It is used to adjust the instrument to read 100% transmittance or 0 absorbance. In use, a 0% transmittance value (infinite absorbance) is established by placing a curtain between the light source and the photocell. Electronic control is then exerted so that the meter will read 0% Transmittance on its scale. The blank sample (containing no solute or dye) is inserted, the curtain opened and the meter readjusted to read 100% transmittance. All other measures are then made by merely inserting the samples into the light path and measuring the % transmittance. Most spectrophotometers have a built in means of direct conversion of this reading to absorbance.