Michael J. Duggin

Imaging polarimetry in scene element discrimination

Recent work has shown that the use of a calibrated digital camera fitted with a rotating linear polarizer can facilitate the study of Stokes parameter images across a wide dynamic range of scene radiance values. Here, we show images of a MacBeth color chips, Spectralon gray scale targets and Kodak gray cards. We also consider a static aircraft mounted on a platform against a clear sky background. We show that the contrast in polarization is greater than for intensity, and that polarization contrast increases as intensity contrast decreases. We also show that there is a great variation in the polarization in and between each of the bandpasses: this variation is comparable to the magnitude of the variation in intensity.

Synthesis of optical polarization signatures of military aircraft

Focal plane wide band IR imagery will be compared with visual wide band focal plane digital imagery of a camouflaged B-52 bomber. Extreme enhancement is possible using digital polarized imagery. The experimental observations will be compared to theoretical calculations and modeling result of both specular and shadowed areas to allow extrapolations to the synthesis of the optical polarization signatures of other aircraft. The relationship of both the specular and the shadowed areas to surface structure, orientation, specularlity, roughness, shadowing and the complex index of refraction will be illustrated. The imagery was obtained in two plane-polarized directions. Many aircraft locations were measured as well as sky background.

Optical enhancement of aircraft detection using polarization

Focal plane wideband infrared digital polarization imagery will be compared with visual wideband focal plane digital imagery of a camouflaged C-130 aircraft to show the extreme enhancement possible using digital imagery. The experimental observations will be compared with theoretical calculations and modeling results of both specular and shadowed areas. The relationship of both the specular and the shadowed areas to surface structure, orientation, specularity, roughness, shadowing, orientation and complex index of refraction will be illustrated. The imagery was obtained in four plane polarized directions with axes oriented vertically, horizontally and at plus and minus 45 degrees to the vertical. Nine locations on the aircraft were chosen (tail, fuselage, wing and propeller as well as five sky locations to establish sky background. Both sunlit and shadowed locations were examined. The direction of the dominant plane of polarization was obtained, but not the existence of circular polarization, which requires a quarter wave plate to resolve temporal coherence. Unpolarized radiation exists in the imagery,but its coherence is not evident without a phase resolving element.

Hyperspectral imaging polarimeter design and calibration

The integration and calibration of a hyperspectral imaging polarimeter is described. The system was designed to exploit subtle spectral details in visible and near-IR hyperspectral polarimetric images. All of the system components were commercial-off-the-shelf. This device uses a tunable liquid crystal filter and 16-bit cooled CCD camera. The challenges of calibrating a hyperspectral polarimeter are discussed.

Vegetative target enhancement in natural scenes using multiband polarization methods

Relatively little work has been performed to investigate the potential of polarization techniques to provide contrast enhancement in natural scenes. Largely, this is because film is less accurate radiometrically than digital CCD FPA sensing devices. Such enhancement is additional to that provided by between-band differences for multiband data. Recently, Kodak has developed several digital imaging cameras which were intended for professional photographers. The variant we used produced images in the green, red and near IR, simulating CIR film. However, the application of linear drivers to read the data from the camera into the computer has resulted in a device which can be used as a multiband imaging polarimeter. Here we examine the potential of digital image acquisition as a potential quantitative method to obtain new information additional to that obtained by multiband or even hyperspectral imaging methods. We present an example of an active on-going research program.

Measurements of polarization of targets of differing albedo and shadow depth

There are many measurements of polarization made with non- imaging polarimeters. Such measurements have been made in the laboratory, of the sky and of the ground. These measurements can be interpreted only when subsidiary information enables identification of the surface under study. Some measurements have been made with imaging polarimeters based upon film, but these were limited in radiometric accuracy by the medium, or by lack of sensitometry. Some investigators fabricated a polarimeter from videcon cameras, but this study was also limited by radiometric fidelity. With the advent of digital cameras with linear focal plane radiometric response, and software retaining this linearity in extracting the image from the camera, greater radiometric accuracy has been achieved. We report here measurements of polarization which we show to be related to scene radiance. The radiance levels covered include a wide dynamic range and facilitate study of low radiance levels in general previously inaccessible to measurement using an imaging device. We also include data from previous measurements with non-imaging devices and show that they are compatible with data collected using a digital camera. There is an inverse linear relationship between the logarithm of the polarization in recorded radiance and the logarithm of the recorded radiance in data obtained with both imaging and with non-imaging polarimeters.

Enhancement of vegetation mapping using Stokes parameter images

Relatively little work has been performed to investigate the potential of polarization techniques to provide contrast enhancement information for vegetation mapping, and for vegetation condition assessment. Largely, this is because film is less accurate radiometrically than digital FPA sensing devices. Since polarization studies necessitate the differencing of images obtained with a linear polarizer rotated about the optic axis of a camera between sequential exposures, and since some of the differences are small, film has generally lacked the radiometric accuracy needed to reliably record such differences. Kodak has developed a high spatial resolution camera, and the development of linear drivers to read the data from the camera into the computer has resulted in a device which can be used as a multiband imaging polarimeter. Here we examine the potential of digital image acquisition as a potential quantitative method to obtain new information uncorrelated with that obtained by more conventional multiband imaging methods. Such information can potentially be used to form more sensitive vegetation indices, to differentiate species, and to penetrate canopy. We present promising examples of an active on-going research program.

Acquisition of multiband polarimetric imagery: calibration considerations

There is evidence that polarimetric as well as intensity contrast differences in images are band-dependent. Some previous evidence is reviewed. We are currently fabricating and testing a hyperspectral imaging polarimeter to assess and to take advantage of subtle spectral detail in hyperspectral polarimetric images. This device uses a tunable liquid crystal filter and a 16-bit camera. New and varied calibration challenges have occurred and are discussed. We consider it important to present the problems as well as the successes.

Comparative merits of multispectral optical polarization to microwave remote sensing

A brief history of RADAR development is followed by an indication of the relevance of LIDAR to ranging and detection of targets. Initially, radiated laser power is discussed. Peak power of 100 kilowatts with a diode pumped solid-state laser appears feasible. Frequency control appears possible with atomic standards controlling the high power laser. Optical characterization of the polarization properties of lasers on targets is being pursued as well as the options. Coherence length of LASER radiation still poses a problem over ranges beyond one hundred meters. Target identification is enhanced using polarization with the aid of higher-resolution focal plane arrays. Coherence applications appear feasible in the near future.

Radiometric sensor performance model including atmospheric and IR clutter effects

The computer code SENSAT developed for radiometric investigations in remote sensing was extended to include two statistical clutter models of infrared background and the prediction of the target detection probability. The first one is based on the standard deviation of scene clutter estimated from scene data, the second one is based on the power spectral density of different classes of IR background as a function of temporal or spatial frequency. The overall code consists of modules describing the optoelectronic sensor (optics, detector, signal processor), a radiative transfer code (MODTRAN) to include the atmospheric effects, and the scene module consisting of target and background. The scene is evaluated for a certain pixel at a time. However, a sequence of pixels can be simulated by varying the range, view angle, atmospheric condition, or the clutter level. The target consists of one or two subpixel surface elements, the remaining part of the pixels represents background. Multiple paths, e.g. sun-ground-target-sensor, can also be selected. An expert system, based upon the IDL language, provides user-friendly input menus, performs consistency checks, and submits the required MODTRAN and SENSAT runs. A sample case of the detection probability of a sub-pixel target in a marine cluttered background is discussed.