A general theory of perturbed light fields, with applications to forward scattering effects in beam transmittance measurements
SIO Reference 58-37. The problem of the measurement of the optical properties of a given medium is complicated by the fact that the act of measurement perturbs the distribution of radiant flux in the immediate vicinity of the measuring process. Consequently, the numbers derived from a measurement process do not faithfully reflect the inherent optical properties of the medium under study, but rather contain along with the information sought both the effects of the presence of the measuring apparatus and its characteristic response to radian-flux. The present note contains a general formulation of the equation of transfer for perturbed radiance field in an arbitrary optical medium. The resultant theory is applied to the problem of the measurement of the volume attenuation function o< (or, equivalently, beam transmittance) in natural waters and the atmosphere. In particular, the theory leads to several new measuring techniques for o< which take into account the perturbation effect on the light field of the standard measuring apparatus used for the determination of o< . In addition the theory provides a means of estimating the relatively elusive forward scattering value, o-o, of the volume scattering function o- . Finally, two criteria are given for estimating the order of magnitude of tho forward scattering effects encountered in beam transmittance measurements.