Absorbance or attenuance?

The following is a 'Letter to the Editor' written by Henry B. F. Dixon and originally published in Biochemical Education [11]. It is reprinted by permission of its editor.

Biochemists previously used a variety of names such as extinction and optical density for the quantity log10(Po/P), where Po is the radiant power incident on a sample, and P is that transmitted by it (see footnote 1). This quantity is also -log10(T), where T is the transmittance.

We have grown used to the name 'absorbance' (symbol: A) for this quantity (see Table 2.7 in [12]), but this is clearly inappropriate for the quantity when the attenuation of the radiation is due to scattering rather than absorption. Indeed, biochemists often measure the quantity with the specific intention of measuring scattering, as when they estimate cell numbers in a bacterial culture, or swelling of organelles, by this quantity.

It is therefore a relief to be given recommendations that give a logical name for the quantity and its relation to absorbance. In the IUPAC Glossary of terms used in photochemistry (see page 1060 in [13]), we find the quantity itself called 'attenuance' (symbol: D), with the remark that attenuance reduces to 'absorbance' when there is negligible scattering or reflection. So we can now use these terms with clarity and precision.

Footnote 1: Earlier practice used radiant intensity rather than radiant power; radiant intensity is radiant power per unit solid angle.

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