transmittance
/ tranz-MIT-uhnce /
Stand behind a window blind that is half open: roughly half the daylight gets through. Transmittance is just that fraction, made exact — the share of light that survives the trip through a sample and reaches the detector on the other side.
Defined as T = I/I0, transmittance is the intensity of light coming out divided by the intensity going in. It runs from 0 (nothing gets through, a black sample) to 1, or equivalently 0% to 100% when written as percent transmittance. A clear, empty cell transmits close to 100%.
Transmittance is the raw quantity an instrument's detector senses most directly, but it is awkward for quantitative work because it does not rise in step with concentration. That is why chemists convert it to absorbance — its logarithm — before calculating concentrations.
A pale sample that lets 80% of the light through has a transmittance of 0.80 (80%T), which corresponds to a low absorbance of only about 0.097.
High transmittance means little is absorbed, hence low absorbance.
Do not confuse the two scales: 0% transmittance equals infinite absorbance, while 100% transmittance equals zero absorbance. They move in opposite directions.