The Light pollution source on the exposure indicator translates satellite radiance into a familiar 1-9 dark-sky class. This tutorial explains the mapping and when the verdict flips.
The thresholds ClearSpot uses
We display artificial night-sky brightness on the Bortle scale (1 = pristine, 9 = inner-city), derived from VIIRS radiance using the Falchi et al. 2016 mapping.
- Bortle 1-3 - "Clear". Excellent to rural skies; the Milky Way is visible to the naked eye.
- Bortle 4 - the default trigger. "Not clear" if the light sensitivity is on; rural-to-suburban transition.
- Bortle 5-9 - always "Not clear". From suburban sky glow (Bortle 5) to inner-city (Bortle 9) where only the brightest stars remain.
In the cog menu you can move the default trigger between Bortle 3 (amateur astronomer preference) and Bortle 6 (tolerant of suburban skies).
Why Bortle and not just "lux"
Lux measures illumination on a surface, not radiance emitted upward. For a satellite-based health indicator, upward radiance (VIIRS nW/cm^2/sr) is the right quantity, and Bortle is the most widely understood translation for non-specialists. A VIIRS reading of 0.17 nW/cm^2/sr corresponds to Bortle 4 in the Falchi mapping - the value where most cohort studies begin to see measurable effects on circadian rhythm and sleep phase.
What the verdict means
"Not clear" indicates artificial night-sky brightness above natural levels for your H3 hex. The practical impact depends on your use case: astronomers and bird-watchers care about Bortle ≤ 3; sleep-sensitive users care about Bortle ≤ 4; most of us simply want to know whether the night sky overhead is still a "night sky" at all.
From radiance to Bortle, then to your Bortle slider
NOAA publishes nW / cm² / sr, Falchi's atlas converts that into sky brightness in mag / arcsec², and the Bortle scale maps those to nine qualitative steps that astronomers already recognise. ClearSpot stops at the Bortle class + a colour chip so you are not forced to juggle three units at once, but the modal still shows radiance and your chosen ceiling.
Example: the Bortle badge in the light modal
Tap the light source row. You should see the Bortle badge (1-9), a short plain-language line ("suburban sky", and so on), a radiance field when we have it, and the threshold tick mark you configured. If the badge is 5 and your personal trigger is 4, the card should read Alert even when the sky still looks "okay" to a holiday photo.
Time coverage
The VIIRS release is an annual stack; transient municipal LED roll-outs only appear on the next yearly composite. For fast local lighting change, pair the map with on-site observation.
Data source
ClearSpot ingests the VIIRS Day/Night Band annual composites produced by the NOAA Earth Observation Group (Colorado School of Mines). For Europe we cross-check against the Falchi 2016 "World Atlas of Artificial Night Sky Brightness", served by light-pollution-map. Readings are aggregated to an H3 hex grid at resolution 3 (roughly 85 km^2 per cell) and updated annually when new VIIRS composites are released.