Measurement of the velocity difference by photon correlation spectroscopy: an improved scheme.

Homodyne photon correlation spectroscopy is used to measure the velocity difference delta v (l) over varying distance l. Different length scales are probed when the magnification factor M and the width S of a slit in the collecting optics are varied. The measured intensity autocorrelation function is found to be of scaling form for different values of M, provided S is kept at a value below the critical width S (c). A new convenient collecting optics is devised to expand the variable range of l up to 2 decades, over which delta v (l) can be accurately measured. The new scheme is useful for the study of turbulent and other self-similar flows.