A spectrophotometer is an analytical instrument that measures the intensity of light as a function of its wavelength. A common tool in physics, spectrophotometers are commonly used to measure light absorption. There are two major measurement classes of spectrophotometers--single beam and double beam spectrophotometers.

There are a few major differences between the single beam and the double beam spectrophotometers. A single beam spectrophotometer measures absolute light intensity, while the double beam spectrophotometer measures the ratio of light intensities on two separate light paths--the reference standard and the sample. Although double beam spectrophotometers were popular in the early days of spectroscopy, it is now thought that the single beam spectrophotometer is more advantageous.

This is because it becomes difficult to recombine the light beam prior to reaching the monochromator with the double beam spectrophotometers. The monochromator is a device for selecting light from a narrow band of wavelengths. The monochromator splits the light into its component wavelengths, so that only the light from the desired wavelengths reach the sample. One example of a monochromator is a prism. The single beam spectrophotometers also have a larger dynamic range than the double beam spectrophotometers.

The oldest form of spectroscopy is ultraviolet/visible spectroscopy. An ultraviolet/visible spectrophotometer works by placing a sample in the spectrophotometer and shining ultraviolet and/or visible light through. Measurements are made depending on how much light was absorbed by the sample.