Active Imaging System

Abstract

The system comprises a first and a second laser illuminator for illuminating a scene including a target. The pulses of light emitted from the first laser illuminator have a relatively wide spectral linewidth and a relatively long pulse duration compared to the pulses of light from the second laser illuminator. The camera system uses a relatively long gate period to receive the return from the first laser illuminator to obtain a first image of the scene, and a relatively short gate period together with the distance information of the target to receive the return from the second laser illuminator to obtain a second image of the target with selective exclusion of non-target elements in the scene. By image processing, the second image is used as a template to selectively remove non-target elements from the first image. In this way, the final image benefits from the highest possible image quality from the return of the first illuminator and good clutter rejection from the return of the second laser illuminator.

Description

[Background technology]

[0001] Observation systems are used to monitor a local environment using a number of sensors for one or more of the purposes of detection (acquisition), tracking, and identification of remote objects.

[0002] 1 shows a schematic diagram of a representative prior art observation system 1 designed to detect objects in a local environment that may be located within a range of 100 m to 20 km or more from the location of the system 1. System 1 may be mounted in an aircraft or other airborne platform, on a land vehicle, or on a sea-going vessel.

[0003] The observation system 1 comprises an interface 2 , a control and processing subsystem 3 and an acquisition and track system 4 .

[0004] The control and processing subsystem 3 outputs system status data as well as processed image, track data and target information via the interface 2 .

[0005] The interface 2 may, for example, comprise a display for presenting images to a human observer for analysis and action. Alternatively or additionally, the interface may comprise an output that provides input to an autonomous system that can process output data from the observation system 1, for example an image or a series of images, and take appropriate action without human intervention.

[0006] The control and processing subsystem 3 controls the acquisition and track system 4 which comprises three systems: a target detection sensor subsystem 5, a coarse tracking subsystem 6, and an active imaging subsystem 7. A support umbilical 8 interconnects the control and processing subsystem 3 to the subsystems of the acquisition and track system 4 and provides power and data cables (buses), as well as thermal management services if needed (e.g., air or liquid cooling).

[0007] The coarse tracking subsystem 6 comprises one or more sensors for tracking targets. The sensors may include wide field of view (WFOV) tracking sensors that can detect a wide range of angles and are useful for quickly detecting targets with limited spatial resolution, and narrow field of view (NFOV) tracking sensors that have high resolution for target classification and possibly identification. The exact values ​​are determined by the system application.

[0008] A wide field sensor may include one or more cameras or detectors operating within one or more of the following bands: ultraviolet (UV), visible, near infrared (NIR), short wave infrared (SWIR), mid wave infrared (MWIR), and long wave infrared (LWIR).

[0009] A narrow field sensor may include one or more cameras or detectors operating within one or more of the following bands: ultraviolet (UV), visible, near infrared (NIR), short wave infrared (SWIR), mid wave infrared (MWIR), and long wave infrared (LWIR).

[0010] The coarse tracking subsystem 6 may also include a range finder, such as a laser range finder, that provides distance information to targets that may aid in prioritizing or identifying targets.

[0011] The coarse tracking subsystem 6 is steerable, for example using motors to control a gimbal system to which the various sensors are fixed. The field of view (FoR) encompasses all possible directions in space that the acquisition and track system 4 can see, while the field of view (FoV) describes a smaller set of directions in space that can actually be seen at any one time.

[0012] The target detection sensor subsystem 5 obtains data from one or more of many possible sensors 5A, 5B, which may be internal 5A and / or external 5B to the observation system 1, in order to derive a direction in space from which the observation system 1 can begin to look for a potential target object. Data from external sensors 5B is received through interface 2. In an alternative arrangement, the functions of the target detection sensor subsystem 5 may be performed by an external system and its input received through interface 2.

[0013] When a potential target is identified from the output of one or more of the external sensors, the control and processing subsystem 3 verifies that the potential target is within the system's ocular field of view (FoR) of the coarse tracking subsystem 6. If not, the control and processing subsystem 3 does nothing or otherwise commands the coarse tracking subsystem 6 to orient itself so that the potential target is within at least one of its sensor's fields of view (FoV). It is important that the measurement error in the target's position is less than the ocular field of view of the NFoV tracking sensor. The WFoV and NFoV tracking sensors are aligned using one of several known processes so that their lines of sight are congruent or co-aligned.

[0014] The external target detection sensors 5B may be intrinsic to the observation system or they may be remote from the observation system, for example on a separate platform or on a nearby separate observation system. The external target detection sensors, with a subset referred to as cameras, or in the case of IR cameras, FLIRS (forward looking infrared sensors), may comprise one or more of a human field of view, a radar system, an infrared (IR) or thermal camera, a shortwave IR camera, a visible light camera, an ultraviolet (UV) sensor and / or camera, and an acoustic sensor.

[0015] An exemplary observation system is an infrared countermeasure (IRCM) system. An IRCM system is often composed of a remote target detection sensor, also known as a missile launch warning system (MLW). The MLW may use UV and IR cameras to detect the missile launch signature and provide the location of the launch site relative to the MLW platform. In this arrangement, the MLW is completely external to the observation system. The MLW reports the location of the launch to a processor acting as the control and processing subsystem 3. The processor commands a turret, located at a separate location on the same platform, to rotate so that its FOV faces the location of the missile identified by the MLW. The turret includes a FLIR equivalent to a coarse tracking module 6. The FLIR can detect the missile plume and therefore acquire and track the missile. The processor analyzes the data from the sensors on the turret.

[0016] Referring again to FIG. 1, typically the target will be present within the WFoV sensor, allowing the control and processing subsystem 3 to make small corrections to the orientation of the coarse tracking subsystem 6 so that the target is centered within the WFoV, allowing the system to continuously track the target. After detecting and starting to track the target, possibly without yet identifying it, the control and processing subsystem 3 analyzes the output from the NFoV sensor, which is co-aligned with the WFoV sensor and therefore can also see the target. Once the spatial resolution provided by the NFoV sensor is good, the control and processing subsystem 3 may be able to identify the target. Identification may be achieved by a suitable automatic target identification algorithm (ATI) in the control and processing subsystem, or by presenting the image to a human user for identification via the interface 2. At this stage, the control and processing subsystem 3 or a human classifies the target as being of interest for the mission or not of interest for the mission. If the potential target is not of interest, the control and processing subsystem 3 classifies the potential target as clutter. If the target is of interest, it commands the coarse tracking subsystem 6 to continue tracking the object until the mission is complete.

[0017] Situations may arise where the NFoV sensor of the coarse tracking subsystem 6 is unable to provide an image of the potential target with sufficient spatial resolution to identify the potential target, in which case the active imaging subsystem 7 is initiated. The active imaging subsystem 7 uses a technique known as active illumination to obtain further images of the potential target.

[0018] Active imaging is a technique where a light source, usually a continuous wave (CW) laser, or more preferably a pulsed laser, is used to illuminate a remote object. A suitable camera images the light reflected by the object. Most realistic targets are complex in that they may be structured and composed of many materials with different surface treatments, so the light reflected from these undergoes specular and diffuse reflection from the same target. The reflected light may also be somewhat depolarized. Advantages of active imaging are: 1. If the laser uses a shorter wavelength than the WFoV and NFoV thermal sensors, it may be possible to obtain images captured with better spatial resolution than from the WFoV or NFoV sensors, which may help improve target recognition. 2. The light reaching the camera can be filtered to a small range of wavelengths around the laser wavelength, which improves the camera sensitivity and helps localize targets within the entire camera FoV. 3. Active imaging allows for improved imaging contrast compared to thermal imaging when limited by local thermal signatures. For example, in early morning and evening, the temperature of the scene can be nearly isothermal, resulting in too low contrast against the background to detect or identify any targets. Active imaging allows for target imaging independent of scene temperature. 4. Active imaging can be used to perform time-gated imaging to remove objects from the image that are closer and / or farther away from the camera compared to the target. This process is often called clutter reduction or clutter removal. This can be used, for example, to allow an observer to see a target within a netting based camouflage or within a room.

[0019] Time-gated imaging is achieved by operating a pulsed laser at a certain time (the laser firing time) and opening the camera gate at a gate opening time that is delayed from the laser firing time. The camera gate is closed at a gate closing time that is further delayed from the laser firing time.

[0020] This type of time-gated imaging provides intensity information in a two-dimensional format with a spatial resolution determined by the camera pixel size, which may be referred to as two-dimensional (2D) imaging.

[0021] If the length of the laser pulse is short compared to the target, it is possible to obtain further information about the target shape that may aid in identification. This feature requires the camera to provide information about the time of arrival of the reflected light on a pixel-by-pixel basis equivalent to a pixel-level range finder, making it possible to generate a distance-resolved point cloud that can be analyzed to provide target size and shape information. Such use of a time-resolved, time-gated camera is often considered to provide three-dimensional information (3D), where the second dimension is the intensity data by pixel and the third dimension is the time of arrival of the signal.

[0022] 2D time-gated cameras can also be used to develop 3D information. One approach is to use several laser pulses to interrogate the target with slightly different delays (gate opening times) relative to the laser firing time.

[0023] CW laser illuminators can also be used to produce 2D images through the natural frame rate of the camera. Each image is integrated over the frame rate of the camera (typically 0.5 to 5 ms or more). All objects illuminated by the laser are visible within the illuminated scene, which may include objects in the foreground or background that are not the intended target. The output of this type of camera does not allow for time-based clutter removal.

[0024] Referring to FIG. 2, the active imaging subsystem 7 comprises an active imaging controller 7A, an illumination laser 7B, a programmable delay generator 7C, and a time-gated camera 7D.

[0025] The active imaging controller 7A is adapted to receive instructions from the control and processing subsystem via bus 8 and then control the first pulsed illumination laser 7B, the programmable delay 7C, and the time-gated camera 7D via an internal bus 7E.

[0026] In response to a signal received from the active imaging controller 7A, the illumination laser 7B illuminates the target 9 with pulsed light 10 through a first aperture 7F of the active imaging subsystem 7. The illumination laser 7B also provides a timing signal to a programmable time delay generator 7C which provides a time delay signal, i.e., an open gate signal, to the gated camera 7D. In response to receiving the open gate signal, the camera 7D opens its gate to collect light 11 reflected from the target 9 through a second aperture 7G. The camera 7D is programmed to close its gate at a predetermined time from the open gate signal.

[0027] Referring to FIG. 3, laser 7B is symbolically designated T FWHM In many applications, T FWHM Values ​​of 5 to 50 ns for λ are typical, but both longer and shorter pulses may be used. For safety reasons, the preferred wavelength of the illumination laser 7B is in the range of 1.5 μm to 1.8 μm, but other wavelengths with good atmospheric transmission may be used. The laser pulse length, measured in time, is determined by the speed of light, c=3×10 8 ms -1 This can be converted to a physical pulse length by multiplying by (approximate value). Physical durations ranging from 5 ns to 50 ns correspond to approximately 1.5 m to 15 m.

[0028] The laser 7B has a laser pulse repetition period T p The pulse repetition period can be varied as required. p Typical values ​​of are in the range of 100 ms to 500 μs. For improved timing accuracy between laser 7B and time-gated camera 7D, optionally a small portion of the laser 7B output is timed to T 0 may be guided to detector 7H, T 0 Detector 7H in response generates a signal synchronized with the emission of laser pulse 10, which provides an input to programmable delay 7C. The emission time of the laser pulse is T 0 It may be referred to as.

[0029] The laser pulse propagates to the target 9 at a distance L. The value of L may be known to the observation system 1, for example, by using a range finder in the coarse tracking subsystem 6. In such a case, the pulse propagates to the target 9 at a distance L. 0 The light 11 reaches the target 9 at +L / c and is reflected at time T o It is known that the time gate camera 7D reaches the target at +2L / c. The controller 7A sets the programmable delay 7c to 2L / ct 0 T 0 , the camera gate opens before the reflected light reaches the camera 7D. The camera gate is set to the time t 0 +t c The reflected light pulse is 2L / c+t so that is the gate opening time. c T 0 The gate time is T 0 To allow for uncertainty in the measurement of the pulse duration, T FWHM It must be longer, but can be minimized to improve the signal to noise of the system.

[0030] The camera 7D comprises a lens system 70 that collects light entering the second aperture 7G and images it onto a focal plane array (FPA) 71. The focal plane array 71 is constructed on a suitably pixelated detector material capable of detecting the laser light with sufficient sensitivity. Each pixel on the focal plane array 71 is processed by electronics on an associated readout integrated circuit (ROIC) 72. The ROIC 72 collects and integrates the photocurrent from each pixel and implements a gating circuit such that the sensitivity of the FPA 71 is effectively time-switched from low to high and back according to the gate-opening time. The image data collected during the gate-opening time are serially numbered and passed to the control and processing subsystem 3 via bus 7E.

[0031] The gated camera D also includes a narrow band filter 73 to minimize the amount of non-laser light reaching the focal plane array 71. For example, the active imaging system 7 may include a solid state laser 7B having a linewidth of less than 1 nm and a narrow band filter 73 having a passband of about 1 nm. Alternatively, a semiconductor laser 7B having a linewidth of less than 25 nm may be used with a filter 73 having a passband of about 25 nm. Both of these example filters compare very favorably with InGaAs-based shortwave infrared detectors, which are typically sensitive in the 0.6 to 1.7 μm spectral range, in terms of background noise.

[0032] The active imaging system shown in FIG. 2 illustrates a bistatic architecture where the laser 7B and the time-gated camera 7D have separate transmit and receive apertures 7F, 7G.

[0033] An alternative optical architecture, known as monostatic, is shown diagrammatically in Figure 4. This variation of the active imaging system 7 comprises a single optical aperture 7G that is used to both transmit the laser light and receive the reflected light from the target. A mirror 7J that is highly reflective at the laser wavelength is used to steer the laser beam 10 towards the aperture 7G. An optical component 7K, which may use known techniques for spatially multiplexing light such as polarization or aperture sharing, combines the paths of the transmitted laser beam 10 and the reflected light 11.

[0034] FIG. 5 shows the process of clutter removal using the system of FIG. 4, but the systems of FIGS. 2 and 3 can be used as well. A target 9 (a tank in this example) is illuminated by a laser 7B, and the return is collected by a time-gated camera 7D, processed, and presented to the user via the interface 2 as a first image 20 or a second image 21. When obtaining the first image 20, the gate-on time is long compared to the laser pulse duration, so the image contains, in addition to a view of the target 9, some foreground and some background detail. Conversely, if the gate-open time is comparable to the physical depth of the target, the laser pulse duration is shorter than the gate-on time, and the target is placed right within its gate-open time, and then the second image 21 is obtained. The target 9 depicted in the second image 21 is neatly separated from the foreground and background. If the physical laser pulse duration is much shorter than the overall target dimension and the camera 7D is 3D capable, structures in the image 21 can be measured, for example the length of a gun barrel if the target 9 is a tank, which can aid in target identification.

[0035] The usefulness of the active imaging system 7 depends on the quality of the resulting image compared to the additional costs incurred by integrating active imaging into the observation system 1. Both the laser 7B and the camera 7D are known to contribute to the quality of the image produced. Furthermore, the choice of laser type, such as between solid-state lasers (e.g., Nd:YAG, Er:Yag, or Ho:Yag), compared to semiconductor lasers, for example, results in different power requirements. Solid-state lasers also result in a relatively fixed wavelength and a relatively narrow linewidth, compared to semiconductor lasers, where the wavelength and linewidth can be varied.

[0036] An ideal laser source for active imaging would have the following characteristics: a. Beams with large line widths, for example 5 nm to 50 nm or more, can be produced to mimic the properties of natural light and reduce the effects of speckle on images. b. Safe to use in long-range viewing applications where the object of interest is located 100 m to 20 km or more from the system location. This requirement is most easily met by using wavelengths in the range of 1.5 μm to 1.8 μm. c. A time-gated camera with appropriate sensitivity to the laser wavelength can be used. d. When used with the line width specified in point a, with the camera specified in point c, it can produce enough energy per pulse to reach the target of interest specified in point b. Typically, such lasers require pulse energies in the range of 10 to 100 mJ per pulse, depending on the exact camera selected for use. The power of such lasers is given by P=E / T p =Ev p may be calculated using the repetition rate of the laser as, where P is the power in watts, E is the pulse energy in joules, and T p is the pulse repetition period, and the pulse frequency is v p =1 / T pand is measured in Hz. For a particular laser choice, increasing the repetition rate from 20 Hz to 2000 Hz can result in a 100-fold increase in output power and power consumption. e. The laser must be electrically efficient enough to minimize power consumption and heat load within the observation system. The optimal wall plug efficiency currently available is greater than 40% from packaged semiconductor lasers (individual semiconductor devices may have greater than 50% electrical to optical power conversion). On the other hand, standard solid state lasers operate with wall plug efficiencies in the range of less than 2 to 5%. Wall plug efficiency is the power derived from an external power source to operate the laser. This figure includes the power used by all component parts of the laser, not just the immediate electrical to optical conversion step. Thus, wall plug efficiency may include, but is not limited to, any electronic control card required to operate the laser subsystem, any thermal control required by parts of the laser, and losses in internal voltage conversion or power conditioning within the laser as well as losses in electrical to optical conversion. Typically, wall plug efficiency varies over the operating temperature range of the system. f. The physical laser pulse duration must be shorter (distance-wise) than the target dimensions so that the target can be cleanly separated from the foreground and background. In many applications, this requires a pulse duration in the range of 10 to 25 ns, which equates to a pulse length of 3 m to 7.5 m. If 3D image data of the target is required, shorter pulse durations, for example in the range of 1 nm to 3 ns, are often required to provide a spatial resolution in the range of 0.3 m to 1 m. h. The laser operates at a relatively high repetition rate (1000 Hz or higher) to minimize image degradation caused by atmospheric turbulence, for example, in long-range imaging. As noted in point d, this represents a significant increase in power consumption over standard laser sources operating at tens of Hz. i) Lasers are low cost.

[0037] Table 1 lists various known commercially available laser types that satisfy condition c and compares their characteristics with the criteria considered for active imaging sources.

[0038] [Table 1]

[0039] It can be seen that there are currently no low-cost lasers that can simultaneously provide short pulses, high electrical efficiency, and large linewidths. There may be short pulse, high repetition rate examples of these and other lasers that appear to meet these requirements, but generally these are laboratory demonstrations and may not be readily available in an affordable, efficient, reliable package. Summary of the Invention

[0040] The present invention has been conceived to provide an improved active imaging system for use in the observation systems described above.

[0041] According to a first aspect of the present invention, there is provided an active imaging system comprising a first laser illuminator and a second laser illuminator, each adapted to illuminate a scene including a target with pulses of light; the pulses of light emitted from the first laser illuminator have a relatively wide spectral linewidth and a relatively long pulse duration compared to the pulses of light from the second laser illuminator; a camera system arranged to receive light from both the first and second laser illuminators reflected by the scene, wherein the camera system is adapted to use a relatively long gating period to receive returns from the first laser illuminator to obtain a first image of the scene including the target, and to use a relatively short gating period together with distance information of the target to receive returns from the second laser illuminator to obtain a second image of the target selectively excluding non-target elements in the scene; and image processing means adapted to use the second image as a template to selectively remove non-target elements from the first image.

[0042] Recognizing that simultaneously meeting the requirements listed in points a through i is not feasible with a single laser of the prior art, the present invention replicates the ideal laser by using two illuminator sources to obtain image data for two source images of the target, and by using imaging processing to combine the image data into a further image that has no foreground and background clutter and has the high image quality of the target provided by the first image.

[0043] Due to its wider linewidth, higher quality images, i.e., images with less speckle, can be produced using the return from the first laser illuminator than from the return from the second laser illuminator. The shorter pulse duration of the second illuminator allows for better clutter rejection and, if short enough, 3D image detail of the target.

[0044] The first laser illuminator may be operated at a higher repetition rate compared to the second laser illuminator so that the first image of the target, and therefore the resulting synthesized image, contains less noise caused by atmospheric turbulence.

[0045] Further advantageously, because the broad linewidth, short pulse duration, and high repetition rate do not need to be produced by a laser alone, two relatively inexpensive lasers can be used which, despite being two, can still be less expensive to implement in an active imaging system than a single laser having all the desired characteristics. For example, a system may include a type B laser (in Table 1), i.e., a Q-switched Nd:YAG with an optical parametric oscillator (OPO), as the first laser, and a type D laser, i.e., multiple co-packaged semiconductor lasers, as the second laser.

[0046] The specific linewidths and repetition rates of the first and second laser illuminators are selected depending on the specific application, but for example, the first laser illuminator may operate at a linewidth of at least 10 nm, preferably at least 25 nm. The first laser illuminator may operate at a repetition rate of at least 1 kHz, preferably at least 2 KHz, to reduce atmospheric noise in the resulting image.

[0047] Atmospheric noise in the second image is not a concern. The second laser illuminator may operate at a repetition rate below 1 KHz, optionally below 100 Hz. This allows for the selection of a relatively inexpensive laser for the second laser.

[0048] The second laser may operate with a pulse duration of 20 nm or less to resolve targets on the order of a few meters or less in size.

[0049] The second laser may operate with a linewidth of 2 nm or less.

[0050] Laser pulse energy may be estimated using standard techniques based on the laser distance equation. Typically, this is controlled by camera sensitivity, target distance, target reflectivity and polarization, background light, e.g., solar illumination within a narrow band filter range and leakage outside this range, laser branching, receiving aperture size, atmospheric transmittance, and losses in filters and other optical components. As a result, pulse energy can vary widely. Practical experience suggests that pulse energies ranging from 1 mJ for short-range applications to 100 mJ for long-range applications provide useful performance.

[0051] The active imaging system may be adapted to operate the first and second laser illuminators at different times.

[0052] The camera system may comprise a single camera to capture returns from the first and second laser illuminators to obtain the first and second images. Alternatively, the system may comprise a first camera adapted to capture returns from the first laser illuminator and a second camera positioned to capture returns from the second illuminator.

[0053] The pulses of light from the first and second laser illuminators may have substantially the same center wavelength. Alternatively, the pulses of light from the first and second laser illuminators may be of different center wavelengths with non-overlapping linewidths.

[0054] If the first and second laser illuminators operate at different central wavelengths, the system may include an optical element, such as a dichroic splitter, to spatially separate the returns from the two lasers so that they can be directed to different cameras.

[0055] If one camera is used, it is preferred that the first and second laser illuminators operate at the same wavelength and the returns are captured in a time multiplexed manner. This allows the camera to be matched to a very narrow band filter, thereby increasing the camera's sensitivity to the laser light and reducing background light from, for example, other emission sources.

[0056] Alternatively, if the first and second laser designating devices operate at different wavelengths with a single camera adapted to capture both wavelengths, or with two cameras each positioned to capture the return from one of the laser illuminators, the pulses and returns may be detected simultaneously.

[0057] The system may comprise a third laser illuminator adapted to illuminate the scene with light pulses of duration smaller than that of the second laser illuminator, for example around 2 ns, to provide spatial resolution on length scales of less than 1 m.

[0058] Returns from the third laser illuminator may be received by one of the first or second cameras, or by the third camera.

[0059] The invention will now be described, by way of example only, with reference to the following figures: [Brief description of the drawings]

[0060] [Figure 1] FIG. 1 is a schematic diagram of a prior art observation system. [Diagram 2] 2 is a schematic diagram of an active imaging system for the prior art observation system of FIG. 1; [Diagram 3] 3 is a schematic diagram of the active imaging system of FIG. 2 showing additional details. [Figure 4] 1 is a schematic diagram of a variant of a prior art active imaging system; [Diagram 5] 1 is a schematic diagram of a variant of a prior art active imaging system illustrating the process of clutter rejection. [Figure 6] Schematic of an active imaging system including two laser illuminators in a bistatic arrangement and a single time-gated camera. [Figure 7] FIG. 13 illustrates an algorithm for applying a filter function to isolate targets in a first image using a second image as a template. [Figure 8] Schematic of an active imaging system with two illuminators and two time-gated cameras in a bistatic arrangement. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] 6 shows an observation system 100 comprising an interface 110, a control and processing subsystem 120, and an active imaging subsystem 130. The control and processing subsystem 120, which comprises processing capabilities, is adapted to execute an improved active imaging algorithm.

[0062] The observation system 100 also typically includes a target detection sensor subsystem and a coarse tracking subsystem, FIG. 1, each of which functions as previously described.

[0063] The active imaging subsystem 130 includes a controller 131, a first pulsed illumination laser 132, a second pulsed illumination laser 133, a first aperture 134, a second aperture 135, a programmable delay 136, and a time-gated camera 137.

[0064] The first laser 132 is adapted to emit laser pulses of relatively long duration compared to the laser pulses emitted by the second laser 133 .

[0065] The first laser 132 is adapted to operate at a repetition rate of at least 2 KHz. The second laser 133 is adapted to operate at a repetition rate below 2 KHz, preferably below 1 KHz, for example below a few hundreds of MHz.

[0066] In operation, pulses of light 150, 160 from each of the first and second lasers 132, 133 are directed to a target (not shown) through a first aperture 134. A second separate aperture 135 collects reflected laser light 150A, 160A from the scene and directs the light onto a focal plane array (not shown) of a gated camera 137 adapted to detect the wavelengths of the returns 150A, 160A of both lasers 132, 133.

[0067] A narrow band filter (not shown) appropriate to the wavelength and linewidth of the first and second lasers is placed in front of the time gated camera 137 .

[0068] The first and second lasers 132, 133 may emit pulses of light having the same or different central wavelengths. If the first and second lasers 132, 133 emit pulses of the same or similar wavelengths, the pulses of the two lasers are interleaved (time multiplexed) so that they are not received simultaneously at the second aperture 135. If the pulses have different central wavelengths, it is still usually preferable to interleave (time multiplex) the pulses, unless the camera is capable of spectral separation and applying different gate times to receive the spectrally separate pulses.

[0069] A programmable delay generator 136 operating under the control of the controller 131 is triggered by the emission from either laser illuminator 132, 133 to time the opening of a gate of a time-gated camera 137 to detect the reflected light return from the respective laser 132, 133. This delay is determined using the distance L to the target, which is known to the observation system 100.

[0070] The gate open time is derived using the laser illuminator pulse duration to minimize the total duration of the gate open time while ensuring that it is longer than the pulse duration. Thus, the gate open time is longer when receiving the return from the first laser illuminator 132 than when receiving the return from laser illuminator 133.

[0071] The controller 301 may operate the lasers 132, 133 such that for every laser pulse generated by the second laser illuminator 133, there may be one or more pulses generated by the first laser illuminator 132.

[0072] The detected images from each return are passed from the time-gated camera 137 through the controller 131 to the control and processing subsystem 120 .

[0073] Due to the relatively short pulse duration of the second laser 133, the image of the target derived from the return from the second laser 2 (hereinafter referred to as the second image or multiple second images) contains relatively little, if any, information of background and foreground features in the scene.

[0074] In contrast, an image of the target derived from the return from the first laser 132 (hereinafter referred to as the first image or first images) contains more information of the background and foreground features of the scene due to the longer pulse time of the first laser, but due to the relatively broad spectral linewidth and high repetition rate of the first laser, the first image contains less spatial noise and therefore may be of higher quality than that from the second laser.

[0075] The control and processing subsystem 120 comprises an image processor 121 adapted to execute an algorithm 200 (see FIG. 7 ) that uses a second image as a template to remove or filter non-target features of the scene from one or more first images that do not appear in the second image.

[0076] Principles of opto-mechanical design well known to those skilled in the art usually allow the system to be constructed and aligned using standard techniques such that the first illuminator is co-aligned with the second illuminator at an acceptable level, which may typically be 1 / 5 to 1 / 20 of the laser divergence angle. Preferably, each illuminator 132, 133 should have the same divergence angle and direction within normal opto-mechanical tolerances, e.g., 1 / 5 of the laser divergence angle. Again, standard techniques for achieving this are known and can reach the required accuracy. Furthermore, the camera itself confirms that this alignment has indeed been achieved, since the illumination spot on the target is clearly visible.

[0077] The firing time of each illuminator laser is synchronized to the camera gate time, as previously described, to ensure that the reflected light reaches the camera 137 while it is open. The gate time may be different for each illuminator (usually longer for the first illuminator 132 than the second illuminator 133). The appropriate firing time for each laser 132, 133 before opening the gate is calculated from the known distance to the target, derived from the laser range finder.

[0078] 7 shows an exemplary algorithm. A specific case is considered where there are maxs first illuminator pulses, where maxs is one or more for every maxn, and maxn is one or more second illuminator pulses, thereby producing a sequence of maxn multiplied by maxs pulses. Typically maxs is greater than maxn.

[0079] At the start of Loop 1, the system acquires an image using a second laser illuminator 133, B(n=1). An image processing function is called to convert this image into a template TB(n=1) using a number of possible image processing techniques.

[0080] After acquiring a template, TB(n=1), a series of maxs images are acquired using the first laser illuminator 132, processed using TB(n=1) to remove background and foreground, and then output as an image.

[0081] After completing a series of maxs images using laser illuminator 1, a second laser illuminator 2 image is acquired and converted to a TB (n=2) which is used to process the next maxs image, and so on.

[0082] To implement this algorithm, the image processor 121 comprises the functions of a template generator 122 and a filter 123 .

[0083] The template generator 122 is adapted to receive a second image 200 from the active imaging subsystem 130 of a scene including a target, and process this image to create a template 250. An exemplary method for this process comprises assigning a value of 1 or 0 to each pixel of the second image. Each pixel that holds a meaningful value is assigned a value of 1, and each pixel that holds substantially no value other than expected background noise is assigned a value of 0. The resulting template 250 may thus be thought of as a silhouette of the target.

[0084] The template 250 is output to the filter 123, which processes a corresponding first image 300 of the same scene produced by the first laser illuminator 312 by pixel multiplication with the template 250. This process zeroes out the values ​​of all pixels in the first image 300 that map to pixels of the template 250 that have a value of 0, thereby removing all background and foreground clutter from the first image 300 leaving only the target in the output image 400. This process does not affect the values ​​of pixels contained within the silhouette of the template 250, and therefore the resulting image 400 retains the high quality of the first image 300.

[0085] The particular template creation process outlined here is for illustrative purposes only, and several other methods of manipulating image pairs may be used, including, for example, an edge detection process.

[0086] Following processing, the final image 400 with background and foreground removed is output via interface 110, for example to a human user or an autonomous system.

[0087] Figure 8 shows a variant architecture of the active imaging subsystem, which differs from the embodiment of Figure 6 in that it includes first and second programmable delays 136, 136' and first and second time-gated cameras 137, 137'.

[0088] The wavelengths of the first laser illuminator 132 and the second laser illuminator 133 are different and their linewidths do not overlap.

[0089] A small portion of the emission from the first laser 132 is received by a first programmable delay 136, and a small portion of the emission from the second laser 133 is received by a second programmable delay 136' to set the gate opening time of the second camera 137'.

[0090] The reflected returns 150A, 160A from the two lasers 132, 133 received at the second aperture 135 are directed to a mirror 138 that has high reflectivity for the wavelength of the second laser illuminator 133 and high transmittance for the wavelength of the first laser illuminator 132. This splits the returns such that the return 150A from the first laser illuminator 132 is received at the first camera 137 and the return 160A from the second laser illuminator 133 is received by the second camera 137'.

[0091] This arrangement allows the first and second laser illuminators 132, 133 to fire simultaneously such that returns 150A, 160A are received simultaneously.

[0092] One or more filters (not shown) may be included to limit the light received at mirror 138 to the wavelengths of the first and second laser illuminators 132,133.

[0093] To provide good filtering, the alignment between the pixels of each camera 137, 137' must be known. For example, if the pixel size in the first time-gated camera is smaller than that of the second time-gated camera, the number and size of pixels can be made equal by creating a rectified image using interpolation or other standard image processing techniques.

[0094] The images derived from the first time gated camera and the second gated camera are processed in the manner described above to generate a template from the return of the second laser illuminator 2 which is used to process the image derived from the first gated camera.

[0095] Ensuring that the FoV of the first gated camera 137 overlaps to an acceptable extent with the FoV of the second time-gated camera 137' can be achieved using design principles well known to those skilled in the art. If the pixel sizes between the cameras are different, optical or processing techniques may be used to interpolate the data so that equal pixels on each time-gated camera can be defined. Finally, it is necessary to ensure that the system knows which pixels in the first gated camera correspond to which pixels in the second time-gated camera. This is difficult to consistently achieve by passive alignment, so an active co-alignment process may need to be used. For example, examples are known where two time-gated cameras can passively view a scene and correlate features appropriate to derive an accurate map of pixel-to-pixel relationships. The following is a summary of the claims as originally filed: [1] An active imaging system comprising: a first laser illuminator and a second laser illuminator, each adapted to illuminate a scene including a target with a pulse of light, wherein the pulse of light emitted from the first laser illuminator has a relatively broad spectral linewidth and a relatively long pulse duration compared to the pulse of light from the second laser illuminator; a camera system positioned to receive light from both the first laser illuminator and the second laser illuminator reflected by the scene, where the camera system: adapted to use a relatively long gating period to receive returns from the first laser illuminator to obtain a first image of the scene including the target; adapted to use a relatively short gating period to receive returns from the second laser illuminator to obtain a second image of the target that selectively excludes non-target elements in the scene; image processing means adapted to use the second image as a template to selectively remove non-target elements from the first image; An active imaging system comprising: [2] The active imaging system of [1], wherein the first laser illuminator is configured to operate at a higher pulse repetition rate than the second laser illuminator. [3] The active imaging system of [2], wherein the first laser illuminator is configured to operate at a pulse repetition rate of at least 2 KHz. [4] The active imaging system of [1] or [2], wherein the active imaging system is adapted to operate the first and second laser illuminators to illuminate the scene at different times. [5] The active imaging system of [4], wherein the first and second laser illuminators operate at substantially the same center wavelength. [6] The active imaging system of any one of [1] to [4], wherein the first and second laser illuminators operate at different center wavelengths. [7] An active imaging system as described in any one of [1] to [6], wherein the camera system comprises a single detector that captures returns from both the first laser illuminator and the second laser illuminator to obtain the first and second images. [8] The active imaging system of [6] or [7], wherein the active imaging system is adapted to operate the first and second laser illuminators to simultaneously illuminate the scene. [9] The active imaging system of [6] or [8], wherein the camera system comprises a first camera adapted to capture returns from the first laser illuminator and a second camera positioned to capture returns from the second laser illuminator.

Claims

1. 1. An active imaging system comprising: a first laser illuminator and a second laser illuminator, each adapted to illuminate a scene including a target with a pulse of light, wherein the pulse of light emitted from the first laser illuminator has a relatively wide spectral linewidth and a relatively long pulse duration compared to the pulse of light from the second laser illuminator; a camera system positioned to receive light from both the first laser illuminator and the second laser illuminator reflected by the scene, where the camera system: adapted to use a relatively long gating period to receive returns from the first laser illuminator to obtain a first image of the scene including the target; adapted to use a relatively short gating period to receive returns from the second laser illuminator to obtain a second image of the target that selectively excludes non-target elements in the scene; image processing means adapted to use the second image as a template to selectively remove non-target elements from the first image; An active imaging system comprising:

2. The active imaging system of claim 1 , wherein the first laser illuminator is configured to operate at a higher pulse repetition rate than the second laser illuminator.

3. The active imaging system of claim 2 , wherein the first laser illuminator is arranged to operate at a pulse repetition rate of at least 2 KHz.

4. 3. The active imaging system of claim 1 or 2, wherein the active imaging system is adapted to operate the first and second laser illuminators to illuminate the scene at different times.

5. The active imaging system of claim 4 , wherein the first and second laser illuminators operate at the same center wavelength.

6. The active imaging system of claim 1 , wherein the first and second laser illuminators operate at different center wavelengths.

7. 7. The active imaging system of claim 1, wherein the camera system comprises a single detector that captures returns from both the first laser illuminator and the second laser illuminator to obtain the first and second images.

8. 8. An active imaging system according to claim 6 or 7, wherein the active imaging system is adapted to operate the first and second laser illuminators to simultaneously illuminate the scene.

9. 9. The active imaging system of claim 6 or 8, wherein the camera system comprises a first camera adapted to capture returns from the first laser illuminator and a second camera positioned to capture returns from the second laser illuminator.

Images