A monochromator is an optical device that separates polychromatic light (such as sunlight or light coming from a lamp) into a range of individual wavelengths (monochromatic light) and allows a narrow band of these individual wavelengths to be selected. The light of the desired wavelength band is then directed onto a sample, a detector or other components of the optical system.
The separation of light into its individual wavelength components is called dispersion. An element with this property is called a dispersive element. In order to select a narrow band of these wavelengths, a slit can be used to block the unwanted wavelengths. The narrower the slit, the narrower the wavelength band. In general, a monochromator consists of a dispersive element, an entrance slit, mirrors to produce a parallel beam similar to sunlight, an exit slit, and mirrors to extract the monochromatic light.
By fixing the slit and rotating the dispersive element, the direction of the dispersed light is turned so that the colour of the resulting monochromatic light changes: as you can see in Figure 1 in the upper panel, the position of the dispersive element causes orange-red light to exit the slit, while in the lower panel the dispersive element rotates so that the light exiting the slit is cyan.
Monochromators can be divided into different types depending on the type of dispersive element used and the optical arrangement of the system:
The dispersive element in prism monochromators is a prism. Prisms have a high light utilization efficiency and do not produce higher order light and very little stray light. However, dispersion is dependent on wavelength (high for UV, low for IR) and temperature.
The dispersive element in grating monochromators is a reflecting diffraction grating. It provides a constant dispersion for all wavelengths and a low dependence on temperature. However, they produce relatively large amounts of scattered light and require the use of filters to block higher order light. Due to their superior dispersion properties, diffraction gratings are often used in modern instruments.
The most popular design for grating monochromators in microplate readers and spectrophotometers is the Czerny-Turner monochromator. This type of monochromator uses curved mirrors so that the light reflected from the mirror is collimated out of the slit (Figure 2).
Monochromators are commonly used in measurement devices such as spectrometers or microplate readers. They are a popular device for wavelength selection in a range of detection technologies, such as absorption and fluorescence intensity.