[3.8] L*(x) (at a point along a ray path)

Rate of increase in luminance per unit path length, at the point and in the direction of the ray, due to “generated” (emitted or scattered) luminance; or the “generated” luminous intensity per unit volume, at the point and in the direction of the ray, by which a distributed source can be characterized.

$L^{*}=dL_{g}/dX=dI_{g}/dV$,

where dx is an element of distance along the ray path, dV is an element of volume at the point, and the subscript g denotes a “generated” quantity.

Note: This quantity is useful in dealing with radiative transfer through a region where significant emission or scattering into the ray, as well as attenuation by absorption or scattering out of the ray, occurs along a ray path; also for evaluating the intensity of an extended volume of emitting or partially transmitting (transparent) material. (For discussion in more detail, see pp. 41-45 of NBS Technical Note 910-2.*)

* Self Study Manual on Optical Radiation Measurements; Part 1 – Concepts; Chapters 4 and 5, NBS Technical Note 910-2. Washington, DC: U.S. Department of Commerce. Feb. 1978.

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