= Absorbance
{title2=$A$}
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<Absorbance> is the <dimensionless> quantity $A=\log_{10}(P_0/P)$, where $P_0$ and $P$ are reference and transmitted radiant <powers> at the chosen <wavelength>. For example, $A=1$ means one tenth of the reference <power> is transmitted. Under the <Beer-Lambert law>, <absorbance> can determine an absorbing species' <concentration>; attenuation by cloudiness or unrelated colored <solutes> needs separate controls. <IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry/Beer–Lambert law>[IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry; IUPAC Gold Book: absorbance and its concentration relation]{ref}
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