When light is released by a liaht source it may be measured and evaluated in various ways. The total light flux could be collected and the quantity indicated in terms of the unit of luminous flux which is the lumen. Luminous flux is the radiant Power of the source evaluated according to the assumed standard sensitivity of the human eye. The area flux density of the lght flux could be measured either incident on a surfae as illuminance (lux or lumens/m2) or leaving a surface as luminous exitance (lumens/m2). The solid angular flux density could be measured as the luminous intensity (candelas or lumens/steradian). The intensity per unit area could be measured as the luminance (cd/m2). Which of these auantities is chosen for measurement would depend upon its purpose. Such measurements can be used to give an indication of the distribution of the emitted light flux in space. However, the effects of the radiation depend not only upon the quantity, intensity and distribution of the radiation but also upon the wavelengths of the radiation present in the light flux. Measurements related to spectral distribution are therefore also required to enable variation of response with wavelength to be taken into account. For visual effects the colour of sources and surfaces are important and so the means of colour specification and measurement can be important for some applications. The measurement of chromaticity coordinates is therefore sometimes reauired. In considering visual effects it is important to appreciate that the relationship between the apparent brightness of a source or surface and the stimulus in terms of luminance is non-linear. Under certain circumstances there is a logarithmic relationship while under others it is a power law relationship. Adaptation and induction effects also modify the practical results and must be considered when interpreting measurements or in setting up experimental procedures.