A colorimeter works by shining a light onto the sample and measuring how much light leaves; this figure is then compared to a standard control amount. A known reagent is introduced to the sample and the color that develops as a result of the reagent is used to determine the sample's transmission of light.
The colorimeter measures light transmission in units of absorbance or percent transmission. Colorimeters follow Beer's Law to provide the units of absorbance. Beer's Law is a chemistry equation used to measure a sample's absorption by looking at the amount of light that enters and leaves, the absorptivity and the curvature of the sample's container.