Perhaps the most common application of spectrophotometers is the measurement of light absorption, but they can be designed to measure diffuse or specular reflectance. Strictly, even the emission half of a luminescence instrument is a kind of spectrophotometer.

The spectrophotometer measures quantitatively the fraction of light that passes through a given solution. In a spectrophotometer, a light from a lamp in a near-ir/vis/uv spectrophotometer (typically a deuterium gas discharge lamp) is guided through a monochromator, which picks light of one particular wavelength out of the continuous spectrum. This light passes through the sample that is being measured. After the sample, the intensity of the remaining light is measured with a photodiode or other light sensor, and the transmittance for this wavelength is then calculated.

Wide spectrum range of spectrophotometer is available in the market that is classified according to the wavelengths they work with. There are two major kind of spectrophotometers- single beam spectrophotometer and double beam spectrophotometer are available in the market. A singe beam spectrophotometer measures the ratio of absolute light intensity and the double beam spectrophotometer measures the ratio of the light intensity on two different light paths. Although ratio measurements are easier, and generally stabler, single beam instruments have advantages; for instance, they can have a larger dynamic range.

The most common spectrophotometer is used in the uv and visible regions of the spectrum, and some of these instruments also operate into the near-infrared region as well. Spectrophotometer that is designed for the main infrared region is quite different because of the technical requirements of measurement in that region. One major factor is the type of photosensors that are available for different spectral regions, but infrared measurement is also challenging because virtually everything emits ir light as thermal radiation, especially at wavelengths beyond about 5 µm.

Historically, spectrophotometers use a monochromator to analyze the spectrum, but there are also spectrophotometers that use arrays of photosensors and, especially in the ir. There are spectrophotometers that use a fourier transform technique to acquire the spectral information in a technique called fourier transform infrared.

This scientific instrument is preferred extensively in colorimetric science and in ink manufacturing, printing companies and by textiles vendors. Uv spectrophotometer and ir spectrophotometer that are designed for infrared region are quite different because of the technical requirement of the measurement in the region.