Page 237 - Photonics Essentials an introduction with experiments

Measurements in Photonics

Measurements in Photonics 231

The monochromator consists of focussing mirrors and a wave-
length-dispersive element. This is usually a grating scribed into a
piece of soft glass. The grating spatially separates light of different
wavelengths, similar to the way that Newton’s prism works (Fig.
10.5).
The mirrors are curved so that they act as convex lenses. The en-
trance slit is treated as a point source and the light is focused into a
parallel beam and directed to the grating. In a good instrument, the
grating is uniformly illuminated by the light. When the light leaves
the grating, it is still parallel and the process is reversed so that light
is now focused on the exit slit.
You can see that the size of the grating and the length that the light
travels to reach the grating define an angle of acceptance for light
that enters the monochromator. If the angle of the entering light
beam lies within this angle, it will strike the grating. If it lies outside,
then it can still enter the monochromator, but some of the light will
not hit the grating and will be scattered around inside the monochro-
mator, generating background noise. The length of this path divided
by the width of the grating defines the f-number of the monochroma-
tor. In a well-designed optical system, all the light from the device is
focused into the instrument and the grating is fully illuminated, lead-
ing to maximum usage of available light. This happens when the f-
number of the lens you choose to focus light into the monochromator
matches the f-number of the monochromator.

10.4 Gratings
Gratings are made by scribing a series of closely spaced lines on a
sheet of glass. To make it easy to do the scribing, the glass is soft but,
as a result, very easily scratched. When the grating is doing its job in
a spectrometer, it is completely illuminated or filled. That means that
each part of the grating is contributing an equal part to the total sig-
nal. So it is easy to understand that if there is a piece of dust, a spot,
or even a small scratch in one part of the grating, the basic perform-
ance will not be affected.
For example, if you are looking at the grating and an eyelash falls out
and lands on the grating, leave it alone. Above all do not ever try any of
the following: touch the grating, wipe the grating with tissue, rinse the
grating with water or alcohol, blow on the grating, or rub the grating
with your finger or any other instrument. All of these actions affect the
entire grating and thus may ruin it forever. You can easily scratch a
grating by rubbing it with your finger or a piece of lens tissue, and you
will scratch the grating over a sizable fraction of it surface.
A grating is characterized by two numbers: one is the number of

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