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Components of Optical
Instruments, Cont…
Lecture 7
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Monochromators
A monochromator is the part of instrument
responsible for producing monochromatic
radiation. It is an essential component of any
spectroscopic instrument and is composed
of a prism or grating, as the l selector, in
addition to focusing elements; like mirrors or
lenses. All these components are contained
in a box that has an entrance and an exit slit.
Two common types of monochromators can
be described:
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Czerney-Turner Grating
Monochromator
This is composed of a grating, two
concave mirrors and two slits. The
following setup can be associated with
this monochromator system:
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Focal
Plane
Grating
Entrance Slit
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Exit Slit
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Bunsen Prism Monochromators
This type of monochromators uses a
prism as the dispersion element in
addition to two focusing lenses and
two slits. The setup can be depicted as
in the figure below:
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Collimating Lens
Focusing Lens
Focal
Plane
Entrance
Slit
Exit
Slit
Prism
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Performance Characteristics of
Grating Monochromators
Four main properties can assess the
performance of grating monochromators.
These include the following:
1. Spectral Purity
If the exiting beam is thoroughly studied, it will
always be observed that it is contaminated
with small amounts of wavelengths far from
that of the instrumental setting. This is
mainly due to the following reasons:
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a. Scattered radiation due to presence of
dust particulates inside the
monochromator as well as on various
optical surfaces. This drawback can be
overcome by sealing the
monochromator entrance and exit slits
by suitable windows.
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b. Stray radiation which is radiation that
exits the monochromator without
passing through the dispersion
element. This problem as well as all
other problems related to spurious
radiation, including scattering, can be
largely eliminated by introducing
baffles at appropriate locations inside
the monochromator, as well as painting
the internal walls of the
monochromator by a black paint.
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c. Imperfections of monochromator
components, like broken or uneven
blazes, uneven lens or mirror surfaces,
etc, would lead to important problems
regarding the quality of obtained
wavelengths.
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2. Dispersion of Grating Monochromators
Dispersion is the ability of a monochromator to
separate the different wavelengths. The
angular dispersion can be defined as the
change in the angle of reflection with
wavelength:
Angular dispersion = dr/dl
We have previously seen that:
nl = d(sin i +sin r)
differentiating this equation at constant angle
of incidence gives:
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n dl = d cos r dr which gives upon rearrangement:
dr/dl = n/d cos r
In fact, we are more interested in the linear dispersion
(change of the distance at the focal plane with
wavelength), D where:
D = dy/dl
Where; y is the distance along the focal plane. If the
focal length of the focusing mirror is F, then:
dy = Fdr substitution in the linear dispersion equation
gives:
D = Fdr/dl
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A widely used parameter for expressing the
dispersion of grating monochromators is the
inverse of the linear dispersion. This is called
reciprocal linear dispersion, D-1 = 1/D
D-1 = dl/Fdr but we have dr/dl = n/d cos r
Therefore, one can write:
D-1 = d cos r/nF
At small reflection angles (<20o) cos r
approximates to unity which suggests that:
D-1 = d/nF or D = nF/d
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3. Resolving Power of a Grating
Monochromator
The ability of a grating monochromators to
separate adjacent wavelengths, with very
small difference, is referred to as the
resolving
power
of
the
grating
monochromator, R.
R = l/Dl where:
Dl is the difference between the two adjacent
wavelengths (l2 – l1) and l is their average
(l1 + l2)/2
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The resolving power can also be defined
as:
R = nN
Where n is the diffraction order and N is
the number of illuminated blazes.
Therefore, better resolving powers
can be obtained for:
a. Longer gratings.
b. higher blaze density.
c. Higher order of diffraction.
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4. Light Gathering Power
The ability of a grating monochromator to
collect incident radiation from the entrance
slit is very important as only some of this
radiation will reach the detector. The speed
or f/number is a measure of the ability of the
monochromator to collect incident radiation.
f = F/d
where; F is the focal length of the collimating
mirror or lens and d is its diameter.
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The light gathering power of a grating monochromator
increases as the inverse square of the f/number.
LGP(1) = 1/(f/n)2
The f/number for most monochromators ranges from 1
to 10.
For Example: If monochromator 1 has an f/1 and
monochromator 2 has an f/2, the light gatehring
power of the two monochromators can be
compared as follows:
LGP(1) / LGP (2) = 22/12 = 4
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This means that the light gathering power of
the monochromator 1 is 4 times
monochromator 2.
If monochromator 1 has an f/2 and
monochromator 2 has an f/8, the light
gatehring power of the two monochromators
can be compared as follows:
LGP(1) / LGP (2) = 82/22 = 16
This means that the light gathering power of
the monochromator 1 is 16 times
monochromator 2.
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Monochromator Slits
Slits of a monochromator are very important for its
performance. It may be primitive to say that multiple
wavelengths hitting the focal plane can emerge from
the exit slit if the exit slit is too wide. On the other
hand, a beam of very low power can emerge from the
exit slit when the slit is too narrow. The first case
leads to bad wavelength selection (bad resolution)
as a mixture of wavelengths is obtained, while the
other case may make it impossible for the detector
to sense the low power beam (bad detectability).
Therefore, the width of the slits should be carefully
adjusted, where some instruments allow such
adjustments.
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However, many instruments have fixed slit
monochromators optimized for general
purpose applications. A slit is machined from
two pieces of metal to give sharp edges that
are exactly aligned (same plane) and parallel.
The entrance slit of a monochromator can be
looked at as a radiation source with an image
that will exactly fill the exit slit at a particular
grating setting. Images from other
wavelengths will align at the focal plane of
the monochromator. An image of interest can
be brought to focus by appropriate rotation
of the grating.
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