If the T of a sample is 0.40, the %T of is 40%. This means that 40% of the photons in the incident light emerge from the sample as transmitted light and reach the photodetector. If 40% of the photons are transmitted, 60% of the photons were absorbed by the sample.
From the transmitance or % transmittance, one can calculate the quantity known as absorbance (A). Absorbance is the amount of light absorbed by a sample. It is calculated from T or %T using the following equations: