Bruker's IFS 125HR spectrometer is designed as an ultra-high resolution FT-IR spectrometer which provides outstanding performance
for R&D laboratory application. But many of its installations are used for highly accurate atmospheric trace gas analysis,
atmospheric emission studies, and remote monitoring of atmospheric pollutants. Measurements of human made atmospheric pollution
are made either in transmittance by using the sun as active IR radiation source or in a passive emission mode.
Permanently Aligned IFS 125HR Interferometer
The IFS 125HR features a precise cube corner Michelson interferometer. The moving cube corner mirror is translated on a hybrid
bearing as the design ensures beam integrity over the course of the scan; no active alignment is needed. The permanently aligned
scanner provides several important advantages: the sensitivity is maximized, the indispensable long-term stability of the
interferometer is inherently maintained, and the spectral range extension is simplified. Beamsplitters can easily be exchanged
by the user.
For atmospheric transmittance measurements, the solar radiation is collected by a solar tracker system, which automatically
follows the position of the solar source. To ensure a very high pointing accuracy, the image of the sun on the spectrometer's
input aperture is observed by a camera. An image processing software (called CamTracker ) calculates the relative position
of the centers of the sun versus the input aperture and gives feedback to the tracker's motors (Figure 1).
Figure 1: A547N Solar tracker accessory for the IFS 125HR. Inset: Image of the sun on the input aperture of the spectrometer.
The two blue circles show the calculated positions of the two objects.
Dedicated Installation Examples for Atmosphere Application
The IFS 125HR dual channel DigiTect™ technology together with dichroitic optical components and the modular design of the
instruments optics bench allow very special instrument configurations to apply the IFS 125HR spectrometer for a broad spectral
range in parallel measurements. The following two installations are mentioned.
a) Meteorological Bureau Shenzhen
At the Meteorological Bureau China a unique setup of three IFS 125HR high resolution spectrometers have been installed recently
placed in a new astronomical observatory which is located in Shenzhen, a small city near Hong Kong. The observatory was built
and sponsored by the local government.
The aim is to monitor atmospheric trace gases in three different configurations: a) facing the sun (using a solar tracker)
to get the absorption features from the upper atmosphere; b) aiming to either the sea -clean background- or c) to the city
-polluted- to get an emission signal from the sky using the same solar tracker optics. The special feature in Shenzhen is
that three compact IFS 125HR instruments— without the standard sample compartment — can be used simultaneously and continuously
to acquire spectral data in the complete MIR, NIR, and VIS-UV spectral ranges, 600 cm-1 through 33,000 cm-1. Any other customer would be forced to change optical components and has to take the measurements of different ranges one
after the other.
Figure 2: Arrangement of three compact IFS 125HR spectrometers installed in the new astronomical observatory at the Shenzen
Meteorological Bureau in China.
As shown in Figure 2, the solar beam is captured by a large Chinese made solar tracker mounted under the dome of the observatory.
The solar radiation is then guided down to a 3 × 120° reflective beam splitter that evenly distributes the radiation to the
input ports of the spectrometers (Figure 3).
Figure 3: Close view onto the 3 × 120° reflective beam splitter that evenly distributes the IR radiation to the input ports
of the IFS 125HR spectrometers.