Every lumen a fixture sends upward or sideways scatters off air molecules and
dust to form a glowing dome over the city — sky glow — that swamps starlight and
disorients nocturnal wildlife. A full-cutoff shield redirects nearly all light
downward onto the road where it's actually needed.
uplight_fraction: shielded ≈ 3%, unshielded ≈ 25%
sky_glow ∝ Σ(lamp_lumens · uplight_fraction) · atmospheric_scatter
visible_stars ≈ max_stars · (1 − sky_glow)
- Brightness — total lumen output per fixture; more light means more potential glow.
- Lamp density — how many fixtures are lit across the district.
- Colour temperature — bluer, cooler light (higher K) scatters more in the atmosphere and disrupts wildlife circadian rhythms more than warm light.
- Shielding — the single biggest lever: a full-cutoff fixture cuts uplight roughly 8×.