absorbance
/ ab-SOR-bunce /
Picture shouting across a quiet room versus a noisy one. Absorbance is a way of scoring how much a sample 'muffles' a beam of light — a high score means a lot of light was swallowed on the way through, a low score means most of it sailed past.
Technically, absorbance (A) is the logarithm of the ratio of light going in to light coming out: A = log(I0/I). The logarithm is a clever choice, because it makes absorbance climb in a straight line with concentration, which is exactly what Beer-Lambert law needs. An absorbance of 1 means only one-tenth of the light got through; an absorbance of 2 means only one-hundredth.
Absorbance is the number chemists actually use for quantitative work because it is proportional to concentration. Its weakness is at the extremes: when almost no light gets through, tiny errors in the faint signal blow up, so very high absorbance readings are noisy and untrustworthy.
If a blue solution lets 10% of the light through, its transmittance is 0.10 and its absorbance is log(1/0.10) = 1.0 — a convenient round figure right in the reliable range.
Transmittance of 10% corresponds to an absorbance of exactly 1.
Absorbance has no units. It is the logarithmic partner of transmittance: as transmittance falls, absorbance rises, but the two carry the same information.