  We consider the problem of smartphone video-based heart rate estimation,
which typically relies on measuring the green color intensity of the user's
skin. We describe a novel signal in fingertip videos used for smartphone-based
heart rate estimation: fingertip contact surface area. We propose a model
relating contact surface area to pressure, and validate it on a dataset of 786
videos from 62 participants by demonstrating a statistical correlation between
contact surface area and green color intensity. We estimate heart rate on our
dataset with two algorithms, a baseline using the green signal only and a novel
algorithm based on both color and area. We demonstrate lower rates of
substantial errors (>10 beats per minute) using the novel algorithm (4.1%),
compared both to the baseline algorithm (6.4%) and to published results using
commercial color-based applications (>6%).
