Multipass Cell Design in the new LGD Compact
Axetris, Laser Gas Detection, Environmental Compliance, Process Control, Research, Medical
The optical path length plays an important role in infrared spectroscopy like we are using in the LGD Compact: According to Beer Lambert's law
(I = I0 e-αL)1 the optical path length in an absorbent substance largely determines the attenuation of the radiation intensity. Thus, the longer the optical path, the better the detection limit for gas detection with the IR Spectrometer.
As part of the product development of the LGD Compact, various designs of multipass cells were tested by Axetris, including the White cell, the Herriott cell and the CMR cell (Circular Multi Reflection). Decisive factors for the choice of a multipath cell were an increased performance capability compared to the LGD F200P2 A, easy production, a robust design against vibrations and temperature fluctuations and a compact design with a low T90 time.
Impressive path lengths of up to several meters can be achieved with the White cell and the CMR cell. Especially the latter scores here with a very good pathlength to cell volume ratio, which, however, results in a significant cost increase. Also, both cell types often need a very precise alignment which makes the cells susceptible to external influences.
To get a robust product design at an affordable price, a Herriott cell was implemented into the new LGD Compact. When comparing the LGD F200P2 platform to the LGD Compact, the cell volume remains approximately the same while the path length doubles. Thanks to several beam reflections, the footprint of the gas cell could be reduced to just 5 cm in length. In combination with the unique optical laser package, a significant increase in performance was achieved with the LGD Compact. With lower gas detection limits and an improved signal-to-noise ratio these advantages can be felt directly in the application.
1 I = Light intensity; I0 = Initial light intensity; α = absorption factor; L = optical path length
Optical Design for a Herriott cell simulated in ZEMAX (top) and spot pattern on the gas cell window in a Herriott cell (bottom)