The scattering of optical radiation from airborn aerosols

In the study of optical radiation scattered from aerosols a controversy has existed as to whether the scattered radiation that results when the incident radiation comes from a laser is different from the radiation which results when the source of incident radiation is a linearly polarized, narrowband thermal source. An experimental system is developed to investigate the nature of the scattered radiation which employs photo-electron counting. It is argued that the scattered radiation may be neither coherent nor totally incoherent. A model for the scattered radiation is constructed from physical considerations which consists of a sum of coherent and incoherent components. Normalized second and third factorial moments of the counting distributions are computed from the experimental data which is obtained by using scattered laser light, scattered thermal light and radiation from a light emitting diode as the radiations illuminating the photomultiplier tube. These are compared and their behaviour with respect to the average intensity is analysed and is explained, and the applicability of the model is discussed. It is concluded that the radiation reaching the detector does contain a small coherent component if the radiation incident upon the scatterers comes from a laser. A few examples are presented showing the significance of the findings on the interpretation of experimental results in light scattering experiments.