photon exitance (at a point of a surface) [*M*_{p}]

1. quotient of the photon flux d*Φ*_{p} leaving an element of the surface containing the point, by the area, d*A*, of that element

2. equivalent definition: integral, taken over the hemisphere visible from the given point, of the expression *L*_{p}cos*θ*d*Ω*, where *L*_{p} is the photon radiance at the given point in the various directions of the emitted elementary beams of solid angle, d*Ω*, and *θ* is the angle between any of these beams and the normal to the surface at the given point

Unit: s^{-1}·m^{-2}

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