The key components of a UV-Vis spectrophotometer are:

A light source that generates a broadband of electromagnetic radiation across the UV-visible spectrum
A dispersion device that separates the broadband radiation into wavelengths
A sample area, where the light passes through or reflects off a sample
One or more detectors to measure the intensity of the reflected or transmitted radiation

Other optical components, such as lenses, mirrors, or fiber optics, relay light through the instrument.

Schematic of the internal layout of an Agilent Cary 5000 UV-Vis-NIR spectrophotometer, showing the main components. Note that this is a high-performance instrument. UV-Vis spectrophotometers for routine measurements have a simpler optical design.

Light sources

See the next question for full details on light sources.

Monochromators

All the light sources produce a broad-spectrum white light. To narrow the light down to a selected wavelength band, the light is passed through a monochromator, which consists of:

An entrance slit
A dispersion device, to spread the light into different wavelengths (like a rainbow) and allow the selection of a nominated band of wavelengths
An exit slit where the light of the nominated wavelengths passes through and onto the sample

A single monochromator spectrophotometer is used for general-purpose spectroscopy and can be integrated into a compact optical system. A double monochromator is typically found in high-performance instruments.

For more details on how monochromators work and on holographic gratings, filters, plus single and double monochromator spectrophotometers, download the Agilent handbook on the Basics of UV-Vis Spectrophotometry.

Sample compartments

The sample compartment of a UV-Vis spectrophotometer is typically a black-colored box with a closing lid. The matt black inside the compartment helps to absorb stray light that may enter the compartment. In the sample compartment, the sample is positioned to allow the beam from the monochromator to pass through the sample. Glass, plastic, or quartz cuvettes are used for liquid samples. Solid samples are held in position by a holder attached to the floor of the sample compartment. The light can also be taken out of the sample compartment using fiber optics.

Detectors

A detector converts the light from the sample into an electrical signal. Like the light source, it should give a linear response over a wide wavelength range, with low noise and high sensitivity.

Each detector has a different sensitivity and wavelength range. For systems with multiple detectors, the system will switch to the detector corresponding to the required wavelength range for the measurement. UV-Vis spectrophotometer detectors include photomultiplier tubes (PMT) and silicon diodes (Si). Indium gallium arsenide (InGaAs) photodiodes and lead sulfide (PbS) detectors are found on high-performance UV-Vis-NIR systems to improve wavelength coverage or sensitivity.

To find out more about these detectors and their uses, download the Agilent handbook on the Basics of UV-Vis Spectrophotometry.