A spectrophotometer is an instrument used to identify a substance or determine its concentration by measuring the intensity of light reflected by or transmitted through the substance. It projects a specific wavelength of light—usually in the visible, ultraviolet (UV), or infrared (IR) region of the electromagnetic spectrum—onto a sample, and any light that passes through is recorded by a detector within the instrument. Spectrophotometers are used for applications such as material and chemical engineering, water quality testing, clinical analysis of biological fluids, and industrial quality control of chemicals. They vary as to the region of the spectrum and the range of wavelengths they test. The most common type is a UV-vis spectrophotometer, which is calibrated for testing both visible and UV light. In some spectrophotometers, the filter or grating may be changed to match the wavelength to the application. A color spectrophotometer, also known as a colorimeter, is a specialized type that measures red, green, and blue wavelengths of visible light for color analysis of video screens, inks and dyes, precious stones, and blood hemoglobin, among other substances.
The light source in a spectrophotometer varies depending upon the spectrum it tests; deuterium is generally used for UV, tungsten halogen for visible, xenon for visible and UV, and infrared elements for IR light. Some instruments project a single beam of light to measure relative intensity before and after a sample is inserted, while others project one beam onto a reference sample and another onto the test sample for simultaneous comparison of intensity. A single-beam spectrophotometer generally offers a wider dynamic range while a double-beam unit provides more stable and straightforward measurements.
Spectrophotometers are available in handheld models for field use and benchtop models for lab applications. Many instruments have a sample compartment that accommodates a cuvette or other container for holding samples. Cuvettes, in particular, are designed for optical uniformity and clarity to provide consistent test results. While there are models with an analog scale, most have a digital readout for more precise, consistent readings. A spectrophotometer may have a USB port for connecting to a computer to save and analyze test results. Certain microprocessor-based units, particularly those designed for testing biological samples, feature various pre-programmed methods of analysis.