with a wavelength lower than 400 nm is called ultraviolet light. This type of light not only tans or burns the skin, but it is also responsible for damaging many plastics, either the basic material or the colourants that have been added. Radiation with a wavelength greater than 700 nm is classified as infrared light and is perceived as thermal radiation.
We all know from everyday experience that a coloured object looks different in the sunlight, under cloudy skies, under fluorescent light or under a normal electric light bulb. It is therefore important to select a light source before defining colour tolerances both for visual assessment and in colourimetry. Colour assessment should be carried out in a colour assessment booth where the light source can be selected.