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28 CHAPTER 2. ATMOSPHERE AND REMOTE SENSING
2.1.11 Applicability of light-emitting diodes as light sources for active
DOAS measurements
Holger Sihler (Christoph Kern, Ulrich Platt)
Abstract The spectral stability of light-emitting diodes (LEDs) was studied in view of their applicability
in Long Path Differential Optical A bsorption Spectroscopy (LP-DOAS). Beside a constant
temperature also a highly accurate current source to drive the LEDs was found to be essential.
intensity of LED [a.u.]
10 18
10 19
10 20
10 21
400 420 440 460 480 500 520
10 22
540
wavelength [nm]
Figure 2.12: Spectral emission of a Luxeon LXHL-LR3C high power 3 W royal blue LED (at 700 mA,
10 ◦ C, blue line) in comparison with the absorption cross sections of two trace gases – NO2 (Voigt
2002, red line) and Glyoxal (Volkamer 2005, black line) – which have significant absoption structures
in this spectral range.
Background To date, high pressure 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 in the required
wavelength region and short lifetime resulting in
high maintenance costs. Modern LEDs potentially
represent a very advantageous alternative
for both LP-DOAS [Kern et al. , 2006] and cavity
enhanced absorption spectroscopy (CEAS) [Ball
et al. , 2004; Langridge et al. , 2006]. Additionally
LEDs are much easier to maintain considering the
risk of explosions and interfering electromagnetic
radiation.
Methods and results As one may notice in
the comparison between the emission of an LED
and the trace gas absorption cross sections (Figure
2.12), the LED spectrum contains some narrowband
structures considered to be etalon structures.
Only a slight variation of these structures
during the measurement process increases
the residual of a DOAS evaluation by one order of
magnitude or even more in comparison to Xenon
arc lamps. Stabilising their emission spectrum is
therefore the key to make LEDs competitive as
DOAS light-sources.
The spectral position of the etalon structure
depends on the chip temperature. A non-zero
heat resistance between chip and heat sink yields
optical density of trace gas [cm2/molecule]
a dependency on both the heat sink tempearture
and the dissipation of electrical energy inside the
chip. In the special case of the LXHL-LR3C an
already attained temperature stabilisation within
0,1 K corresponds to 0,1 % of the current. Hence,
a stabilisation of the LED drive current was found
to be as important as controlling the temperature.
A compact sealed LED housing with a peltiercooled
heatsink and molecular sieve drying agent
was designed to decouple the LED from the ambient
temperature. Together with a standard PIDcontroller
and a current source, the spectral noise
of a LED could be reduced to a value lower than
that of halogen lamps. The stability of an arc
lamp has not been attained yet.
Funding IUP
Outlook/Future work Future experiments
will concentrate on UV-LEDs to detect further
trace gases (e.g., SO2, ClO, or CH2O). Also superluminescent
LEDs (SLEDs) will be studied in
respect to their applicability. The latter provide
very high radiances but are only in the wavelength
region above 650 nm available. Also the application
of an even more precise temperature controller
(TEC) is very promising. A newly customdesigned
current source-TEC combination has to
be tested and compared to industry standard components.