Lux measures how much visible light is landing on a surface: one lumen per square meter. The human eye adapts across an enormous range: full noon sun is around 100,000 lux; candlelight is around 1 lux; the moon at full is around 0.25 lux. The eye does not feel that range linearly, but the body's circadian system does.
For waking activity, IES targets are 300–500 lux for typical office work and 500–1000 lux for detail tasks. OSHA 29 CFR 1926.56 sets workplace minimums. Most residential living rooms run at 50–100 lux in the evening, which is fine for relaxation and bad for reading.
The bigger story is circadian. The retina has a separate non-image-forming pathway that signals "it is daytime" to the suprachiasmatic nucleus, and it needs roughly 1,000+ lux of broad-spectrum light during the day, and very little after sunset, to keep sleep and mood healthy. Most indoor environments invert this: dim during the day, bright at night.
The sensor uses a TI OPT3001 with human-eye-matched spectral response. Watch it for a week and the dashboard will tell you whether your daytime light exposure is high enough, most users find it is not.
References
- IES - The Lighting Handbook www.ies.org
- OSHA 29 CFR 1926.56 - Workplace illumination www.osha.gov
- WHO - Circadian rhythms and health www.who.int
- Texas Instruments - OPT3001 datasheet www.ti.com
