Spectrophotometry is a quantitative measurement of the absorption/transmission or reflection of a material as a function of wavelength. Despite being termed UV-Vis, the wavelength range that is typically used ranges from 190 nm up to 1,100 nm in the near-infrared.

Using a spectrophotometer and carrying out absorption/transmission measurements we can determine the amount (or concentration) of a known chemical substance simply, by studying the number of photons (light intensity) that reach the detector. The more a material absorbs light at a specific wavelength, the higher the concentration of the known substance.

A typical absorption measurement as a function of wavelength is given below: