This volume contains most of the invited papers presented at the International Workshop on Light Scattering by Irregularly Shaped Particles held on June 5-7, 1979. at the State University of New York at Albany (SUNYA). Over seventy participants representing many dis ciplines convened to define some of the ever-increasing number of resonant light-scattering problems associated with particle shape and to relate their most recent investigations in this field. It is obvious from the two introductory papers that an investi gator's primary discipline determines his/her approach to the light scattering problem. The meteorologist, Diran Deirmendjian, advocates an empirical methodology: to model the scattering by atmospheric aerosols, using equivalent spheres as standards, in the most effi cient and simplest manner that is consistent with remote sensing, in situ, and laboratory· data. Because of the almost infinite variety of particle shapes, he questions not only the possibility but even the usefulness of the exact solution of scattering by a totally arbitrary particle. The astrophysicist, J. Mayo Greenberg, is primarily concerned with the information content carried by the scattered light because this radiation is the sole clue to under standing the nature of interstellar dust. What measurements (polar ization, color dependence, etc ••• ) should be made to best determine a given particle characteristic (size, surface roughness, refractive index, etc ••• )? Thus, he considers the physics of the scattering process to be of paramount interest.

This final report summarizes the research activities under the 'Systematic Study of Light Scattering by Irregularly Shaped Particles.' The beginning one and one-half years were devoted to the analysis of existing experimental data and the related theoretical light-scattering studies, in parallel with the reconstruction, renovation and calibration of the microwave analog scattering facility then relocated from the Albany, N.Y. area. The following one and one-half year period was devoted to actual measurement tasks and analysis of data in our original proposal; e.g., the investigations of scattering by interacting spheres and by particles with rough surfaces. Simultaneous with these measurements a number of 2:1 finite cylinders were also measured on their extinction properties when they were either preferentially or randomly oriented in space.

We present light scattering studies of irregularly shaped particles which significantly affect the climate. We built and calibrated our apparatus which was able to measure all six independent scattering matrix elements. Our apparatus detects light from 0.32° to 157° simultaneously. We studied all six scattering matrix elements of irregularly shaped Arizona Road Dust which behave differently than those of spheres. We strongly focused on the most important scattering matrix element -- the phase function, scattered intensity vs. the scattering angle, which we applied Q-space analysis to. Q-space analysis involves plotting the scattering intensity vs. the magnitude of the scattering wave vector q or qR with R the radius of a particle, on a double logarithmic scale. We measured and studied the phase functions of Al2O3 abrasives; compared the scattering from the abrasives with the scattering of spheres. To generalize the study, we collected a large amount of experimental and theoretical data from our group and others and applied Q-space analysis. They all displayed a common scattering pattern. The power law exponents showed a quasi-universal functionality with the internal coupling parameter [rho]ʹ. In situ studies of the soot fractal aggregates produced from a burner were also conducted. A power law exponent -1.85 is seen to imply the aggregates have fractal dimension of D[subscript f]=1.85. The overall work presented shows Q-space analysis uncovers patterns common to all particles: a q-independent forward scattering regime is followed by a Guinier regime, a power law regime, and sometimes an enhanced back scattering regime. The description of the patterns applies to spheres as well, except the power law regime has more than a single power law. These simple patterns give a unified description for all particle shapes. Moreover, the power law exponents have a quasi-universal functionality with [rho]ʹ for non-fractal aggregates. The absolute value of the exponents start from 4 when [rho]ʹ is small. As [rho]ʹ increases, the exponents decrease until the trend levels off at [rho]ʹ[greater-than or equivalent to]10 where the exponents reach a constant 1.75±0.25. All the non-fractal particles fall on the same trend regardless of the detail of their structure.

This is the next volume in series of Light Scattering Reviews. Volumes 1-5 have already been printed by Springer. The volume is composed of several papers ( usually, 10) of leading researchers in the respective field. The main focus of this book is light scattering, radiative transfer and optics of snow.

This fourth volume of Light Scattering Reviews is composed of three parts. The ?rstpartisconcernedwiththeoreticalandexperimentalstudiesofsinglelightsc- tering by small nonspherical particles. Light scattering by small particles such as, for instance, droplets in the terrestrial clouds is a well understood area of physical optics. On the other hand, exact theoretical calculations of light scattering p- terns for most of nonspherical and irregularly shaped particles can be performed only for the restricted values of the size parameter, which is proportional to the ratio of the characteristic size of the particle to the wavelength?. For the large nonspherical particles, approximations are used (e. g. , ray optics). The exact th- retical techniques such as the T-matrix method cannot be used for extremely large particles, such as those in ice clouds, because then the size parameter in the v- iblex=2?a/???,wherea is the characteristic size (radius for spheres), and the associated numerical codes become unstable and produce wrong answers. Yet another problem is due to the fact that particles in many turbid media (e. g. , dust clouds) cannot be characterized by a single shape. Often, refractive indices also vary. Because of problems with theoretical calculations, experimental (i. e. , la- ratory) investigations are important for the characterization and understanding of the optical properties of such types of particles. The ?rst paper in this volume, written by B. Gustafson, is aimed at the descr- tionofscaledanalogueexperimentsinelectromagneticscattering.