Patent Publication Number: US-11641441-B2

Title: Optical gas imaging systems and method compatible with uncooled thermal imaging cameras

Description:
RELATED MATTERS 
     This application is a National Stage filing from International Patent Application No. PCT/US2019/033779, filed May 23, 2019, which claims the benefit of U.S. Provisional Application No. 62/676,700 filed May 25, 2018, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     Infrared imaging cameras are used in a variety of situations. For example, infrared imaging cameras are often used during maintenance inspections to thermally inspect equipment. Example equipment may include rotating machinery, electrical panels, or rows of circuit breakers, among other types of equipment. Infrared inspections can detect equipment hot spots such as overheating machinery or electrical components, helping to ensure timely repair or replacement of the overheating equipment before a more significant problem develops. 
     Depending on the configuration of the camera, the infrared imaging camera may also generate a visible light image of the same object. The camera may display the infrared image and the visible light image in a coordinated manner, for example, to help an operator interpret the thermal image generated by the thermal imaging camera. 
     Visible light and/or infrared imaging has been applied to the field of gas imaging, in which a user will inspect a target scene for the presence of a target gas, for example, to perform leak detection and remediation (LDAR) processes. Various methods include active gas imaging techniques, such as by actively illuminating a target scene with one or more particular wavelengths of light, or by passive gas imaging techniques. 
     Optical gas imaging systems can be difficult to use effectively in the field due to the often small changes that a gas cloud may have on an overall infrared image. Such changes may be too small to observe during typical camera operation. Additionally, without a sufficiently high signal to noise ratio in a thermal camera, noise (e.g., due to sensor/electronics drift, thermal fluctuations, component self-heating or other factors) may obscure subtle changes in the image due to gas, making gas recognition very difficult. In addition, the impact on the infrared image by a gas cloud will depend on the size (e.g., path thickness) and density of the gas cloud, neither of which is necessarily constant throughout a gas cloud. Thus, the degree and shape of the attenuation or augmentation from a gas cloud will vary within a scene as the gas cloud changes shape over time. Such changes in the image can be compounded by camera motion or motion within the scene, making such systems difficult to operate. These difficulties can be problematic since any one of them may cause the camera to display artifacts that can obscure gas within the scene, making it difficult to observe by a user. 
     In addition, gases selectively absorb/emit infrared radiation at specific wavelengths and can make normally invisible gases visible under certain conditions, for instance, by using a camera sensitive to the absorption/emission wavelengths (e.g., infrared wavelengths). Various factors contribute to whether the gas will be visible via infrared imaging, for example, during optical gas imaging, including the temperature difference between the gas and the apparent temperature of the scene background. This temperature difference often has a significant impact on the practical effectiveness of optical gas imaging cameras, for instance, during leak detection inspections. However, the apparent temperature difference can vary widely depending on ambient conditions and the camera operator&#39;s choice of viewing position, often requiring high levels of training for camera operators to ensure a proper viewing position and often contributing to inconsistent results between operators and gas imaging or LDAR surveys taken at different times. 
     Infrared cameras for gas imaging often use cryo-cooled sensor arrays, special spectral filters, and/or other optical devices to provide spectral separation in order to achieve an adequate signal to noise ratio to observe gas in a scene among noise and other image clutter. Uncooled infrared sensor arrays, with their broadband IR response and in view of the typically narrow IR absorption bands of gases often have low or inadequate signal-to-noise and/or gas signal vs background scene contrast (e.g., clutter) for effective optical gas imaging without these special spectral approaches. 
     Additionally, uncooled infrared sensor arrays are often subject to fixed-pattern noise, for example, due to pixel-to-pixel nonuniformity in the sensor array and/or the changing stray IR associated with camera self-heating and/or changes in ambient temperature that can clutter a scene and provide false data and/or obscure the ability to observe gas in a scene. 
     SUMMARY 
     Aspects of the disclosure relate to thermal imaging systems having an infrared camera module, a user interface, a display, a processor in communication with the user interface, the display, and the infrared camera module, and a memory in communication with the processor. The processor can be configured to capture infrared images via the infrared camera module. The memory can include instructions to cause the processor to perform various actions, such as upon a detected actuation from the user interface. 
     In some examples, the processor can be configured to perform a non-uniformity correction process to reduce or eliminate fixed pattern noise from infrared image data from the infrared camera module. The processor can capture infrared images of an infrared scene via the infrared camera module at a plurality of times. Each of the captured infrared images can be subject to the non-uniformity correction and registered via an image stabilization process. 
     The processor can perform an optical gas imaging process to generate optical gas image data emphasizing change in the infrared scene, and generate a display image including the generated optical gas image data for presentation on the display. 
     In some cases, the display image can include a drift indicator that indicates relative motion of the infrared camera module from a reference position. This can assist a user in maintaining a camera module stationary to reduce misalignment among captured image data. In some examples, the drift indicator comprises a first marker that indicates the reference position and a second marker that indicates the position of the infrared camera module relative to the reference position. 
     In some examples, a process of capturing images, registering the images, performing the optical gas imaging process, and generating a display image can be repeated for as long as the user interface is actuated. For example, in some cases, the user interface comprises a depressible trigger, and such steps can be performed and repeated for as long as the trigger is depressed, and stopped when the trigger is released. In some examples, upon release of the trigger, the processor is configured to perform a new non-uniformity correction process and then perform and repeat such a process until the trigger is released. 
     In some aspects, a thermal imaging system can include a processor configured to receive infrared image data from an infrared camera module and determine an apparent background temperature for each of a plurality of regions in the infrared image data. In some examples, the processor can receive data representative of a temperature of a target gas and, for each of the plurality of regions in the received infrared image data, compare the apparent background temperature for that region with the temperature of the target gas to determine a temperature difference value. The processor can compare the temperature difference value to a minimum temperature difference value associated with the target gas, and, if the temperature difference value is below the minimum temperature difference value, consider the region to lack sufficient contrast to reliably observe the target gas within the region. 
     Some systems can include an air temperature sensor in communication with the processor, wherein receiving data representative of the temperature of the target gas can include receiving air temperature information from the air temperature sensor. 
     In some examples, systems can include a display in communication with the processor. In some such embodiments, the processor can generate optical gas image data comprising a plurality of regions corresponding to the plurality of regions in the captured infrared image data. The processor can be further configured to generate display image data including the optical gas image data, the display image data having a plurality of regions corresponding to the plurality of regions in the optical gas image data and the infrared image data. The processor can output the display image for presentation on the display. In some examples, regions in the display image corresponding to regions considered to lack sufficient contrast to reliably observe the target gas within the region are presented in a visually distinguishable way from regions not considered to lack sufficient contrast to reliably observe the target gas within the region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective front view of an example thermal imaging camera. 
         FIG.  2    is a perspective back view of the example thermal imaging camera of  FIG.  1   . 
         FIG.  3    is a functional block diagram illustrating example components of the thermal imaging camera of  FIGS.  1  and  2   . 
         FIGS.  4 A and  4 B  show schematic illustrations of a gas impacting a thermal image. 
         FIGS.  5 A- 5 C  schematically show how infrared imaging can detect the presence of gas in a target scene. 
         FIG.  6    is a schematic illustration of a plurality of image frames captured in a frame buffer. 
         FIG.  7    is a process flow diagram illustrating an exemplary process for generating and updating filtered foreground and background for continued updating of an optical gas image. 
         FIG.  8    is a schematic illustration of a process for generating an optical gas image. 
         FIG.  9    is a schematic diagram illustrating combinations of image data that can be used to generate a display image. 
         FIG.  10    is an exemplary user interface showing user-selectable options for performing gas imaging operations. 
         FIG.  11    shows an exemplary lookup table for determining a required difference between the gas temperature and the apparent background temperature in order to achieve a certain minimum contrast due to a given gas type and gas amount. 
         FIG.  12    is a process-flow diagram showing an exemplary process for determining whether or not a pixel can be reliably used to detect a given gas type and amount and for displaying such determinations. 
         FIG.  13    is a schematic illustration of a process for real-time generation of an optical gas image including information regarding whether or not a given gas type and gas amount is observable at various locations within the optical gas image. 
         FIG.  14    is a process flow diagram illustrating a process for accommodating for camera motion during an optical gas imaging process. 
         FIG.  15    is a process flow diagram illustrating a process for accommodating for thermal change during an optical gas imaging process. 
         FIGS.  16 A and  16 B  are schematic illustrations simulating camera motion during an optical gas imaging process. 
         FIG.  17    is a process-flow diagram showing an exemplary combination of performing a NUC process before and/or during an optical gas imaging process that uses image stabilization. 
         FIGS.  18 A and  18 B  show exemplary display interface indicating to a user an amount of camera drift during an optical gas imaging. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides some practical illustrations for implementing various embodiments of the present invention. Examples of constructions, materials, dimensions, and manufacturing processes are provided for selected elements, and all other elements employ that which is known to those of ordinary skill in the field of the invention. Those skilled in the art will recognize that many of the noted examples have a variety of suitable alternatives. 
     A thermal imaging camera may be used to detect heat patterns across a scene, including an object or objects, under observation. The thermal imaging camera may detect infrared radiation given off by the scene and convert the infrared radiation into an infrared image indicative of the heat patterns. In some embodiments, the thermal imaging camera may also capture visible light from the scene and convert the visible light into a visible light image. Depending on the configuration of the thermal imaging camera, the camera may include infrared optics to focus the infrared radiation on an infrared sensor and visible light optics to focus the visible light on a visible light sensor. 
     Various embodiments provide methods and systems for producing thermal images with reduced noise using averaging techniques. To further improve image quality and eliminate problems that may arise from averaging (e.g. blurring, ghosting, etc.), an image alignment process is performed on the thermal images prior to averaging. 
       FIGS.  1  and  2    show front and back perspective views, respectively of an example thermal imaging camera  100 , which includes a housing  102 , an infrared lens assembly  104 , a visible light lens assembly  106 , a display  108 , a laser  110 , and a trigger control  112 . Housing  102  houses the various components of thermal imaging camera  100 . The bottom portion of thermal imaging camera  100  includes a carrying handle  118  for holding and operating the camera via one hand. Infrared lens assembly  104  receives infrared radiation from a scene and focuses the radiation on an infrared sensor for generating an infrared image of a scene. Visible light lens assembly  106  receives visible light from a scene and focuses the visible light on a visible light sensor for generating a visible light image of the same scene. Thermal imaging camera  100  captures the visible light image and/or the infrared image in response to depressing trigger control  112 . In addition, thermal imaging camera  100  controls display  108  to display the infrared image and the visible light image generated by the camera, e.g., to help an operator thermally inspect a scene. Thermal imaging camera  100  may also include a focus mechanism coupled to infrared lens assembly  104  that is configured to move at least one lens of the infrared lens assembly so as to adjust the focus of an infrared image generated by the thermal imaging camera. Additionally or alternatively, the focus mechanism may move the FPA relative to one or more lenses of the infrared lens assembly. 
     In operation, thermal imaging camera  100  detects heat patterns in a scene by receiving energy emitted in the infrared-wavelength spectrum from the scene and processing the infrared energy to generate a thermal image. Thermal imaging camera  100  may also generate a visible light image of the same scene by receiving energy in the visible light-wavelength spectrum and processing the visible light energy to generate a visible light image. As described in greater detail below, thermal imaging camera  100  may include an infrared camera module that is configured to capture an infrared image of the scene and a visible light camera module that is configured to capture a visible light image of the same scene. The infrared camera module may receive infrared radiation projected through infrared lens assembly  104  and generate therefrom infrared image data. The visible light camera module may receive light projected through visible light lens assembly  106  and generate therefrom visible light data. 
     In some examples, thermal imaging camera  100  collects or captures the infrared energy and visible light energy substantially simultaneously (e.g., at the same time) so that the visible light image and the infrared image generated by the camera are of the same scene at substantially the same time. In these examples, the infrared image generated by thermal imaging camera  100  is indicative of localized temperatures within the scene at a particular period of time while the visible light image generated by the camera is indicative of the same scene at the same period of time. In other examples, thermal imaging camera may capture infrared energy and visible light energy from a scene at different periods of time. 
     Visible light lens assembly  106  includes at least one lens that focuses visible light energy on a visible light sensor for generating a visible light image. Visible light lens assembly  106  defines a visible light optical axis which passes through the center of curvature of the at least one lens of the assembly. Visible light energy projects through a front of the lens and focuses on an opposite side of the lens. Visible light lens assembly  106  can include a single lens or a plurality of lenses (e.g., two, three, or more lenses) arranged in series. In addition, visible light lens assembly  106  can have a fixed focus or can include a focus adjustment mechanism for changing the focus of the visible light optics. In examples in which visible light lens assembly  106  includes a focus adjustment mechanism, the focus adjustment mechanism may be a manual adjustment mechanism or an automatic adjustment mechanism. 
     Infrared lens assembly  104  also includes at least one lens that focuses infrared energy on an infrared sensor for generating a thermal image. Infrared lens assembly  104  defines an infrared optical axis which passes through the center of curvature of lens of the assembly. During operation, infrared energy is directed through the front of the lens and focused on an opposite side of the lens. Infrared lens assembly  104  can include a single lens or a plurality of lenses (e.g., two, three, or more lenses), which may be arranged in series. In some examples, the infrared lens assembly  104  may include lenses having diffractive or reflective properties or elements. Additional optical components such as mirrors (e.g., Fresnel mirrors) and the like may be included within or otherwise proximate to the infrared lens assembly  104 . 
     As briefly described above, thermal imaging camera  100  includes a focus mechanism for adjusting the focus of an infrared image captured by the camera. In the example shown in  FIGS.  1  and  2   , thermal imaging camera  100  includes focus ring  114 . Focus ring  114  is operatively coupled (e.g., mechanically and/or electrically coupled) to at least one lens of infrared lens assembly  104  and configured to move one or both of the FPA and the at least one lens to various focus positions so as to focus the infrared image captured by thermal imaging camera  100 . Focus ring  114  may be manually rotated about at least a portion of housing  102  so as to move the at least one lens to which the focus ring is operatively coupled. In some examples, focus ring  114  is also operatively coupled to display  108  such that rotation of focus ring  114  causes at least a portion of a visible light image and at least a portion of an infrared image concurrently displayed on display  108  to move relative to one another. In different examples, thermal imaging camera  100  may include a manual focus adjustment mechanism that is implemented in a configuration other than focus ring  114 , or may, in other embodiments, simply maintain a fixed focus. 
     In some examples, thermal imaging camera  100  may include an automatically adjusting focus mechanism in addition to or in lieu of a manually adjusting focus mechanism. An automatically adjusting focus mechanism may be operatively coupled to at least one lens of infrared lens assembly  104  and configured to automatically move the at least one lens to various focus positions, e.g., in response to instructions from thermal imaging camera  100 . In one application of such an example, thermal imaging camera  100  may use laser  110  to electronically measure a distance between an object in a target scene and the camera, referred to as the distance-to-target. Thermal imaging camera  100  may then control the automatically adjusting focus mechanism to move the at least one lens of infrared lens assembly  104  to a focus position that corresponds to the distance-to-target data determined by thermal imaging camera  100 . The focus position may correspond to the distance-to-target data in that the focus position may be configured to place the object in the target scene at the determined distance in focus. In some examples, the focus position set by the automatically adjusting focus mechanism may be manually overridden by an operator, e.g., by rotating focus ring  114 . 
     During operation of thermal imaging camera  100 , an operator may wish to view a thermal image of a scene and/or a visible light image of the same scene generated by the camera. For this reason, thermal imaging camera  100  may include a display. In the examples of  FIGS.  1  and  2   , thermal imaging camera  100  includes display  108 , which is located on the back of housing  102  opposite infrared lens assembly  104  and visible light lens assembly  106 . Display  108  may be configured to display a visible light image, an infrared image, and/or a combined image that includes a simultaneous display of the visible light image and the infrared image. In different examples, display  108  may be remote (e.g., separate) from infrared lens assembly  104  and visible light lens assembly  106  of thermal imaging camera  100 , or display  108  may be in a different spatial arrangement relative to infrared lens assembly  104  and/or visible light lens assembly  106 . Therefore, although display  108  is shown behind infrared lens assembly  104  and visible light lens assembly  106  in  FIG.  2   , other locations for display  108  are possible. 
     Thermal imaging camera  100  can include a variety of user input media for controlling the operation of the camera and adjusting different settings of the camera. Example control functions may include adjusting the focus of the infrared and/or visible light optics, opening/closing a shutter, capturing an infrared and/or visible light image, or the like. In the example of  FIGS.  1  and  2   , thermal imaging camera  100  includes a depressible trigger control  112  for capturing an infrared and visible light image, and buttons  116 , which form part of the user interface, for controlling other aspects of the operation of the camera. A different number or arrangement of user input media are possible, and it should be appreciated that the disclosure is not limited in this respect. For example, thermal imaging camera  100  may include a touch screen display  108  which receives user input by depressing different portions of the screen. 
       FIG.  3    is a functional block diagram illustrating components of an example of thermal imaging camera  100 . Thermal imaging camera  100  includes an IR camera module  200 , front end circuitry  202 . The IR camera module  200  and front end circuitry  202  are sometimes referred to in combination as front end stage or front end components  204  of the infrared camera  100 . Thermal imaging camera  100  may also include a visible light camera module  206 , a display  108 , a user interface  208 , and an output/control device  210 . 
     Infrared camera module  200  may be configured to receive infrared energy emitted by a target scene and to focus the infrared energy on an infrared sensor for generation of infrared energy data, e.g., that can be displayed in the form of an infrared image on display  108  and/or stored in memory. Infrared camera module  200  can include any suitable components for performing the functions attributed to the module herein. In the example of  FIG.  3   , infrared camera module  200  is illustrated as including infrared lens assembly  104  and infrared sensor  220 . As described above with respect to  FIGS.  1  and  2   , infrared lens assembly  104  includes at least one lens that takes infrared energy emitted by a target scene and focuses the infrared energy on infrared sensor  220 . Infrared sensor  220  responds to the focused infrared energy by generating an electrical signal that can be converted and displayed as an infrared image on display  108 . 
     Infrared sensor  220  may include one or more focal plane arrays (FPA) that generate electrical signals in response to infrared energy received through infrared lens assembly  104 . Each FPA can include a plurality of infrared sensor elements including, e.g., bolometers, photon detectors, or other suitable infrared sensor elements. In operation, each sensor element, which may each be referred to as a sensor pixel, may change an electrical characteristic (e.g., voltage or resistance) in response to absorbing infrared energy received from a target scene. In turn, the change in electrical characteristic can provide an electrical signal that can be received by a processor  222  and processed into an infrared image displayed on display  108 . 
     For instance, in examples in which infrared sensor  220  includes a plurality of bolometers, each bolometer may absorb infrared energy focused through infrared lens assembly  104  and increase in temperature in response to the absorbed energy. The electrical resistance of each bolometer may change as the temperature of the bolometer changes. With each detector element functioning as a sensor pixel, a two-dimensional image or picture representation of the infrared radiation can be further generated by translating the changes in resistance of each detector element into a time-multiplexed electrical signal that can be processed for visualization on a display or storage in memory (e.g., of a computer). Processor  222  may measure the change in resistance of each bolometer by applying a current (or voltage) to each bolometer and measure the resulting voltage (or current) across the bolometer. Based on these data, processor  222  can determine the amount of infrared energy emitted by different portions of a target scene and control display  108  to display a thermal image of the target scene. 
     Independent of the specific type of infrared sensor elements included in the FPA of infrared sensor  220 , the FPA array can define any suitable size and shape. In some examples, infrared sensor  220  includes a plurality of infrared sensor elements arranged in a grid pattern such as, e.g., an array of sensor elements arranged in vertical columns and horizontal rows. In various examples, infrared sensor  220  may include an array of vertical columns by horizontal rows of, e.g., 16×16, 50×50, 160×120, 120×160, or 650×480. In other examples, infrared sensor  220  may include a smaller number of vertical columns and horizontal rows (e.g., 1×1), a larger number vertical columns and horizontal rows (e.g., 1000×1000), or a different ratio of columns to rows. 
     In certain embodiments a Read Out Integrated Circuit (ROIC) is incorporated on the IR sensor  220 . The ROIC is used to output signals corresponding to each of the sensor pixels. Such ROIC is commonly fabricated as an integrated circuit on a silicon substrate. The plurality of detector elements may be fabricated on top of the ROIC, wherein their combination provides for the IR sensor  220 . In some embodiments, the ROIC can include components discussed elsewhere in this disclosure (e.g. an analog-to-digital converter (ADC)) incorporated directly onto the FPA circuitry. Such integration of the ROIC, or other further levels of integration not explicitly discussed, should be considered within the scope of this disclosure. 
     As described above, the IR sensor  220  generates a series of electrical signals corresponding to the infrared radiation received by each infrared detector element to represent a thermal image. A “frame” of thermal image data is generated when the voltage signal from each infrared detector element is obtained by scanning all of the rows that make up the IR sensor  220 . Again, in certain embodiments involving bolometers as the infrared detector elements, such scanning is done by switching a corresponding detector element into the system circuit and applying a bias voltage across such switched-in element. Successive frames of thermal image data are generated by repeatedly scanning the rows of the IR sensor  220 , with such frames being produced at a rate sufficient to generate a video representation (e.g. 30 Hz, or 60 Hz) of the thermal image data. 
     The front end circuitry  202  includes circuitry for interfacing with and controlling the IR camera module  200 . In addition, the front end circuitry  202  initially processes and transmits collected infrared image data to a processor  222  via a connection therebetween. More specifically, the signals generated by the IR sensor  220  are initially conditioned by the front end circuitry  202  of the thermal imaging camera  100 . In certain embodiments, as shown, the front end circuitry  202  includes a bias generator  224  and a pre-amp/integrator  226 . In addition to providing the detector bias, the bias generator  224  can optionally add or subtract an average bias current from the total current generated for each switched-in detector element. The average bias current can be changed in order (i) to compensate for deviations to the entire array of resistances of the detector elements resulting from changes in ambient temperatures inside the thermal imaging camera  100  and (ii) to compensate for array-to-array variations in the average detector elements of the IR sensor  220 . Such bias compensation can be automatically controlled by the thermal imaging camera  100  or software, or can be user controlled via input to the output/control device  210  or processor  222 . Following provision of the detector bias and optional subtraction or addition of the average bias current, the signals can be passed through a pre-amp/integrator  226 . Typically, the pre-amp/integrator  226  is used to condition incoming signals, e.g., prior to their digitization. As a result, the incoming signals can be adjusted to a form that enables more effective interpretation of the signals, and in turn, can lead to more effective resolution of the created image. Subsequently, the conditioned signals are sent downstream into the processor  222  of the thermal imaging camera  100 . 
     In some embodiments, the front end circuitry  202  can include one or more additional elements for example, additional sensors  228  or an ADC  230 . Additional sensors  228  can include, for example, temperature sensors, visual light sensors (such as a CCD), pressure sensors, magnetic sensors, etc. Such sensors can provide additional calibration and detection information to enhance the functionality of the thermal imaging camera  100 . For example, temperature sensors can provide an ambient temperature reading near the IR sensor  220  to assist in radiometry calculations. A magnetic sensor, such as a Hall Effect sensor, can be used in combination with a magnet mounted on the lens to provide lens focus position information. Such information can be useful for calculating distances, or determining a parallax offset for use with visual light scene data gathered from a visual light sensor. 
     An ADC  230  can provide the same function and operate in substantially the same manner as discussed below, however its inclusion in the front end circuitry  202  may provide certain benefits, for example, digitization of scene and other sensor information prior to transmittal to the processor  222  via the connection therebetween. In some embodiments, the ADC  230  can be integrated into the ROIC, as discussed above, thereby eliminating the need for a separately mounted and installed ADC  230 . 
     In some embodiments, front end components can further include a shutter  240 . A shutter  240  can be externally or internally located relative to the lens and operate to open or close the view provided by the IR lens assembly  104 . As is known in the art, the shutter  240  can be mechanically positionable, or can be actuated by an electro-mechanical device such as a DC motor or solenoid. Embodiments of the invention may include a calibration or setup software implemented method or setting which utilize the shutter  240  to establish appropriate bias levels for each detector element. 
     Components described as processors within thermal imaging camera  100 , including processor  222 , may be implemented as one or more processors, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), programmable logic circuitry, or the like, either alone or in any suitable combination. Processor  222  may also include memory that stores program instructions and related data that, when executed by processor  222 , cause thermal imaging camera  100  and processor  222  to perform the functions attributed to them in this disclosure. Memory may include any fixed or removable magnetic, optical, or electrical media, such as RAM, ROM, CD-ROM, hard or floppy magnetic disks, EEPROM, or the like. Memory may also include a removable memory portion that may be used to provide memory updates or increases in memory capacities. A removable memory may also allow image data to be easily transferred to another computing device, or to be removed before thermal imaging camera  100  is used in another application. Processor  222  may also be implemented as a System on Chip that integrates some or all components of a computer or other electronic system into a single chip. These elements manipulate the conditioned scene image data delivered from the front end stages  204  in order to provide output scene data that can be displayed or stored for use by the user. Subsequently, the processor  222  (processing circuitry) sends the processed data to a display  108  or other output/control device  210 . 
     During operation of thermal imaging camera  100 , processor  222  can control infrared camera module  200  to generate infrared image data for creating an infrared image. Processor  222  can generate a digital “frame” of infrared image data. By generating a frame of infrared image data, processor  222  captures an infrared image of a target scene at substantially a given point in time. That is, in some examples, a plurality of pixels making up the infrared image may be captured simultaneously. In other embodiments, sets of one or more pixels may be captured serially until each pixel has been captured. 
     Processor  222  can capture a single infrared image or “snap shot” of a target scene by measuring the electrical signal of each infrared sensor element included in the FPA of infrared sensor  220  a single time. Alternatively, processor  222  can capture a plurality of infrared images of a target scene by repeatedly measuring the electrical signal of each infrared sensor element included in the FPA of infrared sensor  220 . In examples in which processor  222  repeatedly measures the electrical signal of each infrared sensor element included in the FPA of infrared sensor  220 , processor  222  may generate a dynamic thermal image (e.g., a video representation) of a target scene. For example, processor  222  may measure the electrical signal of each infrared sensor element included in the FPA at a rate sufficient to generate a video representation of thermal image data such as, e.g., 30 Hz or 60 Hz. Processor  222  may perform other operations in capturing an infrared image such as sequentially actuating a shutter  240  to open and close an aperture of infrared lens assembly  104 , or the like. 
     With each sensor element of infrared sensor  220  functioning as a sensor pixel, processor  222  can generate a two-dimensional image or picture representation of the infrared radiation from a target scene by translating changes in an electrical characteristic (e.g., resistance) of each sensor element into a time-multiplexed electrical signal that can be processed, e.g., for visualization on display  108  and/or storage in memory. When displayed on a display  108 , an infrared image can comprise a plurality of display pixels. Display pixels can have any defined relationship with corresponding sensor pixels. In some examples, each sensor pixel corresponds to a display pixel in an image representation of infrared data. In other examples, a plurality of sensor pixels may be combined (e.g., averaged) to provide infrared information for a single display pixel. In still other examples, a single sensor pixel may contribute to a plurality of display pixels. For example, a value from a single sensor pixel may be replicated at nearby pixels, such as in a simple upsampling procedure. In other examples, neighboring or otherwise nearby pixels may be averaged to create a new pixel value, such as in an interpolation procedure. Because relationships between display pixels and sensor pixels are defined with respect to camera operation, the generic term “pixel” may refer to the sensor pixel, the display pixel, or the data as it is processed from the sensor pixel to the display pixel unless otherwise stated. Processor  222  may perform computations to convert raw infrared image data into scene temperatures (radiometry) including, in some examples, colors corresponding to the scene temperatures. 
     Processor  222  may control display  108  to display at least a portion of an infrared image of a captured target scene. In some examples, processor  222  controls display  108  so that the electrical response of each sensor element of infrared sensor  220  is associated with a single pixel on display  108 . In other examples, processor  222  may increase or decrease the resolution of an infrared image so that there are more or fewer pixels displayed on display  108  than there are sensor elements in infrared sensor  220 . Processor  222  may control display  108  to display an entire infrared image (e.g., all portions of a target scene captured by thermal imaging camera  100 ) or less than an entire infrared image (e.g., a lesser port of the entire target scene captured by thermal imaging camera  100 ). Processor  222  may perform other image processing functions, as described in greater detail below. 
     Independent of the specific circuitry, thermal imaging camera  100  may be configured to manipulate data representative of a target scene so as to provide an output that can be displayed, stored, transmitted, or otherwise utilized by a user. 
     Thermal imaging camera  100  includes visible light camera module  206 . Visible light camera modules are generally well known. For examples, various visible light camera modules are included in smartphones and numerous other devices. In some embodiments, visible light camera module  206  may be configured to receive visible light energy from a target scene and to focus the visible light energy on a visible light sensor for generation of visible light energy data, e.g., that can be displayed in the form of a visible light image on display  108  and/or stored in memory. Visible light camera module  206  can include any suitable components for performing the functions attributed to the module herein. In the example of  FIG.  3   , visible light camera module  206  is illustrated as including visible light lens assembly  106  and visible light sensor  242 . As described above with respect to  FIGS.  1  and  2   , visible light lens assembly  106  includes at least one lens that takes visible light energy emitted by a target scene and focuses the visible light energy on visible light sensor  242 . Visible light sensor  242  responds to the focused energy by generating an electrical signal that can be converted and displayed as a visible light image on display  108 . In some examples, the visible light module  206  is configurable by a user, and can provide output, for example, to display  108 , in a variety of formats. Visible light camera module  206  may include compensation functionality for varying lighting or other operating conditions or user preferences. The visible light camera module may provide a digital output including image data, which may include data in a variety of formats (e.g., RGB, CYMK, YCbCr, etc.). 
     Visible light sensor  242  may include a plurality of visible light sensor elements such as, e.g., CMOS detectors, CCD detectors, PIN diodes, avalanche photo diodes, or the like. The number of visible light sensor elements may be the same as or different than the number of infrared light sensor elements. 
     In operation, optical energy received from a target scene may pass through visible light lens assembly  106  and be focused on visible light sensor  242 . When the optical energy impinges upon the visible light sensor elements of visible light sensor  242 , photons within the photodetectors may be released and converted into a detection current. Processor  222  can process this detection current to form a visible light image of the target scene. 
     During use of thermal imaging camera  100 , processor  222  can control visible light camera module  206  to generate visible light data from a captured target scene for creating a visible light image. The visible light data may include luminosity data indicative of the color(s) associated with different portions of the captured target scene and/or the magnitude of light associated with different portions of the captured target scene. Processor  222  can generate a “frame” of visible light image data by measuring the response of each visible light sensor element of thermal imaging camera  100  a single time. By generating a frame of visible light data, processor  222  captures visible light image of a target scene at a given point in time. Processor  222  may also repeatedly measure the response of each visible light sensor element of thermal imaging camera  100  so as to generate a dynamic thermal image (e.g., a video representation) of a target scene, as described above with respect to infrared camera module  200 . In some examples, the visible light camera module  206  may include its own dedicated processor or other circuitry (e.g., ASIC) capable of operating the visible light camera module  206 . In some such embodiments, the dedicated processor is in communication with processor  222  for providing visible light image data (e.g., RGB image data) to processor  222 . In alternative embodiments, a dedicated processor for the visible light camera module  206  may be integrated into processor  222 . 
     With each sensor element of visible light camera module  206  functioning as a sensor pixel, processor  222  can generate a two-dimensional image or picture representation of the visible light from a target scene by translating an electrical response of each sensor element into a time-multiplexed electrical signal that can be processed, e.g., for visualization on display  108  and/or storage in memory. 
     Processor  222  may control display  108  to display at least a portion of a visible light image of a captured target scene. In some examples, processor  222  controls display  108  so that the electrical response of each sensor element of visible light camera module  206  is associated with a single pixel on display  108 . In other examples, processor  222  may increase or decrease the resolution of a visible light image so that there are more or fewer pixels displayed on display  108  than there are sensor elements in visible light camera module  206 . Processor  222  may control display  108  to display an entire visible light image (e.g., all portions of a target scene captured by thermal imaging camera  100 ) or less than an entire visible light image (e.g., a lesser port of the entire target scene captured by thermal imaging camera  100 ). 
     In some embodiments, one or both of infrared  200  and visible light  206  camera modules for acquiring IR and VL image data may be included in an image acquisition module  280 . The image acquisition module may be in wired or wireless communication with a processing module  290  that includes a processor such as  222 . Processing module  290  may receive image data from the image acquisition module  280  and perform subsequent processing steps as will be described herein. In some examples, processing module  290  may include portable processing devices, such as a smartphone, a tablet, a stand-alone computer such as a laptop or desktop PC, or the like. In some such embodiments, various components of front end circuitry  202  may be included in the image acquisition module  280 , the processing module  290 , or both. 
     In these and other examples, processor  222  may control display  108  to concurrently display at least a portion of the visible light image captured by thermal imaging camera  100  and at least a portion of the infrared image captured by thermal imaging camera  100 . Such a concurrent display may be useful in that an operator may reference the features displayed in the visible light image to help understand the features concurrently displayed in the infrared image, as the operator may more easily recognize and distinguish different real-world features in the visible light image than the infrared image. In various examples, processor  222  may control display  108  to display the visible light image and the infrared image in side-by-side arrangement, in a picture-in-picture arrangement, where one of the images surrounds the other of the images, or any other suitable arrangement where the visible light and the infrared image are concurrently displayed. 
     For example, processor  222  may control display  108  to display the visible light image and the infrared image in a combined arrangement. In such an arrangement, for a pixel or set of pixels in the visible light image representative of a portion of the target scene, there exists a corresponding pixel or set of pixels in the infrared image, representative of substantially the same portion of the target scene. In various embodiments, the size and/or resolution of the IR and VL images need not be the same. Accordingly, there may exist a set of pixels in one of the IR or VL images that correspond to a single pixel in the other of the IR or VL image, or a set of pixels of a different size. Similarly, there may exist a pixel in one of the VL or IR images that corresponds to a set of pixels in the other image. Thus, as used herein, corresponding does not require a one-to-one pixel relationship, but may include mismatched sizes of pixels or groups of pixels. Various combination techniques of mismatched sized regions of images may be performed, such as up- or down-sampling one of the images, or combining a pixel with the average value of a corresponding set of pixels. Other examples are known and are within the scope of this disclosure. 
     Thus, corresponding pixels need not have a direct one-to-one relationship. Rather, in some embodiments, a single infrared pixel has a plurality of corresponding visible light pixels, or a visible light pixel has a plurality of corresponding infrared pixels. Additionally or alternatively, in some embodiments, not all visible light pixels have corresponding infrared pixels, or vice versa. Such embodiments may be indicative of, for example, a picture-in-picture type display as previously discussed. Thus, a visible light pixel will not necessarily have the same pixel coordinate within the visible light image as does a corresponding infrared pixel. Accordingly, as used herein, corresponding pixels generally refers pixels from any image (e.g., a visible light image, an infrared image, a combined image, a display image, etc.) comprising information from substantially the same portion of the target scene. Such pixels need not have a one-to-one relationship between images and need not have similar coordinate positions within their respective images. 
     Similarly, images having corresponding pixels (i.e., pixels representative of the same portion of the target scene) can be referred to as corresponding images. Thus, in some such arrangements, the corresponding visible light image and the infrared image may be superimposed on top of one another, at corresponding pixels. An operator may interact with user interface  208  to control the transparency or opaqueness of one or both of the images displayed on display  108 . For example, the operator may interact with user interface  208  to adjust the infrared image between being completely transparent and completely opaque and also adjust the visible light image between being completely transparent and completely opaque. Such an exemplary combined arrangement, which may be referred to as an alpha-blended arrangement, may allow an operator to adjust display  108  to display an infrared-only image, a visible light-only image, of any overlapping combination of the two images between the extremes of an infrared-only image and a visible light-only image. Processor  222  may also combine scene information with other data, such as radiometric data, alarm data, and the like. In general, an alpha-blended combination of visible light and infrared images can comprise anywhere from 100 percent infrared and 0 percent visible light to 0 percent infrared and 100 percent visible light. In some embodiments, the amount of blending can be adjusted by a user of the camera. Thus, in some embodiments, a blended image can be adjusted between 100 percent visible light and 100 percent infrared. 
     Additionally, in some embodiments, the processor  222  can interpret and execute commands from user interface  208 , and/or output/control device  210 . This can involve processing of various input signals and transferring those signals to the front end circuitry  202  via a connection therebetween. Components (e.g. motors, or solenoids) proximate the front end circuitry  202  can be actuated to accomplish the desired control function. Exemplary control functions can include adjusting the focus, opening/closing a shutter, triggering sensor readings, adjusting bias values, etc. Moreover, input signals may be used to alter the processing of the image data that occurs in the processor  222 . 
     Processor can further include other components to assist with the processing and control of the infrared imaging camera  100 . For example, as discussed above, in some embodiments, an ADC can be incorporated into the processor  222 . In such a case, analog signals conditioned by the front-end stages  204  are not digitized until reaching the processor  222 . Moreover, some embodiments can include additional on board memory for storage of processing command information and scene data, prior to transmission to the display  108  or the output/control device  210 . 
     An operator may interact with thermal imaging camera  100  via user interface  208 , which may include buttons, keys, or another mechanism for receiving input from a user. The operator may receive output from thermal imaging camera  100  via display  108 . Display  108  may be configured to display an infrared-image and/or a visible light image in any acceptable palette, or color scheme, and the palette may vary, e.g., in response to user control. In some examples, display  108  is configured to display an infrared image in a monochromatic palette such as grayscale. In other examples, display  108  is configured to display an infrared image in a color palette such as, e.g., amber, ironbow, blue-red, or other high contrast color scheme. Combinations of grayscale and color palette displays are also contemplated. In some examples, the display being configured to display such information may include processing capabilities for generating and presenting such image data. In other examples, being configured to display such information may include the ability to receive image data from other components, such as processor  222 . For example, processor  222  may generate values (e.g., RGB values, grayscale values, or other display options) for each pixel to be displayed. Display  108  may receive such information and map each pixel into a visual display. 
     While processor  222  can control display  108  to concurrently display at least a portion of an infrared image and at least a portion of a visible light image in any suitable arrangement, a picture-in-picture arrangement may help an operator to easily focus and/or interpret a thermal image by displaying a corresponding visible image of the same scene in adjacent alignment. 
     A power supply (not shown) delivers operating power to the various components of thermal imaging camera  100  and, in some examples, may include a rechargeable or non-rechargeable battery and a power generation circuit. 
     During operation of thermal imaging camera  100 , processor  222  controls infrared camera module  200  and visible light camera module  206  with the aid of instructions associated with program information that is stored in memory to generate a visible light image and an infrared image of a target scene. Processor  222  further controls display  108  to display the visible light image and/or the infrared image generated by thermal imaging camera  100 . 
     In some cases, thermal imaging cameras can be used for detecting the presence of infrared-absorbing gases in a scene.  FIGS.  4 A and  4 B  show schematic illustrations of a gas impacting a thermal image.  FIG.  4 A  shows a gas cloud  460  and a source  462  of infrared radiation. Beams  464   a - 464   d  represent infrared radiation from source  462  at a wavelength of light that is not substantially absorbed by the gas in gas cloud  460 .  FIG.  4 B  similarly shows a gas cloud  460  and a source  462  of infrared radiation. Beams  466   a - 466   d  represent infrared radiation from source  462  at a wavelength of light that is absorbed by the gas in gas cloud  460 . As shown in the illustrative example of  FIG.  4 B  using broken lines, after passing through gas cloud  460 , the beams  466   b  and  466   c  are attenuated by absorption by the gas while beams  466   a  and  466   d , which do not pass through gas cloud  460 , are unaffected. It will be appreciated that, while described as radiation being “attenuated” by a gas cloud (e.g.,  460 ), in some instances, the gas cloud can augment radiation of a certain wavelength. For example, in an environment in which a gas is at a higher temperature than the background scene, the gas can add to (augment) radiation at one or more wavelengths with respect to the background scene rather than attenuate such wavelengths. 
       FIGS.  5 A- 5 C  schematically show how infrared imaging can detect the presence of gas in a target scene.  FIG.  5 A  shows a source  572  of infrared radiation emitting at a particular wavelength and having uniform amplitude A. Beams  574   a ,  574   b ,  574   d , and  574   e  of infrared radiation do not pass through gas cloud  570   a , while beam  574   c  passes through gas cloud  570   a . In the illustrated example, the gas cloud  570   a  attenuates the wavelength by a factor of 5. Thus, at plane  578 , the amount of radiation received at regions  580  and  584  emitted by source  572  will still have amplitude A, while the amount of radiation received at region  582  will have amplitude A/2, since the radiation was attenuated by gas cloud  570   a . An infrared image X taken at plane  578  would include regions ( 580 ,  584 ) having amplitude A and other regions ( 582 ) with amplitude A/2. 
       FIG.  5 B  similarly shows a source  572  of infrared radiation emitting at a particular wavelength and having uniform amplitude A, however the gas cloud  570   b  in  FIG.  5 B  is larger than that in  FIG.  5 A  (e.g., due to the gas expanding). In this case, beams  574   a  and  574   e  of infrared radiation do not pass through gas cloud  570   b , while beams  574   b - 574   d  pass through gas cloud  570   b . Since the gas cloud  570   b  attenuates the wavelength from the source  572  by a factor of 2, the amount of radiation received at regions  586  and  591  will still have amplitude A, while the amount of radiation received at region  588  will have amplitude A/2. An infrared image Y taken at plane  578  would include regions ( 586 ,  590 ) having amplitude A and other regions ( 588 ) with amplitude A/2. 
     If source  572  emitted only the wavelength attenuated by the gas cloud ( 570   a ,  570   b ) at uniform amplitude A, either image X ( FIG.  5 A ) or Y ( FIG.  5 B ) would show the presence of the gas cloud, due to the difference in radiation received at plane  578 . For example,  FIG.  5 C  shows image X having a region with amplitude A and a region attenuated by gas cloud  570   a  having amplitude A/2. Similarly, image Y includes a region with amplitude A and a region attenuated by gas cloud  570   b  having amplitude A/2. However, if the source  572  emits additional infrared radiation, such as a typical blackbody, attenuation (or augmentation) of a single wavelength or narrow waveband within the image data would not necessarily be apparent in either image X or Y alone. That is, the attenuation or augmentation of the radiation wavelength(s) would not necessarily produce a noticeable amount of contrast in the image, since the attenuation or augmentation of a small waveband due to the gas may be small compared to the overall radiation amplitude in the image. Thus, in a general gas imaging process, a gas cloud can generally affect the radiation of a wavelength or band of wavelengths due received from a target scene due to attenuation or augmentation of the radiation. 
     However, a difference image X-Y taken by subtracting one image from the other will eliminate any constant background data not attenuated by the gas cloud ( 570   a ,  570   b ). In such an image, the region attenuated by the gas cloud in one image (Y) but not in the other image (X) will appear as a non-zero region (in this case, A/2) in the difference image X-Y, thus indicating at least the presence of gas in the scene. Thus, in some instances, such a difference image can comprise an optical gas image, which can be used to identify a current state of a gas cloud. That is, in some instances, an optical gas image can be generated by determining the difference between a current scene (a “foreground” image) showing the effect of a current gas and a historical scene (a “background” image) showing image data reflecting a previous effect of the gas. With a stationary camera, fixed components in the scene (e.g., non-gas) are likely to be substantially the same in the foreground image and the background, but the gas cloud will have likely evolved and/or moved. Thus, the optical gas image, being generated using the difference between the foreground image and the background image, will include regions in which impact of the gas on the received radiation (e.g., attenuation or augmentation) has changed between the images, but contribution from the fixed components will be absent, since such contribution is present in both the foreground and background image. 
     It will be appreciated that the example described with respect to  FIGS.  5 A- 5 C  is a simplified example illustrating the concept of gas imaging using the absorption properties of the gas relative to at least one infrared wavelength. However, in various examples, the impact on the received radiation by a gas cloud will depend on the size (e.g., path thickness) and density of the gas cloud, neither of which is necessarily constant throughout a gas cloud. Thus, the degree and shape of the attenuation or augmentation from a gas cloud can vary within a scene as the gas cloud changes shape over time. 
     One common way to achieve a gas-detection image is to generally perform the step illustrated in  FIG.  5 C , in which an image taken at one time (the foreground image) is subtracted from an image taken at an earlier time (the background image). The difference in the image will be representative of any changes in the target scene, and since a gas cloud is constantly changing, the gas cloud appears in the image. One way this image subtraction can be performed is by subtracting two images from within a scene buffer, for example, subtracting the earliest image in a buffer from the most-recently acquired image in the buffer. It will be appreciated that the relative amount of emission and attenuation with respect to the amplitude A of the source  572  are arbitrary and used for ease of explanation. Often, the amount of attenuation and/or emission of infrared radiation due to the presence of gas will be very small compared to the magnitude of the infrared radiation emitted from the background (e.g., source  572 ), making it generally difficult to image gas using standard imaging techniques, for example, viewing infrared image data without performing additional processing, such as by performing a subtraction process as generally shown in  FIG.  5 C . 
       FIG.  6    is a schematic illustration of a plurality of image frames captured in a frame buffer. As shown, 120 images (Frame 1-Frame 120) are present in the frame buffer  600 . In an exemplary embodiment, the 120 images in  FIG.  6    can represent 120 frames in a two second buffer acquired at a framerate of 60 Hz. In such an example, Frame 120 is taken approximately two seconds after Frame 1. Thus, subtracting Frame 1 (the historical, “background” image) from Frame 120 (the more recent, “foreground” image) would yield an optical gas image showing changes in the scene over approximately a two second timeframe, which could show the presence of an evolving gas cloud within the target scene. Each time a frame in the buffer  600  is replaced (e.g., Frame 1 is replaced with Frame 121), the first and last frames (now Frame 2 and Frame 121) can be compared to generate a new optical gas image. Thus, the optical gas image can be updated at the same framerate as the images are acquired. 
     As described, the effect of a gas cloud on the infrared radiation from a scene that reaches an imaging plane in a thermal camera is dependent on several features, such as the amount and density of the gas. Additionally, a typical target scene will emit a broad span of infrared wavelengths, some of which will be absorbed or emitted by a gas to a greater degree than others, and the impact on received radiation due to the gas may be very small. As a result, the changes in an image due to the presence of or changes in a gas cloud may be below the noise threshold of a thermal camera. That is, without a sufficiently high signal to noise ratio in a thermal camera, noise (e.g., due to sensor/electronics drift, thermal fluctuations, component self-heating or other factors) may obscure subtle changes in the image due to gas, making gas recognition impossible or impractical. 
     Accordingly, reducing the noise in the infrared images used to observe and/or detect gas can increase the ability to resolve gas from noise in the thermal scene. Some ways of reducing noise in the scene include using a high-sensitivity camera sensor or FPA, such as in a cryo-cooled infrared camera. However, such equipment can be costly and difficult or inconvenient to use. For example, such equipment can be bulky, difficult to carry and/or maneuver, can require extended operating time (e.g., cool down times), and the like. 
     Other techniques to improve the signal to noise ratio of the image data used in establishing a gas image includes generating averages of the image data to reduce the noise present in the foreground and/or background images. For instance, in an exemplary embodiment, with reference to  FIG.  6   , images 1-5 can be averaged together to create a filtered background image, while images 116-120 can be averaged to create a filtered foreground image. This generally reduces the noise in the foreground and background images by a factor of √5 (the square root of the number of averaged frames). The difference between the filtered foreground image and the filtered background image would represent changes in the scene (e.g., in the size, density, etc. of a gas cloud) with reduced noise, such that smaller changes in the scene can be observed due to reduced noise. 
     In general, any of a variety of averaging and/or filtering techniques can be employed to reduce the noise in the foreground and background images used to create an optical gas image. In some embodiments, as described above, the filtered foreground image can be generated from an average of a first plurality of frames while the filtered background image can be generated from an average of a second plurality of frames. In some such examples, the first plurality of frames and the second plurality of frames can be entirely distinct from one another (e.g., Frames 1-5 and Frames 120 as described above). In other examples, the first plurality of frames and the second plurality of frames at least partially overlap. In some embodiments, the median frame in the first plurality of images is captured at a more recent time than the median frame in the second plurality of images. Thus, the filtered foreground image will be generally representative of a more recent point in time than the filtered background image. 
     In some examples, the filtered foreground and/or filtered background images are calculated using a traditional average taken across the first and second plurality of frames, respectively. If performing a traditional average of a plurality of frames, then the first plurality of images and the second plurality of images cannot be the same plurality of images, since otherwise subtracting the filtered foreground image and filtered background image would result in a blank image. 
     In other examples, averaging a plurality of frames can include performing a weighted average of the plurality of frames. For example, in calculating an average of a plurality of frames, greater weight can be placed on more recent frames, less recent frames, or based on any of a variety of parameters. Such weighted averaging can be performed on first and second pluralities of frames in order to generate a filtered foreground and background image, respectively. In some examples, when using a weighted average, the first and second plurality of images can be the same plurality of images. For example, in some embodiments, the filtered background image can be calculated from a weighted average of a plurality of frames wherein frames captured earlier are weighted more heavily than are frames that were captured at a later time. Similarly, the filtered foreground image can be calculated from a weighted average of the same plurality of frames wherein frames that were captured later in time are weighted more heavily than are frames that were captured earlier in time. Thus, the filtered background image generally represents an averaged target scene at an earlier time than does the filtered foreground image. 
     Equations (1) and (2) show a simplified technique for performing a weighted average on a single plurality of N images to calculate a filtered foreground image and a filtered background image. For example, with respect to Equations (1) and (2) a plurality of images includes N frames F 1 , F 2 , . . . , F N , wherein image F 1  is the frame in the plurality that was captured at the earliest time and frame F N  is the frame captured at the most recent point in time. 
     
       
         
           
             
               
                 
                   
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     As shown, according to Equation (1), in calculating the filtered background image F background , earlier images contribute to the average much more than the later images. Similarly, according to Equation (2), in calculating the filtered foreground image, F foreground , later images contribute to the average much more than the earlier images. Equations (1) and (2) illustrate one exemplary process of how images captured at different times can be weighted differently in generating filtered foreground and background images. It will be appreciated that many possible forms of averaging are possible in order to generally average a single plurality of images using different weighting techniques to generate different foreground and background images. Exemplary processes for combining a plurality of buffer frames to generate filtered foreground and background images can include applying one or more finite impulse response (FIR) filters to a plurality of images within the buffer. 
     With respect to  FIG.  6   , in some examples, all frames in a buffer  600  can be averaged together to create foreground and background images for use in an optical gas imaging process. For example, Frames 1-120 can be averaged together via a first averaging technique, such as using Equation (1), to calculate a filtered background image. Similarly, Frames 1-120 can be averaged together via a second averaging technique, such as using Equation (2), to calculate a filtered foreground image. As described elsewhere herein, an optical gas image can be calculated by comparing the filtered foreground and background images, such as via subtraction F foreground −F background , and displayed. The optical gas image can show a representation of a gas cloud as it changes over time in the target scene as described elsewhere herein. 
     In some examples, processing frames from a frame buffer may require an undesired amount of memory and/or processing capability in order to adequately perform certain imaging techniques. For example, in the exemplary embodiment described with respect to  FIG.  6   , to process and display an essentially “real-time” optical gas image to illustrate the presence of gas in a target scene, a camera requires memory to include 120 frames in a buffer  600 . Such a camera would also require sufficient processing power to perform calculations including up to all 120 frames twice in order to calculate both the filtered foreground and filtered background images fast enough to continually calculate and present optical gas images to show a substantially “real-time” updating image. Such requirements can increase the cost and complexity of a camera. 
     In some embodiments, alternative techniques can be performed for determining filtered background and foreground images for calculating an optical gas image that can be used to identify a gas cloud in an imaged scene.  FIG.  7    is a process flow diagram illustrating an exemplary process for generating and updating filtered foreground and background images for continued updating of an optical gas image. The process includes the steps of establishing a filtered foreground image ( 702 ) and establishing a filtered background image ( 704 ). This can include, for example, capturing a single frame and calling the frame both the filtered foreground image and the filtered background image. In other examples, a first image can be captured and established as the filtered background frame and a second image can subsequently be captured and established as the filtered foreground frame. In either case, after each of the filtered foreground image ( 702 ) and filtered background ( 704 ) images are established, as each image is updated via the process of  FIG.  7   , non-zero differences will generally be present between the filtered foreground and background images during subsequent iterations of the method of  FIG.  7   . 
     At this point, two images are stored in memory—a filtered foreground image and a filtered background image. Next, a new image is captured ( 706 ). The process includes the steps of establishing a new filtered foreground image by blending the new image captured at step  706  with the previously established filtered foreground image ( 708 ). Similarly, the method includes the step of establishing a new filtered background image by blending the new image captured at step  706  with the previously established filtered background image ( 710 ). Steps  708  and  710  effectively average the new frame with previous data acting to update each of the filtered foreground and background images by incorporating new image data while maintaining the noise-reduction benefits of filtering/averaging the images. 
     According to the method of  FIG.  7   , the new filtered foreground image (from step  708 ) and the new filtered background image (from step  710 ) are compared to establish a new optical gas image ( 712 ). As described elsewhere herein, comparing the foreground and background images can include subtracting one image from the other. Generally, since changes in the image data are what ultimately represent gas in the image, the optical gas image can be calculated by determining the absolute value of the difference between the background and foreground images or by subtracting either of the foreground image and the background image from the other. 
     After establishing a new optical gas image ( 712 ), a new image is captured ( 706 ). The new image is blended with the previously established filtered foreground image (from the previous iteration through the method) to establish yet another new filtered foreground image ( 708 ). Similarly, the new image is blended with the previously established filtered background image (from the previous iteration through the method) to establish yet another new filtered background image ( 710 ). The new foreground and background images can be compared to establish yet another new optical gas image ( 712 ). 
     In some examples, steps in the process of  FIG.  7    can be performed sequentially, such as establishing initial and new filtered foreground and background images. In other examples, some such steps can be performed in parallel. For example, in some embodiments, filtered foreground and filtered background images can be processed and updated in parallel with one another. In an exemplary embodiment, steps  702  and  704  are performed in parallel. Additionally or alternatively, steps  708  and  710  are performed in parallel. In some such examples, filtered foreground and filtered background images can be generated and updated in parallel before being compared to establish new optical gas image data ( 712 ). 
     In some examples, blending the new image with a previous background or foreground image to recursively update the filtered background and/or foreground image can be performed using one or more infinite impulse response (IIR) filters. In some such examples, the new image and the previous image are weighted by a scaling factor and combined. Equations 3 and 4 provide exemplary calculations for blending a new image with previous foreground and background images to calculate new foreground and background images, respectively, according to an exemplary first order IIR filter.
 
 F   foreground =(α foreground )×( F   new )+(β foreground )×( F   prev_foreground )  (3)
 
 F   background =(α background )×( F   new )+(β background )×( F   prev_background )  (4)
 
wherein
 
α foreground +β foreground =1
 
and
 
α background +β background =1
 
     As shown in Equation 3, the updated foreground image, F foreground , is a blend of a new image, F new , and a previous foreground image, F prev_foreground . The new image is scaled by a factor of α foreground , while the previous foreground image is scaled by a factor of β foreground . Similarly, the updated background image, F background , is a blend of the new image, F new , and a previous background image, F prev_background . The new image is scaled by a factor of α background , while the previous background image is scaled by a factor of β background . 
     It will be appreciated that, while referred to as an infinite impulse response filter, in some examples, due to the limited calculation accuracy (e.g., in digital processing), a contribution from a previous image after several iterations of the filter may amount to zero in a digital filtering process. That is, if a value become sufficiently small compared to the calculation precision, the value may effectively become to zero due to processing limitations. Thus, in some embodiments, an image taken during an IIR filtering process may not in practice contribute to a filtered background or foreground image after a sufficiently high number of iterations of the IIR filter. However, as used herein, an infinite impulse response (IIR) filter will also encompass such filters in which such values may become zero beyond a certain number of iterations due to calculation precision limitations, but conceptually would not necessarily become exactly zero with infinite calculation accuracy. 
       FIG.  8    is a schematic illustration of a process for generating an optical gas image using Equations 3 and 4. As shown, a new image  804  is alpha-blended with a previous filtered foreground image  800  using scaling factors α foreground  and β foreground  to create a new filtered foreground image  820 . Similarly, the new image  804  is alpha-blended with a previous filtered background image  802  using scaling factors α background  and β background  to create a new filtered background image  830 . The new filtered foreground image  820  and new filtered background image  830  can be compared (shown in  FIG.  8    as the absolute value of the difference between the images) to establish an optical gas image  840 . 
     As indicated by arrows  822  and  832 , respectively, the new filtered foreground image  820  becomes the previous filtered foreground image  800  and the new filtered background image  830  becomes the previous filtered background image  802  for subsequent iterations of optical gas imaging  840 . That is, as described with respect to the repetitive method of  FIG.  7   , in some embodiments, after generating an optical gas image  840 , another new image  804  is captured and blended with the previously used foreground and background images to generate updated foreground and background images, which are then used to generate a new optical gas image. 
     As described elsewhere herein, the same averaging process should not be used to determine both the foreground image and the background image, since otherwise, subtracting one image from the other would result in a blank frame. Accordingly, with respect to Equations 3 and 4, α foreground ≠α background  and β foreground ≠β background . This ensures that the background and foreground images are not necessarily the same. Similarly, when using, for example, higher order IIR filters, having at least one filtering parameter different between the foreground and background filtering techniques results in a difference image that is not constantly zero. 
     In still further examples, different filters altogether can be used on the foreground image stream and the background image stream to ensure the foreground and background filtering process does not result in a constantly zero difference image. For example, in some embodiments, the filtered background image can be determined using one of a first order IIR filter, a higher order IIR filter, or a FIR filter, while the filtered background image can be determined using different one of the first order IIR filter, higher order IIR filter, or FIR filter. 
     With further reference to Equations 3 and 4, it is evident that the amount of contribution to a new image from the historical images increases with the β value. That is, a larger 13 value (and accordingly, a smaller a value) corresponds to an image giving greater weight to the previous image (e.g.,  800  or  802  in  FIG.  8   ) than the newly acquired image (e.g.,  804 ). Accordingly, in some embodiments, β background &gt;β foreground  so that the background image is generally weighted more strongly toward historical image data while the foreground image is weighted more strongly toward the current image. 
     Further, as the process is repeated and the foreground and background images are updated more and more times, the effect of the different blending ratios will compound, establishing a difference in time represented by the foreground and background images. 
     Another way of interpreting the foreground and background images as they evolve over time is in view of a time constant. Generally, for a given α or β value, the filtered foreground and background images will have contributions from several “new” images that are successively captured over time. However, the number of images that significantly contribute (e.g., a threshold percentage) to the filtered foreground and background images at a given time will be a function of α and β. Further, if the images are captured at a particular frame rate, the number of images that contribute is analogous to a time span during which the images that significantly contribute were captured. That is, for a given α and β blending ratio, there will be an amount of time, τ, before which acquired images no longer significantly contribute to the filtered image. This can be referred to as the time constant associated with the given α and β. 
     Accordingly, the filtered background image will have a time constant, τ background , such that images captured at a time earlier than τ background  prior to the new captured image will not significantly contribute to the background image. Similarly, the filtered foreground image will have a time constant, τ foreground , such that images captured at a time earlier than τ foreground  prior to the new captured image will not significantly contribute to the foreground image. It will be appreciated that images that do not significantly contribute to the updated images necessarily have no contribution. In some cases, the iteratively updated background and foreground images will include contributions from images captured at more than the time constant earlier than the newly captured image, however, this contribution will be relatively small compared to the overall image (e.g., less than a factor of 1/e, etc.). According to this interpretation, the time constant of the continually updated background image, τ background , will be larger than the time constant of the continually updated foreground image, τ foreground  (τ background &gt;τ foreground ). Thus, the background image generally incorporates images taken over a greater period of time than does the foreground image. Additionally, since a longer time constant (larger τ and β) generally incorporates larger contributions of image data from over a longer period of time, a larger time constant generally amounts to more filtering and noise reduction in the filtered image. 
     In some examples, with respect to Equations 3 and 4, the contribution from a given image will contribute a factor of less than 1/e to the final image after approximately 1/α frames have been added into the filtered image after the given image was added. That is, if α=0.1, then the filtered image will be updated 10 times with newly captured images before a previous image contributes less than a factor of 1/e to the filtered image. If the 1/e threshold is used as determining the time constant, then the time constant can be determined based on the framerate and the value of a according to Equation 5. 
     
       
         
           
             
               
                 
                   τ 
                   ≈ 
                   
                     1 
                     
                       α 
                       × 
                       framerate 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     In an exemplary embodiment:
         β foreground =0.9   α foreground =0.1   β background =0.99   α background =0.01       

     Using Equation 5 and a framerate of 60 Hz,
         τ foreground ≈0.17 seconds   τ background ≈1.67 seconds       

     Thus, in such an exemplary embodiment, the filtered foreground image is approximately a weighted average of the previous 10 frames captured over the previous 0.17 seconds while the filtered background image is approximately a weighted average of the previous 100 frames captured over the previous 1.67 seconds. The difference between the filtered foreground and filtered background images can be determined to calculate an optical gas image of the scene. It will be appreciated that images captured earlier than one time constant prior to the most recent image will still have minor contribution to the filtered image. The contributions from such images will continue to decrease after more time passes, such as after two, three, or more time constants. For example, the contribution of an image after three time constants is approximately 5%. 
     In some embodiments, filter parameters such as α, β, τ, or other higher order IIR filter parameters or FIR filter parameters are applied to the entire image. In other examples, such filter parameters can be applied on a region-by-region or pixel-by-pixel basis within an image. For example, in some embodiments, some pixels of a filtered background image are combined with corresponding pixels in a new image based on a first set of filter parameters, while other pixels of the same filtered background image are combined with corresponding pixels in a new image based on a second set of filter parameters different from the first. 
     The process described with respect to  FIGS.  7  and  8    can be repeated, for example, at the image acquisition framerate of a camera (e.g., 30 Hz, 60 Hz, etc.). Each time a new image is captured, it can be blended with previously established foreground and background images to create an updated filtered image with reduced noise and updated content. The new images can be used to generate updated optical gas images at the same rate that images are captured by the camera. 
     Moreover, in some implementations of such a method, the memory of the camera does not require a buffer of many frames in order to create a filtered foreground image and a filtered background image with reduced noise and sufficiently different to create an optical gas image. In some examples, a camera can include three image streams—a background image stream that is repeatedly updated as new images are captured, a foreground image stream that is repeatedly updated as new images are captured, and an optical gas image stream that is repeatedly updated with the updated foreground and background images. Additionally, only a small amount of processing power is necessary to blend the new image with a previous background image and a previous foreground image to generate new background, foreground, and optical gas images. Thus, in some examples, employing the noise reduction process described in  FIGS.  7  and  8    can reduce the demands for processing power, memory, and/or thermal resolution of a thermal imaging camera for performing adequate optical gas imaging processes. 
     As described elsewhere herein, subtracting foreground and background image data will reveal change in the image (e.g., an evolution of a gas cloud), and will subtract out parts of the image that remain constant. Thus, in some instances, an optical gas image showing gas in an otherwise stationary target scene may exclude important features in the image that are present in both the foreground and background images used to generate the optical gas image and are therefore subtracted out during the generation of the optical gas image. In some such examples, portions of the optical gas image showing an evolving gas cloud will be non-zero, while the rest of the scene, such as any stationary background, equipment, etc. will be approximately zero. 
     Thus, in some examples, the optical gas image can be combined with one or more additional image data streams in order to generate an image that both shows the presence of gas (e.g., from the optical gas image) and also provides context within the scene, such as from infrared and/or visible light image data. For example, in some embodiments, optical gas image data can be combined with visible light image data in order to illustrate the presence/location of gas in a target scene while retaining the scene context provided by visible light image data. Additionally or alternatively, infrared image data can be included to provide thermal information of the scene. In some examples, optical gas image data can be combined with combinations of infrared and visible light image data, such as described, for example, in U.S. Pat. No. 7,994,480, which is assigned to the assignee of the instant application and is hereby incorporated by reference. In various examples, combinations of optical gas image data and infrared and/or visible light image data can be performed using similar techniques as combinations of infrared and visible light image data, such as using alarm thresholds, picture-in-picture display modes, and the like. 
       FIG.  9    is a schematic diagram illustrating combinations of image data that can be used to generate a display image. In the illustrated example, optical gas image data  900 , infrared image data  902 , and visible light image data  904  are subject to image data blending/processing  908  to generate a display image  910 . The image data blending and processing  908  can include a variety of functions, such as combining or overlaying data streams, palettizing/colorizing image data streams, and the like. 
     In some examples, data streams can be processed all at once, such as via a blend of image data. For example, the display image  910  can include a blend of optical gas image data  900 , infrared image data  902 , and visible light image data  904  according to Equation 6:
 
Display=α×OGI+β×IR+γ×VL; α+β+γ=1  (6)
 
     In other examples, optical gas image data  900  can be blended with and/or overlaid onto infrared image data  902 , visible light image data  904 , or a combination thereof. For example, as described elsewhere herein, infrared  902  and visible light  904  image data can be blended together, combined in a picture-in-picture format, or otherwise combined in a variety of ways. Such combined infrared image data  902  and visible light image data  904  can be subsequently combined with optical gas image data  900  for generating the display image  910 . In some such examples, the optical gas image data  900  can be blended (e.g., alpha-blended) with the combined infrared  902  and visible light  904  image data. In other examples, the optical gas image data  900  can be overlaid on top of the combined infrared  902  and visible light  904  image data. 
     In some embodiments, the optical gas image data  900  is palettized according to a palettization scheme prior to combining the optical gas image data  900  with other image data streams such that the gas image data  900  is more easily visible in the display image  910 . For example, in some embodiments, the optical gas image data  900  is palettized according to a palettization scheme that contrasts with the palettization scheme in which the infrared image data  902  is displayed so that the optical gas image data  900  can be readily seen. In other examples, β in Equation 6 is set to zero, so that the optical gas image data is combined with the visible light image data. In some such examples, the optical gas image data can be palettized to contrast with the visible light image data. Additionally or alternatively, the optical gas image data  900  can be combined with some or all of the visible light image data, such as via a picture-in-picture mode. In various examples, the optical gas image data can be overlaid onto the visible light image data and/or blended with the visible light image data. 
     Additionally or alternatively, other image processing techniques on one or more image data streams can be performed. For example, edge enhancement techniques can be employed in order to emphasize the edges of objects and/or a gas cloud in the image data, such as described in U.S. patent application Ser. No. 14/837,757 or 14/222,153, each of which is assigned to the assignee of the instant application and is hereby incorporated by reference. Other image processing techniques can include determining and displaying information regarding the likelihood of observing a gas cloud in a target scene using certain gas detection techniques, such as described in U.S. Pat. No. 9,464,984, which is assigned to the assignee of the instant application and is hereby incorporated by reference. 
     In some examples, a thermal imaging camera can show a real-time optical gas image on a display so that a user may observe the evolution of the gas in the scene over time. For example, in some embodiments, the process of  FIG.  7    is performed every time a new image is captured, and each new optical gas image is presented to the user on the display, for example, as a real-time optical gas video. Such display can be presented as optical gas image data (e.g.,  900 ) alone or as a processed display image (e.g.,  910 ), for example, including visible light and/or infrared image data. In other examples, the thermal imaging camera can present a traditional image on the display, such as an infrared image, a visible light image, or a combination of visible light and infrared image data, while performing optical gas imaging analysis in the background. In some such embodiments, the thermal imaging camera can be used to acquire an optical gas image (or a display image including optical gas image data) even when the real-time display does not necessarily include optical gas image data. 
     In various embodiments, a user may select, for example, via a user interface, whether the real-time display and/or any captured image and/or video data includes optical gas image data. In some examples, capturing image data using a thermal imaging camera comprises capturing infrared image data  902 , visible light image data  904 , and calculated optical gas image data  900  substantially simultaneously. Such image data can be presented in real-time on a display according to a particular blending/processing scheme (e.g., in  908 ), and/or can be combined after the fact in any way desired by the user for display, for example, on the camera itself or a stand-alone work station. In various embodiments, blending/processing techniques employed for real-time display of the display image  910  can be adjustable by a user. In some examples, the available blending/processing techniques are selectable from a series of pre-defined image combination techniques and/or can be fully customizable by a camera operator. 
     Since optical gas imaging typically leverages the ability of the gas to absorb in the infrared spectrum to image otherwise transparent gas, optical gas image data is often generated using infrared image data (e.g.,  902 ). Thus, in some embodiments, visible light image data  904  is not necessarily captured in conjunction with the infrared image data  902  and the generating of optical gas image data  900 . In in exemplary embodiment, optical gas image data  900  is generated according to the process shown in  FIG.  8   . For each iteration of the process of  FIG.  8   , when a new image  804  is captured in order to generate an optical gas image  840 , the new image  804  comprises infrared image data  902  while the corresponding optical gas image  840  comprises the optical gas image data  900  corresponding to the infrared image data  902 . In some such embodiments, visible light image data  904  can be captured along with the infrared image data  902 . Put another way, in some examples, infrared image data  902  and optionally visible light image data  904 , are captured at a framerate (e.g., 60 Hz.). The captured infrared image data  902  is used as the new image  804  in the process of  FIG.  8    for generating an optical gas image  840  including optical gas image data  900 . It will be appreciated that, if visible light image data  904  is used to generate optical gas image data  900 , infrared image data  902  can be excluded and a similar process can be performed using visible light image data  904 . 
     During an exemplary gas imaging process, if the thermal imaging camera is sufficiently stationary (e.g., on a tripod), long time constants τ background  and τ foreground  can be used to increase the amount of filtering and reduce the amount of noise in the filtered foreground and background images. However, as noted elsewhere herein, the time constants τ background  and τ foreground  should be sufficiently different from one another that the difference between the filtered foreground and background images is not approximately zero. 
     Additionally, in some cases, the time constants should not be so long that certain artifacts effectively obscure thermal image data. For example, in some cases, a thermal imaging camera, such as a thermal imaging camera comprising an uncooled sensor assembly, will be subject to internal heating, which can cause the thermal image data to drift over time. In some examples, different image processing techniques are used to reduce such thermal artifacts that appear in the image data. However, if an image filtering time constant is too long (e.g., includes significant contribution from images from a sufficiently long time prior), a combination of scenes with and without, or with different degrees of, thermal drift can make it difficult to compensate for the drift. Accordingly, in some examples, while somewhat long time constants are generally advantageous, time constants can be limited to a maximum value in order to reduce any possible artifacts introduced, for example, by self-heating of the camera. 
     Additionally or alternatively, other events can lead to inaccuracies in the filtered images that include contributions from image data captured over a time period. For example, if the target scene being imaged by a thermal imaging camera changes (e.g., do to camera motion), images representing the previous target scene may still contribute to the filtered images. For instance, if generating filtered background and foreground images that include contributions from image data captured prior to the camera motion, the blending of image data from different scenes will obscure the resulting image. Moreover, comparing such images, for example, the result of subtracting a filtered background image from a filtered foreground image as described elsewhere herein when at least one of the images includes contributions from multiple target scenes will likely be obscured to the point that gas could not be reasonably detected in the difference image. That is, the images would be generally unsuitable for generating an optical gas image until the previous background scene was effectively decayed from the image. 
     Similarly, if a user is holding the camera in his or her hand, the image is likely to be less steady when compared to an image captured by a camera fixed on a tripod. Thus, a long time constant that effectively averages many images into a filtered image is more likely to include images taken from several slightly different perspectives as the camera shakes in the user&#39;s hand. This can also lead to artifacts, clutter, and general inaccuracy in the filtered image(s), and can obscure gas in an optical gas image. 
     Thus, there is a tradeoff between using a enough filtering (e.g., large enough sample of frames, a sufficiently large time constant, etc.) to reduce the noise in the filtered foreground and background images as much as possible, and not using so much filtering to risk imparting artifacts or inaccuracies into a filtered image (e.g., due to camera motion, self-heating, etc.). Moreover, the optimal amount of filtering can depend on a variety of parameters, such as whether the camera is mounted or hand-held, or if the camera moves to image a different target scene. 
     The amount of time required for the previous background scene to effectively decay from the foreground and/or background image depends on the filtering technique employed to generate the filtered foreground and background images. For example, with respect to  FIG.  6   , if frames 1-5 (the frames taken at the earliest time) are averaged to generate the filtered background image from a two second buffer, the background image will include contributions from a scene for up to two seconds after the scene changes. That is, if the camera pointing is changed so that the camera changes from imaging a first target scene to a second target scene, the filtered background image will include contributions from the first target scene for up to two seconds after the camera pointing is changed. Not only will the camera be less capable of performing optical gas imaging for these two seconds, but the generated image will likely be cluttered and difficult to comprehend, since it could be based on image data from two different scenes (e.g., the difference between the first target scene and the second target scene). 
     Similarly, if, for example, Equations 3 and 4 are used to calculate the filtered foreground and background images, respectively, the amount of time after which image data from the first target scene will effectively be decayed from the filtered image is a function of the time constants of the filtering process. As described, generally τ background &gt;τ foreground , and if τ background  is approximately the time it takes for an image to decay to contribute a factor of 1/e into the filtered background image, it will take approximately three time constants (3×τ background ) after changing from the first target scene to the second target scene for the contribution from the previous scene to fall to 5%. In the example described above wherein β background =0.99, this would amount to approximately 1.67×3=5 seconds before the previous scene would decay to a 5% contribution. In some cases, this can become frustrating to the user, since the camera would generally be unable to provide an observable scene or a reliable optical gas image for approximately 5 seconds after the camera is moved. 
       FIG.  10    is an exemplary user interface showing user-selectable options for performing gas imaging operations. In the embodiment of  FIG.  10   , a user may select to enable or disable gas detection mode using ON and OFF buttons. When gas detection mode is off, the thermal imaging camera will not show optical gas image data on the display, for example, when presenting a substantially real-time display of acquired image data. In some examples, when optical gas imaging is turned off, the thermal imaging camera calculates but does not display optical gas image data. In other examples, when the optical gas imaging is turned off, the thermal imaging camera does not calculate optical gas image data. 
     The interface of  FIG.  10    further includes selectable Image Capture or Video Capture modes of operation. The user can select whether saving optical gas image data is accomplished as standalone optical gas images or as a video stream of optical gas images (the optical gas images or video being optionally combined with infrared and/or visible light image data). 
     As described elsewhere herein, motion of the camera during an optical gas imaging process can negatively impact the quality of the image. For example, camera motion can lead to slight changes of the imaged scene over the course of acquiring images that contribute to filtered foreground or background image, which can lead to artifacts present in the image data. In the embodiment of  FIG.  10   , the user is able to select between a tripod mode of operation and a handheld mode of operation. In some examples, each of the tripod mode and handheld mode of operation are associated with predetermined optical gas imaging filtering parameters that affect the amount of contribution of previously captured images. Such predefined filtering parameters can include, for example, the number of frames in buffer  600  that are averaged (e.g., according to Equations 1 and 2) or the time constants τ foreground  and τ background  associated with Equations 3-5 to generate the filtered foreground or background images. 
     Because a thermal imaging camera supported by a tripod will generally experience less inadvertent motion, the time constants and/or number of frames used for generating the filtered foreground and background images can be longer in order to further reduce noise in the image without as many artifacts introduced into the scene due to camera motion. Thus, in some examples, the number of frames from buffer  600  averaged to generate the filtered background image is greater in the tripod mode of operation than in the handheld mode. In other examples, the background and/or foreground time constant (τ background  and/or τ foreground ) associated with the tripod mode is longer than the background time constant associated with the handheld mode of operation. Analogously, the filtering parameter β background_tripod &gt;β background_handheld . 
     Since the foreground time constant is generally shorter than the background time constant, it is generally less prone to developing artifacts over time. Accordingly, in some examples, the foreground time constant (τ foreground ) may be the same for both tripod and handheld modes of operation. In other examples, similar to the background time constant, the foreground time constant is longer in tripod mode than in handheld mode. Similarly, in other examples, the number of images averaged from butter  600  to generate the filtered foreground image in tripod mode can be greater than or equal to the number used in handheld mode. 
     According to the exemplary user interface of  FIG.  10   , a user may select a mode of operation based on the intended use of the thermal imaging camera. The user may disable gas detection mode when using the thermal imaging camera in a traditional thermal imaging mode, and may enable gas detection mode in order to perform an optical gas imaging process. In some examples, the user may elect to operate in image or video modes of gas detection. Further, the user can select whether to operate the thermal imaging camera in tripod or handheld mode, which can dictate which of a predefined set of filtering parameters are used when generating the optical gas image. 
     In an exemplary embodiment, the tripod mode of operation includes blending parameters of β background =0.99 (α background =0.01, τ background ≈1.67 seconds) and the handheld mode of operation includes blending parameters of β background =0.99 (α background =0.01 τ background ≈1.67 seconds). In another example, tripod mode of operation includes blending parameters of β background =0.96875 (31/32) and β foreground =0.9375 (15/16) and the handheld mode of operation includes blending parameters of β background =0.875 (7/8) and β foreground =0.75 (3/4). 
     In some examples, the optical gas detection interface further includes a “custom” option in which a user can customize setting used for performing optical gas imaging. For instance, in some such embodiments, the thermal imaging camera can be configured to perform image filtering during an optical gas imaging process using one or more adjustable filter parameters. Such adjustable filter parameters can include the number of frames averaged from buffer  600  to generate the filtered background and/or foreground images or the blending ratios, α and β, and similarly the time constant, τ, of Equations 3-5. Other filtering parameters, for example, relating to higher order IIR filters or the like, can similarly be adjusted by the user. Additionally or alternatively, a user may be able to select a type of filter used to generate the filtered background and/or foreground images, such as selecting between any one or more of a FIR filter, a first order IIR filter, and a higher order IIR filter. Other user-adjustable parameters can include blending and/or processing parameters (e.g.,  908 ) for generating a display image  910  from the optical gas image data  900  and other image data streams. 
     As described, the camera can be programmed with predefined filtering parameters that can be used when operating in particular modes of operation in which camera motion is more or less likely (e.g., handheld operation vs. tripod operation). However, even within such modes of operation, the camera may move a significant amount, such as panning from one scene to another or as a user moves to a new imaging location. This motion can similarly cause the target scene to change, resulting in possible contributions to a filtered image from different scenes and resulting in a filtered image that is cluttered and inaccurate with respect to the current target scene for the camera. 
     As described elsewhere herein, adjusting the filtering parameters (e.g., a number of frames averaged from a buffer  600 , parameters α, β, τ from Equations 3-5, etc.) can affect the impact of motion in the target scene by reducing the contribution from frames from an earlier time. For example, with respect to  FIG.  6   , averaging frames 60-120 at a framerate of 60 Hz will only average images going back one second, rather than averaging, for example frames 1-120, which will average images going back two seconds. Thus, image contribution from a previous target scene will impact the filtered image for less time when filtering over the more recent portion of the buffer. 
     Similarly, with respect to the filtering described with respect to Equations 3-5, a time constant of approximately 1.67 seconds (β=0.99, α=0.01) will result in a filtered image having contributions of images of approximately 5% or more over approximately the previous 5 seconds. By contrast, a time constant of approximately 0.17 seconds (β=0.9, α=0.1) will result in a filtered image having contributions of images of approximately 5% or more over approximately the previous 0.5 seconds. Thus, reducing β (or analogously, increasing α or decreasing τ) reduces the contributions of images from images going back further in time. 
     In addition to gas absorption and attenuation of infrared radiation, gas present in a target scene also emits infrared radiation that may be detected via an infrared imaging device. For instance, with respect to  FIGS.  5 A and  5 C , beam  574   c  received at X having amplitude A/2 will generally be due to both attenuation of infrared radiation emitted from source  572  (amplitude A) by gas cloud  570   a  and infrared radiation of gas cloud  570   a  itself. 
     However, issues can arise if the gas cloud (e.g.,  570   a ) absorbs and emits infrared radiation equally at a given wavelength, since the impact of the radiation absorption and emission by the gas would negate one another and would be invisible to an infrared imaging camera. For example, with respect to  FIG.  5 A , if the gas attenuates radiation from source  572  by half (e.g., from amplitude A to amplitude A/2), but also emits infrared radiation at amplitude A/2, then the total infrared radiation received at X in region  582  will be A/2 (from scene  572 )+A/2 (emitted from gas)=A, and therefore the gas will have no impact on the imaged scene. Put another way, the gas will provide insufficient contrast from the background to observe gas in the scene, even using various processing techniques such as image subtraction to emphasize the location and or motion of the gas. It will be appreciated that the relative amount of emission and attenuation with respect to the amplitude A of the source  572  are arbitrary and used for ease of explanation. Often, the amount of attenuation and/or emission of infrared radiation due to the presence of gas will be very small compared to the magnitude of the infrared radiation emitted from the background (e.g., source  572 ), making it generally difficult to image gas using standard imaging techniques, for example, viewing infrared image data without performing a background subtraction process such as those described elsewhere herein. 
     In some cases, the amount of contrast caused by gas in an infrared image, or whether the gas will be observable in infrared, is dependent on three main factors. The type of gas impacts the absorption/emission lines within the waveband of the imaging device. The amount of gas, which is dependent on both the concentration of gas within a gas plume and the size (path length) of the gas plume through which the received infrared radiation travels, generally represents the number of gas molecules experienced by the light before the light reaches a camera. In some examples, this can be quantified as a concentration path length (e.g., in ppm-meters). Finally, the difference between the temperature of the gas and the apparent temperature of the scene background can impact the ability to view the gas. The apparent background temperature can be, for example, the calculated temperature of the background based on the received infrared radiation (in some cases, the apparent temperature is calculated based on received radiation and the expected emissivity of the object, and may not be exactly equal to the actual temperature due to variations in the emissivity of the background). Mathematically, in some cases, visual contrast in an infrared image due to gas in the target scene is a function of these features:
 
contrast= f (gas_type,gas_amount,| T   gas   −T   background )
 
     The amount of contrast in an image to qualify as making an object in the image “visible” can be quantified, for example, as a signal to noise ratio, wherein the level of the signal (in some cases, the signal due to gas, for example, in an optical gas image) must be significantly large when compared to the noise present in the image. Put another way, the apparent temperature difference between subtracted pixels in an optical gas image must be large enough with respect to the noise equivalent temperature difference (NETD) in the optical gas image in order to reasonably observe the effect of the gas on the optical gas image data. 
     When viewed in the context of optical gas image data as described elsewhere herein and in U.S. Provisional Application No. 62/507,203, filed May 16, 2017, hereby incorporated by reference in its entirety, wherein a difference between a foreground image and background image is calculated to emphasize motion of gas within the scene, the subtraction eliminates much of the stationary or unchanging signal from the overall scene since the effect of gas on the overall image data is relatively small. Thus, in some such examples, the remaining data in the optical gas image (after the subtraction) generally includes the differences in the scene due to moving gas and remaining noise. The gas will be “visible” if the remaining gas signal is sufficiently large compared to the remaining noise. In some cases, a signal-to-noise ratio of 1.0 is adequate for observing data representing gas in an optical gas image in view of the noise present in the optical gas image, though in some cases, a user may observe the effect of gas at signal-to-noise ratios less than 1. 
     Generally, the amount of contrast provided by the gas increases with the amount of gas and with the difference between the temperature of the gas and the apparent background temperature. Thus, in such examples, as the amount of gas increases, the contrast caused by the gas increases. Similarly, in such examples, as the difference between the gas temperature and the apparent background temperature increases, the contrast caused by the gas increases. Accordingly, in such examples, given a particular type of gas, in order to meet a minimum contrast threshold desired to ensure visibility of the gas, the gas amount and temperature difference must be sufficiently high. Further, in some examples, if there exists a minimum amount of gas that must be made sufficiently visible during gas imaging (e.g., a threshold level of gas above which concentrations may become hazardous), the difference between the gas temperature and the apparent background temperature must be sufficient to ensure that the minimum amount of gas results in a level of contrast that can be observed. 
     Various standards exist in gas imaging practices that require particular types of gas to be visible at particular concentrations or amounts. For instance, the OOOOa (Quad-OA) standard requires optical gas imaging to be able to “see” a 50/50 mix of methane and propane at 10,000 parts-per-million (ppm) leaking from a ¼″ diameter orifice at a flow rate of 60 grams/hour. This standard provides a type of gas (methane and propane) and an amount of gas (10,000 parts-per-million (ppm) leaking from a ¼″ diameter orifice at a flow rate of 60 grams/hour). Per the relationships described above, given these standards, in order for the contrast caused by the gas to reach a level to where the gas is visible, the difference between the gas temperature and the apparent background temperature must be sufficiently large. 
     In some embodiments, a thermal imaging camera performing an optical gas imaging process such as described herein and in U.S. Provisional Application No. 62/507,203, filed May 16, 2017, hereby incorporated by reference in its entirety, continually acquires new images (e.g., step  706  in  FIG.  7   ) for combining with the previous foreground and background images to establish new filtered foreground and background images to compare to establish a new optical gas image. In some examples, each new image acquired via the thermal imaging camera can be radiometrically calibrated to provide an apparent temperature value for each of a plurality of pixels in the image. Thus, by virtue of acquiring new images for performing an optical gas imaging process, the thermal imaging camera also acquires information regarding the apparent scene temperature. This information can be saved in memory, temporarily or for long-term storage, for further analysis, for example, as described below. 
     As described above, the difference between a gas temperature and the apparent background temperature observed by a thermal imaging camera can dictate whether or not a given amount and type of gas can effectively be imaged using the thermal imaging camera. In some embodiments, a thermal imaging camera can be configured to receive a gas temperature input representative of the temperature of a gas located in the target scene. In various examples, the gas temperature input can be received in a variety of ways. For instance, in some embodiments, a thermal imaging camera can include or otherwise be in communication with an air temperature measuring tool, for example, as described in U.S. application Ser. No. 14/855,884, filed Sep. 16, 2015, hereby incorporated by reference in its entirety. In some such examples, a measured air temperature can be used as the gas temperature input when the gas in the environment is likely to quickly equilibrate to approximately air temperature, making the gas temperature and the air temperature approximately equal. Additionally or alternatively, the thermal imaging camera could be used to capture thermal image data representative of a reservoir containing gas that is to be looked for in a gas imaging procedure, wherein radiometric infrared image data of the tank could be used as a gas temperature input. Additionally or alternatively, a gas temperature input value could be entered manually by a user. 
     The thermal imaging camera can be configured to, for example, via the processor, compare temperature values associated with each of a plurality of pixels in the radiometric infrared image data with the gas temperature input value to determine, for each such pixel, the difference between the gas temperature and the apparent temperature of the IR image data. As described above, for a given gas type and amount, whether or not the gas can be observed via the infrared imaging camera (has sufficient contrast) is dependent upon this temperature difference. Accordingly, the camera can be configured to analyze each pixel and determine whether or not the difference between the gas temperature and the apparent background temperature at that pixel is large enough so that the pixel would show a predetermined minimum amount of contrast for the identified gas amount and type. In some embodiments, the thermal imaging camera can be configured to modify the display of pixels lacking sufficient difference between the gas temperature and the apparent background temperature to achieve a minimum contrast in the infrared image data for a given type of gas and minimum amount of gas to be detected. 
     The thermal imaging camera can determine whether or not the difference between the gas temperature and the apparent background temperature is large enough to achieve a desired contrast value can be performed in a variety of ways. In some embodiments, the camera can be programmed with an equation that relates, for a given type of gas, a quantitative measure of a minimum amount of gas to be observed, a quantitative measure of a minimum amount of contrast desired in the image, and a temperature difference. Additionally or alternatively, the thermal imaging camera can include one or more lookup tables in memory that can be used to identify, for a given type of gas and a minimum amount of gas to be observed, the required difference between the gas temperature and the apparent background temperature to achieve a minimum contrast amount to observe the gas. 
       FIG.  11    shows an exemplary lookup table for determining a required difference between the gas temperature and the apparent background temperature in order to achieve a certain minimum contrast due to a given gas type and gas amount. In the exemplary lookup table of  FIG.  11   , N different gas types and M different amounts of gas are included. The intersection in the table of a gas type (e.g., gas type n) and a gas amount (e.g., gas amount m) provides a minimum temperature difference value (e.g., ΔT (n, m)) required in order to achieve sufficient contrast in order to observe the given gas type at the given gas amount. 
     In some embodiments, lookup tables such as the table shown in  FIG.  11    can be generated empirically, for example, during a factory calibration of a thermal imaging camera. In some such examples, a technician can observe various combinations of gas types, gas amounts, and differences between the gas temperature and the apparent background temperature to determine the temperature difference (ΔT) at which a given gas type and gas amount provides sufficient contrast in an optical gas image. In various embodiments, sufficient contrast can be identified subjectively by the user, wherein the user observes the point at which the given gas in the given amount becomes sufficiently salient in the image. In other examples, statistical analysis can be performed on the image data to determine a point at which the contrast due to gas in the optical gas image causes statistically sufficient contrast. In some embodiments, different lookup tables can be used for different amounts of contrast and/or for different methodologies of establishing “sufficient” contrast to be able to easily observe gas. In some embodiments, a thermal imaging camera can provide an end-user with the option to perform a self-calibration, in which the user generates a lookup table such as the one shown in  FIG.  11   . In cameras including a plurality of lookup tables associated with different levels and/or methodologies for determining sufficient contrast to observe gas, a user may be provided with an option to select a table for use. For instance, in some examples, different tables such as the table shown in  FIG.  11    can be used when the camera is in a handheld mode vs. a tripod mode. 
     Once the thermal imaging camera determines, for instance, via the processor, which pixels do and do not include an apparent background temperature sufficiently different from a gas temperature, and therefore which pixels would and would not include sufficient contrast caused by gas in the scene. The processor can be configured to, when generating optical gas image data and/or display image data including optical gas image data, display pixels that do not include sufficient temperature difference in a visibly distinguishable way compared to pixels that do include sufficient temperature difference. That is, those pixels in which a selected amount and type of gas would provide sufficient contrast given the difference between the gas temperature and the apparent background temperature appear differently from those pixels that would not provide sufficient contrast. This displayed information can be presented to a user in substantially real time, such as via a live video. This way, a user may quickly observe where in the scene the desired type and amount of gas would even be possible to view (that is, provide sufficient contrast) and where the gas (e.g., at the minimum desired amount) would not be possible to view (that is, would not provide sufficient contrast). 
       FIG.  12    is a process-flow diagram showing an exemplary process for determining whether or not a pixel can be reliably used to detect a given gas type and amount and for displaying such determinations. The process of  FIG.  12    comprises receiving a selected gas type and gas amount ( 1200 ). In some examples, a thermal imaging camera can be pre-programmed to search for a minimum amount of a predetermined type of gas. In other examples, a camera can be programmed with a list of gas types (e.g., individual gasses, mixtures of gasses, etc.) and/or minimum amounts of gas to be observed via gas imaging from which a user can select the desired gas type and/or amount. 
     In various examples, the “amount” of gas can be identified in a variety of different ways. For instance, in some examples, an amount can correspond to a concentration path length of the gas. In other examples, an amount can include a combination of specific parameters, such as concentration, flow rate, and/or orifice leak size, such as may be defined in a particular gas observation standard (e.g., OOOOa). In some such examples, a single value for an “amount” of gas need not be calculated, but can be a combination of values. Such parameters can be pre-programmed into a camera (e.g., a camera programmed to meet OOOOa standards) or can be entered by a user (e.g., by selecting parameters individually or by selecting a predefined set of parameters, such as defined by OOOOa standards). 
     Additionally or alternatively, other factors can contribute to whether or not a gas will be visible in a scene, such as wind speed, distance from suspected leak source, or the like. In some examples, a thermal imaging camera can include or otherwise be in communication with one or more tools to measure one or more such parameters, such as an anemometer, distance meter, or the like. Additionally or alternatively, some such parameters can be entered manually by a user. Such data may impact the amount of gas that can be scene given a certain temperature difference (ΔT), or similarly, the amount of temperature difference that is necessary to view a given amount of gas. Thus, in some embodiments, such parameters can be used to determine the temperature difference necessary to observe a given gas amount/type. 
     As described, several parameters can be used to determine, for example, an amount of temperature difference necessary to observe a given type of gas at a given amount. In some such embodiments, one or more such parameters (e.g., wind speed, distance, etc.) can be selectively incorporated into the analysis by a user selection. For instance, a user may choose to incorporate a wind speed measurement into the determination of the minimum temperature difference necessary to observe a given gas type in a given amount, or may choose to ignore such data. In various examples, determination of the minimum temperature difference necessary to observe a given gas type in a given amount can be performed using an equation including a plurality of variable or a multi-dimensional lookup table to accommodate for additional parameters. For instance, in some examples, each of a plurality of lookup tables such as that shown in  FIG.  11    can correspond to ΔT values for a given wind speed and/or distance to target. 
     The method further comprises determining a minimum temperature difference (e.g., the minimum difference between a gas temperature and an apparent background temperature) necessary to exceed a contrast threshold in a gas imaging process ( 1202 ). In some examples, the contrast threshold can be a received contrast threshold selection ( 1201 ). In other examples, the contrast threshold can pre-set within the thermal imaging camera, for example, based on a factory calibration or a previous user calibration. Determining the minimum temperature difference to exceed the contrast threshold can be performed, for example, using a lookup table (e.g., as shown in  FIG.  11   ), using an equation, or the like. In an exemplary embodiment, a lookup table used to determine the minimum temperature difference ( 1202 ) corresponds to a selected gas type and/or gas amount. 
     The method further includes the steps of determining a gas temperature ( 1204 ) and acquiring radiometrically calibrated infrared image data of a scene ( 1206 ). As described elsewhere herein, in various embodiments, determining a gas temperature can include measuring a gas temperature, for example, using an air temperature measurement tool, or manually receiving a user-entered gas temperature value. The radiometrically calibrated infrared image data can be captured using the thermal imaging camera. 
     The process includes selecting a pixel from the acquired infrared image data ( 1208 ) and determining the difference between the determined gas temperature and an apparent temperature of the selected pixel ( 1210 ). The method further includes determining if the temperature difference (ΔT) is above the determined minimum temperature difference ( 1212 ). If so, the pixel can reliably display the selected gas type at amounts equal to or greater than the selected amount ( 1214 ), and the pixel is displayed normally according to the display settings of the optical gas imaging process. However, if the temperature difference is below the determined minimum, the pixel cannot reliably display the selected gas present at the selected amount in the image ( 1218 ), and is presented in a distinguishable way in an optical gas image view ( 1220 ). As shown, the process can be repeated for a plurality of pixels in order to identify which of a plurality of pixels may be suitable for performing reliable imaging of the selected gas type at the selected amount. It will be appreciate that, while shown in  FIG.  12    as analyzing a plurality of pixels serially (e.g., determining whether or not the ΔT value is above a minimum for a given pixel, then selecting a new pixel and repeating the analysis), in various examples, such analysis can be performed for a plurality of pixels in parallel. For instance, in some examples, the process shown in  FIG.  12    can be performed for one or more pixels simultaneously or substantially simultaneously. In some such examples, every pixel to be analyzed can be analyzed simultaneously or substantially simultaneously. The process can be repeated temporally to provide a real-time or substantially real-time pixel analysis. 
     During an exemplary use case, with reference to  FIG.  12   , a user wishes to observe methane (type of gas) at amounts equal to or greater than 10,000 ppm-meters (gas amount represented as a concentration path length), and inputs such parameters into a thermal imaging camera that receives the parameters at step  1200 . The camera  1202  determines, for example, via a lookup table, the minimum temperature difference between the gas and the scene in order to achieve sufficient contrast to reliably view the methane at 10,000 ppm-meters. The thermal imaging camera can determine gas temperature information from an air temperature measurement tool in communication (wired or wireless) with the camera, and captures radiometrically calibrated infrared image data of a scene. 
     The camera compares, for each of a plurality of pixels, the temperature associated with the image data at that pixel with the gas temperature, and compares the temperature difference with the determined minimum temperature difference. If, for a given pixel, the difference between the apparent temperature in the image data and the gas temperature is above the minimum, then methane gas at 10,000 ppm-meters (or greater) present at that pixel would provide visible contrast in the optical gas image. On the other hand, if the difference between the apparent temperature in the image data at a given pixel and the gas temperature is below the minimum, then methane gas at 10,000 ppm-meters (or greater) present at that pixel would not provide visible contrast in the optical gas image. 
     In a displayed image, those pixels in which the selected gas at the selected amount may be visible (e.g., due to sufficient contrast) will be displayed in a distinguishable way from those pixels in which the selected gas at the selected amount will not be visible (e.g., due to insufficient contrast). The displayed image that shows those pixels with sufficient temperature difference to observe the selected gas/amount differently from those without sufficient temperature difference to observe the selected gas/amount can include optical gas image data (e.g.,  900  in  FIG.  9   ) and/or a display image (e.g.,  910  in  FIG.  9   ). 
     In an exemplary embodiments, pixels that are determined to include sufficient temperature difference in order to achieve sufficient contrast to observe the selected gas/amount are presented normally according to a prescribed display scheme (e.g., with a certain palettization, a certain blending ratio between different data streams, etc.). In some such examples, pixels that are not expected to show sufficient contrast to observe the selected gas/amount can be replaced by or blended with a predetermined indicator color to clearly convey to the user that such pixels do not include a background scene that is adequate for producing enough contrast to be able to observe the selected gas/amount. 
     In various examples, such a predetermined indicator color can be manually selected by a user or may be automatically programmed into the camera. In some examples, an indicator color is automatically chosen so that the indicator color clearly contrasts with a palettization scheme otherwise used in the image. For instance, when utilizing a grayscale palette for performing a gas imaging procedure, the pixels in which the selected gas/amount would not produce sufficient contrast could be replaced with or blended with a non-gray color, such as red, to clearly indicate the presence of such pixel and the inability to reliably view the selected gas at the selected amount at such pixels. 
     In some examples, a plurality of indicator colors may be used. For example, in some embodiments, a scene analysis mode may be initiated, in which pixels lacking sufficient temperature difference to reliably display the selected gas at the selected amount may include a first indicator color and pixels having sufficient temperature difference to reliably display the selected gas at the selected amount may include a second indicator color. Thus, a user may be able to view a scene and easily distinguish between portions of the scene in which the selected gas at the selected amount can and cannot be reliably viewed. 
     Additionally or alternatively, in some embodiments, any number of indicator colors may correspond to an equal number of temperature difference values to which the difference between the gas temperature and the apparent pixel temperature can be compared. This can be used to establish a range of likelihood for the ability to view a selected gas at a selected amount in a particular scene. 
     With reference to  FIG.  9   , when performing a gas imaging process, a user may view real-time live images on a display in order to inspect for gas in the target scene. In various embodiments, the user may choose to view a blend of optical gas image data  900 , infrared image data  902 , and visible light image data  904  together on the display. In some examples, the indicator color can be blended with or otherwise replace pixels in the optical gas image data  900  prior to any combination with other image streams to produce the display image  910 . In some such examples, the indicator color may be selected (manually or automatically) to contrast with both the optical gas image palettization and the infrared image palettization schemes. In other examples, the indicator color may be blended with or otherwise replace such pixels in the display image  910  after processing/blending  908  of the optical gas image data  900 , infrared image data  902 , and visible light image data  904 . 
     In some examples, during a gas imaging process, the user may choose to exclusively view the optical gas image data  900  on the display. In such examples, the indicator color is combined with or used to replace pixels in the optical gas image data  900  for convenient display. 
     In some embodiments, given a fixed contrast threshold and gas temperature and a given gas type and gas amount, the only contributing factor to whether or not the difference between the gas temperature and the apparent scene temperature for a given pixel meets the minimum temperature difference is the apparent temperature of the background. That is, in some examples, whether or not a selected gas type and amount can produce sufficient contrast to be observed (e.g., step  1212  in  FIG.  12   ) is dependent only on the apparent temperature at a given pixel. Thus, adjusting the positioning of the camera to change the background scene will change which pixels have an apparent temperature that would yield sufficient contrast given the selected gas type and amount. Accordingly, during use according to some embodiments, a user may notice that a large number of pixels on the display include the indicator color, suggesting that such pixels would not provide adequate contrast to show the selected gas at the selected amount whether or not the gas were present. Thus, the user may choose to reposition the camera in order to change the background to increase the number and/or location of the pixels that would provide adequate contrast to show the selected gas at the selected concentration. In doing so, the user may be provided by substantially real-time updates displaying not only the presence of gas in the scene, but also the portions of the scene in which a selected gas may not be visible at a certain amount due to insufficient background temperature contrast. 
       FIG.  13    is a schematic illustration of a process for real-time generation of an optical gas image including information regarding whether or not a given gas type and gas amount is observable at various locations within the optical gas image. Similar to the process illustrated in and described with respect to  FIG.  8   , the process of  FIG.  13    includes using a filtered foreground image and a filtered background image to generate (e.g., via subtraction) an optical gas image  1340 . A new image  1304  is captured and combined with a previous/existing filtered background image  1302  to create a new filtered background image  1330 . The new image  1304  is similarly captured and combined with a previously/existing filtered foreground image  1300  to create a new filtered foreground image  1320 . The new filtered foreground  1320  and background  1330  images can be used to generate an optical gas image  1340 . In some embodiments, the new foreground  1320  and background  1330  image become the previous foreground  1300  and background  1302  images, respectfully, during subsequent iterations of the process when a new image  1304  is captured. Details of how, according to various embodiments, the new image  1304  is combined with the previous/existing foreground  1300  and background  1302  images to arrive at the new foreground  1320  and background  1330  images, as well as how the optical gas image  1340  is created from the new foreground  1320  and background  1330  images, are described elsewhere herein (e.g., with respect to  FIG.  8   ), and are not repeated here. Instead, the process is summarized at a high level for convenience in box  1335  in  FIG.  13   . 
     In some examples, in addition to being used in the optical gas imaging process (shown generally in box  1335 ), the new image  1304  can be processed as described above in order to determine where in the image data a certain type and amount of gas can be observed. In the illustrated example of  FIG.  13   , the apparent pixel temperature for a variety of pixels in a new infrared image  1304  is compared with a gas temperature value  1310  to establish a difference between the gas temperature  1310  and the apparent background scene temperature from new image  1304  at each of a plurality of pixels, generally shown as ΔT  1312 . In some embodiments, the value of the gas temperature  1310  comprises a single value, such as measured via an air temperature sensor, manually entered, or the like, while the apparent pixel temperature from the new image  1304  comprises a two-dimensional array of temperature values within the new image  1304 . In such examples, the temperature difference values at ΔT  1312  will also be a two-dimensional array of values, representing the difference between the gas temperature  1310  and each apparent pixel temperature from new image  1304 . 
     Each value of ΔT  1312  can be compared to a ΔT minimum  1350 , which can be dependent on a particular gas type  1352  and gas amount  1354  as described elsewhere herein, to determine whether or not the value of ΔT is above the minimum value ( 1360 ). In the illustrated example, if the ΔT value  1312  for a given pixel is above the ΔT minimum  1350 , then that pixel will include sufficient contrast when observing the gas type  1352  and amount  1354  associated with the ΔT minimum  1350 . However, if, for the given pixel, the ΔT value  1312  is not above the ΔT minimum  1350 , then the corresponding pixel in the optical gas image  1340  can be made to be visually distinguishable from pixels having a corresponding ΔT value  1312  above the minimum  1350 , for example, by including (e.g., via blending or superimposition of) an indicator color as described elsewhere herein. This process can be performed for each of a plurality of pixels (in series or in parallel processing of two or more pixels) and repeated for each of the plurality of pixels to provide a real-time or substantially real-time optical gas image  1340 . 
     The process shown in  FIG.  13    can be repeated each time a new image  1304  is acquired, for example, at the framerate of the thermal imaging camera. In some such examples, the process is repeated at a sufficient frame rate in order to generate a real-time or approximately real-time display of optical gas image data  1340 , including an indication (e.g., via an indicator color) of which locations in the image data may not be able to adequately display the selected gas and amount. 
     As noted elsewhere herein, a user of the thermal imaging camera viewing an optical gas image (or a display image including optical gas image data) may observe the pixels identified as lacking sufficient temperature difference to show sufficient contrast for a selected gas type and amount, and may choose to adjust the position of the camera in order to update the background scene in an effort to reduce the number of such insufficient pixels. Additionally or alternatively, in some embodiments, the thermal imaging camera, while producing optical gas images or display images including optical gas image data in real time or approximately real time, can analyze the image to determine information related to the number and/or location of pixels deemed to have insufficient temperature difference to observe the selected gas type and amount. In some such examples, the thermal imaging camera can be configured to prompt the user to adjust the location of the camera in order to improve the ability for the thermal imaging camera to image the selected gas at the selected amount. 
     For instance, in an exemplary embodiment, the thermal imaging camera can be configured to track the number of pixels having a difference between the gas temperature and the apparent background scene temperature below the determined minimum temperature difference for a selected type and amount of gas. If the number of pixels having a temperature difference below the minimum is greater than a predetermined threshold (e.g., above a predetermined percentage of analyzed pixels, above a predetermined total number of pixels, etc.) the camera may output an alert (e.g., audio alert, visual alert, vibrational alert, etc.) to the user suggesting that the user reposition the camera to include a different background scene and potentially increase the number of pixels having temperature difference above the threshold. 
     Additionally or alternatively, in some examples, the camera can determine the location of the pixels lacking sufficient temperature difference to be likely to observe the selected gas at the selected amount, and suggest a user reposition the camera if the pixels are located in identified locations. For instance, in some embodiments, if the center of the image data (e.g., a circular, rectangular, or other shaped region of pixels approximately in the center of the image data) includes a sufficiently large number of pixels having temperature difference below the minimum, the camera may suggest the user reposition the camera in an effort to improve the likelihood to observe the selected gas at the selected amount in the center of the image. 
     It will be appreciated that, while often described with respect to determining which pixels include sufficient difference between a gas temperature and an apparent background temperature for performing gas imaging, in some embodiments, such analysis can be performed in a plurality of regions that are not necessarily single-pixel regions. For example, in some embodiments, a region comprising a plurality of pixels can be assigned an apparent temperature (e.g., a minimum temperature, a maximum temperature, or an average temperature of the pixels within the region) that can be compared to the gas temperature. Similar analysis as described above can be applied to the region as a whole rather than each pixel individually, for example, to reduce the number of times the analysis is performed on a given image and save on processing time and requirements. In general, regional analysis can be performed on a plurality of regions, each including one or more pixels, wherein such regions need not be the same size. 
     In some embodiments, a thermal imaging camera can be configured to detect motion of the camera. For example, the thermal imaging camera can include one or more accelerometers or other motion sensors (e.g., MEMs, gyros, etc.) configured to sense motion of the camera. Such sensors can provide data to the processor, which can use the data to determine that the camera has moved or is moving. Additionally or alternatively, the camera can be configured to analyze captured frames to determine inter-scene motion, such as using image registration processing to determine an amount the camera has moved from frame to frame. In some embodiments, the thermal imaging camera can use detected motion data to determine if the camera has moved a sufficient amount to impact the efficacy of the current filtering parameters for generating a high quality optical gas image without clutter or artifacts from a previous target scene. 
     For instance, in some examples, optical gas imaging processes (e.g., as shown in and described with respect to  FIG.  8   ) comprise subtracting images from one another, leaving only the difference remaining. However, if this difference has relatively low magnitude (e.g., due to a small amount of attenuation due to the presence of an absorbing gas in the scene), the difference image may be difficult to observe without amplifying the signal, or analogously in a thermal image data processing sense, decreasing the span of temperatures (e.g., temperature differences) visible in the display palette in order to distinguish the signal from the background. This results in a strong emphasis of motion in the optical gas image, which often is beneficial in attempting to view gas (e.g., a target gas), but also often magnifies undesired movement, such as due to camera motion. 
       FIG.  14    is a process flow diagram illustrating a process for accommodating for camera motion during an optical gas imaging process. The method includes determining the current mode of operation of the camera ( 1400 ), such as, for example, a tripod mode or a handheld mode, and setting filtering parameters for generating filtered background and foreground images ( 1402 ). As described elsewhere herein, such filtering parameters can be predefined parameters based on the current operating state of the thermal imaging camera. 
     The method includes the step of acquiring images and generating optical gas imaging data ( 1404 ). This can include, for example, performing any of the processes described herein, such as using one or more of Equations 1-5 or  FIGS.  7 - 9   . The thermal imaging camera can continue acquiring images (e.g., new image  804  in  FIG.  8   ) and generating optical gas image data (e.g., optical gas image  840 ), for example, at the image acquisition frame rate of the thermal imaging camera. 
     While performing the optical gas imaging process, the camera can analyze motion data of the camera ( 1406 ). Motion data can be provided by any of a variety of sources, for example, one or more hardware components (e.g., accelerometers, etc.), image analysis (e.g., registration processing) or the like. The camera can determine if excess motion is present ( 1408 ). In some embodiments, the camera compares an amount of motion (e.g., an amount of rotation, an amount of linear motion within the image data in consecutive scenes based on registration analysis, etc.) to a first threshold. If no excessive motion of the camera is detected, the camera proceeds to acquire images and generate optical gas imaging data ( 1404 ) according to typical operation. 
     However, if excess motion is detected, the camera can temporarily adjust or disable the filtering parameters used in the optical gas imaging process ( 1410 ). For example, if performing an first order IIR filtering process according to Equations 3-5, upon sensing excessive motion in step  1408 , the camera can reduce the value of β (particularly β background ) in order to reduce or eliminate the contribution of frames acquired prior to the detected excessive motion to the filtered images. Doing so can help eliminate clutter or artifacts in the image that may confuse or annoy the user. 
     In some examples, temporarily reducing the filtering parameters comprises reducing the background and/or foreground filtering parameter β to a predetermined value. In some such examples, the predetermined value is zero so that the optical gas image only comprises the most recently acquired image and does not include historical data from before the camera motion took place. 
     In some examples, such reduction in the filtering parameters occurs for a predetermined period of time, such as for one second, half of a second, or the like. In some such embodiments, after the expiration of such period of time, the filtering parameters are reset according to the normal mode of operation ( 1412 ) such as established in step  1400  and optical gas imaging continues at step  1404 . 
     In other examples, after adjusting the filtering parameters ( 1410 ), the camera can continue to analyze motion data ( 1406 ) as represented by the broken line between  1410  and  1406 , and determine if excess motion is still present ( 1408 ). If so, the camera maintains the reduced filtering parameters and continues to analyze the motion until the detected motion (e.g., between consecutive frames) reduces below a second threshold. In some such examples, the second threshold can be equal to the first threshold, or can be different from the first threshold. If excessive motion is no longer detected at  1408 , then filtering parameters are reset according to the mode of operation ( 1412 ) before optical gas imaging is resumed. 
     In some examples, scene changes that contribute to clutter or confusion in a filtered image (e.g., a filtered background image) are due to changes other than camera motion. Thermal changes in the target scene effectively change the image data captured across a plurality of frames, which can lead to similar errors brought about by camera motion as described elsewhere herein. For example, if a person unexpectedly walks through the target scene, residual thermal artifacts from the person may remain in the filtered images even after the person departs the target scene. In an exemplary embodiment, if a person moves into the target scene, thermal image data representing the person will generally build up in the filtered background image over the course of a few time constants. However, since the foreground time constant is typically shorter than the background time constant, thermal image data representing the person will build up in the filtered foreground image quicker than in the background image. Thus, when comparing the filtered background and foreground images to generate optical gas image data, the differences between the thermal signature of the person in the background and foreground images will make the moving person appear in the optical gas image. 
     The same principle will apply with any substantive thermal change in the target scene. Similar to motion of the camera, changing of the thermal characteristics of the target scene will tend to cause cluttering of the image due to residual artifacts of the previous scene prior to the changes of the thermal data. As with the motion described elsewhere herein, such residual artifacts will typically last for several time constants until eventually being decayed from the filtered image data. 
     Artifacts in the filtered images due to thermal changes in the target scene can be addressed in a similar manner to addressing artifacts due to scene motion (e.g., camera motion/jitter).  FIG.  15    is a process flow diagram illustrating a process for accommodating for thermal change during an optical gas imaging process. The method of  FIG.  15    is similar to that shown in  FIG.  14   . The method similarly includes determining the current mode of operation of the camera ( 1500 ) and setting filtering parameters for generating filtered background and foreground images ( 1502 ). The method includes the step of acquiring images and generating optical gas imaging data ( 1504 ). 
     According to the method of  FIG.  15   , while performing the optical gas imaging process, the camera can analyze thermal data of the target scene being imaged by the camera ( 1506 ). Thermal changes can be caused by any variety of sources, such as an object entering or leaving the scene, or an event occurring in the scene causing a significant thermal change. The camera can determine if excess thermal changes are present ( 1508 ), for example, by comparing detected changes to a threshold value. Threshold values can include, for example, an absolute temperature change exceeding a threshold or a rate of change of temperature exceeding a threshold. In an exemplary embodiment, the camera compares the rate of change of temperatures (e.g., frame-to-frame changes) in the target scene to a first threshold. If no excessive thermal changes are detected, the camera proceeds to acquire images and generate optical gas imaging data ( 1504 ) according to typical operation. 
     However, if excess thermal changes are detected, the camera can temporarily adjust or disable the filtering parameters used in the optical gas imaging process ( 1510 ). For example, if performing a first order IIR filtering process according to Equations 3-5, upon sensing excessive thermal changes in step  1508 , the camera can reduce the value of β (particularly β background ) in order to reduce or eliminate the contribution frames acquired prior to the detected excessive thermal change to the filtered images. Doing so can help eliminate clutter or artifacts in the image that may confuse or annoy the user. 
     As described elsewhere herein, in some examples, filter parameters (e.g., α, β, etc.) can vary within an image during an exemplary filtering process. For example, in a first order IIR filtering process, some pixels or groups of pixels may be updated with a different time constant than other pixels or groups of pixels. With respect to  FIGS.  14  and  15   , certain changes in the detected image data can be localized to certain regions within the image. For example, with respect to  FIG.  15   , in some environments, excessive thermal change ( 1508 ) might occur due to a localized hot spot appearing in an image (e.g., a person entering a portion of the scene) without affecting other portions of the scene. In some examples, the pixels or regions affected by the thermal change (e.g., the pixels including the person entering the scene) can be identified and distinguished from those unaffected by the change. In some such examples, temporarily adjusting or disabling filtering parameters ( 1510 ) can be performed only on the pixels or regions in which the excessive thermal change ( 1508 ) is detected. In an exemplary embodiment, such thermal changes can be detected by analysis of consecutive or nearly consecutive unfiltered (e.g., new) infrared images. Comparisons of such images can be used to determine the presence and location of excessive temperature changes that may negatively impact optical gas imaging. 
     Similarly, with respect to  FIG.  14   , as described elsewhere herein, certain video compression standards (e.g., MPEG) can be analyzed to detect motion within a scene (e.g., via motion vector analysis). In some embodiments, motion of the entire scene (e.g., due to camera motion) can be detected using such video analysis. However, in other examples, such analysis can be used to determine the portions of the image experiencing scene-to-scene motion (e.g., those affected by movement within the scene). Similarly, in some such examples, filtering parameters associated with individual regions or pixels that experience excessive motion ( 1408 ) can be temporarily adjusted or disabled ( 1410 ) while the filtering parameters of other regions (e.g., not experiencing excessive motion) can remain unaffected. In some embodiments, image processing to determine excessive motion can be performed on infrared image data. Additionally or alternatively, such image processing can be performed on corresponding visible light image data. In some examples, visible light image data provides a clearer image in which such motion analysis can be performed more easily than in infrared image data. 
     In still further embodiments, the amount filtering parameters are adjusted due to detected scene changes (e.g., in-scene motion or isolated temperature changes) can be dependent on the amount of scene change detected as such locations. For example, in some embodiments, the amount that filtering parameter of regions or pixels that experience excessive thermal change ( 1508 ) are adjusted by an amount dependent upon the amount of detected thermal change. 
     The processes of  FIGS.  14  and  15    are generally effective to minimize artifacts in an optical gas image due to large amounts of camera motion or temperature change in a scene, such as when a user is moving from scene to scene or when a temporary thermal anomaly appears in the scene. However, other issues may also negatively impact optical gas imaging processes. In some cases, a camera may be consistently subject to small amounts of motion, such as hand jitter of a camera operator. In some examples, such motion may be small enough in magnitude to not constitute “excessive motion” in step  1408  in  FIG.  14   , and so would not result in adjusted filtering parameters according to the method of  FIG.  14   . In other examples, if the hand jitter did constitute “excessive motion” in step  1408 , but was consistently happening, the filtering parameters may be adjusted or disabled too often to perform any adequate gas imaging process. Additionally, operating the camera in a handheld mode as described elsewhere herein can be used to minimize impact of hand motion, but often is accompanied by shorter averaging times and therefore a lower signal-to-noise ratio, reducing optical gas imaging sensitivity. 
     In some examples, small amounts of motion may be compensated for by using image registration and/or stabilization techniques, which can be performed using hardware and/or software solutions. For example, a gyro-stabilized gimbal platform can be used to physically compensate for frame-to-frame displacements of the camera itself in approximately real-time. In some examples, camera displacements can be determined using hardware components, such as accelerometers and/or gyro sensors. 
     Digital corrections can utilize methods that search for a frame-to-frame displacement vector that represents the displacement between a current frame and a previous (e.g., a reference) frame. Some such processes iteratively search for the displacement vector that provides the strongest correlation between the current frame and the previous frame. Inputs from hardware motion sensors (e.g., accelerometers, gyro sensors, etc.) can be used as input “seed” to assist in an image stabilization algorithm. However, various such methods are computationally intensive and costly to employ in a thermal imaging camera. 
     Further, various challenges apply when such stabilization algorithms are applied to background-subtraction infrared optical gas imaging processes, such as one or more processes described herein. Uncooled thermal imaging cameras often experience fixed pattern noise (“FPN”) associated with uncooled IR sensors, which can be due to several factors, such as pixel-to-pixel variations in gain and offset parameters, 1/f pixel noise, internal infrared radiation within the camera, and the like. Various techniques are known for correcting for FPN, such as non-uniformity correction (“NUC”) processes as described, for example, in U.S. application Ser. No. 13/011,846 filed Jan. 21, 2011, which issued as U.S. Pat. No. 8,760,509 on Jun. 24, 2014, hereby incorporated by reference in its entirety. Some techniques include presenting a substantially uniform thermal scene to the infrared sensor of the thermal imaging camera (e.g., a shutter, the sky, etc.) and establishing a NUC subtraction value for each pixel in order to remove the FPN at the pixel. 
     However, FPN can create unique problems when performing optical gas imaging processes utilizing image stabilization techniques, as will be described with respect to  FIGS.  16 A and  16 B .  FIGS.  16 A and  16 B  are schematic illustrations simulating camera motion during an optical gas imaging process.  FIG.  16 A  shows exemplary thermal imaging data at 16 pixels of a thermal imaging camera, arranged into rows a, b, c, and d and columns w, x, y, and z. In general, each pixel will include a unique amount of FPN associated with that pixel. For instance, pixel coordinate (a, w) includes fixed pattern noise FPN a,w  associated therewith. 
     As shown, an object  1600  is located at pixel coordinate (b, z) in  FIG.  16 A . However, in the illustrated example, a short time later, in the image of  FIG.  16 B , object  1600  is located at pixel coordinate (a, y) due to motion of the thermal imaging camera between the time the image of  FIG.  16 A  and the image of  FIG.  16 B  is captured. 
     As described elsewhere herein, an optical gas imaging process can be configured to emphasize change in a scene in order to observe the presence of a gas. Thus, differences not due to gas, such as due to camera motion (e.g., between  FIGS.  16 A and  16 B ) can cause motion artifacts to appear in the optical gas image. As described above, image stabilization and/or registration processes can be used to correct for this difference. For example, if the thermal imaging camera is configured to perform an image stabilization process to register the images of  FIGS.  16 A and  16 B , such a process would generally result in shifting, for example, the image in  FIG.  16 B  rightward by one pixel and downward by one pixel so that object  1600  is in the same location in each captured image. 
     However, if unique FPN exists in the image at different pixel locations, it is likely that pixel coordinate (b, z) and pixel coordinate (a, y) have different amounts of FPN associated therewith. Accordingly, in the image in  FIG.  16 A , the IR intensity value associated with pixel (b, z), I b,z , will be due to the combination of the contribution from object  1600 , I obj , and the FPN associated with the pixel, FPN b,z . That is, with respect to  FIG.  16 A :
 
 I   b,z   =I   obj +FPN b,z .
 
     Similarly, in the image in  FIG.  16 B , the IR intensity value associated with pixel (a, y), I a,y , will be due to the combination of the contribution from object  1600 , I obj , and the FPN associated with the pixel, FPN a,y . That is, with respect to  FIG.  16 B :
 
 I   a,y   =I   obj +FPN a,y  
 
     Accordingly, if the images of  FIGS.  16 A and  16 B  are registered during an optical gas imaging process, the difference in FPN associated with pixel coordinates (b, z) and (a, y) will appear as a difference in infrared image data at a single location (the location of object  1600  in the registered image stack) such as described above with respect to  FIG.  15   . The optical gas imaging process may act to emphasize this apparent change in infrared image data over time, obscuring the ability to accurately observe gas in the optical gas image despite the fact that the scene remained static and the origin of the apparent change is attributable to different FPN values at different pixels. Moreover, while the location of object  1600  in  FIGS.  16 A and  16 B  is described herein for simplicity, each pixel in the pair of infrared images that are registered to correct camera motion will generally have some apparent temperature change due only to the difference in FPN present at the different pixels that imaged like portions of the target scene (e.g., object  1600 ). 
     Eliminating the FPN across a scene, such as by performing a NUC procedure, can mitigate or eliminate such issues arising in optical gas imaging using image stabilization algorithms. However, FPN can evolve over time, and can therefore cause problems during optical gas imaging even if a NUC process is performed. For instance, the amount of FPN needed to negatively impact an optical gas imaging process is typically much lower than the amount of FPN that would be required to negatively impact a typical thermal imaging process. Accordingly, in some such cases, it can be beneficial to perform a NUC process a short time before an optical gas imaging process that utilizes image stabilization and/or periodically throughout such an optical gas imaging process in order to minimize the amount of FPN that appears during the optical gas imaging process. 
       FIG.  17    is a process-flow diagram showing an exemplary combination of performing a NUC process before and/or during an optical gas imaging process that uses image stabilization. The process includes initiating an optical gas imaging process ( 1700 ) and performing a NUC process ( 1700 ). In some examples, a user may initiate an optical gas imaging process ( 1700 ) via a selection on a user interface such as shown in  FIG.  10   . In some embodiments, the thermal imaging camera may automatically perform a first NUC process ( 1702 ) or otherwise instruct a user to perform such a process upon initiating of the optical gas imaging process ( 1700 ). 
     After performing the NUC process, the method includes capturing an image for an optical gas imaging process ( 1704 ). The captured image can include, for example, the new image  804  in  FIG.  8    for use during an optical gas imaging process. Next, an image stabilization process is performed to register the captured image to a reference location ( 1706 ). The reference location can be determined, for example, by the location of a first image captured after the process is initiated. For instance, with respect to  FIG.  8   , the first new image  804  can establish a reference location, and subsequent “new images”  804  captured during the stabilized optical gas imaging process can be registered to that reference location. Thus, if the image is the first captured during a stabilized optical gas imaging process, the registration process may include defining the reference location to use during subsequent stabilization registrations. 
     After registering the image, a stabilized optical gas imaging process can be performed ( 1708 ), such as the gas imaging process shown in  FIG.  8   , for example, while utilizing image stabilization techniques to register new images to the reference location. The combination of the image registration ( 1706 ) and the NUC process ( 1702 ) help compensate for camera motion (e.g., camera jitter) and/or FPN artifacts that may appear during the stabilized optical gas imaging process. 
     In some examples, a thermal imaging camera can be configured to periodically perform a NUC process or otherwise prompt a user to perform a NUC process. In some such examples, the thermal imaging camera does so after a predetermined amount of time and/or a predetermined number of image frames have been captured. In some examples, the amount of time and/or number of captured frames between performed and/or prompted NUC processes can depend on one or more camera settings. For example, in some embodiments, NUC is performed and/or prompted more frequency during a handheld optical gas imaging mode than during tripod optical gas imaging mode, since greater amounts of registration due to handheld jitter may more quickly and/or severely include FPN artifacts into optical gas image data. 
     Thus, the process of  FIG.  17    can include the step of determining whether or not to update the NUC ( 1710 ). If not, then a new image can be captured ( 1704 ) and the stabilization/registration ( 1706 ) and optical gas imaging ( 1708 ) processes can be performed using the captured image, such as according to the process shown in  FIG.  8   . If NUC is to be updated at step  1710 , then a NUC process can be performed ( 1702 ) prior to capturing a new image ( 1704 ) for the optical gas imaging process. 
     In an exemplary embodiment, a user may initiate a process similar to that shown in  FIG.  17   . For instance, a user may initiate a stabilized optical gas imaging process ( 1700 ) by depressing a trigger, such as trigger  112  shown in  FIG.  1   . In an exemplary use case, a user may be performing an optical gas imaging process without using image stabilization (e.g., in handheld mode) prior to initiating a stabilized optical gas imaging process. Upon depression of the trigger, the camera may automatically perform a NUC process ( 1702 ) and then perform a stabilized optical gas imaging process ( 1708 ) while capturing images ( 1704 ) and performing image stabilization ( 1706 ) until the trigger is released. In some such examples, a user may perform such a process a plurality of times. For instance, during an exemplary gas inspection, a user may depress the trigger to perform optical gas imaging and watch for gas (e.g., a selected gas) in a first scene, release the trigger, move the camera to view a second scene, and depress the trigger to perform optical gas imaging and watch for gas in the second scene. 
     Performing optical gas imaging using image stabilization processes may allow for using longer time constants to improve the sensitivity of the optical gas imaging process. For example, image stabilization may eliminate the positioning errors that often make handheld optical gas imaging susceptible to noise and other artifacts due to camera motion. Thus, in some examples, the camera can be configured to, automatically and/or via user selection, use “tripod mode” optical gas imaging time constants while operating in handheld mode with image stabilization. 
     For example, with further reference to  FIG.  10   , in some embodiments, a user may select between tripod mode and handheld mode. In some embodiments, selecting handheld mode leads to another screen in which a user may choose to enable or disable image stabilization. In some such embodiments, enabling image stabilization in the handheld mode results in longer time constants than does disabling image stabilization. In some examples, the time constants for handheld mode using image stabilization are approximately the same as the time constants for tripod mode. 
     In another example, handheld mode automatically includes image stabilization. In some such embodiments, the optical gas imaging time constants are the same when using tripod mode or when using handheld mode with image stabilization. In various examples, a user may manually adjust one or more time constants regardless of the default time constants employed by the camera according to the user settings. 
     During an optical gas imaging process utilizing image stabilization, even if images are properly registered, in some cases, too much camera pointing drift results in very little overlap between the images to be registered, resulting in a very small usable region in which to perform the optical gas imaging process. In some embodiments, the camera can be configured to provide real-time or substantially real-time feedback to a user regarding how much the camera pointing is drifting during the optical gas imaging process. 
       FIGS.  18 A and  18 B  show exemplary display interface indicating to a user an amount of camera pointing drift during an optical gas imaging.  FIG.  18 A  shows an exemplary display  1800   a , for example, presentable on display  108  in  FIG.  1   . The display  1800   a  includes a pointing drift indicator  1810   a  that can be used to indicate to a user an amount and/or direction that the pointing of a camera is drifting from a reference position during an optical gas imaging process (e.g., during a stabilized optical gas imaging process). In the example of  FIG.  18 A , the pointing drift indicator  1810   a  includes a first marker  1820  and a second marker  1830 , slightly smaller than and positioned within first marker  1820 . In some embodiments, the display  1800   a  is updated in real time or substantially real time to indicate camera motion using first  1820  and second  1830  markers. 
     In an exemplary embodiment, the camera is configured to sense motion or pointing drift of the camera, for example, representing the difference in camera aiming between a recently-captured image and a reference frame, such as an optical gas imaging background image. Such pointing drift can be determined, for example, using one or more motion sensors, such as an accelerometer and/or a gyro sensor. In some such examples, the camera processor (or another system processor) can be configured to receive motion data from a motion sensor to determine the amount of camera displacement relative to a previous or reference position. Additionally or alternatively, the amount of motion can be determined via an image registration calculation, such as performed during an image stabilization process. 
     Once the amount of motion has been determined, the first  1820  and the second  1830  marker can be positioned relative to one another to represent the amount of motion. For instance, in an exemplary embodiment, the location of the first marker  1820  is fixed in display  1800   a  and represents a reference location. The location of second marker  1830  is updated with each frame to indicate the position of the captured image relative to the reference location. For example, in some such embodiments, if the camera pointing drifts upward during an optical gas imaging process, the second marker  1830  would similarly move upward with respect to first marker  1820  during the optical gas imaging process. Thus, a user is provided with real time or substantially real time feedback regarding camera position in order to help the user minimize pointing drift during the optical gas imaging process. 
     The pointing drift indicator  1810   a  is shown as being positioned in the upper-left corner in display  1800   a  in  FIG.  18 A . This location can allow the user to generally view the motion of second marker  1830  relative to first marker  1820  using his or her peripheral vision while still observing the center of the display while looking for indications of gas present in the scene. In some examples, first  1820  and/or second marker  1830  can be presented in a color or colors that contrast from the scene palette and/or each other in order to assist the user in observing the motion of the second marker  1830  relative to the first marker  1820  on the display  1800   a.    
       FIG.  18 B  shows an exemplary display  1800   b , for example, presentable on display  108  in  FIG.  1   . The display  1800   b  includes a pointing drift indicator  1810   b  that can be used to indicate to a user an amount and/or direction that the pointing of a camera is drifting from a reference position during an optical gas imaging process. In the example of  FIG.  18 B , the pointing drift indicator  1810   b  includes a first marker  1840  and a second marker  1850 , slightly smaller than and positioned within first marker  1840 . In some embodiments, similar to display  1800   a  described with respect to  FIG.  18 A , the display  1800   b  is updated in real time or substantially real time to indicate camera motion using first  1840  and second  1850  markers. 
     In general, first  1840  and second  1850  markers in  FIG.  18 B  can act similar to markers  1820  and  1830  in  FIG.  18 A , wherein second marker  1850  moves relative to the first marker  1840  to indicate motion of the camera relative to a reference position. However, rather than being positioned in a corner of the display, first  1840  and second  1850  markers surround much of the scene, allowing a user to observe the center of the display  1800   b  and observe motion of the second marker  1850  at the edge of the screen. 
     While shown as rectangles in  FIGS.  18 A and  18 B , the pointing drift indicators can generally include any one or more shapes that the user may observe moving with respect to one another as the pointing of the camera drifts during an optical gas imaging process. 
     Limiting the pointing drift of the camera during an optical gas imaging process utilizing image stabilization, such as by using pointing drift indicators  1810   a  or  1810   b , can improve the efficiency of the image stabilization process. For example, embodiments including one or more pointing drift indicators to help the user manually correct for any camera displacement can reduce the search area for the optimal displacement vector between a captured image and a reference position during the stabilization process. For instance, in an exemplary embodiment, when performing an image stabilization process, the camera may utilize data from one or more position sensors and/or the relative locations of pointing drift indicators (e.g., markers  1820  and  1830 ) to identify an approximate displacement vector between a reference location and the location of the camera at the time an image was captured. Additionally or alternatively, a camera may limit a search area for an optimal displacement vector to those vectors which would result in the pointing drift indicator indicating a maximum pointing drift. For instance, in an exemplary embodiment with respect to  FIG.  18 A , a camera performing an image stabilization process may limit the analyzed displacement vectors to only those vectors that would result in displacement that retains the second marker  1830  entirely within the boundary of the first marker  1820 . Thus, in such an embodiment, as long as the user maintains the second marker  1830  within the first marker  1820  during operation, the displacement vector best suited for registering a newly-captured image to a reference position will be within the analyzed displacement vectors. 
     In some examples, limiting the search area for an optimal displacement vector when performing an image stabilization process reduces the processing load required to still produce a real-time or substantially real-time optical gas image using the stabilization process. In some embodiments, the reduced processing load enables additional or more computationally demanding processing steps to be performed while still producing real-time or substantially real-time image data. For instance, in some embodiments, image stabilization techniques can utilize sub-pixel image registration techniques to achieve further improved registration of a captured image to a reference position during real-time or approximately real-time optical gas imaging. 
     Additionally or alternatively, the computational demand of image stabilization can be reduced by performing the stabilization process using a subset of available pixels for registration. In some such examples, a user can define a sub-region in the image in which to perform stabilization calculations. In various embodiments, this can include drawing an outline of an area or an image or overlaying a predefined shape onto the image to identify the area to be used during image stabilization. 
     Utilizing a sub-set of image data for image stabilization can reduce the processing time and/or resources necessary for achieving accurate registration. Additionally or alternatively, in some examples, if a portion of the scene includes moving objects (e.g., clouds), the stabilization process could be restricted to a certain region of the image that generally does not include such moving objects that could otherwise interfere with the registration process. Further, in some examples, lenses used in generating image data (e.g., infrared image data) provide a degree of image distortion (e.g., barrel distortion) in the resulting image data. In some such cases, the optimal displacement vector for performing image stabilization varies over the field of the image due to such distortion, leading to unreliable registration and/or increased computational demand for determining the optimal displacement vector. In some such cases, image stabilization calculations can be restricted to being performed in an image area least likely to be impacted by distortion, such as a small, centrally-located region. 
     In various examples, areas for use during image stabilization calculations can be selected in a variety of ways. In some embodiments, a user may choose an area from a predefined list of options, such as, for example, the full image, an area predefined to reduce the impact of lens barrel distortion, an area defined by a pointing drift indicator, such as first marker  1820 , or the like. Additionally or alternatively, as noted elsewhere herein, a user may define a custom area in which to restrict the image stabilization process, such as by positioning and/or sizing a predefined shape over the image or drawing a custom shape over the image. 
     Image filtering, stabilization, and optical gas imaging techniques described herein can be used to reduce the noise of thermal images so that changes between scenes due to the presence of an infrared absorbing gas in the target scene can be resolved from the noise. Similarly, the adaptability of the filtering techniques can accommodate use in both hand-held and tripod-mounted cameras, maximizing the noise reduction without inducing false residual artifacts into the image data. 
     The filtering and corresponding optical gas imaging techniques can be used, for example, in a thermal imaging camera including an uncooled sensing array and without the need for expensive optical notch and/or bandpass filters, greatly reducing the cost and complexity of portable systems used for performing such processes. Similarly, such uncooled cameras can themselves be smaller, lighter, and easier to transport from location to location than more complex, cooled thermal imaging cameras, increasing the ease and capability of performing high quality optical gas imaging processes in a plurality of locations using a single camera. 
     Processes described herein, such as generating filtered background and foreground images and optical gas image data, can be performed using one or more processors, such as processor  222  embedded in a thermal imaging camera that is configured to capture images that are filtered. In some embodiments, processors configured to perform some or all of such processes can be located separately from the thermal imaging camera(s) that acquire the image data. 
     For example, in some embodiments, a thermal imaging camera can be fixedly positioned in place for continued thermal analysis of equipment under test. The thermal imaging camera can be configured to acquire thermal image data of a target scene and communicate the thermal image data to an external device for subsequent processing, such as a workstation, the cloud, a technician&#39;s mobile device (e.g., smartphone, tablet, etc.) or the like. In some such examples, various processing steps can be taken at the external device, such as via software operating on the workstation, an application (“app”) running on the technician&#39;s mobile device, cloud computing, or the like. 
     In some examples, substantially real-time optical gas imaging processes can be performed by processing the captured images at the external device. In such examples, the thermal imaging camera used to acquire the image data can be inexpensive and include minimal processing power, but can capture images used for optical gas imaging analysis. Additionally or alternatively, in some examples, processing steps in various processes described herein can be divided among a variety of components, such as a thermal imaging camera that captures thermal images, an external device, or the like. 
     Various embodiments have been described. Such examples are non-limiting, and do not define or limit the scope of the invention in any way. Rather, these and other examples are within the scope of the following claims.