Do phone heart rate monitors work how

The operative term here is photoplethysmograph. It's the term for what your phone is acting as.

Basically, your blood vessels rhythmically dilate and contract with every heart beat. This is measurable as either a change in reflectance or transmittance (depending on how the photoplethysmograph is configured. In the case of your phone, it's measuring reflectance, as the light is adjacent to the camera.

In medical instruments, it's generally done with infrared light, because blood absorbs shortwave infrared more then red or white light, and your tissue is more transparent to infrared. However, there is no reason it wouldn't work with a phone camera.

If done properly. it should be as or more accurate then manual measurement. However, a phone is not a proper photoplethysmograph, so the realistic achievable accuracy is likely dependent on a number of factors, including camera performance, proximity of the illuminating LED to the camera lens, and the algorithm used to extract the pulse from the video return.

However, you need to be aware that all photoplethysmographs are very sensitive to mechanical movement. It is essential that you hold your finger as still as possible, and avoid moving the phone relative to the finger.