Georges R. Fournier

Analytic phase function for ocean water

Using a modified form of the anomalous diffraction approximation we have been able to derive in closed form an analytic expression for the phase function of Mie scatterers integrated over an inverse power law (Junge) size distribution. The analysis explains the apparent singularity seen experimentally at the forward scattering angle. Simple relationships are also derived that relate the inverse power law as a function of scattering angle in the near forward direction to the power law of the size distribution. The parameters of the formula are the relative index of refraction and the inverse power of the size distribution. A comparison is given between the analytic formula and exact integration of the Mie scattering for spheres. This new phase function is used in the analysis of forward angle transmissometer- nephelometer data collected by DREV in the Arctic, Atlantic, and Pacific.

Range-gated underwater laser imaging system

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.

In Harbor Underwater Threat Detection/Identification Using Active Imaging

We present results from trials of the LUCIE 2 (Laser Underwater Camera Image Enhancer) conducted in Halifax Harbor, Nova Scotia, Canada and Esquimalt Harbor, Victoria, British Columbia, Canada. LUCIE 2 is a new compact laser range gated camera (10 inches in diameter, 24 inches in length, and neutrally buoyant in water) originally designed to improve search and recovery operations under eye safe restrictions. The flexibility and eye safety of this second generation LUCIE makes it a tool for improved hull searches and force protection operations when divers are in the water attempting to identify bottom lying objects. The camera is equipped with a full image geo-positioning system. To cover various environmental and targets size conditions, the gate-delay, gate width, polarization and viewing and illuminating angles can be varied as well. We present an analysis on the performance of the system in various water conditions using several target types and a comparison with diver and camera identification. Coincident in-situ optical properties of absorption and scattering were taken to help resolve the environmental information contained in the LUCIE image. Several new capabilities are currently being designed and tested, among them a differential polarization imaging system, a stabilized line of sight system with step-stare capability for high resolution mosaic area coverage, a precision dimensioning system and a diver guided and operated version.

Imagery-derived modulation transfer function and its applications for underwater imaging

The main challenge working with underwater imagery results from both rapid decay of signals due to absorption, which leads to poor signal to noise returns, and the blurring caused by strong scattering by the water itself and constituents within, especially particulates. The modulation transfer function (MTF) of an optical system gives the detailed and precise information regarding the system behavior. Underwater imageries can be better restored with the knowledge of the system MTF or the point spread function (PSF), the Fourier transformed equivalent, extending the performance range as well as the information retrieval from underwater electro-optical system. This is critical in many civilian and military applications, including target and especially mine detection, search and rescue, and diver visibility. This effort utilizes test imageries obtained by the Laser Underwater Camera Imaging Enhancer (LUCIE) from Defense Research and Development Canada (DRDC), during an April-May 2006 trial experiment in Panama City, Florida. Imaging of a standard resolution chart with various spatial frequencies were taken underwater in a controlled optical environment, at varying distances. In-water optical properties during the experiment were measured, which included the absorption and attenuation coefficients, particle size distribution, and volume scattering function. Resulting images were preprocessed to enhance signal to noise ratio by averaging multiple frames, and to remove uneven illumination at target plane. The MTF of the medium was then derived from measurement of above imageries, subtracting the effect of the camera system. PSFs converted from the measured MTF were then used to restore the blurred imageries by different deconvolution methods. The effects of polarization from source to receiver on resulting MTFs were examined and we demonstrate that matching polarizations do enhance system transfer functions. This approach also shows promise in deriving medium optical properties including absorption and attenuation.

Target discrimination of man-made objects using passive polarimetric signatures acquired in the visible and infrared spectral bands

Surveillance operations and search and rescue missions regularly exploit electro-optic imaging systems to detect targets of interest in both the civilian and military communities. By incorporating the polarization of light as supplementary information to such electro-optic imaging systems, it is possible to increase their target discrimination capabilities, considering that man-made objects are known to depolarized light in different manner than natural backgrounds. As it is known that electro-magnetic radiation emitted and reflected from a smooth surface observed near a grazing angle becomes partially polarized in the visible and infrared wavelength bands, additional information about the shape, roughness, shading, and surface temperatures of difficult targets can be extracted by processing effectively such reflected/emitted polarized signatures. This paper presents a set of polarimetric image processing algorithms devised to extract meaningful information from a broad range of man-made objects. Passive polarimetric signatures are acquired in the visible, shortwave infrared, midwave infrared, and longwave infrared bands using a fully automated imaging system developed at DRDC Valcartier. A fusion algorithm is used to enable the discrimination of some objects lying in shadowed areas. Performance metrics, derived from the computed Stokes parameters, characterize the degree of polarization of man-made objects. Field experiments conducted during winter and summer time demonstrate: 1) the utility of the imaging system to collect polarized signatures of different objects in the visible and infrared spectral bands, and 2) the enhanced performance of target discrimination and fusion algorithms to exploit the polarized signatures of man-made objects against cluttered backgrounds.

A new passive polarimetric imaging system collecting polarization signatures in the visible and infrared bands

Electro-optical imaging systems are frequently employed during surveillance operations and search and rescue missions to detect various targets of interest in both the civilian and military communities. By incorporating the polarization of light as supplementary information to such electro-optical imaging systems, it may be possible to increase the target discrimination performance considering that man-made objects are known to depolarize light in different manners than natural backgrounds. Consequently, many passive Stokes-vector imagers have been developed over the years. These sensors generally operate using one single spectral band at a time, which limits considerably the polarization information collected across a scene over a predefined specific spectral range. In order to improve the understanding of the phenomena that arise in polarimetric signatures of man-made targets, a new passive polarimetric imaging system was developed at Defence Research and Development Canada - Valcartier to collect polarization signatures over an extended spectral coverage. The Visible Infrared Passive Spectral Polarimetric Imager for Contrast Enhancement (VIP SPICE) operates four broad-band cameras concomitantly in the visible (VIS), the shortwave infrared (SWIR), the midwave infrared (MWIR), and the longwave infrared (LWIR) bands. The sensor is made of four synchronously-rotating polarizers mounted in front of each of the four cameras. Polarimetric signatures of man-made objects were acquired at various polarization angles in the four spectral bands. Preliminary results demonstrate the utility of the sensor to collect significant polarimetric signatures to discriminate man-made objects from their background.

Underwater laser imaging system with large field of view

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.

Interpretation of scattering by oceanic particles around 120 degrees and its implication in ocean color studies

Field observations and theoretical studies have found that the volume scattering functions (VSFs) of oceanic particles exhibit minimum variability at angles near 120°. However, its physical interpretation is still unknown. We find this minimum variability angle represents the intersection of two backscattering-normalized VSFs, one representing particles of sizes smaller than the wavelength of light and the other larger than the wavelength of light. This also suggests that the VSFs of oceanic particles at angles between 90° and 180°, which play a critical role in ocean color study, can be modeled by linear mixing of these two end members. We further validate this mixing model using measured VSFs in coastal and oceanic waters around the US and develop a two-component model predicting the backward shapes of the VSFs.

Development of performance metrics to characterize the degree of polarization of man-made objects using passive polarimetric images

Spectral sensors are commonly used to measure the intensity of optical radiation and to provide spectral information about the distribution of material components in a given scene, over a limited number of wave bands. By exploiting the polarization of light to measure information about the vector nature of the optical field across a scene, collected polarimetric images have the potential to provide additional information about the shape, shading, roughness, and surface features of targets of interest. The overall performance of target detection algorithms could thus be increased by exploiting these polarimetric signatures to discriminate man-made objects against different natural backgrounds. This is achieved through the use of performance metrics, derived from the computed Stokes parameters, defining the degree of polarization of man-made objects. This paper describes performance metrics that have been developed to optimize the image acquisition of selected polarization angle and degree of linear polarization, by using the Poincare sphere and Stokes vectors from previously acquired images, and then by extracting some specific features from the polarimetric images. Polarimetric signatures of man-made objects have been acquired using a passive polarimetric imaging sensor developed at DRDC Valcartier. The sensor operates concomitantly (bore-sighted images, aligned polarizations) in the visible, shortwave infrared, midwave infrared, and the long-wave infrared bands. Results demonstrate the improvement of using these performance metrics to characterize the degree of polarization of man-made objects using passive polarimetric images.

NEARSCAT full-spectrum narrow forward-angle transmissometer-nephelometer

The crucial parameters required for the design of underwater optical systems are optical absorption and scattering as a function of location, depth, and wavelength in the ocean. DREV has developed, built, and deployed an underwater probe (NEARSCAT), whose sole purpose is to gather information about the underwater light field in the waters of interest to Canada. The instrument is unique in that it can scan all wavebands in the visible spectrum from 400 nm to 700 nm. It can also continuously sample up to 6 arbitrarily chosen wavelength bands simultaneously with a resolution of 10 nm. The instrument can separate the absorption and scattering components of seawater. This instrument was deployed at 16 locations along the East Coast of Canada, ranging from the north of Baffin Island to Cabot Strait. It was also deployed at 21 stations on the West Coast of Canada. The water column was sampled to a maximum depth of 300 m. The data was found to be extremely consistent and of high quality. We found that the waters were much less absorbing than was previously believed. A strong scattering layer was found to exist near the surface, and extending to a depth of 40 meters. This layer does not strongly absorb. The lack of absorption, the strong layering of scattering, and the predominance of narrow-angle forward scattering has important consequences for optical underwater systems as it implies that signal recovery techniques will be effective in the underwater environment and allow much better performance than was previously thought to be possible.