[5.9.4] The relationship between the luminances of an object and its immediate background. It is equal to (L_{1}-L_{2})/L_{1} , or (L_{2}-L_{1})/L_{1}, or \left | \Delta L/L_{1} \right | , where L_{1} and L_{2} are the luminances of the background and object, respectively. The form of the equation must be specified. The ratio \Delta L/L_{1} is known as Weber’s fraction.

Note: See note under luminance. Because of the relationship among luminance, illuminance, and reflectance, contrast is often expressed in terms of reflectance when only reflecting surfaces are involved. Thus, contrast is equal to (\rho _{1}-\rho _{2})/\rho _{1}, or (\rho _{2}-\rho _{1})/\rho _{1}, where \rho _{1} and \rho _{2} are the reflectances of the background and object, respectively. This method of computing contrast holds only for perfectly diffusing surfaces; for other surfaces it is only an approximation unless the angles of incidence and view are taken into consideration. (See reflectance.)

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