A Cost-Effective Method for the Spectral Calibration of Photoplethysmography Pulses: The Optimal Wavelengths for Heart Rate Monitoring

Figures

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  Figure 1. Gel-based optical phantoms with various hemoglobin (Hb) concentrations for PPG measurement. (a) Homogenous phantoms (D = 0); (b) two-layered phantoms with top layer thickness D = 0.20 mm; (c) Hb concentrations of phantoms are selected to simulate one PPG pulse so that each phantom represent one data point; (d) measured absorption coefficient, μa, spectra of selected Hb concentrations; and (e) μa and μs′ spectra of ink 1% v/v and lipid 1% w/v, respectively.

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  Figure 2. A schematic drawing of the customized reflectance spectroscopy system. The fiber probe is mounted normally on the sample surface. In all the phantom measurements, the fiber probe slightly touches the phantom’s surface. In the spectralon measurement, the fiber probe is positioned above the sample surface at a distance so that maximum intensity is collected at the spectrometer. An MATLAB v2024a graphical user interface is used to control the spectrometer and to collect and analyze the PPG pulses.

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  Figure 3. (a) A schematic of the source (S) and detectors on the surface of the tissue at SDD = 1, 2, 3, and 4 mm (center to center); (b,c) normalized energy fluence for the diastole (b) and the systole (c).

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  Figure 4. Experimental measurement of optical phantoms when (a–c) D = 0 and when (d–f) D = 0.20 mm. (a,d) Diffuse reflectance signal (DRS) as function of wavelengths when Hb concentration is varied from 2.0 to 3.1 mg/mL, (b,e) corresponding PPG waveform at selected wavelengths, (c,f) and AC/DC ratios. Due to current instrument limitation, all measurement was performed at SDD = 0.5 mm.

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  Figure 5. (a,b) AC/DC vs. MOP when epidermal thickness D = [50, 100, 150] µm and SDD = [1–4] mm at wavelengths of 523 nm (a) and 945 nm (b); (c) AC/DC as function of SDD when D = 100 µm at both wavelengths.