A spectrometer consists of three main parts:
- an emission source which produces the spectrum,
- an optical system which collimates and disperses the spectrum and
- the detecting device to measure the emitted lines intensities.
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.
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,
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.
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.
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 itís no longer used for application with electronic
First published on the web by Richard Payling: 15 May 2000.