Geometry for the definition of Intensity. When speaking of irradiance, one should be careful both to describe the surface and to indicate at which point on the surface the irradiance is being evaluated, unless this is very clear in the context of the discussion, or if the irradiance is known or assumed to be constant over the whole surface.įIGURE 7. Irradiance is a function of position in the surface specified for its definition. Since there is no mathematical or physical distinction between flux incident upon, passing through, or leaving a surface, the term irradiance is used throughout this article to describe the flux per unit area in all three cases. (The term emittance, related to the emissivity, is reserved for use in describing a dimensionless optical property of a material's surface and cannot be used for emitted irradiance.) The irradiance leaving a surface can be called the exitance and can be given the symbol M, to distinguish it from the irradiance incident on the surface, but it has the same units and defining equation as irradiance. The flux can also be that leaving the surface in any direction in the hemispherical solid angle of emergence from the surface.
(The subscript “o” is used to indicate that this area is in an actual surface and is not a projected area.) The flux incident on a point in a surface can come from any direction in the hemispherical solid angle of incidence, or all of them, with any directional distribution. Where dΦ is an infinitesimal element of radiant flux and ds o is an element of area in the surface. A famous example comes from the classic experiment of Hecht, Shlaer, and Pirenne (1942), who estimated that only about 10 photons must be absorbed in the visual pigment for an observer to reliably detect a small, brief flash under optimal conditions. This is because the visual effect of a stimulus that is smaller than the eye's spatial summation area and shorter than the eye's integration time does not depend on how the light is distributed in space and time. It is sometimes used to describe very small and brief stimuli. In visual science, radiant energy is not a commonly used quantity because it does not tell us how concentrated the light is in space or time. The equivalent quantity in actinometry is the number of photons in the stimulus. Radiant energy is simply a measure of the total amount of light. The subscript e indicates a radiometric quantity. The most fundamental radiometric quantity is radiant energy, Q e, expressed in joules (J). Williams, in The Science of Color (Second Edition), 2003 2.8.1.1 Radiant energy Gamma correction is considered further in Chapter 4. Luma is the term used to denote the combination of these corrected components that produces the gamma-corrected luminance signal, according to the specification of the color space used. These outputs are normally labeled as R', G', and B'. However, before these emerge from the camera, they are usually nonlinearly mapped according to the response of the HVS, using a technique known as gamma correction. LumaĪn image sensor normally produces output values proportional to radiance, with the incident light typically filtered into R, G, and B bands. It is normally assessed subjectively by an observer with reference to a white area adjusted to have the same brightness as the target viewed.
Brightnessīrightness is the perception elicited by the luminance of a visual target and, as such, represents a nonquantitative indication of the physiological sensation of light. CIE luminous efficiency curve (publicly available: ).