Editing Reflectance (section)

===Directional reflectance===
The ''directional reflectance'' of a surface, denoted ''R''<sub>Ω</sub>, is defined as<ref name="ISO_9288-1989" />
<math display="block">R_\Omega = \frac{L_{\mathrm{e},\Omega}^\mathrm{r}}{L_{\mathrm{e},\Omega}^\mathrm{i}},</math>
where
*{{math|''L''<sub>e,Ω</sub><sup>r</sup>}} is the [[radiance]] ''reflected'' by that surface;
*{{math|''L''<sub>e,Ω</sub><sup>i</sup>}} is the radiance received by that surface.

This depends on both the reflected direction and the incoming direction. In other words, it has a value for every combination of incoming and outgoing directions. It is related to the [[bidirectional reflectance distribution function]] and its upper limit is 1. Another measure of reflectance, depending only on the outgoing direction, is ''I''/''F'', where ''I'' is the radiance reflected in a given direction and ''F'' is the incoming radiance averaged over all directions, in other words, the total flux of radiation hitting the surface per unit area, divided by π.<ref>{{cite journal |first1=Jeffrey |last1=Cuzzi |first2=Lindsey |last2=Chambers |first3=Amanda |last3=Hendrix |title=Rough Surfaces: is the dark stuff just shadow? |journal=[[Icarus (journal)|Icarus]] |date=Oct 21, 2016 |volume=289 |pages=281–294 |doi=10.1016/j.icarus.2016.10.018 |pmid=31708591 |pmc=6839776 }}</ref> This can be greater than 1 for a glossy surface illuminated by a source such as the sun, with the reflectance measured in the direction of maximum radiance (see also [[Seeliger effect]]).