Introduction

Photoplethysmography (PPG) forms the basis of many optical monitoring technologies used in both clinical practice and consumer health devices. Although the influence of skin pigmentation on the accuracy of pulse oximetry and related light–tissue interaction methods was first discussed more than four decades ago,1–5 recent clinical evidence has brought renewed attention to this issue. In a large retrospective study, Sjoding et al.6 reported systematic differences in pulse oximetry performance between Black and White patients, with Black patients experiencing nearly three times the frequency of undetected hypoxemia under hypoxic conditions. Such findings have reinforced concerns that skin pigmentation may alter optical signals. Given the widespread use of PPG-based techniques, there is a need to improve the physical understanding of how skin pigmentation-related absorption and scattering influence PPG signals and potentially contribute to observed disparities.

Understanding how skin pigmentation influences PPG signal formation is challenging in in vivo studies, where optical effects are coupled with physiological variability such as perfusion, tissue composition, and vascular dynamics.7 As a result, it is difficult to isolate the contribution of pigmentation-related optical properties from other confounding factors. In vitro tissue phantoms offer a controlled alternative, enabling systematic investigation of light–tissue interactions under reproducible conditions. When combined with Monte Carlo simulations of photon transport, phantom-based studies provide a powerful approach for linking PPG signals to underlying optical energy deposition, photon path distributions, and absorption profiles.

Controlled experimental approaches have previously been used to investigate PPG signal formation using optical tissue phantoms, primarily in reflectance configurations.8–13 Rodriguez et al. used an anatomically inspired optical finger phantom to evaluate PPG signal formation under controlled conditions with an embedded vascular network.8 Similarly, Chen et al. fabricated a multilayer phantom with tunable optical properties across the visible and near-infrared spectrum, providing a well-characterized tool for optical validation.9 Bhusal et al. evaluated a pulsatile, channel-based finger phantom and demonstrated that epidermal melanin layers influenced reflectance PPG waveforms.10 Jenne and Zappe developed a multilayer tissue phantom with expandable vascular structures that replicate the optical and mechanical properties relevant to PPG signal.11 Previous work from the Research Centre for Biomedical Engineering (RCBE) similarly designed and fabricated a multilayer vascular finger phantom to investigate the effect of skin pigmentation on reflectance PPG signals.12 Despite these advances, existing phantom-based studies have predominantly focused on reflectance-only measurement geometries, have often relied on simplified representations of vascular anatomy, and have not linked observations to depth photon transport metrics.

In parallel to these experimental efforts, Monte Carlo models have been widely used to investigate light transport in tissue and to assess how variations in optical properties, including melanin concentrations, influence photon propagation and detected signals. These simulations have provided valuable insights into depth sensitivity and absorption behavior across tissue layers.14–17 However, Monte Carlo approaches have largely been applied independently of phantom-based experiments and often have not been integrated with in vitro studies for the evaluation of skin pigmentation effects on PPG signals. As a result, the combined influence of skin pigmentation, physiologically representative vascular network, and photon transport on PPG signal formation remains incompletely characterized.

To address these gaps, this study combines an advanced multilayer vascular finger phantom with Monte Carlo simulations to investigate the effect of skin pigmentation on PPG signals in both reflectance and transmittance measurement modes. By directly linking experimentally measured PPG signals to depth-dependent photon transport metrics, this work aims to improve understanding of how skin pigmentation-related optical properties influence light–tissue interaction and PPG quality.