Mister Clarke said that any sufficiently advanced technology is indistinguishable from magic. Optical heart rate measuring comes pretty close. You can read one's heart rate by flashing beams of carefully calibrated red and green light through one's skin.
Like any magic, this one is also a delicate trick to pull. Optical heart rate sensors require careful calibration. For that you need flexible hardware and Heart rate 3 click offers just that. It has two separate components: a photo-diode heart-rate sensor that acquires the data, and a bio-sensing specific IC that filters it and converts it into a usable signal.
The result is a flexible platform that can be configured for different use-cases. For example, skin color is a major issue with optical heart rate sensing. But so is ambiental lighting the touch pressure and so forth.
This click will require some trial-and-error for you to figure out the optimal positioning of your finger on the sensor, but with patience you will be able to get good results.
All the relevant details and links are available on the product page.