A logarithmic measure of how much light a sample stops: 0 means all of it passes, 1 means a tenth passes, 2 means a hundredth.
The base-10 logarithm of the reciprocal of transmittance, A = −log₁₀ T, also called optical density. It has no unit and is quoted at a stated wavelength and path length. For a clear solution it is proportional to the concentration of the absorbing substance and to the path length (the Beer–Lambert law).
Absorbance is not the fraction of light absorbed. That fraction is absorptance, which cannot exceed 1; absorbance has no upper limit.
Beer colour in SRM is the absorbance of the beer at 430 nm through 1 cm, multiplied by 12.7.
Colour grades for beer, wine, honey, syrups, oils and spice extracts are absorbance readings at fixed wavelengths, and dye strength is checked the same way.
Absorbance is the base-10 logarithm of the reciprocal of transmittance: A = −log₁₀ T. The CIE's vocabulary calls the same quantity transmittance optical density, and it has no unit. A spectrophotometer shines a beam through the sample in a cell of fixed path length, compares it with the beam through a blank, and reports the result at one wavelength or as a curve. For a clear solution the Beer–Lambert law makes absorbance proportional to both concentration and path length, which is why it is the quantity used to find how much colourant is present.
| Light transmitted | Light removed | Absorbance |
|---|---|---|
| 100% | 0% | 0.000 |
| 50% | 50% | 0.301 |
| 10% | 90% | 1.000 |
| 1% | 99% | 2.000 |
| 0.1% | 99.9% | 3.000 |
Each whole unit of absorbance is a further factor of ten: 1 means a tenth of the light gets through, 2 a hundredth, 3 a thousandth. Halving the light adds only 0.301. The scale is logarithmic so that two layers stack by simple addition, which is why instruments report it in preference to the percentage transmitted.
How this was calculated: Absorbance is the base-10 logarithm of the reciprocal of transmittance, A = −log₁₀ T, computed for five round transmittance values.
A beer, a wine or a syrup is graded for colour.
The grade is an absorbance at a set wavelength and path: beer colour in SRM is the absorbance at 430 nm through 1 cm multiplied by 12.7. Haze adds to the reading, so an unfiltered sample grades darker than it looks, and measuring in the wrong cell size gives a wrong grade by a fixed factor.
A dye bath or an ink is checked for strength by measuring a concentrated sample.
The proportional relationship breaks down at high concentrations and in turbid liquids, so the reading no longer tracks the strength. The sample is diluted by a known factor until it falls in the instrument's working range.