A spectrophotometer is an instrument which is used to measure the intensity of electromagnetic radiation at different wavelengths. Important features of spectrophotometers are spectral band with and the range of absorption or reflectance measurement. Then, add the solute which would be the blank for the experiment. 2  It is important not to add water to the blank so that it won’t affect the accuracy of the results. 3  As a next step, the spectrophotometer is set to the desired wavelength. 4  Insert the blank cuvette, making sure that the arrow is aligned each time.