Absorbance and transmittance
Move between a logarithmic reading and the fraction of light transmitted.
Keep the fraction and percentage separate
Transmittance T is the transmitted-to-incident power ratio. In a calculation, 50% transmittance means T = 0.50. Entering 50 directly into −log₁₀(T) mixes percentage notation with a fraction.
Three anchors to remember
A = 0 gives 100% transmittance. A = 1 gives 10%. A = 2 gives 1%. Going the other way, 50% transmittance gives A = −log₁₀(0.5) ≈ 0.30103.
Doubling A from 0.5 to 1 changes transmission from about 31.62% to 10%. The relationship is logarithmic, so doubling absorbance does not halve transmission.
The endpoints need care
At exactly 0% transmission, no finite absorbance satisfies the equation. A displayed value rounded to 0% can instead represent a very small positive result. At extreme values, the computer can also reach its numerical limit.
Our idealised converter accepts A ≥ 0 and 0 < %T ≤ 100. A negative measured absorbance can occur relative to a blank. This input rule does not diagnose the instrument or sample.
Sources & further reading
- IUPAC Gold Book — Absorbance
- IUPAC Gold Book — Beer–Lambert law
- Agilent — The Basics of UV-Vis Spectroscopy
Original educational material. Content review records a source and consistency check; it is not specialist peer review or laboratory method validation.