Spectrometers
Spectrometer
A spectrometer consists of three main parts:
- an emission source which produces the spectrum,
- an optical system which colimates and disperses the spectrum and
- the detecting device to measure the emitted lines intensities.
Sources
There are different kinds of sources which range from arc to glow discharge and differ from each other according to the way of operating (continuous or transitory), the composition (variable or constant), and whether or not they are homogeneous.
Imaging system
The imaging system has 3 main features: aperture, transmittance and adaptation to environment. The aperture depends on the dimension of the source and the characteristics of the spectrometer. The transmittance depends on the spectral range and the optical components. The adaptation to environment takes into account the arrangement of components, their material and the environment, such as humidity, pressure etc.
The optical components of the system include the following: spherical and cylindrical lenses, flat and spherical mirrors, parallel planes; optical path under vacuum or controlled nitrogen atmosphere; dispersers, which can be gratings or prisms; optical cables.
A conventional lens configuration is formed of two lenses (see picture below): the first lens set at the entrance getting the image source, the second one at the slit dispersing the element. There are other different mounts which can include condensers or crossed cylinder lenses to make light beam homogeneous, or can create intermediate images for selecting a fraction of the source.
Common spectrometer optical systems consists of a polychromator or a monochromator which scatters the spectrum and isolates the analytical lines of the elements to be analysed. A common spectrometer optical system consists of a polychromator which scatters the spectrum and isolates the analytical lines of the elements to be analysed. As shown below, a Paschen Runge-type polychromator consists of an entrance slit, a concave grating and exit slits. The slits and the centre of the grating are located on a circle, known as the Rowland circle. The radius of the Rowland circle equals the focal length of the grating. The grating is curved at twice the radius of the Rowland circle so that the light from the entrance slit is focused onto the exit slits.
First published on the web by Richard Payling: 15 May 2000.
revised and extended 19.11.2007
Authors: Aranka Derzi and Giovanni Lotito. The text is based on a lecture given by Thomas Nelis at the first Gladnet training course in Antwerp Sept. 2007
TOP
The prism:
In optics, a prism is a transparent optical element with flat, polished surfaces. A prism can be used to break light up into its constituent spectral colours (the colours of the rainbow). A very important feature of the prism is the refraction index which varies with the wavelength. The dispersion depends on the variation of the refraction index with the wavelength. There is typically a large non linear dispersion in the UV and a small one in the visible spectral range. The effective spectral range is therefore limited by the absorption. The prism was used as spectrograph with a fixed incident angle and a photographic detector. Nowadays it’s no longer used for application with electronic detectors.
TOP
Polychromators
A polychromator's performance depends on essential features such as brightness, stability, spectral purity and resolution.
A polychromator can be mounted with two gratings as shown below, with the second grating taking its light from the zero order diffraction (direct reflection) of the first grating. The photomultipliers can be mounted in different veritical and horizontal positions using mirrors and filters and other optical devices to optimise space. An example of the inside of a polychromator with a 1 metre focal length for high resolution is shown below:
Inside the circular spectrometer, light enters from the top on the left hand side, the grating is mounted inside the bottom left hand side, and the photomultiplier tubes are mounted on the Rowland circle near the top and right hand side.
First published on the web: 15 May 2000. Last modified:
20 November, 2007
Author: Richard Payling
TOP
Scanning Polychomators
Normally polychromators are used with fixed wavelengths. But it is possible to scan each of the fixed polychromator channels over limited ranges of wavelength.
One early way to do this was by rotating a transparent plate after the entrance slit to form slightly differing entrance angles onto the grating. The wavelength range that could be scanned was limited to only about ±0.1 nm before defocussing and other aberrations became too severe.
A better way, first introduced by Jobin-Yvon Emission (PolyScan®), is to move the entrance slit around the Rowland circle. This limits defocussing and extends the scanning range to about ±2 nm.
Scanning of polychromators is now used routinely for:
- alignment of the primary slit
- verifying the alignment of secondary slits
- checking spectral interferences
- background measurements off peak
- doing multiple lines on one polychromator channel
PolyScan® is a registered trade mark of Horiba Jobin-Yvon, France.
First published on the web: 15 May 2000. Last modified:
20 November, 2007
Author: Richard Payling
TOP