Photoplethysmography has many advantages, such as small size, low power consumption, rapid response, etc., thus has also been gradually introduced into applications like fitness trackers, home care equipment and automotive monitoring devices, says Joanne Wu, research manager of LEDinside. However, some consumers still question about the reliability of devices, so the manufacturers are committed to improving the accuracy of sensors in order to gain consumer confidence.

Photoplethysmography require high linearity, high sensitivity and good signal-to-noise ratio. The products incorporate emitter and receiver. The emitter needs to match the requirements of algorithm programs, so the wavelength of the LED products varies from brand to brand. The common design uses green LED paired with infrared LED to detect the amount of blood flowing through your wrist at any given moment, and to count the heartbeats. If pairing red LED with infrared LED, the sensor can detect the amount of deoxygenated hemoglobin (Hb) and oxygenated hemoglobin (HbO2) during any given time to calculate the oxygen saturation levels. The main suppliers of infrared LED include OSRAM OS and Epistar.

On the other hand, receivers mostly use photodiodes, especially large-area photodiode with low dark current, which has short response time and can receive LED energy completely. The next generation of Apple Watch is also expected to use larger-sized photodiodes, and the number of photodiodes used may be four to five times more than previous models. The current major supplier of photodiodes is Opto Tech.