An optical heart-rate monitor will detect the pulse by shining a light through the skin to measure blood flow.
Times, Sunday Times
The optomechanical heart-rate sensors in these headphones shine light against the skin in your ear to detect your pulse.
Times, Sunday Times
Any 10 m target between 100 and 300 m using a similar or better system would be able to detect the pulse but would not be detected by the emitter.
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