physics.opticsSep 27, 2026

Multi-Aperture PPG with MAPIS: Spatial Optical and Temporal Coherence Fields

Authors: Shuguang Wang, Yuanjing Wang

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Abstract

Conventional reflectance photoplethysmography (PPG) typically reduces tissue optical responses to one or a few detector channels. Matrix Pinhole Image Sensing (MAPIS) instead simultaneously samples multiple aperture-dependent optical signals. We analyzed 44 recordings of 30 seconds each from a single MAPIS device to characterize the spatial relationships among DC intensity, pulsatile AC, perfusion index (PI), and temporal coherence across Red and infrared channels. AC increased sublinearly with DC at both wavelengths, resulting in higher PI toward lower-DC apertures, consistent with a first-order relative path-sensitivity interpretation. A second counterintuitive spatial trend was observed in temporal coherence: lower-DC and lower-AC apertures exhibited higher autocorrelation and greater cardiac-periodic spectral concentration. IR signals also showed consistently higher temporal coherence than corresponding Red signals. These observations demonstrate that multi-aperture PPG resolves reproducible spatial relationships among optical intensity, fractional pulsatile sensitivity, and waveform coherence that are largely averaged together in conventional single-channel PPG. The present study establishes within-device relationships and motivates controlled studies of the underlying photon-path mechanisms.

Figures & tables

Figure 1: Layout of the MAPIS aperture array and light sources. The Red (655 nm) and IR (940 nm) LEDs are located beneath the aperture array within a compact shared package.
Figure 2: Schematic cross-sectional view of the MAPIS optical geometry. Light propagating through the target tissue is sampled by different imaging apertures on the image sensor. The illustrated curves represent effective photon-path ensembles rather than individual photon trajectories.
Figure 3: Equal-record mean spatial maps obtained from 44 MAPIS recordings of 30-seconds each. Despite the lower DC and AC levels in more weakly illuminated apertures, PI and ACF increase in these regions, revealing spatial relationships that are not evident from signal amplitude alone. The distinct Red and IR patterns further indicate wavelength-dependent optical transport. These maps illustrate spatial relationships that are not directly accessible from a conventional single-channel PPG measurement.
ObservationInterpretation
AC decreases more slowly than DC across the aperture fieldPI increases toward lower-DC apertures, consistent with the asymptotic optical model.
PI forms a structured two- dimensional spatial fieldPI reflects relative path sensitivity rather than simple radial distance or absolute photon path length.
PI partially opposes the DC spatial gradientThis compensation explains the sublinear AC–DC relationship.
Lower-DC and lower-AC apertures exhibit higher temporal coherenceTemporal coherence appears to depend on optical sampling, not simply on signal amplitude or additive-noise SNR.
IR exhibits consistently higher temporal coherence than RedA possible explanation is stronger spatial averaging of local non-cardiac perturbations by the IR photon-path ensemble.
Table 1: Principal Observations

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