Reflection and Transmission of Light by Thin Films of Nonhaemolysed Blood

Abstract

The reflectance of light by nonhaemolysed blood has been studied previously only in thick layers. Some reflection oximetric techniques assumed an exponential relationship between oxygen saturation and reflection, whereas others assumed an inverse linear relationship. Reflectance and transmittance of thin films of flowing nonhaemolysed blood were measured, using an integrating sphere. Reflection and transmission were found to be linearly related, demonst,rat>ing bhat reflection is exponentially related to the extinction coefficient of haemoglobin. Confirmation was obtained of the observations that reflection increases and becomes asymptotic as sample depth increases and that reflection bears a parabolic relationship to the concentration of red blood cells.

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“…S ₀ may be estimated from data presented by Anderson and Sekelj [129, 130]. They measured the reflectance of a thin-layer whole-blood sample over a wide range of hemoglobin concentrations.…”

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“…and does not change with further increases (2,44). Reflectance is also inversely related to the absorption coefficient of hemoglobin and the magnitude of this change increases with increasing sample depth.…”

“…The intensity of reflected red light is proportional to the relative oxygenation (53) and parabolically related to the hemoglobin concentration (2,38). Thus, single-wavelength oximeters are sensitive to changes in erythrocyte concentration (18,67).…”

“…Others have modi fed commercial photometers (65) or spectrophotometers (2) for study of reflection properties of blood. These provide relatively narrow bandwidths, but are bulky, and of course two instruments are required for two-wavelength oximetry (65).…”

“…As the dimensions of the particles are reduced the mean depth of penetration of the medium by incident light is reduced, so that diffuse reflectance becomes relatively greater for the poorly absorbed wavelength than for the one strongly absorbed (78). Several workers report a hyperbolic increase in reflectances of blood with increasing PCV (2,21,38). With fiberoptic oximeters a decrease in reflectance occurs above 50 to 60% PCV (21,38).…”