 radiometric measurement and luminous for photometric measurement. The subscript e is used to indicate a radiometric symbol and the subscript V for a photome- I steradian (sr) tric symbol. A physical visualisation of the terms to be defined is of solid angle given in Figure 7.3. Figure 7.4 illustrates the concept of solid angle required in the visualisation of Figure 7.3. 7.4.1 Radiometric terms and units These are as follows: 1. Radiant flux or radiant power, The time rate of flow of radiant energy emitted from a light source. Expressed in Js-1 or Surface area 2. Irradiance, Ec. The radiant flux density incident on a surface. equal to p2 Usually expressed in W cm-2. 3. Radiant intensity, Ic. The radiant flux per unit solid angle travelling in a given direction. Expressed in W sr Figure 7.4 Diagram illustrating the steradian (from Zaha, 4. Radiant excitance, Mc. The total radiant flux divided by the surface area of the source. Expressed in W cm ². 1972) Units of measurement 7/5 7.4.2 Photometric terms and units The equivalent photometric terminologies to the radiometric ones 3. footlambert. defined above are as follows: 1. Luminous flux or power, . The time rate of flow of luminous Table 7.1 provides a summary of the radiometric and photometric energy emitted from a light source. Expressed in 1m. terms with their symbols and units. Some typical values of natural 2. Illuminance or illumination, Ev. The density of luminous power scene illumination expressed in units of Im/m² and footcandles are incident on a surface. Expressed in Im cm-2. Note the follow- given in Table 7.2. Table 7.3 gives some approximate values of ing: 1 Im cm-2 = 1 phot; 1 Im m-2 = 1 lux; 1 luminance for various sources. footcandle. 3. Luminous intensity, Iv. The luminous flux per unit solid angle, Table 7.1 Radiometric and photometric terms travelling in a given direction. Expressed in 1 m sr ¹ ¹. Note that 1 1m sr = 1 cd. 4. Luminous exitance, Mv. The total luminous flux divided by the Quantity Symbol Unit(s) surface area of the source. Expre