CasebackWhat it measures

What it measures

What the disc senses

Three small LEDs, green, red and infrared, shine into the skin under the disc, and a photodiode next to them measures how much of that light comes back. Blood absorbs light and the amount of blood under the sensor changes a little with every beat, so the returning light dips and recovers with the pulse.

An accelerometer measures movement. It gives the steps and the sleep stages, and it tells the pulse algorithm which part of the signal is arm motion. A thermistor on the skin side reads skin temperature.

There is no ECG. An ECG needs two contacts on different parts of the body, and a disc under a watch touches one wrist.

SkinDiscLEDsPhotodiodeWhat the photodiode reads08 s
  • Green light
  • Red light
  • Infrared light
Schematic. Distances and depths are not to scale; longer wavelengths reach deeper.The waveform is a finger recording from a hospital monitor (BIDMC, subject 36, public data). It is not from the disc.

Three answers and what they come from

The app opens on three answers: Recovery and Sleep, both from 0 to 100, and Movement, your steps against a usual day of yours.

Each is built from signals the disc records. Recovery compares your resting heart rate, HRV, last night's sleep and skin temperature with your own last weeks. Sleep comes from motion and heart rate, and Movement from motion. Blood oxygen is shown on its own, as a trend.

A filled mark is shown as a value and an open mark as a trend. Caseback is not a medical device and does not diagnose anything.

AnswersSignalsRecovery0 to 100Sleep0 to 100Movementsteps againstyour usual daySkin temperatureHRVHeart rateMotionBlood oxygen
AnswersSignalsRecovery0 to 100Sleep0 to 100Movementsteps againstyour usual daySkintemperatureHRVHeart rateMotionBlood oxygen

Filled mark: shown as a valueOpen mark: shown as a trend