Physica Scripta. Vol. T105, 98–100, 2003
UV, Visible and IR Spectrum of the Cs High Pressure Lamp
G. Pichler1,, V. Živčec1, R. Beuc1, Ž. Mrzljak2, T. Ban1, H. Skenderović1, K. Günther3 and J. Liu3
1
Institute of Physics, P.O. Box 304, HR-10001 Zagreb, Croatia
Factory of Electric Lamps, Zagreb, Croatia
3
OSRAM, Berlin, Germany
2
Received February 21, 2003; accepted February 21, 2003
PACS Ref: 32.70.Fw, 42.72. g, 52.80.Yr
Abstract
High pressure pulsed discharge with cesium vapor could become a very
promising lamp, because of its white light properties. It has considerable losses
in the infrared spectral region and negligible contribution in the near UV
spectral region. We shall present the results of extensive spectral studies of such
a lamp with a special emphasis on the near infrared and near UV portions of
emission spectra for a variety of experimental conditions. We shall discuss the
origin of atomic emission lines and some peculiar molecular bands, which
contribute to the overall emission in these spectral regions.
1. Introduction
The history of light sources development is very rich in
surprises and novel technological solutions. Since the time
electricity became the main driving force of modern light
sources several very interesting phases have been seen.
From incandescent lamps using carbon filament to
tungsten filament is certainly an important early development (Edison, Just and Hanaman, Coolidge). Then the
electric discharge lamps received considerable attention
mainly because of their much larger efficiency to produce
white light. Low and high pressure light sources with
different gas and material fillings can nowadays reach
between 100 and 200 lumens per watt [1,2]. The highest
efficiencies are obtained with low pressure sodium lamps,
but they have a very poor color reproduction capability
(poor color rendering index, CRI). The high pressure
sodium lamps attain better CRI, but their efficiency is
typically about 100 lumens per watt. Metal-halogen lamps
attain very high CRI factors and have acceptable efficiency.
In the modern world approximately 20% of the total
electrical energy is used for lighting purposes [3]. It is not
surprising to observe several new developments toward
more efficient, less toxic, white light sources with excellent
CRI factor approaching the value of 100, which can be
attained under normal sunshine. High pressure sodium
lamps are widely employed light sources for outdoor
purposes. However, they have relatively weak blue part of
the spectrum and strong absorption in a wide region
around the center of the sodium D lines. High sodium
pressure causes extended collision-induced line wings
which spread into green and red spectral regions. Recently,
there was a serious attempt to develop pulsed high-pressure
cesium discharge lamps [4,5], but the development went up
to almost 50 lm/W efficiency. We shall present a few of the
latest spectroscopic results of these high pressure pulsed
cesium discharge lamps in order to point to some possible
e-mail: [email protected]
Physica Scripta T105
directions in the future development of this new alkali light
source.
2. Experiment
In all our emission measurements we used several different
scanning monochromators with appropriate photomultipliers sensitive in ultraviolet, visible and near-infrared
spectral regions. The output from the photomultipliers was
amplified and processed by a boxcar averager and recorded
in a laboratory computer. We found two regions of voltage
where the lamp could operate in a stable mode. The first
was in the pulsed mode of operation at 220 V. A stable
pulsed light signal was achieved for electric currents
around 10 A and pulse repetition of about 286 Hz. The
second region was at about 110 V where the electric current
was around 6 A and the pulse repetition decreased to
170 Hz. As will be shown below, the emission spectra from
the lamp at 286 Hz current pulse repetition (voltage 220 V)
differs strongly from the emission spectra at 170 Hz current
pulse repetition (voltage reduced to 110 V).
3. Results
In Fig. 1 we present an overall spectrum at 286 Hz
repetition, obtained by a boxcar averager with the gate
placed at the maximum of the pulses. The high pressure
discharge in the alumina burner of a cesium lamp exhibits a
strong spectrum in the visible, and after profound
absorption around the cesium resonance lines (800–
960 nm), it also shows considerable emission in the infrared
spectral region. At this high electric current all other
atomic spectral lines appear almost entirely in absorption.
In Fig. 2 a spectrum of the same lamp is shown, but now at
a voltage of only 125 V, and pulse repetition of 170 Hz. All
atomic spectral lines now appear mostly in emission. There
are also some molecular spectral phenomena, which can be
attributed to bound excited Cs2 molecules, and some of
them require additional studies [6,7,8,9].
A Cs partial energy term diagram is shown in Fig. 3. We
denote spectral transitions that have been observed in the
lamp emission spectrum at lower running voltage. The
transitions from higher energy levels are not explicitly
shown, although they can be readily observed in Fig. 4,
where we denote their positions by crosses. This portion of
the spectrum from 300 to 625 nm, has been taken by means
of a 1m McPherson scanning monochromator. The UV
portion of the cesium pulsed high-pressure lamp exhibits a
few principal series lines of cesium, some forbidden lines
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UV, Visible and IR Spectrum of the Cs High Pressure Lamp
Fig. 1. Overall spectrum of the high-pressure pulsed cesium lamps at
higher repetition rate and at voltage of 220 V.
Fig. 2. Overall spectrum of high-pressure pulsed cesium lamps at lower
repetition rate and at a voltage of 125 V.
Fig. 3. Energy term diagram of cesium with a few spectral transitions.
(6s-nd and 6s-ns transitions) and accompanying recombination continuum. In the visible part of the spectrum the
limits of nd3=2 6p1=2 and nd5=2;3=2 6p3=2 are shown by
two vertical lines close to 500 nm.
In Fig. 5 we present an interesting comparison between
two lamps having different burners, both at 110 V. It
appears that the alumina burner (PCA) was much stronger
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99
Fig. 4. Ultraviolet and portion of the visible spectrum of the cesium lamp
at a voltage of 110 V.
Fig. 5. Comparison of the overall spectrum of two cesium lamps at 110 V,
having a PCA burner of 3 mm in diameter, and a sapphire burner with
5 mm diameter.
than the burner made out of pure sapphire crystal. It is
certainly the consequence of a narrower cylinder in the case
of alumina burner. Maya and coworkers [5] showed that
the lamp efficiency increases for decreasing diameter of the
burner. This causes stronger plasma constriction, which
increases the plasma temperature and thus the light
efficiency.
Finally, we show what kind of lamp tailoring we need in
order to increase the cesium lamp efficiency. We definitely
need to cut the near infrared and infrared portions of the
spectrum, whereas the blue-green portion should also be a
bit more increased. The near infrared part could be
efficiently cut away by using a mixture of rubidium and
cesium in the burner. The transmission spectrum of dense
rubidium vapor exhibits strong resonance line absorption
valleys, as shown in Fig. 6. This strong absorption in the
spectral region 740–830 nm might presumably increase the
mixed Rb-Cs lamp efficiency by about 10–20%.
In our future work we shall concentrate on the study of
the infrared portion of the pulsed cesium discharge from 1
to 5 microns, in order to assess the losses and eventually to
find remedies that will increase the lamp efficacy.
Physica Scripta T105
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G. Pichler et al.
Lamp tailoring may include coating of the bulb inner
glass walls which enables infrared reflection and transmission of the visible photons (hot mirror). Furthermore, the
composition of the arc plasma may be even more complex,
adding elements whose strong resonance lines may enrich
certain parts of the visible spectrum.
We believe that better energy saving may be achieved
and with lower light pollution, mainly because of pulsed
mode of lamp operation.
5. Conclusion
Fig. 6. Transmission spectrum of dense rubidium vapor at a temperature
of about 350 C, where the strong and broad absorption of the first
resonance rubidium lines is readily visible.
4. Discussion
It is interesting that we found two stable regimes of the
high pressure pulsed cesium discharge having entirely
different spectra. At a current of 10 A and repetition rate
of 286 Hz, the cesium lamp of 80 W has a temperature of
about 3850 K, and the visible radiation can be fitted to a
black-body radiation curve. However, the lamp efficacy
was only up to 45 lm/W. On the other hand, the color
rendering index (CRI) is very high and can reach values
above 98. The radiance in the ultraviolet is almost
negligible and there are no relevant dangerous substances
that could enter the environment by recycling. Changes
in the input voltage do not influence much the CRI
factor, and therefore it possesses excellent dimming
properties [5]. However, the infrared spectrum might
appear much more sensitive to voltage changes than the
visible part.
We found one satellite band peaking at 945 nm, a second
peaking at 980 nm and a third peaking at 1055 nm. These
observations have been performed for burners made of
PCA and pure sapphire. We calculated the profiles of these
infrared satellite bands using ab initio potential curves with
appropriate transition dipole moments and found interesting correspondence with observed infrared satellite bands.
Several avoided crossings at shorter interatomic distances
could only be observed in emission since they lie above the
repulsive lowest triplet potential energy curve [10].
High pressure pulsed cesium lamp offers interesting
subjects for the study of atomic and molecular spectral
features at large atom densities and elevated temperatures,
especially when the current is about 6 A and the repetition
rate falls to 170 Hz only.
The light intensity depends on the diameter of the
burner, and in the case of PCA burner it was about 3 mm,
whereas for the sapphire burner it was about 5 mm. The
smaller the diameter of the burner the stronger is the
emission because of the tighter constriction of the
discharge plasma. It would be of considerable interest to
further study even smaller constrictions in special capillary
cesium discharges.
Physica Scripta T105
The endeavor to increase the pulsed cesium lamp efficiency,
will certainly contribute to better energy savings, better
color rendering index, and less toxic plasma composition.
However, bright, white and efficient new light sources may
contribute to so-called light pollution. Changing the dark
nights into illuminated surfaces of modern cities leads to
many changes in the natural life of the planet. Pulsed lamp
regime may be used to diminish light pollution by means of
appropriate switching according to the need for illumination of certain streets, bridges etc. only when traffic is
present.
For entirely new light sources we definitely need ideas
that may be extracted from the vast body of atomic and
plasma physics, but also from solid state physics [11]
European Cost 529 project under the title ‘‘Lighting for the
21st century’’ will eventually transform into a Network of
Excellence within the Framework 6 Program. We may
expect that new light sources will emerge in the near future
with unprecedented efficacy in excess of 200 lm/W.
Acknowledgement
This work was supported by the Ministry of Science and Technology of the
Republic of Croatia (Projects 0035002 and 0035004) and by OSRAM GmbH,
Berlin. This work is also connected with COST 529: Light Sources for the 21st
century.
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