According to the results of this study published by Elsevier, Welltory's measurement accuracy is on par with ECG monitors. The measurement process goes as follows.

We illuminate the finger with the flash and use the camera to take a video that captures changes in the color of your blood vessels, then use it to count your heartbeat.

Video frame brightness is associated with variations in the light absorption of the blood. When the heart contracts, the capillaries in the fingertip are full of blood, more light is absorbed and the frame becomes darker. When the heart expands and the capillaries in the fingertip are full of less blood, less light is absorbed, and the frame becomes brighter. PPG signals are obtained by calculating the intensity changes of the frames. Heart rate is extracted from them.

Note! On some smartphones, the flash is very hot and can burn. We recommend keeping your finger 2-3 mm away from the flash.