This paper outlines a simplified theory to explain the behaviour of photoelectric plethysmographs used for the measurement of blood flow in the skin. The paper reports the finding that light diffusing through blood, from a source outside a blood vessel, can diffuse preferentially in the direction of motion of the blood, and that this preferential diffusion is likely to arise from a combination of the plasma skimming which occurs at the vessel wall and from the orientation effects of the erythrocytes in motion.
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