Vincent Larochelle

Standoff sensing of bioaerosols using intensified range-gated spectral analysis of laser-induced fluorescence

In atmospheric sensing, one application that has demonstrated several impressive successes over the last two decades is the light detection and ranging (LIDAR). Elastic LIDAR has shown an important capability in providing aerosol density and spatial distribution from a standoff position. However, it provides limited information on the material composition of the aerosol component. On the other hand, inelastic LIDARs (including laser-induced fluorescence and Raman LIDARs) measure the spectrally distributed returned signal that may contain important clues about the nature of the scatterers. In order to investigate the capability of these LIDARs in characterizing bioaerosols from a standoff position, Defence Research & Development Canada initiated a three-year program in spring 1999, named SINBAHD (Standoff Integrated Bioaerosol Active Hyperspectral Detection). The aim of the program was to investigate the sensitivity and discrimination capabilities of an inelastic LIDAR based on the intensified range-gated spectral detection of laser-induced fluorescence. An exploratory prototype based on this technique has shown sensitivity of a few living bioaerosol particles per liter of air for a range of 1.4 km at night. Furthermore, based on spectral signatures measured during open-air releases, good discrimination capabilities were obtained between Bacillius subtilis var globiggi (BG) and Erwinia herbicola (EH). These results agree well with a performance model using Raman returns from atmospheric nitrogen as a calibration tool.

Range-gated active-imaging system for search-and-rescue and surveillance operations

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.

Estimating atmospheric extinction for eyesafe laser rangefinders

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 m and α a . We present simple expressions for α m and α a 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.

Enhanced Military Target Discrimination using Active and Passive Polarimetric Imagery

Surveillance operations often make use of electro-optic (EO) imaging systems to detect civilian and military targets. To increase the overall target detection performance, such active/passive EO sensors could exploit the polarization of light as additional information to discriminate man made objects against different backgrounds. The target contrast enhancement obtained by analyzing the polarization of the reflected light from either a direct polarized laser source as encountered in active imagers, or from natural ambient illumination, can be used for such target discrimination scheme. This paper reports results from field experiments exploiting polarization-based imaging sensors to enhance the detection of man made objects. Active and passive polarimetric signatures of objects have been acquired at wavelengths in the near and long-wave infrared bands. Results demonstrate to what extent and under which illumination and environmental conditions the exploitation of active/passive polarimetric images is suitable to enable target discrimination.

Evaluation of active and passive polarimetric electro-optic imagery for civilian and military targets discrimination

Electro-optic (EO) imaging systems are commonly used to detect civilian and military targets during surveillance operations and search and rescue missions. Adding the polarization of light as additional information to such active and passive EO imaging systems may increase the target discrimination performance, as man made objects are known to depolarized light in different manner than natural background. However, while the polarization of light has been used and studied in the past for numerous applications, the understanding of the polarization phenomenology taking place with targets used in cluttered backgrounds requires additional experimentations. Specifically, the target contrast enhancement obtained by analyzing the polarization of the reflected light from either a direct polarized laser source as encountered in active imagers, or from natural ambient illumination, needs further investigation. This paper describes an investigation of the use of polarization-based imaging sensors to discriminate civilian and military targets against different backgrounds. Measurements were carried out using two custom-designed active and passive imaging systems operating in the near infrared (NIR) and the long-wave infrared (LWIR) spectral bands. Polarimetric signatures were acquired during two distinct trials that occurred in 2007, using specific civilian and military targets such as cars and military vehicles. Results demonstrate to what extent and under which illumination and environmental conditions the exploitation of active and passive polarimetric images is suitable to enable target detection and recognition for some events of interest, according to various specific scenarios.

Two generations of Canadian active imaging systems: ALBEDOS and ELVISS

Search and rescue and general surveillance mission pose a serious challenge to conventional imaging systems used by actual aircraft crews. These systems must often work in low- light and low-visibility conditions to find the identify targets. A new airborne imaging technology has been developed to overcome several deficiencies encountered with common CCD cameras, image intensified system and thermal imaging sensors. The recent developments in laser diode arrays, laser diode beam collimation and gatable micro- channel plate intensifier have made possible the construction of a compact active imagin system, called the Airborne Laser-Based Enhanced Detection and Observation Systems (ALBEDOS). This system proved particularly efficient at night and in degraded weather conditions. In addition, it was demonstrated that range gating, besides eliminating most of the light backscattered by aerosols, provided to some extent immunity to blooming effects specific to highly sensitive cameras. The system was installed on a helicopter and tested in various scenarios in October 1995 to demonstrate its potential. To enhance the surveillance capability over large areas of coverage, to optimize detection of humans and small objects and to improve the effectiveness of the search aircraft, a second-generation payload is presently developed and combines the benefits of two complementary imaging sensors. The Enhanced Low-Light level Visible and IR Surveillance System (ELVISS) consists of an improved range-gated active imager and a high-quality thermal imager, installed in two separate airborne platforms slaved together and controlled by a single user interface. It is expected that such a sensor systems will have a direct impact on improving the response time in finding those in need of assistance or simply in increasing the performance, reliability and efficiency of crews involved in general surveillance operations. This paper explains the concept of range gating, details a preliminary performance model and describes the two generations of Canadian active imagers: ALBEDOS and ELVISS.

ALBEDOS: an airborne laser-based enhanced detection and observation system for coast guard and maritime patrol application

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.

Active range-gated spectrometric standoff detection and characterization of bioaerosols

In atmospheric sensing, one application that has demonstrated several impressive successes over the last two decades is LIght Detection And Ranging (LIDAR). With elastic signal returns, this technique remotely provides information such as the particle density and, for a multiple field of view LIDAR, the distribution in size of the aerosols as a function of the range along the probing laser beam. For this type of application, the return signal has the same spectrum than the laser source. Some specific techniques, such as Raman or resonant LIDARs, collect the return signal at wavelengths other than the source. However, these signals are usually narrow spectrally and are collected with a single bandpass spectral filter. Recently, the Canadian Defence Research and Development Branch has initiated the evaluation of a novel LIDAR concept which opens the possibility of collecting simultaneously the detailed spectral information contained in spectrally wide return signals. One drawback with this approach is the loss of simultaneous information at multiple ranges, i.e., the spectral information is available only for a specific range. Nevertheless, there are applications where the partial loss of range information is compensated by the gain resulting from the spectral information. This paper describes the concept and reviews the general model predicting the capability of this technique for the standoff detection of bioaerosols. It shows a numerical simulation of the anticipated spectral profiles collected with the proposed active range-gated fluorescent LIDAR for a particular bioaerosol as a function of ranges, and for both day and night operational scenarios.

Performance assessment of various imaging sensors in fog

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.

Airborne far-IR minefield imaging system (AFIRMIS): description and preliminary results

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.