= A. P. Shepherd
{c}
\b[A. P. Shepherd] is the first named coauthor of a 1989 instrument paper that separated <hemoglobin> concentration from <oxygen saturation> in a sample of whole <blood>. The published byline uses these initials. The apparatus illuminated a blood-filled capillary tube with red and infrared <light-emitting diodes>; scattered light travelled along the tube to a <photodetector>, followed by amplification, digitization and microprocessor calculation.<An oximeter for measuring hemoglobin concentration and oxygen content by A. P. Shepherd and others>[Abstract and byline, American Journal of Physiology 257, H1705–H1711, DOI 10.1152/ajpheart.1989.257.5.H1705]{ref}
The important distinction is <hemoglobin-concentration and saturation separation>. <Oxygen saturation> measures the fraction of available <hemoglobin> oxygen-binding capacity that is occupied. In a simplified accounting model, the amount of oxygen bound to <hemoglobin> per unit <blood> volume is $C_{\mathrm{bound}}=\kappa c_{\mathrm{Hb}}S$, where $c_{\mathrm{Hb}}$ is <hemoglobin> mass concentration, $S$ is fractional <oxygen saturation> and $\kappa$ is oxygen-binding capacity per unit <hemoglobin> mass. At the same $S=0.98$, stipulated concentrations of $150$ and $75\,\mathrm{g\,L^{-1}}$ therefore give a twofold difference in bound oxygen content. The example omits dissolved oxygen and illustrates why <oxygen saturation> alone cannot determine the oxygen carried per unit <blood> volume.
The paper's <instrument calibration> addressed this separation optically: the microprocessor used a nonlinear function of infrared intensity for total <hemoglobin> concentration and a ratio of logarithms of red and infrared intensities for <oxygen saturation>. Those are distinct calibrated transformations of the detector signals; a two-colour reading is not automatically a concentration-independent saturation measurement.<An oximeter for measuring hemoglobin concentration and oxygen content by A. P. Shepherd and others>[Abstract: infrared concentration function, logarithmic intensity ratio and concentration-related calibration shifts]{ref}
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