Detection means for the selective detection of gases, based on o

Details Detection means for the selective detection of gases, based on o Detection means for the selective detection of gases, based on o

1982-10-25

1985-12-10

Evans, F. L.

Optics: measuring and testing

By shade or color

With color transmitting filter

356 51, 250343, G01N 2117

Patent

active

045576033

DESCRIPTION:

BRIEF SUMMARY
The invention deals with the field of optical spectroscopy, as well as geometrical optics. It relates to a relatively uncomplicated detection means for the selective detection of gases. The detection means essentially comprises a thermally or mechanically modulated incandescent body arranged in an almost closed ellipsoid reflector and whose light is supplied to a detection cell. The ellipsoid reflector either directly contains the gas to be measured or it is evacuated or filled with an inert gas which is non-absorptive in the measuring gas absorption range. ln the former case, the light emanating from the ellipsoid is supplied to a wide-band detector combined with a narrow-band interference filter or a narrow-band detector. In the latter case of the non-light-absorbing ellipsoid filling, after passing through a narrow-band interference filter, the light is irradiated into a measuring gas-containing photoacoustic cell. In this case, measurement takes place with the photoacoustic cell completely or almost completely closed. Acoustic decoupling during measurement and the exchange of the gas in the photoacoustic cell can be ensured by capillaries in the cell wall, liquid barriers, or gravitation-controlled ball valves together with a gas supply device, which is in turn based on a small loudspeaker, or by a cut-off delivery device comprising a combination of diaphragm pumps and disk valves.
The microphone signal is detected and processed by means of a lock-in amplifier operating in a digital off-line manner and which is controlled by a microprocessor. A single processor unit can control in parallel or in series the gas scavenging and signal processing of a plurality of detection units.
The production of an almost parallel light beam from light irradiated by an almost punctiform light source is known from the general teachings of geometrical optics. In general, parabolic or spherical mirrors and/or spherical or aspherical lens systems, i.e. optical condensers are used for this purpose. If interest is attached to a high radiation efficiency of the light source and if the light beam is also to have a small divergence, it is indispensible to have large geometrical dimensions of the optical system producing the light bundle. In the construction of optical spectrometers, this is disadvantageous and of decisive importance. Thus, a good resolution requires on the one hand in the case of the conventionally used dispersion or interference monochromator systems a small divergence of the bundled light beams at the monochromator (this requirement also applies when using a narrow-band interference filter) and on the other hand every effort is generally made to ensure that the maximum proportion of the radiation from the light source can be used.
In the case of infrared spectrometers, account must be taken of the atmospheric absorption in the beam path, particularly due to water vapour, this more particularly being achieved by scavenging with dry air or evacuating the complete unit. Thus, for these reasons alone, in certain applications there is a desire for an optical system which, with small geometrical dimensions, supplies an intense light beam with small divergence.
The requirements regarding divergence are not too high when using interference filters. To avoid an undesired change to the band pass characteristics of the interference filter, half the angular opening of the light bundle should be below 15 angular degrees. Even when the unit is not closed, it is desirable for the aforementioned reasons for the light beam to be guided over the minimum distance through the surrounding atmosphere. These requirements are only inadequately fulfilled by conventional light/condenser systems.
It is certainly necessary for the light source to be intensity-modulated and this is generally carried out mechanically by means of a rotary sector disk. Since, however, inexpensive detectors (e.g. pyroelectric elements or photoacoustic cells) are often operated in optimum manner at low frequency, the light source can often be directly thermall

REFERENCES:
patent: 3562524 (1971-02-01), Moore et al.
patent: 3946239 (1976-03-01), Salzman et al.
patent: 4188543 (1980-02-01), Brunsting et al.

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