From the linked article: Apple has been revealed to be the largest customer of the British electronics start-up Rockley Photonics, The Telegraph reports.

Rockley Photonics has developed non-invasive optical sensors for detecting multiple blood-related health metrics, including blood pressure, blood glucose, and blood alcohol levels, many of which are only normally detectable with more invasive dedicated medical equipment. Rockley's sensors beam infrared light through a user's skin, much like the existing sensors on the back of the Apple Watch for detecting heart rate and blood oxygen levels.

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