CCD BASED APPROACH TO HIGH-PRECISION SIZE AND REFRACTIVE-INDEX DETERMINATION OF LEVITATED MICRODROPLETS USING FRAUNHOFER-DIFFRACTION

We describe a fast and convenient method of high precision size and re fractive index determination of electrodynamically levitated microdrop lets using Fraunhofer diffraction. The diffraction data were obtained with a 16-bit, unintensified charge coupled device (CCD) camera, and c onverted into angle-resolved elastic scattering intensity patterns by means of a carefully determined set of transformation parameters. The angular scattering patterns were analyzed without any a priori estimat e of the droplet size and only a nominal estimate (approximate to 2%) of the refractive index. Experimental angular scattering patterns were fit to calculated patterns from Mie theory using a graded step-size a nd scaling algorithm and optimized with respect to both droplet diamet er and refractive index (real part only) with a precision of less than or equal to 3 parts in 10(4) and 1 part in 10(3), respectively. Poten tial application to quantitative fluorescence and Raman spectroscopy, as well as mixture analysis in microdroplets is discussed. (C) 1997 Am erican Institute of Physics.