A spectrophotometer is based on the Beer-Lambert law, which states that absorbance (amount of light absorbed) of the solution has a linear relationship with the length of light and the concentration of a sample. Spectrophotometer techniques are mostly used to measure the concentration of solutes in solution by measuring the amount of the light that is absorbed by the solution in a cuvette placed in the. A spectrophotometer is a laboratory equipment that can measure the number of photons (the intensity of light) absorbed after passing through the solution of the sample. When light passes through a sample, the molecules in the sample absorb some of it, and the rest passes through.