A cutoff luminaire is a light fixture designed to limit the light it emits at and near the horizontal, directing most of its output downward where it is needed.
The Illuminating Engineering Society (IES) long classified fixtures by how much light they emit at high angles. Under those definitions a cutoff fixture allows no more than 2.5 percent of its lamp lumens at or above horizontal and no more than 10 percent at or above 80 degrees, while a full-cutoff fixture allows none above horizontal. Many standards now use the IES BUG rating (backlight, uplight, glare) instead. See non-cutoff luminaire.
Classic IES categories
- Full cutoff: no light at or above horizontal; limited light just below it.
- Cutoff: very little light above horizontal.
- Semi-cutoff: somewhat more light permitted at high angles.
- Non-cutoff: no limits.
Why it matters for parking
- Less light pollution and sky glow from surface lots and garage roofs.
- Less light trespass onto neighbors.
- Less glare for drivers and pedestrians.
- Compliance with dark-sky ordinances and green building programs. See DarkSky International.
The Parkonomics view
Light that goes up lights nothing. Full-cutoff fixtures on a surface lot or a garage roof are among the simplest good-neighbor choices an owner can make.
Related terms: Non-Cutoff Luminaire, Glare, Light Pollution, Light Trespass, DarkSky International, Light Standard