High resolution angular-resolved measurements of the light Scattered by red blood cells in suspension

C.V.L. Pop, I. Turcu, S. Neamtu

National R&D Institute for Isotopic and Molecular Technologies, P.O.Box 700, 400293 Cluj-Napoca, Romania

There are a great variety of experimental techniques able to capture the light scattered by cells in suspensions extending from a simple photo-detector to sophisticated devices using high quality CCD cameras.
While the light-scattering properties of individual red blood cell (RBC) are relatively well known, the time-varying speckle pattern of light intensity scattered by RBCs in suspension is extremely complex and carry encrypted a great amount of information about the RBCs structure, shape and dynamics. 
The mean size of RBCs is about ten times larger than the wavelength of the incident radiation, and the relative refractive index of the internal and external media is close to one. The scattered light is densely packed in the forward direction, the scattering process being extremely anisotropic.
For dilute suspensions the probability that the photons scattered once are scattered again by a second cell is relatively small, the scattering process being dominated by single scattering events. In this domain the forward scattering anisotropy is very high established mainly by the individual contribution of RBCs. When the concentration is high enough so that the rate of multiple scattering events becomes comparable with the rate of single scattering, the system switches to the multiple scattering regime. The scattered light looses gradually its anisotropy and consequently the amount of light scattered at small angle decreases.
The main purpose of this contribution is to investigate experimentally the angular dispersion of the scattered light upon interaction with suspensions of RBCs, over a wide range of haematocrits.
A standard experimental set-up has basically three main components: the laser, the investigated sample and the detector. Since our interest focus on the domain of small scattering angle, the angular resolution was improved by using a CCD camera as detecting device. Because the experiment was performed with a coherent laser beam a supplementary difficulty was encountered: the appearance of the fluctuating interference speckle pattern. The difficulty introduced by interference was removed by capturing images with extended exposure time. By increasing the exposure time the images lose their contrast and practically, an averaged value of the scattered light can be detected by each pixel.
The effective anisotropy parameter was obtained by fitting the Henyey-Greenstein function to the collimated transmission datasets obtained for CCD camera measurements. The results confirm the theoretical predictions: at small haematocrits where single scattering is dominant the anisotropy is high and is almost independent on haematocrit while in the multiple scattering domain the anisotropy has a power low decrease with respect to the sample haematocrit.