Hazardous air pollutants monitor

• US 5,451,787 A
• Filed: 10/11/1994
• Issued: 09/19/1995
• Est. Priority Date: 05/04/1993
• Status: Expired due to Term

• Current Assignee: Westinghouse Electric Company LLC (Brookfield Asset Management Incorporated)
• Original Assignee: Westinghouse Electric Company LLC (Brookfield Asset Management Incorporated)
• Inventors: Taylor, Lyle H.
• Primary Examiner(s): Fields, Carolyn E.
• Assistant Examiner(s): Glick, Edward J.
• Application Number:
  US08/321,228
• Time in Patent Office: 343 Days
• Field of Search: 250/338.5, 250/339.13, 250/343
• US Class Current: 250/338.5
• CPC Class Codes: G01N 2021/395 using a topographic target
• CPC Class Codes: G01N 2021/398 CO2 laser
• CPC Class Codes: G01N 21/314 with comparison of measurements at specific and non-specific wavelengths dual wavelength spectrometry G01J3/427
• CPC Class Codes: G01N 21/3504 for analysing gases, e.g. multi-gas analysis
• CPC Class Codes: G01N 21/39 using tunable lasers

Abstract

Gases such as pollutants are discerned in a sample, preferably using a laser to emit an infrared light beam along a sight path for illuminating the gases. The beam is directed along the sight path and collects light from the gases. An optical tunable filter selects a particular optical wavelength or band, and the filtered wavelength is focused on a detector coupled to an analyzer. The analyzer can include a processor that can pulse the laser or coordinate collection of data from sample gases and from a reference cell containing known gases, e.g., using an optical chopper wheel. The processor analyzes the light levels as a function of wavelength to discriminate for the presence of selected gases by determining a characteristic pattern of light absorption and light emission by the gases. The tunable filter has an acousto-optical crystal of Tl₃ AsSe₃, in which an RF acoustic wave is generated for varying diffraction of light by the filter, thereby selecting a wavelength. A nonlinear output crystal can in inserted controllably to selectively generate harmonics for increasing wavelength coverage, and can also be Tl₃ AsSe₃. A broad band infrared illumination source can be used. When the infrared excitation beam is off, the filter acts on infrared emissions of the gases. Sensitivity to narrow emission lines is increased by modulating the RF drive to the tunable filter, producing derivatives of the spectra.

Claims

1. An apparatus operable to detect the presence of selected gases in a sample, comprising:

an infrared source operable to emit an infrared beam along a sight path intersecting the sample, the infrared beam exciting gases in the sample, including at particular excitation wavelengths, thereby causing the gases to emit light at certain emission wavelengths, and to absorb light at certain absorption wavelengths, the emission wavelengths and the absorption wavelengths being characteristic of particular component gases in the sample;

viewing means directed along the sight path for collecting light from the gases in the sample responsive to the infrared beam;

an acousto-optical tunable filter coupled to the viewing means, the tunable filter being operable controllably to diffract light collected by the viewing means for selecting particular optical wavelengths and to direct said particular optical wavelengths onto a detector means; and

,

an analyzer coupled to the detector means and to the tunable filter, the analyzer being operable to control the tunable filter for selecting a plurality of said particular optical wavelengths and to discriminate for presence of said particular component gases by determining a characteristic pattern of the emission and absorption wavelengths detected.

2. The apparatus according to claim 1, wherein the infrared source comprises an infrared laser.

3. The apparatus according to claim 2, wherein the analyzer comprises a processor, and further comprising means coupled to the processor operable to tune the output from the laser to a selected excitation wavelength.

4. The apparatus according to claim 1, wherein the infrared source comprises a broad band infrared source and a collimator.

5. The apparatus according to claim 2, further comprising an output crystal between the laser and the sample, the output crystal producing a harmonic of a wavelength of the laser beam such that the gases are excited at a plurality of wavelengths.

6. The apparatus according to claim 5, further comprising means coupled to the analyzer, for switching the output crystal controllably into and out of the sight path.

7. The apparatus according to claim 6, wherein the analyzer includes a processor operable to control the laser for providing one of a pulsed illumination and a continuous wave illumination along the sight path in an infrared band, the analyzer discriminating for both the absorption wavelengths and the emission wavelengths in the infrared band.

8. The apparatus according to claim 7, further comprising a controllable oscillator controlled by the processor and coupled to the tunable filter for selecting said particular optical wavelengths directed onto the detector means, and wherein the processor is operable to discriminate for a plurality of emission and absorption spectral patterns characteristic of different gases.

9. The apparatus according to claim 8, wherein the acousto-optic tunable filter comprises Tl₃ AsSe₃.

10. The apparatus according to claim 6, wherein the means for switching the output crystal includes a gimballed mirror.

11. The apparatus according to claim 1, wherein said detector means comprises two detectors and further comprising means for generating simultaneously two orthogonal acoustic waves in the acousto-optical filter at different frequencies, said two detectors receiving light from diffraction at the different frequencies, respectively, such that said two detectors measure two wavelengths at once.

12. The apparatus according to claim 1, further comprising a reference cell containing known gases and means for also directing the infrared beam through the reference cell, the analyzer being operable to compare a response of the sample to a response of the reference cell for at least one of baseline correction and assessment of gas concentrations in the sample.

13. An apparatus operable to detect the presence of selected gases in a sample, comprising:

an infrared source operable to emit an infrared beam along a sight path intersecting the sample, and means for switching the infrared beam between an on-state and an off-state, the infrared beam exciting gases in the sample at particular excitation wavelengths when in the on-state, thereby causing the gases to emit light at certain emission wavelengths, and to absorb light at certain absorption wavelengths, the emission wavelengths and the absorption wavelengths being characteristic of particular component gases in the sample;

viewing means directed along the sight path for collecting light from the gases in the sample responsive to the infrared beam;

an acousto-optical tunable filter coupled to the viewing means, the tunable filter being operable controllably to diffract light collected by the viewing means for selecting particular optical wavelengths and to direct said particular optical wavelengths onto a detector;

an analyzer coupled to the detector and to the tunable filter, the analyzer being operable to control the tunable filter for selecting a plurality of said particular optical wavelengths and to discriminate for presence of said particular component gases by determining a characteristic pattern of the emission and absorption wavelengths detected; and

,

wherein the analyzer is operable to collect emission wavelengths during the off-state of the infrared source.

14. The apparatus according to claim 13, wherein the infrared source comprises an infrared laser.

15. The apparatus according to claim 14, wherein the analyzer comprises a processor and the infrared laser is switchable under control of the processor.

16. A method for measuring atmospheric gases in a sample, comprising the steps of:

directing an infrared beam through the sample, so as to excite the sample and cause the sample to emit and to absorb light at particular wavelengths that are characteristic of component molecules in the sample;

receiving light from the sample and applying the light to an acousto-optical tunable filter having means for applying an acoustic wave at a predetermined frequency for diffracting the light into a spectrum, and applying the light to a detector means;

varying the predetermined frequency to select among a plurality of wavelengths of absorption and emission at the particular wavelengths that are characteristic of the component molecules;

measuring a level of the light at said particular wavelengths;

analyzing a pattern of the light received from the sample, at the particular wavelengths, for detecting the component molecules in the sample.

17. The method according to claim 16, wherein the infrared beam is obtained from an infrared laser, and further comprising the step of generating harmonics in the laser beam so as to excite the sample at a plurality of wavelengths.

18. The method according to claim 16, wherein the infrared beam is obtained from an infrared laser, and further comprising repetitively selecting for output from the laser a sequence of particular illumination wavelengths of a plurality of wavelengths, and further comprising analyzing successively for particular wavelengths characteristically absorbed by at least one pollutant gas.

19. A method for measuring atmospheric gases in a sample, comprising the steps of:

providing an exciting infrared light beam and means to emit the light beam at selected wavelengths along a beam path intersecting the sample such that the selected wavelengths of the infrared light beam excite component molecules in the sample;

receiving infrared radiation from the sample, the infrared radiation having absorption and emission spectra characteristic of the component molecules;

applying the infrared radiation to an acousto-optical tunable filter and an associated detector means, and tuning the filter by applying to the filter an acoustic wave at an predetermined frequency to diffract the infrared radiation such that a selected wavelength is applied to the detector;

measuring a level of said infrared radiation at the selected wavelength;

tuning the filter by successively applying additional frequencies, measuring the level at the additional selected wavelengths, and proceeding through a set of frequencies sufficient to define the absorption and emission spectra of the sample; and

,

determining the component molecules of the sample from the absorption and emission spectra.

20. The method according to claim 19, wherein the selected wavelengths are obtained by at least one of tuning a tunable infrared light source, doubling an infrared laser light source and modulating a tunable output crystal inserted along the light beam.

21. The method according to claim 19, further comprising varying an output wavelength of the exciting infrared light beam so as to select an illumination wavelength, tuning the filter and defining the absorption and emission spectra under illumination at a plurality of illumination wavelengths.

22. The method according to claim 21, wherein said detector means comprises two detectors and the method further comprises applying to the filter a second, orthogonal acoustic wave;

simultaneously diffracting the infrared radiation along separate paths to the two detectors; and

measuring the level at two different selected wavelengths simultaneously.

23. The method according to claim 21, further comprising inserting an output crystal into the beam path and directing the exciting infrared light beam through the output crystal for obtaining a plurality of simultaneous wavelengths of illumination.