Fraunhofer diffraction by arbitrary-shaped obstacles.

We consider Fraunhofer diffraction by an ensemble of large arbitrary-shaped screens that are randomly oriented in the plane of a wavefront and have edges of arbitrary shape. It is shown that far outside the main diffraction peak the differential scattering cross section behaves asymptotically as theta(-3), where theta is the diffraction angle. Moreover, the differential scattering cross section depends only on the length of the contours bordering the screens and does not depend on the shape of the obstacles. As both strictly forward and total diffraction cross sections are specified by obstacle area only, the differential cross section of size-distributed obstacles is expected to be nearly independent of obstacle shape over the entire region of the diffraction angles.

Lidar returns from multiply scattering media in multiple-field-of-view and CCD lidars with polarization devices: comparison of semi-analytical solution and Monte Carlo data.

Quite recently, a semi-analytical approach to the sounding of multiply scattering media (clouds, seawaters) using multiple-field-of-view and CCD lidars with polarization devices was developed. The angular distributions of polarized components of the lidar returns from multiply scattering media computed on the basis of this theory using the small-angle approximation are presented and discussed. The semi-analytical nature of the solution makes the computation procedure faster. The obtained data are compared with results provided by the most advanced Monte Carlo algorithms for simulation of modern lidar performance. The good agreement between data provided by the semi-analytical approach and Monte Carlo computations assures one that these approaches can serve as a reliable theoretical base for interpretation and inversion of cloud lidar sounding data obtained with polarized lidars, including polarized multiple-field-of-view and CCD lidars.

Possibilities of warm cloud microstructure profiling with multiple-field-of-view Raman lidar.

The possibilities of cloud characteristics retrieval with multiple-field-of-view Raman lidar are considered. It has been shown that the Raman lidar return is sensitive to two cloud characteristics; the scattering coefficient and the effective droplet size. This sensitivity is studied and the optimal receiver fields-of-view (FOVs) for cloud sounding are recommended. The optimal FOV values are estimated to be approximately R/H (R, the collecting optics radius, H, the cloud altitude) to measure the scattering coefficient profiles, and approximately 0.01z/H for the droplet size measurements (z, the cloud thickness). The algorithm based on the iterative scheme and singular value decomposition as a regularization procedure is presented and verified using computer simulation. The recommendations for profile retrieval with variable altitude resolution are given.

Demodulation techniques for the amplitude modulated laser imager.

A new technique has been found that uses in-phase and quadrature phase (I/Q) demodulation to optimize the images produced with an amplitude-modulated laser imaging system. An I/Q demodulator was used to collect the I/Q components of the received modulation envelope. It was discovered that by adjusting the local oscillator phase and the modulation frequency, the backscatter and target signals can be analyzed separately via the I/Q components. This new approach enhances image contrast beyond what was achieved with a previous design that processed only the composite magnitude information.

Simple model of the optical characteristics of bubbles and sediments in seawater of the surf zone.

The development of a simple model of the seawater inherent optical properties (IOPs) associated with bubbles and sediments would represent a great advance in surf zone optics. We present one solution for this problem using a combination of geometrical optics and Fraunhofer diffraction. An analytic model of the IOPs of bubbles and sediments (the extinction and absorption coefficients, and phase function) is developed in terms of the moments of the particle size distribution and the complex refractive index of particles.

Amplitude-modulated laser imager.

Laser systems have been developed to image underwater objects. However, the performance of these systems can be severely degraded in turbid water. We have developed a technique using modulated light to improve underwater detection and imaging. A program, Modulated Vision System (MVS), which is based on a new theoretical approach, has been developed to predict modulated laser imaging performance. Experiments have been conducted in a controlled laboratory environment to test the accuracy of the theory as a function of system and environmental parameters. Results show a strong correlation between experiment and theory and indicate that the MVS program can be used to predict future system performance.

Retrieving seawater-backscattering profiles from coupling Raman and elastic lidar data.

We propose a technique for retrieving seawater-backscattering profiles that is based on the joint use of elastic and Raman lidar returns. We suggest using two lidar channels: the Raman channel and the elastic channel with a light frequency equal to a half-sum of initial and Raman-shifted frequencies of the Raman channel. These specific wavelengths provide the same attenuation laws for elastic and Raman signals if absorption and scattering spectra can be approximated by a power law. In particular, seawater supplies such a possibility in the region of 400-500 nm if extremely bioproductive waters are not considered and the chlorophyll absorption peak at 440 nm does not come out of the background of dissolved organic matter absorption. With these specific initial wavelengths, the elastic and Raman lidar returns differ only in the backscattering coefficients. Because the Raman-backscattering coefficient is constant along the profile, the (elastic-to-Raman) ratio of these lidar returns directly produces the profile of the elastic-backscattering coefficient. This technique stays valid even under multiple-scattering conditions, which is of great importance for seawater sounding.

Scattering optics of snow.

Permanent snow and ice cover great portions of the Arctic and the Antarctic. It appears in winter months in northern parts of America, Asia, and Europe. Therefore snow is an important component of the hydrological cycle. Also, it is a main regulator of the seasonal variation of the planetary albedo. This seasonal change in albedo is determined largely by the snow cover. However, the presence of pollutants and the microstructure of snow (e.g., the size and shape of grains, which depend also on temperature and on the age of the snow) are also of importance in the variation of the snow's spectral albedo. The snow's spectral albedo and its bidirectional reflectance are studied theoretically. The albedo also determines the spectral absorptance of snow, which is of importance, e.g., in studies of the heating regime in snow. We investigate the influence of the nonspherical shape of grains and of close-packed effects on snow's reflectance in the visible and the near-infrared regions of the electromagnetic spectrum. The rate of the spectral transition from highly reflective snow in the visible to almost totally absorbing black snow in the infrared is governed largely by the snow's grain sizes and by the load of pollutants. Therefore both the characteristics of snow and its concentration of impurities can be monitored on a global scale by use of spectrometers and radiometers placed on orbiting satellites.

Analytical modeling of Raman lidar return, including multiple scattering.

An analytical approach to modeling Raman lidar return with multiple scattering in presented. This approach is based on a small-angle quasi-single-scattering approximation developed earlier for elastic lidar sounding. An approximation of isotropic backscattering for the Raman-scattering case is proposed and tested. The computed results are presented and compared with known data. The approximation was found to be quite simple and provided a high accuracy of Raman lidar return calculations.