Monochromator – Working Principles, Application and Uses

What is a monochromator? A monochromator is used to isolate a specific wavelength of light. This process is expertly explained in the video below. Monochromators consist of a grating, an entrance slit and mirrors to produce a parallel beam similar to sunlight. The grating then disperses the parallel light into a spectrum with a narrow peak at the nominal wavelength selected.

A monochromator is an optical device that separates light with different wavelengths into a range of narrow bands. It can be used to select a particular wavelength of interest from a wide spectrum, and it can also be used to control the transmission bandwidth of a light source.

There are many different types of monochromators, but they all use the same basic principles. They all use the phenomenon of optical dispersion (prisms) or diffraction (gratings) to spatially separate colors of light and then have a mechanism for directing the selected wavelength to an exit slit.

For prism monochromators, the incoming light is first focused into a narrow optical slit. Then the slit is rotated to move the position of the dispersive element, changing the direction that the output light takes. This is why the color of the monochromatic light that exits the slit changes, as shown in Figure 1 above.

The dispersive element in a prism monochromator is the difference in the index of refraction between different elements of the prism. This causes the light to slow and bend in a way that splits it into different colors. For example, blue light will be slowed and bent more than red light when it passes through the prism. This will cause the blue light to have a longer wavelength than the red light, resulting in the monochromatic output from the slit.

There are other ways to disperse light, such as using a diffraction grating instead of a prism. However, a diffraction grating does not have as high of a light utilization efficiency and will produce a higher level of stray light. As a result, a grating monochromator typically requires more power to operate.

How Does a Monochromator Work?

A monochromator uses the phenomenon of optical dispersion (the variation in speed of different wavelengths through a prism material) or that of diffraction using a grating to separate incoming light into its component colors. A mechanism then directs only the selected color to an exit slit. A wide range of instruments may employ a monochromator, including fluorescence, transmission and reflection spectrographs, for testing solar cells or sensors, and in polarimetry.

Prism monochromators use the index of refraction of prism materials to split colors of light, because the varying speeds cause the prism to bend differently in response to different wavelengths (blue, for example, slows more than red). Grating monochromators also depend on the diffracted order of a diffraction grating and instrument focal length to disperse incoming light. They also rely on the diffraction efficiency of the grating to minimize power losses, and this is typically measured by the diffraction angle (the amount the grating bends in response to the applied voltage).

The choice of material for the prism or grating and its design can dramatically affect performance. The highest resolution monochromators are very large and use a combination of mirrors with long focal length and a diffraction grating with a larger area. They must therefore have high mechanical and thermal stability, while their mountings must be very precise to ensure accuracy.

There are two primary types of monochromators: broad-band and narrow-band. Broad-band monochromators work by allowing the passage of more than one wavelength at a time, which allows them to operate with lower power levels and still provide adequate results. The narrow-band monochromators, on the other hand, require more energy to operate because they block a larger range of wavelengths.

Application

The monochromator separates the incoming electromagnetic radiation into wavelength bands that can be directed toward a detector, such as a photomultiplier tube (PMT). In the most common form of a monochromator, a prism or diffraction grating is used to spatially separate the different colors of light. The resulting separated band of wavelengths is collimated using mirrors, allowing it to be directed towards the PMT or other detector. The selected wavelength of radiation can be set by varying the tilt angle of the grating or prism, enabling one particular wavelength to be isolated.

The slit in the entrance of the monochromator blocks out any unwanted wavelengths and is the limiting factor for how narrow or broad a specific spectral line can be. The dispersive element, in this case a grating, takes the parallel light emitted by the slit and diverges it into many different paths. This is a result of the fact that the diffraction pattern on a grating differs for each wavelength, and the fact that some of these wavelengths are reflected at the edge of the grating. The individual wavelengths that reach the exit slit are then able to be identified and extracted.

A good monochromator will also include optical systems that image the light coming in through the entrance slit onto the exit slit, and into a detector in the case of a spectrograph. This is a critical function that can have a significant impact on the performance of the monochromator. The imaging optical system needs to be able to precisely control which wavelengths reach the detector. This is usually done by using a source that produces well-defined spectral lines, and adjusting the position of the grating until those spectra appear in the output beam.

Uses of a Monochromator

Monochromators separate polychromatic light into a range of wavelength bands and allow only the desired ones to pass through. They are used in many applications including fluorescence spectroscopy, reflection spectroscopy and transmission spectroscopy, for testing sensors and solar cells, as well as in polarimetry and refraction.

The main advantage of a monochromator is its ability to isolate light according to the exact wavelength specifications unique to your application. For example, a stage lights monochromator allows the precise control of the color of the light that is directed at a specific area of the stage or set of equipment. This can be useful for industrial, research or even for determining precisely the correct light setting in professional photography.

A monochromator uses the phenomenon of optical dispersion (using a prism) or diffraction using a grating to spatially separate colors and direct them to an exit slit. A typical monochromator design consists of a dispersive element, an entrance slit, mirrors to produce a parallel beam similar to sunlight and another to direct the light toward a detector or sample.

Spectrometers and microplate readers are common examples of measurement devices that use a monochromator. In addition to a diffraction grating, these monochromators can also contain lenses, spherical mirrors and other elements to improve wavelength resolution.

Generally speaking, a monochromator that relies on diffraction has the best wavelength resolution and performance but it can be more difficult to build because it requires a complex arrangement of mirrors. It is also difficult to achieve very narrow bandwidths with a diffraction grating alone because other unwanted wavelengths will continue to pass through the slit and reach the detector. This is referred to as the stray light level and it is important to minimize it for most applications.

Repair and Servicing of a Monochromator

In a monochromator, the light from one or more different sources is passed through an order sorting filter. This filters out higher diffractive orders to allow only the desired wavelengths to pass through the monochromator to detectors. This allows multiple wavelengths to be simultaneously detected by a single detector, and is useful for analysing samples that react differently to different wavelengths of light. Depending on the application, the monochromator can be placed either directly after the light source or immediately prior to the detectors. In the former case, the monochromator may be optimised for a specific light source and in the latter for a particular wavelength range.

Monochromators are available with various grating options and coatings, which affect both the dispersion and core spectral characteristics. They can also be fitted with varying width slits, which are typically micrometer-adjustable. In addition, many monochromators can be fitted with motorised or replaceable gratings to provide flexibility in spectral tuning.

There are two main types of monochromators: Fastie-Ebert and Czerny-Turner. In the Fastie-Ebert configuration, a large spherical mirror first collimates the light which will fall on the plane grating and then focuses the dispersed image of the entrance slit in the exit plane. This configuration produces a flattened spectral field with good coma correction and astigmatism at a single wavelength.

The Czerny-Turner monochromator consists of a pair of concave mirrors and a planar diffraction grating. The geometry of the mirrors can be asymmetrical or symmetrical, and the asymmetry may be used to compensate for astigmatism and spherical aberration. In general, the geometry of a Czerny-Turner monochromator is more flexible than that of the Fastie-Ebert instrument. The asymmetrical geometry can produce a flattened spectral field and good coma correction at more than one wavelength.