a) Luminance has technical as well as colloquial definitions. The generic flux from a light-emitting or lightreflecting surface; the subjective response to luminance is brightness. The quotient of the luminous flux 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. b) The luminous flux may be leaving, passing through, and arriving at the surface or both. The luminance for each element of a surface within the field of view is defined as the ratio of luminous flux per solid angle to the unit projected area of the surface. Units are candelas per square meter, foot lamberts, nits.