The possibilities and limitations of Photon Correlation Spec-particle diameter less than 0.5 pm reliable particle sizes can be troscopy as a practical tool for particle sizing are revisited. In obtained. In the range of 0.5 to 1 pm this was only possible a first step, well characterized monodisperse polystyrene latices within a very narrow range of concentration of dispersed with particle sizes ranging from about 30 nm to about 2 pm material. For the largest size investigated (about 2 pm) the PCS were examined in five different industrial and academic measurements were less reliable which was reflected in a conlaboratories. Particular attention was paid to the effect of parti-siderable variation of the results. cle concentration and scattering angle. It turned out that for a ' Ing. N. De Jaeger, Ing. R. Sneyerx,

The application of the algorithm of Zeiger and McEwen to the analysis of noisy photon correlation data is investigated. For the particular case where the data are sampled at equidistant time intervals a complete solution is given allowing reliable reconstruction of the spectrum of exponential decay rates without any a priori knowledge. A particular attractive feature of the method is that the singular value analysis of the Hankel matrix of autocorrelation functions offers a practical criterion for the decomposition of the data into a signal and a noise part. Some tests of the method are illustrated with experiments on monodisperse latices, gold sols, and binary mixtures of monodisperse latices. In the latter case comparable and even better results are obtained in significantly shorter computing time when compared to an analysis with Contin and the maximum entropy method. Since the present method does not require any a priori parameter setting, it is also complementary to these methods.

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The application of photon correlation spectroscopy on mammalian eye lenses in vivo is revisited. It is shown that the use of a short wavelength laser type and a logarithmic correlator improves the signal-to-noise ratio to such an extent that shorter measurement times are possible without impairing the information content of the correlation function. Experimental correlation functions obtained in vivo on a rabbit eye lens, are analyzed with several techniques. The histogram approach is most successful for the determination of the distribution function of relaxation processes in the correlation function and proposes four different populations of components in the lens. This result is comparable to that from in vitro measurements on highly concentrated solutions of alpha-crystallins and of fiber cell cytoplasm, the former proteins being the main scattering components both in vivo and in vitro in the eye lens system. Our results indicate that the application of photon correlation spectroscopy on eye lenses in vivo offers new perspectives to use this technique as a fast, noninvasive tool to study relaxation phenomena in normal and cataractous lenses. The sensitivity of the method allows it to be used as an important analytical technique in the study of prevention and treatment of cataract.

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