Absorbance and Transmittance

A detector measures transmittance: the fraction of the incident beam that makes it through the sample. Absorbance is the negative base-10 logarithm of that fraction, and the logarithm is what makes absorbance — unlike transmittance — directly proportional to concentration. The scale is worth memorising: 100 %T is A = 0, 10 %T is A = 1, 1 %T is A = 2. Each whole absorbance unit means another factor of ten of light swallowed.

Old grating spectrophotometers had a linear %T dial with a cramped, non-linear absorbance scale printed beneath it, and generations of students learned the hard way that reading 0.5 %T off that squeezed end carried enormous uncertainty. The practical sweet spot is roughly 20–65 %T (A ≈ 0.2 to 0.7), where the photometric error is smallest. A worked case: a solution transmitting 50.0% of the light has A = −log₁₀(0.500) = 0.301 — half the light gone costs you almost exactly 0.3 absorbance units, the same 0.301 that turns up in every decibel and pH calculation.