Cerny Turner Design
A monochromator is a spectrometer capable of measuring a single wavelength which can be scanned through a wide wavelength range.
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.
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.
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.
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.
Geoff Tyler, Horiba Jobin-Yvon, France; Richard Payling, Surface
Analytical, Australia and Thomas Nelis, EMPA Materials Science and
Technologoy, Thun, Switzerland.
Last modified :
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