Optical properties of human whole blood were measured in vitro at 633 nm with a double integrating sphere set-up. The blood was kept at constant flow through a flow cell while slowly heating the blood from approximately 25°C to 55°Cin a heat exchanger. The results show a small but distinct decrease in the g-factor of 1.7±0.6% and a similar increase in the scattering coefficient, p, of 2.9±0.6% at approximately 45-46°C. When studying the thermal effect on the blood cells under a white-light

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... mission microscope, the changes in the scattering properties could be correlated to a sudden change in the shape of the red blood cells, from disc-shaped to spherical, at approximately the same temperature. Furthermore, a continuous manifest increase in the absorption coefficient, was seen with temperature rise, on average 83.8±68.1% when reaching the temperature 50°C. This might be due to heat-induced haemolysis of the red blood cells, resulting in free light absorbing haemoglobin in the surrounding plasma and thus higher effective light absorption.