Diffuse Reflectance of Whole Blood: Model for a Diverging Light Beam

Previous theories describing the optical properties of whole blood have assumed a collimated light source in the derivation of reflectance and transmittance equations. However, many practical instruments which measure blood oxygen rely on noncollimated LED's and optical fibers as light sources. In addition, in the case of blood-perfused cuvettes, the light source is not in direct contact with the blood. Thus, many theories have limited applicability to practical instruments. Therefore, we present here an existing photon diffusion theory that we have generalized to account for the effects of a diverging light source and a thickness of transparent material between the light source and the blood. Comparisons to experimental data are made for both the new equation and the previous one. Better results are obtained with the new equation when applied to a cuvette geometry which is more accurately described by the generalized parameters. Limitations of both the new equation and the photon diffusion approximation in general are discussed.