On the effective refractive index of blood

We calculated the real and imaginary parts of the effective refractive index n(eff) of blood as functions of wavelength from 400 to 800 nm; we employed van de Hulst's theory, together with the anomalous diffraction approximation, for the calculation. We modelled blood as a mixture of plasma and erythrocytes. Our results indicate that erythrocyte orientation has a strong effect on n(eff), making blood an optically anisotropic medium except when the erythrocytes are randomly oriented. In the case in which their symmetry axis is perpendicular to the wave vector, n(eff) equals the refractive index of plasma at certain wavelengths. Furthermore, the erythrocytes' shape affects their contribution to n(eff) in an important way, implying that studies on the effective refractive index of blood should avoid approximating them as spheres or spheroids. Finally, the effective refractive index of blood predicted by van de Hulst's theory is different from what would be obtained by averaging the refractive indices of its constituents weighted by volume; such a volume-weighted average is appropriate only for haemolysed blood. We then measured the real part of the refractive index of various blood solutions using two different experimental setups. One of the most important results of our expriment is that n(eff) is measurable to a good degree of precision even for undiluted blood, although not all measuring apparatuses are appropriate. The experimental data is self-consistent and in reasonable agreement with our theoretical calculations.