Post #4354909

🟢 Your smartwatch's green light has a 90-year-old secret origin. That tiny LED is photoplethysmography (PPG), an optical trick from 1937. Physiologist Alrick Hertzman at St. Louis University pointed a light bulb and photocell at a finger and watched the pulse show up in the changing light passing through the blood. Same principle, different scale: it now runs hospital pulse oximeters and every fitness wearable. Green light near 520-530nm is strongly absorbed by hemoglobin, so when your heart beats and floods the wrist with blood, less light bounces back to the sensor. Count those dips and you have a heart rate. One bit of history worth knowing: the rabbit's-ear version of this experiment belongs to Molitor and Kniazuk at the Merck Institute in 1936. Hertzman's landmark 1937 paper measured human fingers and toes, not a rabbit. The tech also keeps shrinking. In April 2025, SCIVAX and TSLC in Japan announced the world's smallest PPG sensor head at about one square millimeter, a tenth the conventional size. Miniaturization like that is why rings like Oura can get so thin. Read more: https://journals.sagepub.com/doi/abs/10.3181/00379727-37-9630 https://www.businesswire.com/news/home/20250402019875/en/The-Worlds-Smallest-PPG-Sensor-Head https://pmc.ncbi.nlm.nih.gov/articles/PMC4822420/ #wearables #science #history