Fig. 1.

Principle of operation of PO. (a) Representative example of a finger clip of a PO, comprising light detectors and light emitters. Reprinted and modified from Kozlov,⁶⁰ with permission from the publisher. (b) Light scattering distributions in the finger tissue for two wavelengths typically used in PO (red: 660 nm, infrared [IR]: 940 nm). Reprinted and modified from Chatterjee and Kyriacou.⁶¹ (c) Components of light absorption in PO (visualization redrawn from Moyle⁶²). (d) Hb absorption spectra (data source: Biomedical Optics Research Laboratory, UCL, UK). The isosbestic point and two typical PO wavelengths are added. (e) Normalized light transmittance for two wavelengths as functions of ${\mathrm{SaO}}_2$ (data: simulation by Chatterjee and Kyriacou⁶¹). A typical PO calibration curve (data source:⁶²) with visualization of the amplitudes of the blood volume pulsation in the two wavelength ranges (red and IR). AC: alternating current (variable signal component), DC: direct current (constant signal component), ${\mathrm{O}}_2\mathrm{Hb}$: oxyhemoglobin, HHb: deoxyhemoglobin, ${\mathrm{SaO}}_2$: arterial blood oxygenation, ${\mathrm{SpO}}_2$: peripheral arterial hemoglobin ${\mathrm{O}}_2$ saturation.