Spectrophotometers are modern spectrometers that use a monochromatic light that passes through a sample and a photodetector detects the output light. The changes in the output light compared to the source light allow the instrument to plot an output graph of the absorbed frequencies.

The graph indicates the characteristic transitions in the material sample. They are an advanced version of the spectrometers and contain a spectrometer and photometer combined into a single device. The spectrometer here splits the light source into its component wavelengths and the slit can be adjusted in such a way that the light of the required wavelength is passed into the sample and the photometer measures the intensity of light coming out of the sample. The sample is placed between the spectrometer and the photometer in a spectrophotometer. The photometer measures the amount of light that passes out of the sample and delivers the reading in the form of voltage signals. The voltage changes with the light absorption by the sample. 

Spectrophotometers are mainly of two types: single beam and double beam. In a single beam configuration, a single beam measurement is possible at a time. It requires the reference sample and test sample to be measured separately.

In a double beam configuration, the light from the source is split into two different beams before passing through the sample. One beam is passed through the test sample while the other is used as a reference beam. This configuration allows sample and reference reading to be done simultaneously.