Absorbance and transmittance are measurements used in spectrophotometry. Spectrophotometry measures how much radiant energy a substance absorbs at varying wavelengths of light. The technique is useful for determining the identity of an unknown substance as and, with the use of a set of standards, determining a substance’s concentration in a sample.

A transmittance to absorbance table enables fast conversion from transmittance values to absorbance in the lab or in the field. Download a copy of this chart for your classroom or laboratory.

Absorbance can be calculated from percent transmittance (%T) using this formula:

Absorbance = 2 – log(%T)

Transmittance (T) is the fraction of incident light which is transmitted. In other words, it’s the amount of light that “successfully” passes through the substance and comes out the other side. It is defined as T = I/Io, where I = transmitted light (“output”) and Io = incident light (“input”). %T is merely (I/Io) x 100. For example, if T = 0.25, then %T = 25%. A %T of 25% would indicate that 25% of the light passed through the sample and emerged on the other side.

Absorbance (A) is the flip-side of transmittance and states how much of the light the sample absorbed. It is also referred to as “optical density.” Absorbance is calculated as a logarithmic function of T: A = log10 (1/T) = log10 (Io/I).