Patent Application
20190302011
The present subject matter relates generally to remote characterization of visible emissions, and more particularly, to a compact, optical sensor which can remotely measure the opacity of a visible emission plume from a stationary source while allowing eye-safe operation. Assessing visible emissions is important for compliance with environmental regulations and to support the regulatory reporting needs of Federal and State inspectors. The system and method may also be applied to non-stationary sources.
This invention addresses the need for a visible emission characterization system capable of determining the opacity of remote stationary emission sources. The opacity of an attenuating medium, such as a smoke plume, is defined as one minus the transmittance of that medium. The transmittance of an attenuating medium is the fraction of incident radiant energy that remains after passing through that medium. The opacity of an emission source indicates whether it meets regulatory compliance. An emission source has a maximum permissible opacity threshold, and an opacity value beyond that threshold indicates non-compliance with U.S. Environmental Protection Agency (EPA) emissions regulations. Accurate measurement of this opacity is the goal of the present invention.
Methods established by the EPA for the remote measurement of the opacity of emissions from stationary sources include visual determination (via EPA Method 9), determination by lidar (via EPA Method 9, Alternate Method 1), and determination by camera (via EPA Alternative Method ALT-082, ASTM D7520-09). Of these methods, only the lidar method allows opacity determination during both daytime and nighttime hours because it contains its own light source or transmitter and it is not dependent on ambient light contrast conditions. The lidar method is real-time, provides post-observation data/evidence, and is theoretically the most accurate method. Currently, the most common method of determining opacity is by visual determination (EPA Method 9). This method introduces human bias since it relies on the human eye as a sensor, involves extensive labor costs to train personnel and repeat field certification every six months, and does not provide good post-observation data/evidence. While superior, lidar methods have not gained popularity or been in practice so far because existing lidar instruments are bulky, not easily transportable, not eye-safe, and consume significant power. This invention describes an instrument that incorporates state-of-the-art technologies as well as signal processing techniques resulting in a compact, easily portable, potentially eye-safe, energy efficient, battery-operable lidar system for the determination of opacity.
A basic lidar system consists of an optical transmitter, an optical receiver, and associated signal processing and control electronics. In a pulsed lidar system, the optical transmitter sends optical pulses in a collimated light path through the atmosphere towards a target of interest. A small fraction of that transmitted light is backscattered to the optical receiver by atmospheric constituents or objects within light path. The receiver converts the collected optical signal into an electronic signal. The temporal response of this signal corresponds to a distance from the lidar system, since light must travel from the transmitter to a distance or range R and back to the receiver. The level of the atmospheric return with an emission source in the light path compared to the level of the atmospheric return with no emission source in the light path allows determination of the opacity of the emission source.
The present disclosure is directed to a novel opacity measurement system that is compact, handheld or portable and includes all hardware in one unit. More specifically, the present disclosure is directed to an opacity measurement system that can include a laser transmitter, a receiver with one or more detectors, the processing hardware, and power supply in one compact, portable unit. Historically, high-energy pulsed laser sources were large table-top units; however, with recent technological advances the laser transmitter are much more efficient and can be very compact. By processing the lidar return signal with low-power analog circuitry or low-power, low-sample rate digital acquisition hardware, the need for power hungry, high-sample rate, signal acquisition hardware is eliminated. The combination of efficient lidar system and low-power acquisition method allows significant reduction in power consumption of the system. With power consumption reduced by several orders of magnitude, the entire opacity measurement system can be packaged in a compact, handheld or portable, battery-powered unit. This is the basis of this invention.
In general, the disclosed invention relates to, but is not limited to, the measurement of the opacity of a visible emission plume from a stationary source. Specifically, this disclosure relates to the design and construction of an apparatus that uses backscatter for making such measurements from a remote location. As with any lidar system, the lidar system in the present disclosure includes an optical transmitter, an optical receiver, and associated signal processing and control electronics. Taken individually, the components of the disclosed lidar system are not necessarily unique; however, their combination and method of use comprise the novel opacity measurement system outlined in this disclosure. Specifically, the use of both a laser source with a low average power and a low-power signal processing system results in significant reduction in power consumption and allows construction of a battery-operable handheld or portable opacity measurement system for practical use.
The optical transmitter in the present disclosure consists of a pulsed laser source with a relatively low average power to keep system power consumption low. The specifications of the laser source, namely wavelength, pulse width, pulse energy, and pulse repetition frequency (PRF), determine the power consumption of the laser source. The wavelength of the laser source used for an opacity measurement system needs to be non-resonant with the various molecular constituents of the atmosphere and would be preferably ultraviolet or infrared to provide added eye-safety margin. The desired measurement range of the opacity measurement system, which includes opacity dynamic range and sensitivity over a desired distance, determines the necessary laser energy per pulse; the larger the range the higher the pulse energy. Thus, to keep the power consumption low, the PRF of the laser source is kept low to offset the desired high pulse energy. Keeping the power consumption of the laser system low is an integral part of this patent disclosure.
The optical receiver in the present disclosure consists of collection optics to receive the backscattered light and direct it onto one or more detectors. To reduce the contribution of the atmospheric return from non-laser wavelengths an interference filter is placed in the light path before a detector. More than one detector would be needed when implementing polarization separation or when using both low- and high-sensitivity detection schemes. The separation of a light beam into multiple components is known to those skilled in the art. The collected optical signal is converted into an electronic signal with detectors such as photomultiplier tubes or photodiodes operating in analog mode.
It is also possible to use multiple light sources with different wavelengths to realize wavelength-dependent backscatter responses in the system and may distinguish between different types of scattering sources. When multiple light sources with different wavelengths are used, the light from individual laser sources is combined into one transmitted light beam using methods known to those skilled in the art. The receiver collects all the necessary wavelengths and then separates them via wavelength dispersion or wavelength selection elements using methods known to those skilled in the art.
In order to keep the system power consumption low, the signal processing hardware must also be low-power. The signal processing methods may be implemented as either analog, digital, or a combination thereof. Our preferred approach consists of analog integration over a portion of the backscatter signal with subsequent sampling by a digitizer and storage for later processing. A purely digital approach consists of digitizing and storing the backscatter signal and then numerically integrating the necessary portion. For the above or any other signal processing approaches, the value obtained from a single signal acquisition may be used directly or averaged with values from other signal acquisitions. Averaging of multiple signal acquisitions is typically used to improve the accuracy of the result. Typically, the analog processing consumes less power than the ultrafast digital processing.
Following the procedure outlined in EPA Method 9, Alternate Method 1, the opacity of an emission plume is determined by comparison of two different signal acquisitions, one aimed at the emission plume and one clear of the emission plume. The opacity measurement system should first be aimed away from the emission source to obtain a reference signal. The results from the reference signal are then stored. Next the opacity measurement system is aimed at the emission source to obtain the plume signal. The opacity of the emission source can then be determined from these two measurements following EPA Method 9, Alternate Method 1. As per the EPA Method, additional reference signal acquisitions may also be required after the plume signal acquisition.
The combination of a laser with low average power and a low-power signal processing system results in significant reduction in total power consumption and allows construction of a battery-operable opacity measurement system. With appropriate selection of laser parameters, the opacity measurement system can also meet laser safety standards for eye exposure.
The above, as well as other objects and advantages of this disclosure, will become readily apparent to those skilled in the art from reading the following description of an embodiment of the invention. The description and drawings illustrate exemplary embodiments of the invention and serve to enable one skilled in the art to make or use the invention and are not intended to limit the scope of the invention in any manner. With respect to the methods disclosed and illustrated, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical.
The present disclosure is built upon a typical lidar system which includes an optical transmitter, an optical receiver, and associated signal processing and control electronics. In
As the light pulse is transmitted into the atmosphere towards the target, a backscatter signal is returned. The temporal response of this signal corresponds to a distance from the lidar system, since light must travel from the transmitter to a distance or range R and back to the receiver. This correspondence is given by R=c t/2, where c is the speed of light and t is the round-trip transient time from source to the plume. The lower part of
Laser wavelengths used in an opacity measurement system can range from 0.2 μm to 11 μm where certain wavelengths are preferable over others. The wavelength of the laser source used for an opacity measurement system needs to be non-resonant with the various molecular constituents of the atmosphere, otherwise the attenuation will not be dominated by the atmospheric aerosol content. When dealing with open or public areas where people may intersect the path of the laser beam, the beam must meet the laser safety standards ANSI Z136.1 and IEC 60825-1 for eye and skin exposure. For pulsed laser sources, these standards specify the maximum permissible exposure (MPE) in units of energy per unit area. In
To keep the system eye-safe, the energy density must be less than the MPE rating. For a given laser source, this means the transmitter area must be large enough to keep the energy density at any location below the MPE/pulse. The table above shows the minimum beam diameters for a laser pulse with a Gaussian intensity profile, pulse energy of 100 μJ, pulse duration of 5 ns, and a pulse repetition rate (PRF) of 5 Hz. This table indicates that visible and near-infrared wavelength laser sources need their diameters significantly expanded to be considered eye-safe. It should also be noted that while the MPE rating applies to one single pulse, the rating does depend on the number of pulses per second (the PRF).
The optical transmitter for the opacity measurement system in this disclosure is a high-energy pulsed laser with low average power. For this application, the pulse duration of the pulsed laser source is typically short, usually less than 50 nanoseconds. The power consumption of a pulsed laser system is determined from the product of its pulse energy, pulse repetition frequency (PRF), and laser efficiency. Since the efficiency typically depends on the type, or wavelength, of the laser source, the pulse energy and PRF become the adjustable parameters. To maintain the same relative sensitivity, every doubling of the measurement range quadruples the necessary pulse energy, leading to high pulse energies for large measurement ranges. Since high pulse energies are typically desired this means the PRF must be kept low. A method to keep low power consumption of the laser system is an integral part of this patent disclosure. Current state-of-the-art high pulse energy laser sources that can meet these requirements include diode lasers, fiber lasers, and diode pumped solid state lasers.
The optical receiver of the opacity measurement system has the flexibility to be implemented with one or more optical detectors with a telescope. To reduce the contribution of the atmospheric return from non-laser wavelengths, an interference filter, which transmits light in a passband around the laser wavelength and suppresses light outside the passband, is placed in the light path before a detector. More than one detector would be needed when implementing polarization separation or when using both low- and high-sensitivity detection schemes. Polarization separation, although not absolutely necessary, allows possible refinement of the opacity measurement and obtains additional information. To acquire the different polarization components of the received backscattered light, a polarization splitter could be used to separate the components and direct each toward separate detectors. Detectors with different sensitivities allows for extended range operation or automated target range detection. When using both low- and high-sensitivity detection, a beam splitter could be used to separate the incoming light into two light paths and direct each toward separate detectors. The separation of a light beam into multiple components, as described above, is known to those skilled in the art. Detectors are used to convert a collected optical signal into an electronic signal. Signal detection is typically realized with photomultiplier tubes or photodiodes (either regular or Avalanche types). For weak backscatter signals, a detector may be operated in Geiger mode where individual photons are counted, but this also results in significant power consumption in the subsequent signal processing hardware. For strong backscatter signals, a detector may be operated in analog mode, and this mode is preferred in this disclosure.
The second aspect of keeping the system power consumption low for a compact system is low-power signal processing method. Many different signal processing methods may be implemented, which can be classified as analog approaches, digital approaches, or a combination of both. In each case, the detected signal may be optionally range-corrected to remove the 1/R2 nature of the response. For a digital approach the range-correction may be implemented either in analog before digitization or digitally after digitization, but for an analog approach the range-correction must use an analog implementation. Analog range-correction has been performed in prior art, see A. W. Dybdahl, 1981, and digital range-correction is straightforward, so they will not be described further. The preferred low-power approach described herein, is a combination approach that consists of analog integration over a portion of the backscatter signal with subsequent sampling by a digitizer. This digitized signal level is then stored for later processing. A purely digital approach would consist of digitizing and storing the backscatter signal and then numerically integrating the desired portion. For the above or any other signal processing approach, the value obtained from a single signal acquisition may be used directly or averaged with values from other signal acquisitions. Averaging of multiple signal acquisitions is typically used to improve the accuracy of the result. Typically, an analog processing approach consumes less power than a digital processing approach.
The ultimate goal of this disclosure is the realization of a compact, handheld or portable, battery-operable system for the remote measurement of opacity. Each of the three components of the system, the transmitter, the receiver, and the signal processing hardware, must have low power consumption. Typical receiver systems are low power and are battery-operable. The transmitter and signal processing hardware usually consumed significant power in prior art.
This written description uses examples to disclose the invention and also enables any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The claims define the patentable scope of the invention, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
The invention described herein was made under a contract from U.S. Air Force contract number FA8222-14-C-0005. The government may have rights under this invention.
