How does it measure blood pressure?, page-4

Thanks - had a look at the article. It's a very credible journal but as the accompanying editorial comment says this is very early technology that was measured under very standardised conditions, lighting, etc:

"However, the reported results may not necessarily translate in practice. As acknowledged by the authors, the smartphone video camera was employed in a highly controlled setting in which the phone was placed on a tripod with 2 external light sources side-by-side at a fixed distance from still subjects for a 2-minute period (see Figure 2). Furthermore, the subject population was almost exclusively East Asian, and the BP of the cohort was restricted by design to the normotensive range (100–139 mm Hg for systolic BP and 60–89 mm Hg for diastolic BP). Room temperature was also fixed at one study site but not the other. In the wild, head and phone motion, variable position and orientation of the camera with respect to the user, poor lighting, cold temperature, and dark skin are all real concerns that can make it virtually impossible to extract meaningful video PPG waveforms"