Sky Glow Comparison Tool
Kocifaj Radiative Transfer Model · PNNL-SA-138348
v1.0
Scenario Parameters
Baseline Source
5%
Fraction of light emitted above horizontal
Comparison Sources
100%
Output relative to baseline
0%
Select Comparison SPDs
Results
Uplight Sensitivity
Sky Glow SPDs
SPD Library
About
Relative Sky Glow — Baseline = 1.0
ⓘ method
Sky glow = Σ(SPD(λ) × SF(λ)) integrated over 380–770 nm. Scattering function SF depends on observer, atmosphere, uplight %, and weighting.
Source Comparison Table
Source CCT (K) Sky Glow Index (rel. units)ⓘModeled sky glow per lumen: Σ(SPD·SF) ÷ Σ(SPD·V(λ)), × lumen output %. Arbitrary model units (not cd/m² or mag/arcsec²) — meaningful only relative to the baseline. See the "vs Baseline" column. vs Baseline
Select sources to compare
Spectral Power Distribution (Normalized)
ⓘ
Each source's spectrum is peak-normalized to show its shape. A lamp's spectrum does not change with Lumen Output % or Uplight % — those affect how much light and sky glow it produces (see Relative Sky Glow and the Sky Glow SPDs tab) — so these curves stay fixed as you adjust them.
Uplight Sensitivity — Relative Sky Glow vs % Uplight
Assumes constant lumen output across the sweep — only the fraction of light emitted above horizontal changes. (Total output is set separately by the Lumen Output % control.)
Weighted Sky Glow SPD (SPD × Scattering Function)
All Spectral Power Distributions in the System
SourceCCT (K)TypeIn Use
Add Custom SPD

Paste a spectral power distribution as wavelength, power pairs (380–780 nm), or just 79 power values at 5 nm steps (380–770 nm). Added sources become available in the Baseline Source dropdown and the Select Comparison SPDs list.

About This Tool

This tool helps lighting professionals, communities, and researchers compare the sky glow impact of different light sources — supporting better lighting decisions that protect the night sky.

It replicates the Sky Glow Comparison Tool (PNNL-SA-138348) developed by Pacific Northwest National Laboratory for the US Department of Energy, based on the Kocifaj radiative transfer model.

Reference: Kocifaj, M. Light-pollution model for cloudy and cloudless night skies. Appl. Opt. 46 (2007).

Parameters
Observer:Near (inside city) or Distant (~30 km outside city) Atmosphere:Clear Low Particulate, Clear High Particulate, or Cloudy Weighting:Unweighted (photopic) or Scotopic (dark-adapted eye) Uplight %:Fraction of luminaire output emitted above horizontal (0–10%) Lumen Output:Total flux relative to baseline source

Default baseline: The tool opens with a High Pressure Sodium (HPS) source at 5% uplight as the baseline — the historical uplight allowance once permitted for legacy “semi-cutoff” street-lighting fixtures. It represents a typical older installation, giving a meaningful reference for comparing modern fully-shielded (full-cutoff, 0% uplight) and lower-CCT sources against it. Change it any time in the Baseline Source panel.

Calculation: Sky Glow Index = Σλ[SPD(λ) × SF(λ, atm, obs, uplight, weighting)] × Lumen%

A ratio above 1.0 means more sky glow than the baseline; below 1.0 means less. Higher CCT (bluer) light sources generally produce disproportionately more sky glow under scotopic weighting due to Rayleigh scattering — a key reason warm-white and amber LEDs are preferred in dark sky–friendly lighting.

Validation: the sky-glow equations and CCT method are verified to match the original PNNL spreadsheet — see VALIDATION.md (equation comparison + validation tables).

Learn more about dark sky–friendly lighting at DarkSky.org

A DarkSky.org tool · Kocifaj model · PNNL-SA-138348 · Methodology · © DarkSky International