Filter
Top Products
Model T101 Instruction Manual Principles Of Operation
249
10.2.4. OPTICAL FILTERS
The Model T101 analyzer uses two stages of optical filters to enhance performance. The
first stage conditions the UV light used to excite the SO
2
by removing frequencies of
light 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.
10.2.4.1. 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 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
214.3
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
214.3
330.3
SO
2
* FLUORESCENT
SPECTRUM
UV SOURCE OPTICAL FILTE
R
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).
10.2.4.2. PMT Optical Filter
The PMT used in the Model T101 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.
07266B DCN6485