The smartwatch and smart bracelet use the PPG to measure heart rate.
Blood contains heme, and the degree of absorption and reflection of light by heme with different oxygen levels also varies. When the heart contracts, the heme density is high and it absorbs more green light. When the heart relaxes, the heme density is low and more green light is reflected. Wearing smartwatches and bracelets on the wrist can easily leak light. The light emitted by fluorescent lamps and energy-saving lamps can cause data errors. The relative movement between optical sensors (LEDs and photodetectors) and the skin can also reduce the sensitivity of light signals.