[2.9] L = d2 Φ/[dω(dA • cosΘ)] = dI/(dA • cosΘ)[note]Properly, this should be a second partial derivative since area and solid angle are independent variables; however, the symbol “d” is used due to the convenience in printing and typing. For this specific use, no possible errors or confusion are foreseen. This practice is in accordance with the International Lighting Vocabulary (CIE S 017/E 2011) and the practice of the national Institute of Standards and Technology.[/note] (in a given direction at a point on the surface of a source, of a receiver, or of any other real or virtual surface)

The quotient of the radiant flux leaving, passing through, or arriving at an element of the surface surrounding the point, and propagated in directions defined by an elementary cone containing the given direction, by the product of the solid angle of the cone and the area of the orthogonal projection of the element of the surface on a plane perpendicular to the given direction.

Note: In the defining equation, Θ is the angle between the normal to the element of the surface and the given direction

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