Periodically alternating path and alternating wavelength bridges for quantitative and ultrasensitive measurement of vapor concentration
First Claim
1. An apparatus for measuring optical absorption by a constituent gas in a measurement cell, said apparatus comprising:
- radiation means for emitting broad band infrared radiation along one of a measurement lightpath and a reference lightpath, the measurement cell being positioned in said measurement lightpath so that said radiation passes therethrough;
alternating means for alternately and periodically directing said radiation along one of said lightpaths at a first frequency;
attenuating means for balancing the relative power of said radiation in each of said lightpaths in the absence of optical absorption of the constituent in the measurement cell;
sensor means for sensing radiation from each of said lightpaths and for producing a sensed signal, said sensed signal exhibiting modulation thereupon at said first frequency only when there is optical absorption of the constituent in the measurement cell; and
synchronous detection means for demodulating said first frequency from said sensed signal so as to extract a DC signal, said DC signal being proportional to the optical absorption of the constituent in the measurement cell.
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Accused Products
Abstract
An apparatus for ultrasensitive detection of low concentrations of constituents using a broad band LED source is described. The apparatus provides enhanced resolution by synchronously detecting an imbalance between two analog signals due to absorption in a measurement cell at a wavelength of interest. Elements are provided to balance the signals in the absence of absorption to provide a reference point for concentration calculations. In accordance with several embodiments of the present invention, a time alternating signal for synchronous detection is achieved by combining signals from each of two alternating paths, only one of which travels through the measurement cell and which may have absorption. If there is absorption in the measurement cell, then the signal created by combining the alternating paths will exhibit residual modulation at the path alternation rate. In accordance with another embodiment of the present invention, a time alternating signal is achieved by combining signals from each of two alternating ultranarrow wavelength channels, each of which travels through the measurement cell but only one of which is on an absorption line of a constituent believed to be in the measurement cell, into a single photosensitive device. If there is absorption in the measurement cell, then the signal from the photosensitive device formed by combining the alternating wavelength channels will exhibit residual modulation at the wavelength channel alternation rate.
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Citations
36 Claims
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1. An apparatus for measuring optical absorption by a constituent gas in a measurement cell, said apparatus comprising:
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radiation means for emitting broad band infrared radiation along one of a measurement lightpath and a reference lightpath, the measurement cell being positioned in said measurement lightpath so that said radiation passes therethrough; alternating means for alternately and periodically directing said radiation along one of said lightpaths at a first frequency; attenuating means for balancing the relative power of said radiation in each of said lightpaths in the absence of optical absorption of the constituent in the measurement cell; sensor means for sensing radiation from each of said lightpaths and for producing a sensed signal, said sensed signal exhibiting modulation thereupon at said first frequency only when there is optical absorption of the constituent in the measurement cell; and synchronous detection means for demodulating said first frequency from said sensed signal so as to extract a DC signal, said DC signal being proportional to the optical absorption of the constituent in the measurement cell. - View Dependent Claims (2, 3, 4, 5, 6)
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7. An apparatus for measuring optical absorption by a constituent gas in a measurement cell, said apparatus comprising:
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radiation means for emitting broad band infrared radiation along one of a measurement lightpath and a reference lightpath, the measurement cell being positioned in said measurement lightpath so that said radiation passes therethrough; modulating means for modulating said emitted radiation at a first frequency; alternating means for alternately and periodically directing said radiation along one of said lightpaths at a second frequency; attenuating means for balancing the relative power of said radiation in each of said lightpaths in the absence of optical absorption of the constituent in the measurement cell; sensor means for sensing radiation from each of said lightpaths and for producing a sensed signal; demodulating means for demodulating said first frequency from said sensed signal and extracting a detected signal, said detected signal exhibiting a residual modulation thereupon at said second frequency only when there is optical absorption of the constituent in the measurement cell; and synchronous detection means for demodulating said second frequency from said detected signal so as to extract a DC signal, said DC signal being proportional to the absorption by the constituent in the measurement cell. - View Dependent Claims (8, 9, 10, 11, 12, 13, 14, 15, 16)
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17. An apparatus for measuring optical absorption by a constituent gas in a measurement cell, said apparatus comprising:
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radiation means for emitting broad band infrared radiation along one of a measurement lightpath and a reference lightpath, the measurement cell being positioned in said measurement lightpath so that said radiation passes therethrough; modulating means for modulating said emitted radiation at a first frequency; alternating means for alternately and periodically directing said radiation along one of said lightpaths at a second frequency; attenuating means for balancing the relative power of said radiation in each of said lightpaths in the absence of optical absorption of the constituent in the measurement cell; first and second sensor means for sensing radiation from each of said lightpaths and for producing a pair of sensed signals; filtering means positioned ahead of said first and second sensor means for passing only an ultranarrow range of wavelengths from each of said lightpaths to said first sensor means and reflecting the remainder of said radiation band to said second sensor means; demodulating means for demodulating said first frequency from said pair of sensed signals and extracting a pair of detected signals, said pair of detected signals exhibiting a residual modulation thereupon at said second frequency only when there is optical absorption of the constituent in the measurement cell; and synchronous detection means for demodulating said second frequency from each of said pair of detected signals so as to extract a pair of DC signals, the one of said DC signals from said first sensor means being proportional to the optical absorption of the constituent in the measurement cell and the other of said DC signals from said second sensor means being proportional to any broad band absorption in the measurement cell. - View Dependent Claims (18, 19, 20, 21)
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22. An apparatus for measuring optical absorption by a constituent gas in a measurement cell, said apparatus comprising:
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radiation means for emitting broad band infrared radiation along a measurement lightpath, the measurement cell being positioned in said lightpath so that said radiation passes therethrough; modulating means for modulating said emitted radiation at a predetermined frequency; filtering means positioned behind the measurement cell for optically splitting said radiation into an absorption channel and a reference channel, said absorption channel having only an ultranarrow range of wavelengths from said lightpath, said reference channel having the remainder of said radiation band; a first sensor means for sensing radiation in said absorption channel and for producing a first sensed signal; a second sensor means for sensing radiation in said reference channel and producing a second sensed signal; attenuating means disposed in one of said absorption and reference channels for balancing the relative power of light therein in the absence of optical absorption of the constituent in the measurement cell; demodulating means for demodulating said predetermined frequency from each of said first and second sensed signals and extracting a respective pair of detected signals; analog-to-digital convertor means for converting said pair of detected signals to a respective pair of digital detected signals; and a digital differential amplifier having a pair of differential inputs connected to said pair of digital detected signals and having an output proportional to the optical absorption of the constituent in the measurement cell. - View Dependent Claims (23, 24, 25, 26, 27, 28, 29)
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30. An apparatus for measuring optical absorption by a constituent gas in a measurement cell, said apparatus comprising:
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radiation means for emitting broad band infrared radiation along a measurement lightpath, the measurement cell being positioned in said lightpath so that said radiation passes therethrough; sensor means for sensing radiation from said lightpath and for producing a sensed signal; filtering means positioned ahead of said sensor means for passing only an ultranarrow range of wavelengths, the relative angle of said filtering means with respect to said lightpath influencing which ultranarrow range of wavelengths is passed by said filtering means; vacillating means for alternately and periodically positioning said filtering means at an angle of one of θ and
θ
±
Δ
θ
degrees with respect to said lightpath, said vacillating means vacillating at one predetermined frequency;said filtering means passing wavelengths centered on λ
A when said filtering means is positioned at θ
degrees and passing wavelengths centered on λ
B when said filtering means is positioned at θ
±
Δ
θ
degrees, Δ
θ
being chosen so that only one of λ
A and λ
B is centered on an optical absorption line of the constituent in the measurement cell; andsynchronous detection means for demodulating said one predetermined frequency from said sensed signal so as to extract a DC signal, said DC signal being proportional to the optical absorption of the constituent in the measurement cell. - View Dependent Claims (31, 32, 33, 34, 35, 36)
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Specification