Ever listen to a train, or airplane, or helicopter as it approaches, passes, and then starts going away? The sounds are higher pitched when approaching, and lower pitched as the source of the sound moves away. The reason for this is that relative to your stationary position, the sound waves get “bunched up” on approach, then “stretched out” on departure, thus changing their pitch. It’s called the Doppler Effect. The same thing happens to light waves, which is how astronomers calculate how fast distant galaxies are moving toward or away from our own.
The amazing thing is that sensors are now sensitive enough to detect tiny shifts in wavelength even for relatively slow moving object, such as blood cells in your body’s blood vessels. Kyocera has announced a new miniature chip uses a tiny laser and a light sensor to measure blood motion below the skin. The skin is stationary relative to the moving blood, and the difference in the wavelengths of the reflected light creates interference patterns that can be detected and used to measure the speed difference. This tiny chip is 3.2 by 1.6 by 1 mm — one-eighth of an inch long — and is designed to be incorporated in devices ranging from smartphones to earbuds.
The blood flow data can be used to infer other useful information, such as body hydration or heatstroke. This new sensor will be available in sample quantities this spring, with the expectation that it will appear in commercial products in 2018.