The value of polarization in camera-based photoplethysmography

Abstract

Camera-based photoplethysmography (cbPPG) is a novel measuring technique that permits the remote acquisition of cardiovascular signals using video cameras. Research still lacks in fundamental studies to reach a deeper technical and physiological understanding. This work analyzes the employment of polarization filtration to (i) assess the gain for the signal quality and (ii) draw conclusions about the cbPPG signal's origin. We evaluated various forehead regions of 18 recordings with different color and filter settings. Our results prove that for an optimal illumination, the perpendicular filter setting provides a significant benefit. The outcome supports the theory that signals arise from blood volume changes. For lateral illumination, ballistocardiographic effects dominate the signal as polarization's impact vanishes.

PulseCam: a camera-based, motion-robust and highly sensitive blood perfusion imaging modality

“…However, a non-contact camera-based system that operates from a distance records a significantly weaker blood volume waveform from any imaged skin region or internal tissue compared to a contact-based PulseOx. Therefore, prior [26][27][28][29] camera-only blood pulsation imaging system usually suffers from the challenge of significantly smaller signal-to-noise ratio. The small blood volume waveform, i.e., the signal of interest, rides on top of a much larger (approx.…”

“…Moreover, the surface reflection can change drastically with even a slight movement of the skin, making the small blood perfusion-related signal recovery even more difficult in the presence of motion. As a result, recent attempts to obtain spatial maps of pulsatile blood perfusion using camera-only approaches have resulted in noisy and motion-corrupted perfusion estimates [26][27][28][29] with limited clinical value.…”

Contactless monitoring of the blood-flow changes in upper limbs

“…Light emitted by each pair of LEDs propagates through film polarizers with parallel transmittance axes assembled ahead of each LED. The cross-polarized analyzer was installed ahead of the respective camera lens to increase SNR [12,24]. The whole setup was covered with opaque fabric to reduce the influence of the ambient light.…”

“…In imaging photoplethysmography (IPPG) the subject's skin is typically illuminated by green polarized light, and the video camera captures the live image through a cross-polarized filter attached to the lens [10][11][12], so it actually measures and records the quantity of the light scattered and reemitted from the dermis tissue including the shallow capillary bed located just below the epidermis. Any changes in the blood microcirculation parameters can be detected as changes in the amount of light reemitted from the skin [13,14].…”

Association of remote imaging photoplethysmography and cutaneous perfusion in volunteers

“…The emergence of iPPG signals is however still far from clear 14 , 15 . A varying light absorption by haemoglobin due to the cutaneous blood volume pulse is considered the key mechanism 16 and indicates the sensitivity of iPPG for cutaneous perfusion.…”