What is the difference between absorbance and transmittance?

Absorbance is the measure of the quantity of light that a sample neither transmits nor reflects and is proportional to the concentration of a substance in a solution. On the meter of the Spectronic 20™, the absorbance is displayed on the bottom scale, which is logarithmic.

The transmittance of a sample is the ratio of the intensity of the light that has passed through the sample to the intensity of the light when it entered the sample ( T = I_out / I_in ). The transmittance is displayed as a percentage on the top scale of the meter of the Spectronic 20™.

To convert between the absorbance and transmittance scales, use the equation: