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28 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
2.1.10 Applicability of light-emitting diodes as light sources
for active DOAS measurements
Participating scientists Christoph Kern, Sebastian Trick, Ulrich Platt
Abstract Light-emitting diodes (LEDs) were tested in respect to their applicability in long path
DOAS (LP-DOAS) measurements. Measurements of NO2 and NO3 were conducted in Heidelberg
using the novel method. Temperature stabilization of the LEDs was found to be essential.
Figure 2.10: (a) Estimated spectral radiance distribution of the Osram 450 W/2 XBO xenon arc lamp,
a Luxeon LXHL-LR3C high power 3W royal blue LED, and a conventional tungsten halogen lamp
(20W). Note that the halogen lamp spectrum is multiplied by 10 in this figure. (b) Spectral radiance
normalized to the electrical input power of the individual sources.
Background To date, xenon arc lamps have established
themselves as the most common light
sources for active DOAS instruments. However,
these have several disadvantages including poor
power efficiency and low lifetime resulting in high
maintenance costs. Modern LEDs potentially represent
a very advantageous alternative. [Ball et al.
, 2004] have already performed first cavity enhanced
absorption spectroscopy (CEAS) measurements
with LED light sources, and we conducted
the first LP-DOAS measurements here.
Funding Diplomarbeit therefore “not applicable”
Methods and results The radiative properties
of a variety of LEDs were characterized,
and parameters such as spectral shape, spectral
range, spectral stability, and how these could
be influenced by environmental factors were analyzed.
A study on the radiative properties of modern
high-power LEDs revealed that despite their
much lower power consumption, they posses spectral
radiances comparable to those of xenon arc
lamps at their peak wavelengths (see Figure 2.10).
The spectra of several LEDs were found to contain
Fabry-Perot etalon-induced spectral structures
that interfered with the DOAS evaluation,
in particular when a constant temperature was
not maintained. It could be shown that LEDs
can successfully be used as light sources in active
DOAS experiments measuring NO2 and NO3
around 450 and 630 nm, respectively. Average detection
limits of 0.3 ppb and 16 ppt, respectively,
were obtained using a 6 km light path in the open
atmosphere.
Outlook/Future work In future LED-DOAS
experiments, emphasis must be put on achieving
higher temperature stability. Also, LEDs are becoming
brighter and more cost-effective, and there
has been considerable recent interest in the development
of UV-LEDs. Devices with emission wavelengths
as low as 250 nm were already demonstrated
in the laboratory. Soon, these will be
available as DOAS light sources, thus enabling the
measurement of many further trace gases such as
BrO, SO2, HCHO, and aromatic hydrocarbons.
Main publications [Kern, 2004],
[Kern et al. , 2005]