PHOTOMETRIC IMMERSION REFRACTOMETRY - A METHOD FOR DETERMINING THE REFRACTIVE-INDEX OF MARINE MICROBIAL PARTICLES FROM BEAM ATTENUATION

Photometric immersion refractometry is a technique for determining the refractive index of particulate material. In this technique, the atte nuation of light by a suspension of particles is measured as a functio n of the refractive index of the immersion medium. A minimum attenuati on occurs at the refractive index of the medium equal to that of the p articles. This technique can serve as a benchmark method for the refra ctive index determination because it is independent of assumptions inv oked by other techniques, such as those based on the inversion of the spectral attenuation data. We present a rigorous model of the photomet ric immersion refractometry based on the anomalous diffraction approxi mation for the attenuation efficiency of particles. This model permits one to determine the average value of the real part of the refractive index of the particles, its variance, and the average imaginary part of the refractive index of the particles. In addition, the fourth mome nt of the particle size distribution can be determined if the concentr ation and shape of the particles are known. We analyze the sensitivity of this model to experimental errors and discuss the applicability of photometric immersion refractometry to marine microbial particles. We also present experimental results of this technique as applied to het erotrophic marine bacteria. The results indicate that the refractive i ndex of these bacteria was narrowly distributed about the average valu e of 1.3886. (C) 1997 Optical Society of America.