Spectrophotometry is the quantitative measurement of the absorption, reflection, diffraction, or transmission of electromagnetic radiation through a material as a function of the intensity and wavelength of the radiation. A spectrophotometer is a device used to measure the change in the radiation intensity caused by its interaction with the material.

The essential components of a spectrophotometer include a source of electromagnetic radiation, a slot for placing a material to be analyzed, and a detector to measure the intensity of the incident radiation from the source and the radiation exiting from the material (transmitted, reflected, or diffracted radiation). The difference between the intensities of incident radiation and transmitted radiation reaching the detector is used to calculate the intensity of radiation absorbed by the material. The sample is exposed to radiation of different wavelengths, often one at a time. The spectrometer provides a series of data with absorption details in each attempt, a process known as scanning. Since the material energy levels are finite, absorption occurs only at specific wavelengths.

The plot of absorbed radiation against the corresponding wavelength is known as the absorption spectrum. Similarly, the plots of reflected, diffracted, and transmitted radiation as a function of wavelength are the reflection, diffraction, and transmission spectra, respectively. These spectra provide information about the specific wavelength at which the interaction between the material and radiation occurs.

The wavelength range of radiation determines the type of transition that occurs in the material. For example, UV–vis spectra show UV–vis radiation absorption by the material. The absorbed radiation causes an electronic excitation in the material. In contrast, infrared spectra show the excitation of vibration levels of a specific bond in the material. The specific absorption wavelength details the molecule's chemical structure inside a range of wavelengths. For example, the specific wavelength of infrared radiation absorbed by a hydroxyl bond in a molecule is different from the wavelength of radiation absorbed by the carbonyl bond. So, infrared spectra can be used to identify functional groups in the material.

Unlike other chemical characterization methods like titration, spectroscopy is a nondestructive characterization technique, and sample material can be recovered after analysis.