ssed in Im cm-2. Radiant flux 5. Luminance, Lv. The luminous intensity per unit area, leaving, passing through or arriving at a surface in a given direction. The Luminous flux surface area is the projected area as seen from the specified Irradiance W cm-2 direction. Expressed in im cm-superscript(1) or cd cm-2. Mathematically if the area of an emitter has a diameter or diagonal Illuminance m cm ² ² phot dimension greater than 0.1 of the distance of the detector it can be considered as an area source. Luminance is also called the photome- tric brightness, and is a widely used quantity. In Figure 7.5 the projected area of the source, A varies directly as the cosine of 0 i.e. Radiant intensity is a maximum at 0° or normal to the surface and minimum at 90°. Luminous intensity m sr 1 ==d Thus Equation 7.2 may be obtained. Radiant exitance W cm-2 Ap = A cos 0 (7.2) Luminous exitance Im cm-2 Radiance Luminance is then the ratio of the luminous intensity (IV) to the projected area of the source (A), as in Equation 7.3. Luminance (Photometric brightness) cd cm-2-stilb luminance = projected - 1, (7.3) per unit area Most extended sources of radiation follow what is known as Lambert's Law, as in Equation 7.4, where I is the intensity of an incremental area in a direction at an angle from the normal to the Since one Im sr-superscript(1) = one cd, depending on the units used for the surface and I is the intensity of an incremental area in the direction area we have: of the normal. 1. cd cm-2 1 stilb. Table 7.2 Approximate levels of natural scene illumination Direct sunlight 1.0 - 1.3 x 10 1.0 - 1.3 x 105 Full daylight 1 - 2 x 104 Overcast day Very dark day Twilight Deep twilight Full moon Quarter moon Starlight Figure 7.5 Diagram illustrating the projected area Overcast starlight 7/6 Optics and vision response provided must accurately follow a cosine law in order to Table 7.3 Approximate levels of luminance for various mimic the geometric response of the eye. sources (Zaha, 1972) A typical photometer head for illuminance measu