Because heart rate forms the foundation of the Basis platform, the device's optical heart-rate sensor pulls the heavy load. The technology takes advantage of a well-studied technique for monitoring cardiac activity known as photoplethysmography (PPG), which uses light to measure variation in the volume of a part of the body. The light comes from a light-emitting diode, or LED, situated on the underside of the Basis watch face. The LED shines light onto the wearer's wrist, and a sensor detects how much comes back.
Here's how it works. As the light passes through the skin and into the underlying tissue, some is reflected, some is scattered and some is absorbed, depending on what it encounters. Blood absorbs light more readily than surrounding tissue, so if there's more blood in the area, less light will come back to the sensor. With each heartbeat, blood surges through arteries and veins and then ebbs, meaning blood volume first increases, then decreases. The Basis B1 sensor "sees" these heartbeats as changes, or pulses, in the reflected light.
Interestingly, PPG works differently with different wavelengths of light. Longer wavelengths penetrate more deeply than shorter wavelengths, so infrared radiation can be used to assess blood volume in deep tissue, while light in the visible range is ideal to measure blood just beneath the skin. The Basis B1 optical monitor relies on green light to count heartbeats. Not only that, it comes to know a user's unique skin topography. If a person has a hairy wrist, for example, special Basis algorithms make adjustments, accounting for how the hair fibers affect light transmission. It makes similar allowances for body type and skin tone.