Open-path gas detector with synchronous flicker detection

Abstract

An open circuit gas detector for detecting the presence of a target gas (12) in the presence of fog or water vapor. An emitter (30) emits a flash (32) of optical energy along a path (10) in a monitored area, including energy at a sample wavelength region absorbed by the target gas (12), at a reference wavelength region not significantly absorbed by the target gas, and at a sync wavelength region different from the sample and reference wavelengths. A receiver (40) includes a sample channel responsive to optical energy at the sample wavelength region, a reference channel responsive to optical energy at the reference wavelength region, and a third sync channel responsive to optical energy at the sync wavelength region. The receiver (40) detects the target gas (12) and uses the output of the sync channel to synchronize operation of the receiver (40) with the flash (32) of optical energy from the emitter.

Description

Background Technology

[0001] Today, open-circuit gas detectors are used to detect target gases, such as hydrocarbon gases. Such detectors consist of a transmitter that emits infrared energy along a path in the monitored area, and a receiver that receives the emitted energy. If a hydrocarbon gas is present in that path, the energy is absorbed at a wavelength specific to the gas type (e.g., 2.3 μm). The receiver determines the presence of the gas by detecting attenuation at a specific frequency, typically associated with a reference channel. In many detectors, the transmitter flashes at a given rate with a low duty cycle.

[0002] The path length can be quite long, for example, tens or hundreds of meters. Alignment of the transmitter and receiver is a problem. Another issue is uptime or useful detector operation during adverse weather conditions such as fog or sandstorms. At certain times, fog or dust density often forces open-circuit infrared detectors offline, i.e., reduces uptime. This invention addresses the problem of improving detector uptime under adverse environmental conditions. Attached Figure Description

[0003] When read in conjunction with the accompanying drawings, those skilled in the art will readily understand the features and advantages of this disclosure from the following detailed description, in which:

[0004] Figure 1 This is a schematic diagram of a typical open-circuit detector used to measure or detect the presence of a target gas (e.g., methane) in a monitored path.

[0005] Figure 2 This is a graph showing the absorption curve of methane near 2.3 μm and the spectral position of the signal that can be used to measure the gas.

[0006] Figure 3 This is a block diagram of an OPIR emitter that uses a xenon flash.

[0007] Figure 4A and 4B The flash characteristics of a xenon flash lamp are shown. Figure 4A The light intensity of a typical pulse as a function of time is shown, while Figure 4B The spectral content of the flashes was described.

[0008] Figure 5 The range of flash rates that can be used in a typical OPIR transmitter is shown graphically.

[0009] Figure 6 The spectral responses of three signals used in an exemplary embodiment of the invention are depicted, as well as the IR absorption curves of methane gas superimposed on the relative output of a xenon flash lamp.

[0010] Figure 7 The typical atmospheric extinction coefficient for dense fog is shown as a function of wavelength.

[0011] Figure 8 This is a simplified schematic block diagram of an exemplary embodiment of a receiver optical block for an OPIR detector according to various aspects of the present invention.

[0012] Figure 9 It is shown by Figure 8 A schematic diagram of an exemplary implementation of receiver signal processing.

[0013] Figure 10 An exemplary process flowchart illustrating the operation of one exemplary embodiment of the detector is shown.

[0014] Figure 11 An example of a noise measurement signal that does not benefit from any aspect of the present invention is shown graphically.

[0015] Figure 12 This demonstrates how synchronized flash detection according to various aspects of the present invention improves the signal-to-noise ratio level of the measurement signal. Detailed Implementation

[0016] In the following detailed description and several figures, the same elements are denoted by the same reference numerals. These figures are not drawn to scale, and for illustrative purposes, relative feature dimensions may be exaggerated.

[0017] like Figure 1 The typical open-circuit detector shown includes a transmitter module 30 that emits an optical flash 32, and a receiver module that receives the emitted flash. If the target gas 12 is within the monitored path 10 of the detector, the spectrum at the receiver module will change due to the IR absorption of the target gas. IR absorption bands are typically narrow spectral lines, so changes in this region compared to nearby bands indicate the presence and quantity of the gas.

[0018] The spectral region in which the target gas absorbs IR is called the sample spectral region. The nearby spectral region unaffected by the target gas is called the reference spectral region. The ratio of the sample signal to the reference signal determines the presence and quantity of the gas. When there is no target gas in the monitored path, the sample signal level and the reference signal level often change in the same way because the received flash signal varies due to factors such as adverse environmental conditions, contaminated optical components, and flash lamp aging.

[0019] In one exemplary implementation, the reference wavelength is chosen to be as close as possible to the sample wavelength but outside the gas absorption wavelength. In practice, each specific gas may have one (or more) complex or simple absorption lines, so the reference may be slightly affected by that gas, and this will be excluded during calibration. Gas calculations still involve dividing the sample by the reference.

[0020] Figure 2 This study depicts how a typical target gas, methane, absorbs the sample spectral signal near the 2.3 μm spectral region, while the nearby 2.1 μm reference spectral signal does not fall within the IR absorption band of methane. The 2.3 μm spectral region is considered the sample spectrum and is an approximation because each type of hydrocarbon has a very unique absorption curve near 2.3 μm.

[0021] Similarly, the 2.1µm reference signal is an approximation and represents a wavelength close to but outside the absorption spectrum of the target gas. The change in the ratio between the sample signal and the reference signal indicates the presence and quantity of gas in the monitored path. Using the ratio between the sample and the reference is a proven and effective method.

[0022] Figure 3 A typical open-circuit detector emitter module 30 is shown, which utilizes a xenon flash lamp 30D to generate a broadband flash 32. In the context of the emitter flash in this exemplary embodiment, "broad" spectrum refers to wavelengths from ultraviolet to infrared, or wavelengths from ~0.2µm to 4µm. The emitter module includes a power supply circuit 30A, a timing circuit 30B, and a high-voltage generator circuit 30C for driving the flash lamp.

[0023] Figure 4A and 4B The flash characteristics of a xenon flash lamp are shown. Figure 4A The light intensity of a typical pulse as a function of time is shown, while Figure 4B The spectral content of the flashes was described.

[0024] Figure 5 The results show that while the flash duration of a typical open-circuit detector is short, the time between flashes is relatively long. This translates to a very low duty cycle flash period, which becomes very noisy when the signal level becomes severely attenuated during adverse environmental conditions such as dense fog, making synchronization at the detector receiver difficult.

[0025] One method for increasing signal level is through averaging, where N values ​​are added together and the sum is then divided by N to obtain the average value of the signal. If the signal is a waveform, time-synchronized averaging can be performed by synchronizing with a specific point in that waveform (e.g., the fast rising edge of a transmitter flash).

[0026] Synchronous averaging is an effective technique for averaging signals in the time domain, but it requires a synchronous signal that has a precise time relationship with the signal to be averaged. Currently available open-circuit detectors perform synchronous averaging, but are limited by the lack of a precise synchronous signal.

[0027] Synchronous averaging can include "time-synchronous averaging", "complex averaging", "time-domain averaging", and "vector averaging". They all require a synchronization signal, provided by the synchronization signal according to various aspects of the present invention.

[0028] A traditional method for synchronization involves the receiver synchronizing with the transmitter's flash by tracking the time between flashes. This requires a fixed flash rate and low-skew clocks at both ends. Even so, the receiver's time base will never perfectly match the transmitter's time base, so time-lapse is inevitable. When prolonged adverse environmental conditions cause the received flashes to become severely attenuated and almost undetectable, just when the system most needs time synchronization averaging to keep running, the receiver is likely to lose synchronization.

[0029] Another traditional method for synchronization in some open-circuit detectors requires a cable connection between the transmitter and receiver modules, allowing the transmitter to send an electrical signal as it flashes. This has known drawbacks, as the signal propagation time from transmitter to receiver depends on the cable length, cable type, and even factors such as water ingress. This method impacts installation costs because it requires a cable between the transmitter and receiver, expensive high-quality signal cable, accurate measurements of the cable length, and conduits to prevent water ingress. Because the flash propagates through free space faster than the electrical signal traveling along the cable, a small timing error is introduced. This timing error reduces the noise reduction effect of accurate time synchronization averaging.

[0030] To increase the uptime of an open-circuit gas detector, the signal-to-noise ratio (SNR) of the system will be increased. According to various aspects of the invention, a technique for flash synchronization with an OPIR transmitter utilizes the unused portion of the flash spectrum so that precise time-synchronized averaging can be used to reduce the noise levels of the sample and reference signals used to perform gas measurements, thereby increasing the system's SNR and uptime.

[0031] An exemplary embodiment of the invention uses a third signal, referred to as “synchronization,” derived from the unused portion of the spectrum of the emitted flash, to provide a precise time-synchronized signal, particularly when the detector is operating under adverse environmental conditions such as fog or sandstorms.

[0032] Figure 4A and 4B The temporal characteristics and spectral output of a typical xenon flash lamp are shown. It is important to note that the flash has a rapid rise time on the order of approximately 1 microsecond and an exponential decay on the order of approximately 10 microseconds. It is also noteworthy that the spectral output peaks at around 1.5 μm in the near-IR region, while significantly decreasing in the 2.3 μm region used for gas measurements. Other types of light sources can be used if a broad spectrum is desired, depending on how the energy is distributed in the spectrum.

[0033] The rapid rise time and 1.5µm spectral output peak in this exemplary embodiment allow for the implementation of a synchronization signal. A third signal, referred to as "synchronization," centered at a wavelength of 1.5µm, can be used to generate the signal required for synchronization. The "synchronization" optical channel is included in an exemplary receiver embodiment of an open-circuit gas detector.

[0034] Figure 6 The spectral responses of the three signals used in this exemplary embodiment are shown because they overlap with the IR response of methane and the spectral output of a typical xenon flash lamp. As can be seen in the figure, within the dashed line labeled "Synchronization Spectral Response," the synchronization spectral response overlaps with a larger portion of the unused spectrum, giving it an energy level advantage relative to the reference and sample spectral responses, which have the limited bandwidth required for stable gas measurements. In this exemplary embodiment, the synchronization spectral response is provided by a synchronization channel photodiode 40A-2 (… Figure 8 The bandwidth is determined because, in this example, there is no filter in front of the synchronous photodiode. The reference bandwidth and sample bandwidth are in the range of 0.05 μm to 0.150 μm, where the actual or exemplary bandwidth is selected based on the specific gas being measured and the optical design. In one exemplary embodiment, a nominal bandwidth of 0.10 μm can be used as the bandwidth for both the reference and sample spectral responses. Figure 6 The description.

[0035] Although Figure 6 An example with a single reference spectral response is shown, but in other cases, the reference wavelength can actually consist of two wavelengths on either side of the sample wavelength. In this case, there will be two reference spectral responses. In that case, the reference channel filter is designed to allow both wavelengths to enter, thus there is only one reference photodiode. The use of dual reference wavelengths is known in the art, and the reason for using it is that, depending on environmental conditions, one wavelength will tend to increase while the other tends to decrease, thus providing a certain balance.

[0036] Another advantage of using a synchronization signal derived from a 1.5µm spectrum relates to how near-IR spectra respond to fog. Figure 7 A derived plot of the atmospheric extinction coefficient for dense fog is shown, indicating that under certain conditions, longer wavelengths attenuate so much more relative to shorter wavelengths. This tends to occur more frequently in dense fog than in light fog. Dense fog is defined as weather conditions where a transmitter flash decreases to 5% of its original value at 30 meters.

[0037] Figure 7 This means that under dense fog conditions, the light in the 2.3µm region will attenuate more than the light in the 1.5µm region. This is precisely when a synchronization signal is needed, because it is known that open-circuit detectors often go offline in fog, and what actually happens is that the signal in the 2.3µm region is reduced to a level below which the detection of the received flash becomes unreliable, making gas measurements impossible. Since the 1.5µm synchronization signal can be used to process the 2.3µm region synchronously, the SNR is improved to a level where reliable gas measurements can be performed.

[0038] Larger spectral bandwidth of the synchronization signal (in) Figure 6 The sum of the values ​​from approximately 1.0 μm to approximately 1.6 μm in the example shown, plus the higher output of the xenon flash in the 1.5 μm region, and considering that dense fog tends to reduce light intensity in the 2.3 μm region, leads to the conclusion that the sync signal is often the largest of the three signals. Furthermore, lens gain for the sync optical path, as well as the sample and reference optical paths, further supports this conclusion. Lens gain refers to the light-gathering ability of the objective lens. The larger the lens, the more light it can gather. 40A-1 and 40A-3 ( Figure 8 () is an objective lens for capturing light for a photodiode. Although in this exemplary embodiment, the synchronization lens 40A-1 is smaller than Figure 8 The sample and reference lenses 40A-3 shown in the figure, along with other factors such as spectral flash intensity, optical bandwidth, beam splitting, and atmospheric extinction, still indicate that the synchronization signal is greater than the sample and reference signals.

[0039] The receiver module includes a separate synchronization channel to provide synchronized flash detection, which triggers the acquisition of data for the sample and reference signals for subsequent averaging, even when the measurement signal is too noisy to be reliably detected on its own.

[0040] Figure 8 An exemplary embodiment of a receiver optical block or module 40A that implements the optical elements of the receiver is shown.

[0041] The sync lens 40A-1 captures the received flash signal and focuses it onto the sync photodiode 40A-2.

[0042] Synchronous photodiode 40A-2 converts photons into electrons in a 1.5µm region to generate a synchronization signal for detecting emitter flashes. As previously mentioned, there is no filter in front of lens 40A-1. The spectral bandwidth of the synchronization channel is set by the spectral response of the selected photodiode. In one exemplary embodiment, the photodiode is designed to operate in the 1.5µm region.

[0043] The main lens 40A-3 captures the received flash signal and focuses it onto the beam splitter 40A-4.

[0044] The beam splitter 40A-4 splits the received flash signal into a sample path and a reference path.

[0045] The sample optics 40A-5 includes a filter that blocks all spectra except for the sample spectrum used for the sample photodiode. The sample optics 40A-5 comprises several optical components, including the filter and a field lens.

[0046] The sample photodiode 40A-6 converts photons into electrons to generate a sample signal. In this embodiment, the photodiode is designed to operate in the 2.3µm region.

[0047] The reference optics 40A-7 includes a filter that blocks all spectra except for the reference spectrum used for the reference photodiode. Like the sample optics, the reference optics comprises several optical components, including the filter and a field lens.

[0048] The reference photodiode 40A-8 converts photons into electrons to generate a reference signal. In this exemplary embodiment, the same photodiode design as the sample photodiode design can be used.

[0049] Figure 9 This is a schematic diagram of an exemplary embodiment of the receiver module 40 according to various aspects of the present invention. Figure 9 An exemplary optical block 40-A is shown, which has a receiver signal chain and processing that results in a gas measurement using a synchronization signal for synchronous flash detection.

[0050] A synchronization signal is used to detect the occurrence of a transmitter flash, thus synchronizing the receiver measurement with the transmitter flash. The sample signal responds strongly to the presence of hydrocarbons in the monitored path. Ideally, the reference signal should not change with the presence of hydrocarbons in the monitored path and should be located spectrally close to the sample signal. The ratio of the sample signal to the reference signal determines the amount of hydrocarbons present in the monitored path.

[0051] The following is about Figure 9Description of each component.

[0052] The 40A-1a synchronous photodiode detects energy in a 1.5µm region and is used to detect emitter flashes, thereby enabling synchronous averaging of the sample and reference signals.

[0053] The sample photodiode 40A-1b detects the energy in a sample region located where the target gas has a strong IR absorption line.

[0054] The reference photodiode 40A-1c detects the energy in a reference region that is near the IR absorption line of the target gas but is not affected by the target gas.

[0055] Amplifiers 40-2a, 40-2b, and 40-2c (TIA) are transimpedance amplifiers that convert photodiode current into voltage.

[0056] Amplifiers 40-3a, 40-3b, and 40-3c (PGA) are programmable gain amplifiers used to amplify analog signals, thereby providing additional gain for signals within the range of an ADC (analog-to-digital converter).

[0057] ADC 40-4a, 40-4b, and 40-4c are analog-to-digital converters.

[0058] FPGA 40-5 is a field-programmable gate array (FPGA) that provides a flash correlation function and synchronously captures waveforms when a flash is detected.

[0059] In this exemplary embodiment, processor 40-6 performs time averaging of the sample signal and the reference signal and calculates the gas measurement value based on the ratio of the sample signal to the reference signal. In one exemplary embodiment, the processor may be implemented by a microprocessor or a microcomputer with memory. Other processor implementation tools may also be implemented. At least the processor functionality can be implemented remotely from a receiver (e.g., at a central station).

[0060] Figure 10 An exemplary process flowchart illustrating the operation of the OPIR detector described above is shown. At 102, the transmitter (30) generates several optical flashes per second. The receiver processes... Figure 10 It is shown at positions 104-116. At position 104, the transimpedance amplifiers (TIA 40-2a, 40-2b, 40-2c) are shown. Figure 9The signals from each photodiode (40A-1a, 40A-1b, 40A-1c) are amplified. Programmable gain amplifiers (PGA 40-3a, 40-3b, 40-3c) amplify the photodiode signals to bring them within the range of ADC40-4a, 40-4b, 40-4c. The three amplified signals are the synchronization signal, the sample signal, and the reference signal.

[0061] At position 106, the ADC digitizes all three signals simultaneously. The ADC continuously provides a digitized data stream to the flash correlation block 40-5.

[0062] At position 108, flash correlation blocks 40-5 use the stored ideal flash waveform to calculate the sliding flash correlation product of all three digitized signals. At position 110, if the flash correlation product exceeds a calculated threshold, the digitized data is considered to contain a flash waveform. If no flash waveform is detected, the operation returns to position 108. Alternatively, other procedures can be used to detect flash waveforms in the digitized data.

[0063] Once a flash is detected, at 112, for each of the three signals, the digitized data associated with the flash is stored in the memory of processor 40-6. In one exemplary embodiment, since all three signals are digitized simultaneously, at 114, the waveforms can be synchronously averaged sample by sample using a moving average algorithm implemented by processor 40-5. Alternatively, other methods can be used to average the reference and sample signals. Averaging reduces irrelevant noise while enhancing the relevant flash waveform.

[0064] At position 116, the target gas level is calculated by processors 40-6 based on the ratio of the average sample signal peak value to the average reference signal peak value. Other methods exist for calculating gas levels based on the ratio of the sample signal to the reference signal. For example, the gas level can also be determined by the ratio of the areas under the curves of both the sample and reference signals (i.e., the sample spectrum and the reference spectrum). It can also be based on the correlation product from 10⁸.

[0065] The advantages of simultaneous flash detection and averaging can be compared. Figure 11 and Figure 12 To prove it. Figure 11 These are waveform captures of exemplary sample and reference signals that have not undergone any averaging. Figure 12 The setup is exactly the same, but a synchronization signal is added to detect the transmitter flash, which is then used for time-synchronized averaging of the sample and reference signals. It can be seen that the noise in both the sample and reference signals is significantly reduced, thus demonstrating the benefits of this invention.

[0066] As mentioned above, the synchronization signal is a stronger signal, which is an advantage under unfavorable conditions. In clear weather and / or with shorter path lengths, a synchronization signal may not be necessary because, in those cases, both the sample and reference signals are strong, and the sample signal can be used to detect flashes.

[0067] Although the foregoing has described and illustrated specific embodiments of the subject matter, those skilled in the art can make various modifications and changes to it without departing from the scope and spirit of the invention.

Claims

1. An open circuit gas detector for detecting the presence of a target gas with improved uptime, comprising: a transmitter unit comprising a light source configured to emit a flash of light energy along a path in a monitored area, the transmitter configured to generate energy at a sample wavelength in a sample spectral region where light energy is absorbed by the target gas along the path, at a reference wavelength in a reference spectral region not significantly absorbed by the target gas, and at a synchronization wavelength in a synchronization spectral region different from the sample and reference wavelengths; and a receiver for receiving the emitted energy; the receiver including a sample channel responsive substantially only to energy at the sample wavelength to provide a sample signal, a reference channel responsive to energy at the reference wavelength to provide a reference signal, and a synchronization channel responsive to energy at the synchronization wavelength in the synchronization spectral region to provide a synchronization signal, wherein the synchronization spectral region is selected so that the synchronization signal is greater than the sample and reference signals; and a receiver responsive to the sample, reference, and synchronization signals, the receiver configured to detect the presence of the target gas responsive to the sample and reference signals, and wherein the receiver is configured to use the synchronization signal to synchronize operation of the receiver with the flash of the transmitter.

2. The detector of claim 1, wherein the target gas is a hydrocarbon and the sample wavelength is nominally 2.3 um.

3. The detector of claim 2, wherein the reference wavelength is nominally 2.1 um.

4. The detector of claim 1, wherein the light source comprises a xenon flash lamp.

5. The detector of claim 1, wherein the receiver is further configured to capture outputs of the sample and reference channels in synchronization with a particular point on a waveform of the output of the synchronization channel.

6. The detector of claim 5, wherein the particular point is a fast rising edge of the flash of the transmitter detected by the synchronization channel.

7. The detector of claim 5, wherein the receiver comprises a processor configured to perform a synchronized average of the received optical flash, wherein a sum of N values of the captured sample and reference signals is taken and the sum is divided by N to obtain an average of the sample and reference signals.

8. The detector of claim 1, wherein the receiver comprises a processor to determine the presence of the target gas by calculating a ratio of the sample signal to the reference signal.

9. The detector of claim 1, wherein the receiver comprises a detection circuit responsive to the sample, reference, and synchronization signals, the detection circuit configured to detect the flash of the transmitter and trigger capture of the sample and reference signals.

10. The detector of claim 1, wherein the light source is configured to produce a broad spectrum flash of light energy covering wavelengths from ultraviolet to infrared.

11. The detector of claim 1, wherein the light source is configured to produce a spectral output peak intensity in the near infrared region of about 1.5 microns, and which falls to less than one fourth of the peak intensity in the 2.3 micron region.

12. The detector of claim 11, wherein the synchronization signal is centered at about 1.5 microns.

13. The detector of claim 1, wherein the flash has a rise time of about one microsecond and an exponential decay within ten microseconds.

14. An open-path gas detector for detecting the presence of a target hydrocarbon gas with improved uptime in the presence of adverse environmental conditions including fog, comprising: a transmitter unit including a flash lamp configured to emit a flash of broadband optical energy along a path in a region under surveillance, the transmitter configured to generate energy at a sample wavelength in a sample spectrum along the path where optical energy is absorbed by a target gas, at a reference wavelength in a reference spectrum not significantly absorbed by the target gas, and at a synchronization wavelength in a synchronization spectrum region different from the sample and reference wavelengths; wherein the synchronization spectrum is selected so that the flash lamp intensity is greater at the synchronization wavelength than at the sample wavelength; and a receiver for receiving the emitted energy; the receiver including: an optical module including a main objective for capturing and focusing the received energy onto a beamsplitter, the beamsplitter splitting the received energy from the main objective into a sample path and a reference path, sample optics in the sample path for blocking all spectra except the sample spectrum for a sample photodiode, reference optics in the reference path for blocking all spectra except the reference spectrum for a reference photodiode, a synchronization objective for capturing and focusing the received energy onto a synchronization photodiode, receiver circuitry responsive to signals from the sample, reference, and synchronization photodiodes to amplify and digitize the respective signals; flash detection and waveform capture circuitry responsive to the respective sample, reference, and synchronization signals, wherein the synchronization signal is used to detect the transmitter flash and capture waveforms representative of the sample photodiode signal and the reference photodiode signal; a processor responsive to the captured waveforms to detect the presence of the target gas; and wherein the receiver is configured to use the synchronization signal to synchronize operation of the receiver with the transmitter flash.

15. The detector of claim 14, wherein the receiver is further configured to capture the sample and reference signals in synchronization with a particular point on the waveform of the synchronization signal.

16. The detector of claim 15, wherein the particular point is a fast rising edge of the transmitter flash detected by the flash detection and waveform capture circuitry.

17. The detector of claim 14, wherein the processor is configured to perform a synchronous average of the received optical flash, wherein a sum of N values of the captured sample signal and the reference signal is taken, and the sum is divided by N to obtain an average of the sample signal and the reference signal.

18. The detector of claim 14, wherein the processor is configured to determine the presence of the target gas by calculating a ratio of the sample signal to the reference signal.

19. The detector of claim 14, wherein the flash lamp is configured to produce a broad spectrum of light energy flashes covering wavelengths from ultraviolet to infrared.

20. The detector of claim 14, wherein the flash lamp is configured to produce a spectral output peak intensity in the near infrared region of about 1.5 microns, and which falls to less than one fourth of the peak intensity in the 2.3 micron region.

21. The detector of claim 20, wherein the synchronization signal is centered at about 1.5 microns.

22. The detector of claim 14, wherein the flash has a rise time of about one microsecond and an exponential decay within ten microseconds.

23. An open path gas detector for detecting the presence of a target gas with improved uptime, comprising: a transmitter unit including a light source configured to emit a flash of light energy along a path in a region under surveillance, the transmitter configured to generate energy at a detection spectral region for detecting the presence of the target gas, the detection spectral region including: a sample spectral region in which light energy is absorbed by the target gas, and a reference spectral region in which light energy is not absorbed by the target gas, and a synchronization spectral region not used by the detection; and a receiver for receiving the emitted energy, including: a sample channel and a reference channel that each provide a sample signal and a reference signal, respectively, in response only to received energy within the detection spectral region; and a synchronization channel that provides a synchronization signal in response to energy within the synchronization spectral region, wherein the synchronization spectral region is selected so that the synchronization signal is greater than the sample and reference signals; and a receiver configured to detect the presence of the target gas in response to the sample and reference signals, and wherein the receiver is configured to use the synchronization signal to synchronize operation of the receiver with the transmitter flash.

24. The detector of claim 23, wherein the light source includes a xenon flash lamp.

25. The detector of claim 23 or claim 24, wherein the receiver is further configured to capture the sample and reference signals synchronously with a particular point on a waveform of the synchronization signal.

26. The detector of claim 25, wherein the particular point represents a fast rising edge of the transmitter flash.

27. The detector of claim 25, wherein the receiver is configured to perform a synchronous average on the received optical flash, wherein a sum of N values of the captured sample signal and reference signal are taken, and the sum is divided by N to obtain an average of the sample signal and reference signal.

28. The detector of any of claims 23-24 and 26-27, wherein the target gas is a hydrocarbon, and the sample spectral region encompasses a 2.3 um wavelength.

29. The detector of claim 28, wherein the reference spectral region encompasses a 2.1 um wavelength.

30. The detector of any of claims 23-24, 26-27, and 29, wherein the receiver includes a processor to determine the presence of the target gas by calculating a ratio of the sample signal to reference signal.

31. The detector of any of claims 23-24, 26-27, and 29, wherein the receiver includes a detection circuit responsive to the sample signal, reference signal, and synchronization signal, the detection circuit configured to detect the emitter flash and trigger capture of the sample signal and reference signal.

32. The detector of any of claims 23-24, 26-27, and 29, wherein the light source is configured to produce a broad spectral light energy flash covering wavelengths from ultraviolet to infrared.

33. The detector of any of claims 23-24, 26-27, and 29, wherein the light source is configured to produce a spectral output peak intensity in the near infrared region of about 1.5 microns, and which falls to less than a quarter of the peak intensity in the 2.3 micron region.

34. The detector of claim 33, wherein the synchronization signal is centered at about 1.5 microns.

35. The detector of any of claims 23-24, 26-27, 29, and 34, wherein the flash has a rise time of about one microsecond and an exponential decay within ten microseconds.

36. The detector of any of claims 23-24, 26-27, 29, and 34, wherein: the sample channel includes a sample photodiode configured to detect light energy at the sample spectral region; and a sample optical filter configured to block light energy outside the sample spectral region; the reference channel includes a reference photodiode configured to detect light energy at the reference spectral region; and a reference optical filter configured to block light energy outside the reference spectral region; and the synchronization channel includes a synchronization photodiode configured to detect light energy at the synchronization spectral region, and does not include an optical filter that limits the bandwidth of the synchronization channel.

Images