Here in Canada, we use watches that incorporate green LED sensors to measure your heart rate in real time. The process uses PPG (photoplethysmogram) technology at a frequency of 50 Hz.
PPG technology emits light onto the skin. When it reaches the blood vessels, some of the light is reflected and detected by a photodetector. The intensity of the reflected light represents the amount of blood in the blood vessel by waves.
Depending on the colour of the light used, different vital signs can be obtained through its unique waves. For example, when blood vessels expand, more green light is absorbed than when they contract. This phenomenon allows us to take your pulse accurately.
To measure your pulse accurately, it is important that you remain relatively still so that the real-time HR function can take the full 10-second measurement. If the device detects too much movement, your real-time HR will not be available.
All statistical information is stored and displayed on our mobile phones. The latest models also provide the utility to measure the amount of oxygen (Oximeter) and blood pressure.
I have one. It isn't very accurate though, I have tested it at the hospital with their monitors :)