You can choose to measure either absorbance or transmittance by pressing the A/T button, the display indicates which has been chosen.

Transmittance measures the light energy passing through the sample and is expressed as percent (T%).

Absorbance is a logarithmic function (A = 2 – log T%) and will usually be the preferred unit since absorbance is directly proportional to the concentration of the absorbing substance (Beer’s Law).