Editing Absorbance (section)

===Attenuance===
Absorbance is a number that measures the ''attenuation'' of the transmitted radiant power in a material. Attenuation can be caused by the physical process of "absorption", but also reflection, scattering, and other physical processes. Absorbance of a material is approximately equal to its attenuance{{clarify|reason=This term desperately needs a definition, otherwise the whole point of the fine distinctions in this section will be lost. Is attenuance a numerical measure of the physical process of attenuation, or are they also, like absorbance and absorption, unexpectedly distinct?|date=April 2015}} when both the absorbance is much less than 1 and the emittance of that material (not to be confused with [[radiant exitance]] or [[emissivity]]) is much less than the absorbance. Indeed,

<math display="block">\Phi_\mathrm{e}^\mathrm{t} + \Phi_\mathrm{e}^\mathrm{att} = \Phi_\mathrm{e}^\mathrm{i} + \Phi_\mathrm{e}^\mathrm{e}\,,</math>

where
* <math display="inline">\Phi_\mathrm{e}^\mathrm{t}</math> is the radiant power transmitted by that material,
* <math display="inline">\Phi_\mathrm{e}^\mathrm{att}</math> is the radiant power attenuated by that material,
* <math display="inline">\Phi_\mathrm{e}^\mathrm{i}</math> is the radiant power received by that material, and
* <math display="inline">\Phi_\mathrm{e}^\mathrm{e}</math> is the radiant power emitted by that material.

This is equivalent to

<math display="block">T + \mathrm{ATT} = 1 + E\,,</math>

where
* <math display="inline">T = \Phi_\mathrm{e}^\mathrm{t} / \Phi_\mathrm{e}^\mathrm{i}</math> is the transmittance of that material,
* <math display="inline">\mathrm{ATT} = \Phi_\mathrm{e}^\mathrm{att} / \Phi_\mathrm{e}^\mathrm{i}</math> is the {{em|attenuance}} of that material,
* <math display="inline">E = \Phi_\mathrm{e}^\mathrm{e} / \Phi_\mathrm{e}^\mathrm{i}</math> is the emittance of that material.

According to the [[Beer–Lambert law]], {{math|1=''T'' = 10<sup>−''A''</sup>}}, so
* <math>\mathrm{ATT} = 1 - 10^{-A} + E \approx A \ln 10 + E, \quad \text{if}\ A \ll 1,</math>
and finally
* <math>\mathrm{ATT} \approx A \ln 10, \quad \text{if}\ E \ll A.</math>