Simulation of Photoplethysmography Signal in Reflection Mode Using Three-layer Skin Optical Modeling

Photoplethysmography (PPG) is an optical, noninvasive method widely applied to detect changes in blood volume within the skin. Accurate modeling of light-tissue interaction is essential for improving PPG sensor design and signal interpretation.

In this study, the three main layers of human skin were simulated using the nonsequential mode of Zemax optical software. Each layer was assigned specific optical properties to accurately represent physiological conditions. A light source with a wavelength of 600 nm was directed onto the modeled skin, and the propagation of light through the tissue to the photodetector was simulated. The direct current (DC) component of the PPG signal was first modeled, representing the baseline optical response. Then, the alternating current (AC) component (pulsatile component linked to cardiac blood-volume changes) was added by simulating periodic changes in blood volume within the dermal vessels during cardiac cycles.

The light transport from the source through the skin tissue was successfully simulated, modeling the interaction of photons within the three skin layers. Subsequently, the resulting signal reaching the photodetector was simulated, capturing both DC and AC components of the PPG waveform. Two of the model's output PPG signals were compared with experimental data, showing good agreement in waveforms.

Using a three-layer skin tissue model combined with the Henyey-Greenstein scattering model to calculate light's angular distribution, along with accurate optical properties of skin layers and blood volume changes during each cardiac cycle, the PPG signal was successfully simulated. This simulation marks a significant step toward developing PPG sensors for the physiological measurement of blood components on the skin.