Light Pollution Calculator

The Bortle Scale is a 9-level numeric scale that measures the brightness of the night sky at a given location. Class 1 represents the darkest possible skies — perfect for deep-sky observation — while Class 9 describes the washed-out skies of a city center where only the Moon and brightest planets are visible. Most suburban observers fall between Class 5 and 7.

SQM stands for Sky Quality Meter, a device that measures sky brightness in magnitudes per square arcsecond (mag/arcsec²). Higher SQM values mean darker skies — a pristine dark site measures around 21.5–22, while a city center may read 16 or lower. The Bortle Scale is a qualitative description, while SQM provides a precise numeric measurement. The two scales correlate closely and this calculator converts between them.

Light pollution raises the sky background brightness, which competes with the faint light from nebulae and galaxies. To achieve the same signal-to-noise ratio as a dark sky site, you need to expose significantly longer — for example, 1 hour at Bortle 1 requires roughly 9–10 hours at Bortle 5. This calculator shows you the exact exposure multiplier for your conditions.

NELM is the faintest star magnitude visible to the unaided eye under a given sky. Under perfect dark skies (Bortle 1), experienced observers can see stars as faint as magnitude 7.6. Under typical suburban skies (Bortle 5–6), NELM drops to around 4.5–5.0, meaning thousands of stars simply become invisible. The higher the magnitude number, the fainter (and more numerous) the stars you can see.

Artificial light at night disrupts the circadian rhythms of birds, insects, sea turtles, and many mammals. It disorients migrating birds, reduces firefly populations, interferes with insect reproduction, and disrupts nocturnal predator-prey relationships. Blue-rich LED lighting is particularly harmful because it more strongly suppresses melatonin production in both animals and humans.

Blue-rich white LED lights cause significantly more sky glow and ecological disruption than warm-toned alternatives. This is because shorter (blue) wavelengths scatter more in the atmosphere. High-pressure sodium lamps (the orange street lights) produce less sky glow per lumen. Fully shielded, downward-directed fixtures with warm-spectrum bulbs are the most light-pollution-friendly choice.

Look for areas with low Bortle ratings on light pollution maps — national parks, forests, and designated Dark Sky Preserves are excellent choices. Drive at least 50–100 km away from urban centers, seek elevated terrain that puts city glow below the horizon, and plan trips around new moon phases when the moon doesn't add background glow to the sky.

Yes. For Milky Way photography, a Bortle class of 4 or lower is generally recommended. Use the exposure multiplier output to understand how much longer you'll need to expose compared to a pristine dark sky, and combine this with moon phase planning. Shooting from a Bortle 1–3 site dramatically reduces required exposure time and reveals far more galactic detail.