What is a monochromator?

A monochromator isolates a certain wavelength from incoming broadband electromagnetic radiation (e.g. light, x-ray radiation). The incoming electromagnetic radiation is sent through an entrance slit, collimated by optics and directed onto a prism or diffraction grating. This separates the electromagnetic radiation according to wavelength. A second collimator then maps the light from one direction and therefore only in one wavelength onto the exit slit. The isolated wavelength emerges at the exit slit. The desired wavelength can be set by turning the prism or grating. The design, in which a reflective diffractive grating is used, is also called a Czerny–Turner monochromator.

On the one hand, monochromators can be used to analyze light by measuring the power of the light of a certain wavelength (with a given bandwidth) behind the exit slit. On the other hand, with a monochromator and a broadband light source, monochromatic light can be generated over a wide spectral range (tunable light source).

For which applications are monochromators used?

Among the uses of monochromators are in fluorescence spectroscopy, reflection spectroscopy and transmission spectroscopy, for testing sensors and solar cells, in polarimetry, as well as in refractometry.

What types of monochromators are there?

There are several types of monochromators that differ in the dispersive element used and the optical arrangement. The main types are grating monochromators, prism monochromators and monochromators with variable filters.

Grating monochromators (Czerny–Turner monochromator)

Grating monochromators use a diffraction grating as the dispersive element. The grating reflects the light in different directions, depending on the wavelength. These monochromators offer a constant dispersion for all wavelengths. Since refractive elements can be completely dispensed with, grating monochromators can be used over a very wide range of wavelengths. Filters are needed to block higher order light.

The most important specifications of a monochromator are the bandwidth of the monochromatic light at the exit slit, wavelength range over which it can be used, and the efficiency with which it can isolate a wavelength.

Figure 1: Grating monochromator with mirrors as collimators

Prism Monochromators

Prism monochromators use a prism to disperse light. The prism breaks the light into its spectral components. Prisms have a high efficiency and produce little stray light. However, the dispersion is strongly wavelength-dependent.

Monochromators that work with variable filters

These monochromators use linear variable filters (LVF) that select different wavelength ranges by shifting them against each other. They offer high light transmission and a continuously adjustable bandwidth. However, they are severely limited in their wavelength range.

What are the main specifications for monochromators?

The most important specifications of a monochromator are the bandwidth of the monochromatic light at the exit slit, wavelength range over which it can be used, and the efficiency with which it can isolate a wavelength.

What are the sources of stray light in monochromators?

In grating monochromators, the scattered light is mainly generated at the optical gratings, but the optical elements used for collimation can also cause stray light. Stray light causes small amounts of light with wavelengths to be suppressed to be emitted at the output of the monochromator.

When are double monochromators used?

Double monochromators are used when a particularly high rejection of stray light is required. They consist of two monochromators connected in series. Both monochromators are set to the same wavelength. If the two monochromators suppress the unwanted wavelength by a factor of 1/1,000 (one to a thousand), then the double monochromator has a suppression of 1/1000,000 (one in a million).

Discover our Monochromators for tunable light sources:

Monochromator Tool

Calculates the aperture, maximum possible wavelength and bandwidth of a monochromator.