Geoscience Reference
In-Depth Information
the measurement, including time-resolved techniques, polarization, and position sensitiv-
ity; however, the most common approach is based on wavelength discrimination by either
dispersive or nondispersive means.
Monochromators and spectrographs are the most widely used dispersive instruments.
They consist of a dispersive element, such as a prism or diffraction grating (
www.newport.
com
), and image transfer optics, which separate a small wavelength from a polychromatic
source (e.g., xenon lamp). Detectors used with monochromators are usually single-channel
large-area devices. Single spectrographs use a fixed grating geometry to monitor a spectral
range dispersed over a linear array that is made up of multiple detector elements. Grating-
based monochromators and spectrographs are available in a wide range of configurations
for applications in the 10-nm to 20-μm range. The manufacture of commercially available
prism-based monochromators is on the decline and as a result the availability of such sys-
tems is scarce or specialized.
A diffraction grating is a plane or concave element with closely spaced grooves. The
grating acts as a multislit source when illuminated by collimated radiation (
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com
). Different wavelengths are diffracted and constructively interfere at different angles.
Most modern spectrometers use reflection gratings with groove densities from 75 to 3600
grooves per millimeter, depending on the spectral range and resolution required. Grating
types are separated by their method of production, “ruled” or “holographic.” Replicating a
master grating prepared by a high-precision ruling engine makes ruled gratings. Projecting
an interference pattern onto a photoresist plate and developing this to produce the pattern
makes holographic gratings. Holographic gratings have essentially perfect groove patterns,
almost perfect elimination of false lines or ghosts, and significantly improved stray light
rejection compared with ruled gratings (
www.horiba.com
)
.
One of the most common configurations of a monochromator is the Czerny-Turner
configuration (
Figure 5.11
). Although many other designs are also available most adopt
the same operating principle (
Figure 5.11
). A Czerny-Turner monochromator. Light (
A
)
is focused onto an entrance slit (
B
) and is collimated by a curved mirror (
C
). The colli-
mated beam is diffracted from a rotatable grating (
D
) and the dispersed beam refocused by
a second mirror (
E
) at the exit slit (
F
). Each wavelength of light is focused to a different
position at the slit, and the wavelength that is transmitted through the slit (
G
) depends on
the rotation angle of the grating.
The incident light is passed through the entrance slit and hits a collimating mirror that
produces a parallel polychromatic light beam onto the diffraction grating. The grating
rotates around a plane through the centre of its face and spatially separates the spectrum in
the incident light. The resulting diffracted light is focused onto the exit slit using a focusing
mirror. Each wavelength is incident upon the exit plane at a specific angle and, by rotating
the grating position, one can scan these wavelengths across the exit slit and discriminate
between each wavelength. The grating equation specifies the angle required to bring each
wavelength through the exit slit (
Figure 5.12
):
sin
αβ
+
sin 0
=
−
6
kn
λ
(5.5)
