  Photoplethysmography (PPG) devices are widely used for monitoring
cardiovascular function. However, these devices require skin contact, which
restrict their use to at-rest short-term monitoring using single-point
measurements. Photoplethysmographic imaging (PPGI) has been recently proposed
as a non-contact monitoring alternative by measuring blood pulse signals across
a spatial region of interest. Existing systems operate in reflectance mode, of
which many are limited to short-distance monitoring and are prone to temporal
changes in ambient illumination. This paper is the first study to investigate
the feasibility of long-distance non-contact cardiovascular monitoring at the
supermeter level using transmittance PPGI. For this purpose, a novel PPGI
system was designed at the hardware and software level using ambient correction
via temporally coded illumination (TCI) and signal processing for PPGI signal
extraction. Experimental results show that the processing steps yield a
substantially more pulsatile PPGI signal than the raw acquired signal,
resulting in statistically significant increases in correlation to ground-truth
PPG in both short- ($p \in [<0.0001, 0.040]$) and long-distance ($p \in
[<0.0001, 0.056]$) monitoring. The results support the hypothesis that
long-distance heart rate monitoring is feasible using transmittance PPGI,
allowing for new possibilities of monitoring cardiovascular function in a
non-contact manner.

Optics;Computer Vision and Pattern Recognition