When light passes through or is reflected from a sample, the amount of light absorbed is the difference between the incident radiation (\(I_0\)) and the transmitted radiation (\(I\)). The amount of light absorbed is expressed as absorbance.
Transmittance, or light that passes through a sample, is usually given in terms of a fraction of 1 or as a percentage and is defined as follows:
\[
T = \frac{I}{I_0}
\]
or
\[
\%T = \frac{I}{I_0} \times 100
\]
Absorbance is defined as follows:
\[ A = -\log T \]
For most applications, absorbance values are used since the relationship between absorbance and both concentration and path length is normally linear as per the Beer-Lambert law.