Chapter 25: Lighting

25.1.1 The Nature of Light

Light is a form of electromagnetic radiation similar in nature and behaviour to radio waves at one end of the frequency spectrum and X-rays at the other. The range to which the human eye is sensitive covers a waveband of approximately 380-760 nanometers (nm), a nanometer being a wavelength of one millionth (10-6) of a millimetre. Within these limits there are differences in colour, white light being that in which an approximately equal amount of power occurs at all frequencies within the waveband.

25.1.2 Frequency and Colour

The human eye is more sensitive to frequencies in the middle of the spectrum (green and yellow light) than those at the ends (red and violet). Consequently, more power must be expended to obtain the same effect on the eye from red or violet light than from green or yellow.

The regions of radiation just outside the visible spectrum are known as the ultraviolet (UV) and infrared (IR). Both are present in natural daylight and they are emitted in varying amounts by artificial light sources. IR gives the effect of heat, while UV radiation is used to excite fluorescence in the powders in fluorescent lamps. Short-wave UV causes skin tanning and can damage the eyes, but the long-wave UV, used for theatrical or display purposes and in discotheques, is virtually harmless.

25.1.3 Basic Lighting Units and Terms

The average illumination on a surface will diminish as the square of its distance...