The depth of penetration (DOP) of a helium-neon (He-Ne) laser (wavelength of 633nm) in whole blood had been widely speculated to range from 0.5mm to 1.5mm. However, due to technical difficulties in determining the DOP, one of which is the problem of detecting the forward scattered light in blood, these speculations had not been quantitatively verified. A knowledge of the DOP as a function of haematocrit and oxygen saturation level is important in the development of a dual-fibre laser Doppler velocimeter for in vitro flow measurements when it is desirable to project the laser beam as far away from the fibre tip as possible. Using two multimode optical fibres, we were able to set up a simple system to measure the DOP in whole blood. The two fibres, potted and polished, were aligned inside a tube filled with blood and with its ends sealed. One fibre admits light from a 15mW He-Ne laser into the blood and the other detects the forward scattered light. The DOP is defined as the separation between the two fibres such that the signal, due to diffusion of red blood cells, disappears in the noise spectrum. Two properties of blood were varied separately while keeping the other constant. Haematocrit (HCT) was varied by dilutions with sodium chloride (saline) solutions and blood oxygenation varied by bubbling air into the sample. The DOP was found to vary with haematocrit in an approximately exponential manner with penetration at HCT 40 of ≈ 1.9mm and at HCT 5 of ≈ 6mm. The DOP also varies with oxygen saturation with a typical range of 1.6mm for desaturated blood to 3mm for saturated blood. These distances will enable sufficient penetration of a two fibre system to allow measurement of blood velocity especially in the arterial circulation.
S. C. Tjin,
"Factors Influencing The Depth Of Penetration Of He-Ne Laser In Whole Blood", Proc. SPIE 1067, Optical Fibers in Medicine IV, (15 June 1989); doi: 10.1117/12.952107; https://doi.org/10.1117/12.952107