Teledyne 100E - Figure 10-4: Excitation Lamp UV Spectrum Before;After Filtration

Theory Of Operation Model 100E Instruction Manual
168 045150102 Rev XB1
that are not needed to produce SO
2
*. The second stage protects the PMT detector from
reacting to light not produced by the SO
2
* returning to its ground state.
UV Source Optical Filter
Zinc-vapor lamps output light at other wavelengths beside the 214nm required for the
SO
2
# SO
2
* transformation including a relatively bright light of the same wavelength at
which SO
2
* fluoresces as it returns to its SO
2
ground state (330 nm). In fact, the
intensity of the of light emitted by the UV lamp at 330nm is so bright, nearly five orders
of magnitude brighter than that resulting from the SO
2
* decay, it would drown out the
SO
2
* fluorescence.
LAMP OUTPUT
(Arbitrary Untis)
10
5
10
4
10
2
202.5
307.6
481.1
10
3
10
1
1
100 200 300 400 500
WAVELENGTH (nm)
0
213.9
330.3
275.6
SO
2
*
Fluorescent
Spectrum
LAMP OUTPUT
(Arbitrary Untis)
10
5
10
4
10
2
10
3
10
1
1
100 200 300 400 500
WAVELENGTH (nm)
0
213.9
330.3
SO
2
* FLUORESCENT
SPECTRUM
UV SOURCE OPTICAL FILTER
BANDWIDTH
BEFORE AFTER
Figure 10-4: Excitation Lamp UV Spectrum Before/After Filtration
To solve this problem, the light emitted by the excitation UV lamp passes through a
bandpass filter that screens out photons with wavelengths outside the spectrum required
to excite SO
2
into SO
2
*. (Figure 10-4).
PMT Optical Filter
The PMT used in the Model 100E reacts to a wide spectrum of light which includes much
of the visible spectrum and most of the UV spectrum. Even though the 214 nm light used
to excite the SO
2
is focused away from the PMT, some of it scatters in the direction of the
PMT as it interacts with the sample gas. A second optical bandpass filter placed between
the sample chamber (see Figure 10-2) and the PMT strips away light outside of the
fluorescence spectrum of decaying SO
2
* (see Figure 10-5) including reflected UV form
the source lamp and other stray light.