Fediverse

🟢 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

Good morning. ☕

@writingslowly His answer is here actually :

https://lazybea.rs/ovr-077 😉

@alex And thanks for your time 🙏

@fastmail I'm a Fastmail user and e.g. my kernel-org account forwars there. Can I migrate my account to EU?

It's the first Monday of the month, which is a great time to document something for the first time with a README file https://github.com/ddbeck/readme-checklist

HPE: Here is the firmware update ISO for your Gen8 Proliant - it is UEFI so you need to activate it

Also HPE: Btw, that generation does not support UEFI, Good Luck :)

@riverpunk @lususlumi

Thank you for the shoutout 🙏

And yes, avoiding technical language is really important, there's almost always a non-technical way of saying the same thing!

@AwetTesfaiesus But also: knowing that people with such deep knowledge and experience are here on the Fediverse, sharing said knowledge and experience — that makes me feel good :)