High-Quality Photoplethysmography Signal Enabled by Narrowband Red Phosphor Converted Light-Emitting Diodes for Accurate Blood-Pressure Monitoring.

Abstract

Photoplethysmography (PPG) is widely used for noninvasive cardiovascular monitoring, yet raw-signal quality is often limited by the broadband emission and thermal spectral drift of conventional red LEDs, increasing reliance on complex downstream processing. Here, we develop a red light source by integrating an Rb₂LiGaF₆:Mn⁴⁺ (RLGF:Mn⁴⁺) microcrystal phosphor with a blue InGaN chip to form a narrowband phosphor-converted LED (pc-LED, FWHM = 13.4 nm) as a replacement for a commercial red module. The crystallized RLGF:Mn⁴⁺ microcrystals deliver a high external quantum efficiency (EQE = 52%) and enable clearer structure-emission correlation via experimentally resolved dual-site Mn⁴⁺ emission. The resulting pc-LED exhibits high electro-optical efficiency (η = 32.03%) and minimal thermal spectral drift (peak shift ≤ 0.3 nm from 20°C-100°C). Using this narrowband, thermally stable light source in a wearable PPG prototype, we quantitatively demonstrate improved signal quality and more stable VPG/APG feature localization compared with a commercial red module. Finally, we provide a proof-of-concept demonstration of blood-pressure estimation by lightly training a basic convolutional neural network, achieving mean absolute errors of 2.86 mmHg for SBP and 2.40 mmHg for DBP on an independent test set. This work establishes a materials-to-device pathway for improving PPG quality through optical spectral engineering.