The technology described in this document can be embodied in a method that includes receiving optical data including information associated with a subject, and determining from the optical data, a first dataset and a second dataset. The first dataset represents time-varying color change at a first body part of the subject, and the second dataset represents time-varying characteristics at a second body part of the subject. The method includes identifying a first point in the first dataset, and a second point in the second dataset. The first point represents a time at which a pulse pressure wave traverses the first body part of the subject, and the second point represents a time at which the pulse pressure wave traverses the second body part of the subject. A pulse transit time (PTT) between the first and second body parts can be calculated as a difference between the first and second points.
