A monochromator is an optical component which is used to produce a beam of light with a very narrow bandwidth - effectively light of a single colour. The way in which this is achieved depends on the application - the most common method is to bend the unwanted wavelengths away from the original path so they can be blocked.
This method is preferred for most technical scenarios, as the wavelength which is transmitted can be tuned - this provides a simple and more affordable alternative to a tunable laser.
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Figure 1. Schematic of a Czerny-Turner configuration. Redrawn from Horiba Scientific's Optics Tutorial
Monochromator Designs
Czerny-Turner monochromators consist of a plane diffraction grating, two small slits and two concave mirrors. The incoming white light passes through the first slit, is collimated (beams of light are made to run parallel) by the first mirror, then reflects off the plane grating.
The surface of the diffraction grating is designed specifically to reflect different wavelengths of light at slightly different angles, spreading out the white light (older designs used a prism to achieve this). The second mirror reflects the light onto the second slit, focusing a single, narrow band of wavelengths onto the slit so that the other wavelengths cannot pass through.
Fastie-Ebert monochromators consist of a single spherical mirror, rather than two smaller ones, along with a plane grating. Two separate portions of the larger mirror are used to collimate the incoming light and focus the outgoing light respectively.
The single mirror makes this design cheaper to manufacture than Czerny-Turner monochromators, but results in reduced image quality due to abberations.
Tuneable Monochromator; Spectroscopy
Figure 2. This video explains the operation of a tunable Czerny-Turner monochromator in a scanning absorption spectrometer.
Monochromator Abberations
Abberations are distortions in the light emitted from a monochromator, caused by the physical characteristics of the monochromator - defects in the surfaces or incorrectly adjusted components can make the abberations stronger, but the distortions can rarely be removed entirely.
Abberations manifest as a loss of resolution at certain wavelengths, or increased noise. The main types of abberation which are encountered regularly when working with monochromators are:
- Astigmatism - a beam with non-negligible width in two dimensions, incident on a surface at an angle to the normal, will produce two focal points in the exiting beam.
- Defocusing - the rays exiting the monochromator are focused on a point away from the detector
- Spherical Abberation - rays from the centre of a curved surface have a different focal point to those from the edges of the surface
- Coma - rays are skewed in the plane of dispersion, raising the noise level on one side of a spectral line
Spectrometers, spectrographs, and other pieces of optical equipment which makes use of monochromators have to correct for these abberations in order to achieve good, accurate measurements across the whole spectrum. A good example is the IsoPlane spectrograph from Princeton Instruments, which is designed to correct for all abberations inherent in the monochromator design.
Applications of Monochromators
Monochromators are used extensively in spectroscopy and spectrophotometry. A light detector in place of the exit aperture can capture all of the light in one go, for rapid measurements of the spectral profile from a sample. Alternatively, a detector placed to capture light coming out of the exit slit, paired with scanning capability, can achieve extremely high resolution. Wavelength scanning is most often achieved by rotating the plane grating. Both of these methods are common in fluorescence and absorption spectroscopy.
Monochromators can also be used to provide tunable narrow-bandwidth light in situations where a tunable laser is too expensive or impractical.
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