A spectrophotometer or UV spectrophotometer is an instrument used in spectrophotometry; the study of electromagnetic spectra. It is more specific than the broader term electromagnetic spectroscopy in that spectrophotometry deals with visible light, near-ultraviolet and near-infrared. Electromagnetic spectroscopy includes time-resolved spectroscopic techniques, while spectrophotometry does not.

Spectrophotometry entails measuring the intensity of light as a function of the color. More specifically, it measures the wavelength of light. Significant features of spectrophotometers are measurement of spectral bandwidth and linear range of absorption. Perhaps the most common application of spectrophotometers is the measurement of light absorption, but they can also be designed to measure diffuse or specular reflectance. Even the emission half of a luminescence instrument can be considered a kind of spectrophotometer.

The use of the UV spectrophotometer or conventional spectrophotometers is not limited to the field of physics. It is also frequently used in other scientific fields such as chemistry, biochemistry and molecular biology. It is widely used in many industrial applications, including printing and forensic examination.

The two major classes of spectrophotometers are the single beam and the double beam. A double beam instrument compares the light intensity between two light paths by splitting the light source into two separate beams. One beam contains a reference sample and the other contains the test sample. A single beam instrument, by contrast measures the relative light intensity of the beam before and after a test sample is introduced. Although comparison measurements from double beam instruments are easier and more stable, single beam instruments can have a larger dynamic range and are optically simpler and more compact.

A visible region spectrophotometer is used extensively in colorimetry science, as opposed to a UV spectrophotometer, which is used in measuring UV-visible wavelengths. Ink manufacturers, printing companies, textiles vendors and many others need the data provided through colorimetry. Readings in the region are taken every 10- 20 nanometers along the visible region. The readings produce a spectral reflectance curve or a data stream for alternative presentations. These curves can be used to test a new batch of colorant to insure that it matches specifications, such as ISO printing standards.

UV spectrophotometry (Ultraviolet-Visible) involves the spectroscopy of photons in the UV-visible region. This means that it makes use of light in the visible and adjacent ranges (including near ultraviolet (UV) and near infrared (IR). The UV spectrophotometer is the instrument used, rather than a visible region spectrophotometer. It measures the intensity of light passing through a sample and compares it to the intensity of light before it passes through the sample. The ratio is called the transmittance and is usually expressed as a percentage. The absorbance is based on the transmittance. The absorption in the visible ranges directly influences the color of the chemicals being used. In this region of the electromagnetic spectrum, molecules experience electronic transitions. This technique is complementary to fluorescence spectroscopy, since fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state.

The primary parts of a spectrophotometer or UV spectrophotometer are a light source, a   sample  holder and a diffraction grating or monochromator to separate the different wavelengths of light and a detector. The light sources are frequently a Tungsten filament, a deuterium arc lamp which is continuous over the ultraviolet region or more recently, light emitting diodes (LED) and Xenon Arc Lamps for visible wavelengths.