Exploring Near- and Far-Field Effects in Photoplethysmography Signals Across Different Source–Detector Distances

Figures

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  Figure 1. (A) Radiation curve of SFH 7013 datasheet [20]. (B) Diagram of radiance in the far field using a rectangular light source. Adapted with permission from Optica Publishing Group [17]. (C) Radiance pattern of a Lambertian source versus a non-Lambertian source.

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  Figure 2. (A) Top view of optical front-end used for the near-field experiment. The multi-chip SFH 7013 LED is placed in the center with 12 SFH 2703 PIN photodiodes (PD) distributed in three rings of radius 6 mm, 9 mm, and 12 mm around it. A light blocker is placed to prevent direct light from the LEDs from reaching the photodiodes. (B) SFH 7013 dimensions, showing the active areas of the green (G), red (R), infrared (IR) LEDs, and the protective layer. From datasheet [20]. (C) Complete optical setup used for the near- and far-field experiment. A diffuse reflector is placed at 2 cm from the optical front-end to reflect light back into the photodiodes homogeneously.

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  Figure 3. Setup of in-vivo investigation performed in a controlled climatic chamber. Photo used with permission from the subject.

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  Figure 4. (A) Normalized DC values (0–1) recorded with the photodiodes rings (6 mm, 9 mm, and 12 mm) during the near- and far-field lab experiment. Results are sorted by source–detector distance (row) and optical window choice (column). Each row–column combination contains the readout of the four different photodiodes within a particular ring and glass choice using green, red, and infrared light and the LEDs off. (B) Coefficients of variation (CV) calculated for each quartet of photodiodes for a given wavelength, source–detector distance, and glass type.

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  Figure 5. (A) Mean DC-PPG amplitudes recorded for all participants at 6 mm, 9mm, and 12 mm s–d distances using green, red, and IR. Subjects are sorted by skin type A–G, and the TIA feedback resistance is displayed next to it. (B) The mean and standard deviation of coefficients of variation are calculated for all subjects for each quartet of photodiodes for a given wavelength and source–detector distance.

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  Figure 6. (A) SpO2 calculated from all photodiodes with varying source–detector distances (6 mm, 9 mm, and 12 mm) and spatial distribution. Participants are sorted by skin type A–G. (B) The mean and standard deviation of coefficients of variation of obtained SpO2 calculated for all subjects for each quartet of photodiodes for a given wavelength and source–detector distance.