Glow in the Dark: User Perceptions of PPG Sampling and the Purkinje Effect in Mesopic Low-Light Environments

Abstract

Photoplethysmography (PPG) is a popular sensing technology in wearable devices like activity trackers and smartwatches, promising continuous health monitoring and improved sleep quality. However, PPG requires active illumination with visible light. Inadequate device fit can cause light leakage, creating a visible 'glow' that becomes perceivable in dark (scotopic) or low-light (mesopic) environments, and can result in user discomfort. We developed a wrist-worn device to simulate various PPG sampling patterns. A user study (N = 26) was conducted where subjects wore the device and verbalized their perceived brightness, disturbance, and qualitative thoughts in a think-aloud protocol. Our findings confirm the relevance of the Purkinje effect in this context. Green light was consistently perceived as brighter, more disturbing, and more spatially pervasive than localized red light. Furthermore, sampling patterns significantly affected user perception, as regular ones were deemed less disturbing than irregular ones. Despite a lower total radiant flux, irregular sampling had a disproportionately higher impact on perceived brightness and, in particular, disturbance. Even minor leakage can degrade comfort and sleep quality. Therefore, we suggest that PPG sampling patterns have to transcend technical optimization, requiring deliberate Purkinje-aware, user-centric design strategies for low-light environments and night sleep tracking.