Monochromator: components, function, types

White light, for example, is made up of a set of wavelengths (colors), corresponding to the visible area of ​​the electromagnetic spectrum; as seen in the different colors of the rainbow. The water droplets refract light, producing a range of different colors.

A similar phenomenon occurs in the monochromator. This device is in contact with a tungsten lamp (visible light) or with a deuterium lamp (visible-ultraviolet light), both of which produce the light that is introduced into the monochromator through the entrance slit.

Monochromator Components

The classic Czerny-Turner monochromator consists of the following parts: an entrance slit, a collimating mirror, a diffraction grating or grating, a second mirror, and the exit slit.

Entrance slit

The entrance slit allows the entry of white light (polychromatic), coming from the illumination lamp, into the monochromator. The width of this slit can be adjusted in order to control the intensity of light passing through it.

Collimating mirror

The function of this mirror is to transform the beams of light that converge on it into parallel rays of light. This is a necessary requirement for diffraction to occur.

In the case of the drops of water that produce the rainbow, the sun’s rays that fall on them behave as parallel due to the great distance that separates the sun from the Earth.

Grating or diffraction grating

It is a flat structure on whose surface a series of grooves separated by a certain distance have been made. When the light coming from the collimating mirror falls on the grooves, it decomposes into the wavelengths that make it up.

Focus mirror

The function of this mirror is to direct the light rays, coming from the diffraction grating, towards the exit slit.

Exit slit

This slit is what allows the exit of the monochromatic light coming from the interior of the monochromator. Its width is conditioned by the intensity of the emerging light and the width of the band that contains the desired wavelength.

Monochromator function

Monochromator on an X-ray line. Advanced Photon Source, Argonne National Laboratory. Los Alamos National Laboratory, Attribution, via Wikimedia Commons

The monochromator is used in numerous optical devices and instruments used in teaching, research laboratories, and clinical laboratories.


Monochromators are present in spectrophotometers, devices that are used in laboratories for different purposes; for example, in determining the activity of different enzymes, in the concentration of metabolic substances, in the concentration of macromolecules, etc.

The monochromator supplies a light of a certain wave that is absorbed specifically by a substance, thus being able to determine its concentration due to a relationship between the concentration of the substance and its absorption of light.

To do this, a calibration curve is constructed, based on the Beer-Lambert Law. Light absorption can be expressed as transmittance, indicating 100% transmittance when light is not absorbed by substances. Meanwhile, a lower transmittance will indicate light absorption.


There are substances that are capable of emitting fluorescence, the concentration of which can be obtained with the use of a spectrofluorometer.

This device has two monochromators: one is used for the excitation of a substance, which allows the substance to emit fluorescence; and the other allows to use the fluorescence produced to determine the concentration of the examined substance.

Atomic absorption spectrophotometer

It is a piece of equipment used to determine the concentration of metals, such as sodium, potassium, iron, copper, magnesium, calcium, etc., and can be used as an absorption or emission technique.

In the atomic absorption technique, the sample in solution is vaporized with a mixture of acetylene and oxygen.

The light from a lamp, specific for the ion whose concentration is determined, passes through the flame and part of the light is absorbed. But another part of the light reaches a monochromator that is part of a spectrophotometer, which allows the determination of the concentration of the vaporized ion in the flame.

Circular dichroism spectrophotometer

This equipment, used in the investigation of the behavior of biological macromolecules, is equipped with a monochromator. Circular dichroism is used, among other studies: in determining the secondary structure of proteins, the conformation of proteins, and their chemical or thermal denaturation.

Microplate reader

It determines the fluorescence of biochemical or immunological processes that occur in the concavities of the microplates, therefore resorting to the use of a monochromator.

The so-called ELISA technique is frequently used in the quantification of proteins, nucleic acids, enzymatic activity assays, as well as in numerous immunological procedures.

Monochromator types

There are two types of monochromators depending on the diffraction device used: grating or grating, and prism monochromators.

Grating or grating monochromators

In these monochromators the diffraction is carried out by a diffraction grating. This is a flat surface that has numerous parallel and identical grooves. For example: a grid for UV light has between 300 and 2,000 slots per mm.

The light coming from the collimating mirror falls on the grating slots and is decomposed into radiations of different wavelengths, which emerge from the gratings at different angles.

By means of a control, the inclination of the grating is modified to make the light of the desired length fall on the exit slit.

Prism Monochromators

An optical prism can refract sunlight in the colors of the rainbow. When light falls on the face of a prism, it is refracted at an angle in relation to the air-crystal interface of the prism, the refraction being dependent on the refractive index.

Also, the refractive index depends on the wavelength of the light. This causes the white light incident on a prism to be decomposed into different wavelengths, corresponding to the different colors.

A control shines the desired light from the prism onto the exit slit of the monochromator.

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