Monochromators

Monochromator

Introduction to monochromators

Unlike polychromators monochromators are used to select a single wavelength of an emission spectrum. When recording of a larger part of the spectrum is required they are operated in the sequential mode, i.e. the intensity of one wavelength at the time is measured. This can be done in either the hopping mode, jumping from one wavelength of interest, such as an emission line of an atom, to the next. A different mode of operation is the scanning mode where are selected area of the spectrum is sequentially acquired by tuning the monochromator. Monochromators are an interesting option when high spectral resolution and light throughput is required. When designing a polychromator for the simultaneously acquiring large parts of the optical spectrum compromises have be to be made in terms of the lay-out of the optical components. Monochromators need be optimised for only one wavelength at the time. This allows the use of larger gratings, mirrors etc leading to a better optical qualitiy, at the price of lower speed.

A compromise to combine the optical quality of a typical monochromator with the possibility of simultansesouly acquiring segment of the spectrum can be acchieve by replacing the combination of the exict slit and the detector by a array detector, typically a diode array or a CCD-chip.

Author: Thomas Nelis, EMPA Materials Science and Technologoy, Thun, Switzerland.

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Fastie-Ebert configuration

The Fastie-Ebert configuration is a rather simple and inexpensive configuration. A spectrometer in this configuration consists of a single large spherical mirror and one plane diffraction grating

The entrance slit is placed in the focal plane of the spherical mirror. Light passing through the entrance slit will be reflected to form a parallel beam, directed towards the plane grating. At the grating the light is dispersed, and ‘reflected’ back towards the mirror. This reflected, or dispersed parallel beam, will again be collimated and form an image, or several coloured images of the entrance slit. The exit slit, again situated in the focal plan of the spherical mirror, serves to select one of the these coloured images, allowing a photo detector to measured the trasmitted light intensity, spectral intensity.

The opening angel q , or rather 2qis an important parameter of the Fastie-Ebert configuration. It determined by the distance between entrance and exit slit, and the focal length (radius) of the spherical mirror. The larger this angle, the more the spherical mirror is used in an off axis configuration, which detoriates the optical quality of the system.

Fastie-Ebert instruments are inexpensive and commonly used design, but exhibit some limitation in their ability to maintain image quality, due to system aberrations such as spherical aberration, coma, astigmatism, and a curved focal field. These aberrations are rather strong due to the off-axis operation of the system .

First published on the web: 19.11.2006

Authors of the latest version: Aranka Derzsi and Giovanni Lotito. The text is based on a lecture given by Thomas Nelis at the first Gladnet training course in Antwerp Sept. 2007

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Cerny Turner Design

A monochromator is a spectrometer capable of measuring a single wavelength which can be scanned through a wide wavelength range.

Schematic Cerny-Turner

A common form of monochromator is the Czerny-Turner design. It is, similar in principle to the Fastie-Ebert mount but offers more flexibility in the design and positioning of the reflector mirror. The Czerny-Turner instrument consists of fixed entrance and exit slits, fixed focussing mirrors and a rotatable diffraction grating. The entrance slit is in the focus plane of the first focussing mirror. This first mirror consequently reflects the light as a parallel beam, towards the grating. The grating in this case is a flat reflection grating. As the grating rotates a different wavelength is reflected or diffracted towards the second focussing mirror. This second mirror focusses the parallel beam from the grating towards the exit slit, where it produces a "monochromatic" image of the entrance slit. Wavelength close to the selected wavelength will be imaged closely to the right or left of the central image, creating a series of images of the entrance slit, each having a different wavelength. The exit slit, now, selects one of these images to transmit the light to the detector positions behind the exit slit.

Although the two mirrors function in a similar fashion as the single spherical mirror of the Fastie-Ebert configuration, i.e., first collimating then focusing the dispersed light from the grating, the geometry of the mirrors in the Czerny-Turner configuration is flexible. This allows Czerny-Turner configuration to be designed for producing a flattened spectral field and good coma correction at one wavelength. Other imaging problems such as spherical aberration and astigmatism will remain at all wavelengths. The Cerny-Turner configuration, even when disigned symmetrically, also allows more easiy for larger systems, where a single spherical mirror would be a little bulky.

Cerny Turner

The wavelength range of a monochromator varies with the choice of grating, but commonly they can scan from 160 nm to 500 nm or ever wider ranges. The spectral resolution depends on the widths of the slits, the choice of grating and focal length, but commonly can be less than 10 pm for high resolution OES. A key to the performance of monochromators is the design of the grating movement: the grating is placed on a large drive wheel with motor control, allowing fine and precise positioning of the grating.

This configuration exists in many variations and dimensions. Larger versions are often used in Raman spectrometers, demanding both high resolution and high light throughput.

First published on the web: 15 May 2000.

Authors: Geoff Tyler, Horiba Jobin-Yvon, France; Richard Payling, Surface Analytical, Australia and Thomas Nelis, EMPA Materials Science and Technologoy, Thun, Switzerland.

Last modified : 010.12.2007 by Aranka Derzsi and Giovanni Lotito. The new text is based on a lecture given by Thomas Nelis at the first GLADNET training course in Antwerp Sept. 2007

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