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<p>or "decadic absorbance" is used instead of optical depth: the <a href="page.php?w=common_logarithm">common logarithm</a> of the ratio of incident to transmitted radiant power through a material. It is the optical depth divided by , because of the different logarithm bases used.</p>

<p><big> Mathematical definitions </big></p>
<p><big> Optical depth </big></p>
<p>The optical depth of a material, denoted , is given by:where<br/>
*  is the <a href="page.php?w=radiant_flux">radiant flux</a> received by that material;<br/>
*  is the <a href="page.php?w=radiant_flux">radiant flux</a></p><p>
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