Laser active imager are introducing a new paradigm in the domain of surveillance. Because they provide the capacity to
image objects based on their reflectivity and not their emissivity, and because they provide a capacity to see through
glass. Moreover, because that being based on high-performance gated intensified tubes, they can operate in adverse
atmospheric conditions, and are becoming looked at as a very valuable tool to gather precise identification information at
long ranges. On the other hand, the laser source making this technology so interesting must offer a safe operational mode
In this paper, we will show the most recent results that this technology can achieve in ship identification and discuss how
to implement safety features to make the laser active imager an eye safe new tool for long range observation whatever its
Night vision capability has become an indispensable tool for military and civilian surveillance operations. Low-light- level television (LLLTV) and Forward-Looking-IR (FLIR) devices have long been used for these applications. Nevertheless, both have their shortcomings when the identification of the target is essential for the success of the mission. LLLTV cannot provide god image resolution in ultra low-light level conditions and is very sensitivity to parasitic light. FLIR system have poor resolution when the temperature difference contrast conditions are not met.
In minefield detection, two main types of operation can be identified. First, there is the detection of surface-laid minefield. This scenario is encountered largely in tactical operations (troop movement, beach landing) where the speed at which the minefield is deployed or the strategic barrier that they represent exceed the need to bury them. Second, there is the detection of buried minefield which is encountered mainly in peacekeeping missions or clearance operations. To address these two types of minefield detection process, we propose an airborne far-infrared minefield imaging system (AFIRMIS). This passive and active imaging system fuses the information from the emissivity, the reflectivity and the 3-dimensional profile of the target/background scene in order to improve the probability of detection and to reduce the false alarm rate. This paper describes the proposed imaging system and presents early active imaging results of surface-laid mines.
All systems operating in the visible and infrared bands of the spectrum are subject to a severe performance degradation when used in adverse weather conditions like fog, snow or rain. This is particularly true for active systems as rangefinders, laser designator, lidars and active imaging sensors where the laser beam will suffer attenuation, turbulence and scattering from the aerosols present in the atmospheric path. This paper presents the ALBEDOS active imaging performance in fog which was determined by observing reference targets through a 22-m controlled-environmental chamber, where fogs with various densities and droplet sizes were generated in a calibrated manner. ALBEDOS is an acronym for Airborne Laser-Based Enhanced Detection and Observation System and is based on a compact, powerful laser diode illuminator and a range-gated intensified CCD camera. It is capable of detecting and identifying people or objects in complete darkness and, to some extent, in adverse weather conditions. In this paper, we compare the efficiency of the range-gated active imager in fog with those of a far-infrared thermal imager and of a low-light level camera operating in a continuous mode.
Search and rescue (SAR) and general surveillance missions pose a number of challenges to imaging system. These systems must work often in low-light level, low-visibility conditions to find and identify targets. A new airborne payload has been developed to overcome several deficiencies encountered with conventional or low-light level cameras as well as thermal imaging sensors. The recent developments in laser diode arrays, laser diode beam collimation and gatable micro-channel plate intensifier have made it possible to build a compact active imaging system. This Airborne Laser Based Enhanced Detection and Observation System (ALBEDOS) is particularly efficient at night and in degraded weather conditions. ALBEDOS is based on a powerful laser diode array illuminator and a range-gated low-light-level TV camera. Therefore, it is immune to the blooming effect specific to highly sensitive cameras and eliminates most of the light backscatter caused by the presence of aerosols. It was proven to detect small retroreflective tapes over many kilometers. In October 1995, the system was installed on a Bell 412 helicopter and tested in various scenarios.
KEYWORDS: Mass attenuation coefficient, Laser range finders, Atmospheric modeling, Signal to noise ratio, Aerosols, Fiber optic gyroscopes, Humidity, Visibility, Visibility through fog, Atmospheric particles
Extinction of laser rangefinder (LRF) pulses by the atmosphere depends on the laser wavelength, weather conditions, and the aerosol concentration along the optical path. The total atmospheric extinction α is the sum of the molecular and aerosol contributions α<sub><i>m</i></sub> and α<sub><i>a</i></sub>. We present simple expressions for α<sub><i>m</i></sub> and α<sub><i>a</i></sub> for wavelengths near 1.44, 1.54, 2.1, and 10.6 μm, which are eyesafe for most LRF applications. Also included are results for 1.06 μm, which although not an eyesafe wavelength is used extensively for LRF applications. The expressions allow the extinction coefficient to be estimated as a function of standard meteorological parameters, assuming horizontal beam propagation at sea level and a homogeneous atmosphere. Measurements of the signal-to-noise ratio of LRF returns from a calibrated target are presented for various weather conditions and wavelengths.
Two years ago we designed, built, and tested a ROV mounted range-gated imaging system. Given that the target covers at least one pixel at the maximum range of interest the model predicts that for the same laser power and under the condition where the field of illumination is matched to the field of view there is no performance penalty in increasing the field of view. In order to test this result we have built and deployed a second generation underwater imaging system whose field of view and field of illumination are matched and continuously variable from 60 mr to 600 mr in water. The laser source was also upgraded in power by a factor of 10 to a water cooled, 2-kHz, 400 mw doubled Nd:YLF laser. The light is collected by a 7-cm diameter zoom lens. The detector is a gated image intensifier with a 7-ns gate and a gain which is continuously variable from 500 to 1,000,000. An on-board image processor has been added to the system. It allows us to frame integrate in real-time and thus further improve system performance.
Search and rescue (SAR) and maritime patrol missions pose a number of challenges for an imaging system. Systems must work in low light level, low visibility conditions to find and identify small targets for both search and rescue and law enforcement roles. Passive low light and thermal imaging systems are often unable to discriminate small targets against sea backgrounds due to low thermal contrast and non-cooperative targets. Active gated television (AGTV) as implemented in the ALBEDOS system, enhances the reconnaissance, surveillance, and SAR capabilities of maritime organizations by generating high resolution video imagery regardless of ambient light and conditions or target thermal properties. Active television uses a laser source to illuminate a scene being viewed by a low light television (LLTV) camera, can filter out unwanted light sources, and also limit the image depth of field. AGTV systems generate video for display or recording under conditions that are typically difficult for other sensors. AGTV systems have demonstrated their ability to provide long range detection of SAR targets, to allow the positive identification of people, and to read license plates and ship or aircraft markings covertly at long ranges. This paper summarizes the advantages of AGTV for reconnaissance and surveillance missions, briefly discusses the theories of operation, and compares AGTV performance to that of conventional sensors. A compact airborne AGTV configuration being developed for trials by the Canadian Forces is described.
Extinction of laser rangefinder (LRF) pulses by the atmosphere depends on the LRF wavelength, weather conditions, and the aerosol concentration along the optical path. The total atmospheric extinction (alpha) ((lambda) ) is the sum of the molecular and aerosol contributions, (alpha) <SUB>m</SUB>((lambda) ) and (alpha) <SUB>a</SUB>((lambda) ). We present simple expressions for (alpha) <SUB>m</SUB>((lambda) ) and (alpha) <SUB>a</SUB>((lambda) ) for the LRF sources: Er:glass, Ho:YAG, and CO<SUB>2</SUB> which operate near 1.54, 2.1, and 10.6 micrometers respectively. Also included are results for Nd:YAG which may be made to lase at the eyesafe wavelength of 1.444 micrometers . The expressions give an estimate of (alpha) ((lambda) ) as a function of standard meteorological parameters, assuming horizontal beam propagation. The effect of forward scattering on the received LRF signal is also discussed.
A careful analysis of a scattering and absorption database of the waters off the coasts of Canada shows that a laser-assisted camera system will have a significantly improved viewing performance over conventional systems. The laser underwater camera image enhancer system is a range-gated laser system that can be mounted on a remotely operated vehicle. The system uses a 2-kHz diode-pumped frequency-doubled Nd:YAG laser as an illumination source. The light is collected by a 10-cm-diam zoom lens. The detector is a gated image intensifier with a 7-ns gate and a gain that is continuously variable from 500 to 1,000,000. The system has been tested in a water tank facility at Defence Research Establishment Valcartier and has been mounted on the HYSUB 5000 remotely operated vehicle for sea trials. In the strongly scattering waters typical of harbor approaches, this system has a range of from three to five times that of a conventional camera with floodlights.
A careful analysis of the scattering and absorption data base of the waters off the coasts of Canada has persuaded us that a laser assisted camera system will have a significantly improved viewing performance over conventional systems. With this purpose in mind, we designed and built the laser underwater camera image enhancer system (LUCIE). The system uses a 2 kHz diode-pumped frequency-doubled Nd:YAG laser as an illumination source. The light is collected by a 10 cm diameter zoom lens. The detector is a gated image intensifier with a 7 ns gate and a gain which is continuously variable from 500 to 1,000,000. The gate delay is adjusted to the focal distance of the lens system. This ensures that only the scattering occurring near the target is seen by the camera system. In the strongly scattering waters typical of harbor approaches this system has a range of from 4 to 6 times that of a conventional camera with floodlights. The system has been tested in a water tank facility at DREV and has been mounted on the HYSUB 5000 remotely operated vehicle (ROV) for sea trials. The images from the system are sent to the surface via a high performance analog link with a bandwidth of 8 MHz. The images are processed to remove the effect of marine snow. This processing and the high repetition rate of the laser, which ensures a lack of speckle, both contribute significantly to the clarity of the images. The NEARSCAT transmissometer- nephelometer system is operated simultaneously with the LUCIE system and this allows us to have the fundamental data necessary for evaluating the performance of the imaging system and validating transmission, scattering, and imaging models.