Photoplethysmography (PPG) is an optical sensing technique that measures blood volume changes and underpins devices ranging from hospital-grade pulse oximeters to consumer wearables that track heart rate, sleep, and oxygenation. Despite its widespread use, PPG accuracy can vary significantly across individuals, particularly by skin tone. Darker skin contains more melanin, which absorbs and scatters light, often leading to less reliable readings. This disparity has been linked to inaccuracies in blood-oxygen measurements among people with more melanin.
Most efforts to improve PPG accuracy rely on software-based solutions, such as advanced filtering or machine learning, to clean up noisy signals caused by motion or poor sensor contact. However, these approaches work on low-quality data rather than addressing the root cause: the interaction of light with tissue. In a recent study, published in Biophotonics Discovery , researchers at Brown University introduced a new approach that directly addresses the PPG signal quality that is at the level of the light–tissue interaction.
The team developed a wearable, polarization-sensitive PPG sensor that uses the orientation of light's electric field to favor signals from deeper blood vessels over superficial layers rich in melanin. The device splits light into two channels: one detects co-polarized light (parallel to the incoming beam), and the other detects cross-polarized light (perpendicular). This design helps filter out superficial scattering and capture stronger signals from deeper tissue.
In tests with volunteers representing light, medium, and brown skin tones, the cross-polarized condition consistently produced higher perfusion index (PI) values—a measure of signal strength—at both red (655 nm) and infrared (940 nm) wavelengths. The improvement was most pronounced for darker skin at the red wavelength.
While the authors caution that the results are preliminary and note that a larger study will be carried out, the approach may reduce bias in PPG-based technologies, laying the groundwork for more inclusive medical and consumer wearables.
"Most PPG devices focus on innovations in the digital signal-processing algorithms," said senior author Kimani C. Toussaint, Jr. "Instead, as optics researchers, we're focusing on what can be achieved by engineering the light itself; we think we're scratching the surface in what could be a new, and more accurate way to obtain better quality PPG signals."
For details, see the original Gold Open Access article by R. Jakachira et al., " Evaluation of a polarization-sensitive, dual-wavelength wearable photoplethysmography sensor across a range of skin tones ," Biophoton. Discovery 3(1), 012509 (2025), doi: 10.1117/1.BIOS.3.1.012509
