A spectrophotometer is an instrument that can pass light of a single wavelength through a solution and measure the amount that passes through. First, the spectrophotometer must be zeroed by passing light of the chosen wavelength (500 nm for example) through a blank containing only the solvent. spectrophotometer can be set to measure either the percent transmittance or the absorbance of a solution. In lab, you will be using a spectrophotometer called the Spectronic 20 or Spec 20 to measure absorbance.