Spatial resolution for two-beam spectroscopy: a new mechanism.

With the help of interference effects, two-beam and multiple-beam spectroscopy detect in the pairs of beams (bundles of rays selected by the optical system) phase correlations due to certain fluctuations in optically thin distributions of incoherent light sources. Originally spatial resolution along the line of sight was expected for multiple-beam spectroscopy because of the limited region of intersection for pairs of beams. Here more general analysis shows another mechanism of spatial resolution allowing use of broader overlapping beams. Thus a simpler two-beam spectroscopy configuration (to be discussed in more detail elsewhere) capable of making more efficient use of emitted light proves to offer the same localized measurement of spatially harmonic fluctuations in the appropriate light source distributions.

Limits of optical transmission measurements with application to particle sizing techniques.

Considerable confusion exists regarding the applicability limits of the Bouguer-Lambert-Beer law of optical transmission. We review the derivation of the law and discuss its application to the optical thickness of the light-scattering medium. We demonstrate the range of applicability by presenting a method for determining particle size by measuring optical transmission at two wavelengths.

Application of the small-angle approximation to ocean water types.

The small-angle approximation to the radiative transport equation is applied to particle suspensions that emulate ocean water. A particle size distribution is constructed from polystyrene and glass spheres with the best available data for particle size distributions in the ocean. A volume scattering function is calculated from the Mie theory for the particles in water and in oil. The refractive-index ratios of particles in water and particles in oil are 1.19 and 1.01, respectively. The ratio 1.19 is comparable to minerals and nonliving diatoms in ocean water, and the ratio 1.01 is comparable to the lower limit for microbes in water. The point-spread functions are measured as a function of optical thickness for both water and oil mixtures and compared with the point-spread functions generated from the small-angle approximation. Our results show that, under conditions that emulate ocean water, the small-angle approximation is valid only for small optical thicknesses. Specifically, the approximation is valid only for optical thicknesses less than 3.

Limits of the small-angle approximation to the radiative transport equation.

The small-angle approximation to the radiative transport equation is used extensively in imaging models in which the transport medium is optically thick. The small-angle approximation is generally considered valid when the particles are very large compared with the wavelength, when the refractive-index ratio of the particle to the medium is close to 1, and when the optical thickness is not too large. We report results showing the limits of the validity of the small-angle approximation as a function of particle size and concentration for a particle-to-medium fixed refractive-index ratio of 1.196. This refractive-index ratio is comparable with that of minerals or diatoms suspended in water.