Method for spectroscopy using two Fabry-Perot interference filte

Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer

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356346, G01B 902

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active

053573404

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BRIEF SUMMARY
The invention relates to a method for Fabry-Perot spectroscopy under utilization of a spectrometer in accordance with the preamble of the claim.
Spectroscopic methods for measuring gases and liquids are very important in the fields of process and ecological technology. Accordingly, numerous measuring methods have been developed with which, from the point of view of central importance as far as the present case is concerned, those methods are considered which are suitable for liquid spectroscopy in the near infrared range, that is, about 1 .mu.m to 3 .mu.m wavelength.
In the following, several known methods for liquid spectroscopy will be considered as well as spectrometers operating in accordance with these methods.
The conventional method for liquid spectroscopy in the near infrared range uses a splitting up of the radiation by means of a grating monochromatic device. The detection of the radiation spectrum is usually carried out here by rotating the dispersion grating which requires a relatively expensive mechanical precision drive. This is particularly disturbing in those cases when measuring is carried out in a rather rough industrial environment. It is usually desirable to avoid mechanically moving parts entirely for such applications of the method and to use, rather, robust devices. Process spectrometers which are available on the market and pertaining to this type have to be classified in a cost or price category as having "high performance". For a simple, robust and economic spectrometer, which can still be operated in a rather rough industrial environment, one cannot use the aforementioned method.
Another method for liquid spectroscopy in the near infrared range is based upon splitting the radiation by means of interference filters. A device is known for the multi-component spectroscopy published in the journal "Automatisierungstechnische Praxis atp" Vol. 32, 1990 pp. 338-342. Herein at the most 14 different interference filters can be pivoted into the radiation path, which interference filters are arranged on the diaphragm-like wheels.
Here is it disadvantageous that indeed still mechanically moving parts are used for receiving the spectrum. Another disadvantage is that the entire spectral range is not available in a gapless fashion. The particular process spectrometer available on the market and pertaining to this type is therefore also to be classified in the "high performance" price category. Again, for a simple robust and economic spectrometer that is to be operated in a rough industrial environment, this described method cannot be used.
Another arrangement for receiving spectra is described in the European Patent Application EP OS 0 180 449. The dispersion of light is carried out here by means of an opto-acoustic crystal. The dispersed light is subsequently imaged upon a photo-detector array. Indeed, the arrangement does not use movable parts, it has, however, the disadvantage that the manufacture of the optic-acoustic crystal is very expensive from a technological as well as monetary point of view. In addition, one should consider that upon applying this device to the infrared spectrum, the detector array by itself is a very expensive item. Again, a simple and economic process spectrometer along this line is out of the question.
The multi-component process analyzer system disclosed in German Patent Application 35 25 490 also operates with a detector array. The dispersion of light is carried out in this case by means of a rather simple dispersing optic, however, the above-mentioned drawbacks concerning a detector array are also applicable here.
Another possibility of liquid analysis in the near infrared range is the so-called Fourier-transform infrared method.
German printed patent application 35 42 161 describes a two-beam interferometer of the Michelson type wherein, by means of a movable mirror, the interferogram of an unknown radiation to be analyzed is produced and converted in a detector into an electrical signal. Through subsequent Fourier transformation, one obtains the radiation spe

REFERENCES:
patent: 4999013 (1991-03-01), Zoechbauer

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