Monochromator

A monochromator (Greek: mono = a chroma = color) is an optical device for spectral isolation of a specific wavelength of an incident amount of electromagnetic radiation ( eg light, x-rays, synchrotron radiation ). Electromagnetic radiation is no treatment - due to their origin - polychromatic, ie from different wavelengths assembled (poly = many ). With the use of a monochromator, the unwanted portion of the radiation is absorbed or deflected. For different wavelength ranges of electromagnetic radiation, the operation differs from monochromators.

Example light

For light, ie electromagnetic radiation in the visible wavelength range or the near side areas, the following principles can be exploited.

Dispersing elements

The incident light is projected in front of the monochromator on the entrance slit which serves as a monochromator for the secondary light source. The light is then within the monochromator depends on its wavelength ( see also: Electromagnetic wave) fanned continuously. This is done by a dispersing element (for example, a prism ), or an optical grating (where the light is not dispersed, but bent ).

By means of a further slit, the exit slit is allowed to pass on a small wavelength range ( = spectral ) of this flared light with the desired wavelength. This gap serves as a secondary light source for the remainder of the measuring arrangement. The undesirable part of this radiation is absorbed by the panel. In order for this selection is as pure as possible (usually concave mirrors ) mapped the entrance slit on the exit slit by optical means.

The width of the entrance and exit slit is usually set manually, they are normally situated on a same width ( typically 0.5 to 2 mm). The optimal width is determined as a compromise of required intensity of light (not too narrow) and the required spectral resolution (ie, not too wide ). The spectral slit width indicates which wavelength range is scanned from the left to the right edge of the exit slit, so the wavelength resolution and the spectral resolution. In addition to the gap width, the gap heights sometimes by an additional adjustable diaphragm pair ( typically 10 to 20 mm). Depending on the application area and the intensity of the incident light, the aperture must be cooled because the absorption of light causes heating of the absorber.

A prism is given to using, when a large wavelength range is to be covered. Depending on the wavelength range is given to prisms of glass ( visible VIS, near IR, near-UV ) or rock salt (NaCl, for far IR ) a. The latter require because of their hygroscopicity of intensive care.

The lattice of the deflection angle depends in contrast to the prism via a sine function of the wavelength. The spectral resolution depends not only on the wavelength-dependent deflection angle also depends on the gap width. ( The result is additionally higher-order light. ) So-called holographic gratings, the last -mentioned disadvantages are avoided more and more. From the holographic gratings there are now versions that are themselves also shaped like a concave mirror (English concave blazed holographic gratings ); This deletes all other imaging elements ( mirror ) between the inlet and outlet gap obsolete, eliminating losses and aberrations are further reduced.

To set the desired wavelength, the dispersing element (sometimes also one of the other imaging elements such as a mirror ) is usually mounted on a turntable which is driven via a shaft and a mechanical drive from the outside. At this wave is outside during automatic operation (manual mode is also still possible ) An electric motor drive and a rotary encoder (eg, multi-speed analog potentiometer ) for detecting the current position flanged.

In order to reduce the scattered light also double monochromators are built, consisting of two virtually back to back mounted Einzelmonochromatoren in a common housing. They have a third optical gap in the middle and have the two rotary drives for wavelength setting coupled backlash. Due to the series connection of identical Mononchromatoren the spectral resolution does not increase. Scattered light is light of a different wavelength which does not fall within the wavelength interval of the monochromator. By a non-ideal dispersive element (e.g., an optical grating ) is wrongly represented and this scattered light is detected on the output slit. By twice applying the frequency selection with the double monochromator the disturbing stray light is reduced. Applies the double monochromator, for example, in spectroscopy of Raman scattering.

Calibration

Prior to measurement, the relationship between the mechanical position of the dispersive element and the wavelength selected here must be determined, so the calibration. For this purpose, usually uses a light source with a known narrow-band spectral and measures the intensity distribution after the monochromator, in dependence on the Monochromatorposition. The light source here lends itself to the mercury vapor lamp, as it has well-known lines throughout the visible and UV range.

Between the support points found by this lines are interpolated later the calibration curve in the form of a smooth curve as possible to obtain the mapping also at any intermediate points. You can even carefully extrapolate something about the range of observed spectral lines also.

Interferometer

Interferometer used as tunable, extremely narrow-band interference filter. In particular, the Fabry-Perot interferometer is used in many areas as a monochromator for spectroscopy.

Example, X-rays

A monochromator for X-rays, that is electromagnetic radiation in the wavelength range from 10 to 0.002 nm, in principle fulfill the same function, only in a different manner: At a suitable crystal at an appropriate angle, the radiation is reflected by the Bragg condition. Since the X- rays penetrate into the crystal, the radiation will be reflected not only on the crystal surface, but in a large number of lattice planes of the crystal lattice. A beam which is reflected on the outermost grid plane, defining a shorter distance than a ray reflected from a plane within the crystal. This distance difference is called retardation. Through this path difference occurs interference of the beams. Due to the high number of different path differences as well as by the high number of reflecting lattice planes almost find all wavelengths destructive interference! Only that wavelength which satisfies the Bragg condition in the given angle, interferes constructively. Usually curved Kristallmonochromatoren be used for X-ray measurements from the rounding was milled out. Such a monochromator, the focusing or the Präkollimation a divergent X-ray beam used.

Find application Kristallmonochromatoren for X-ray measurements. for example:

• Röntgenpulverdiffraktometer
• X-ray fluorescence analysis
• Photoelectron spectroscopy
• Absorption processes, EXAFS, XANES, AAS.