What BUG measures
BUG stands for backlight, uplight and glare. The rating was introduced by the Illuminating Engineering Society in IES TM-15 to describe where an outdoor luminaire sends light that does not land in front of it. It is calculated from the luminaire's photometric data by adding up the lumens that fall in defined zones around it.
Each of the three letters gets a score from 0 to 5, and lower is better. A rating is written, for example, as B1 U0 G1.
B, U and G
Because the scores are based on lumens, the rating depends on output as well as optics. The same housing and optic can rate higher at a larger lumen package, so check the rating for the exact configuration you are specifying.
- Backlight (B): light behind the luminaire, which can spill onto neighbouring property as light trespass
- Uplight (U): light emitted above the horizontal, the fixture's direct contribution to sky glow
- Glare (G): light at high angles, which can be uncomfortable and reduce visibility
Why U0 matters
U0 means the luminaire sends essentially no light above the horizontal. Light emitted upward does nothing for the ground below; it only brightens the sky and wastes energy.
U0 alone does not make a site dark-sky friendly. Light reflected off the ground still adds to sky glow, so the amount of light used matters too.
BUG and full cutoff
Full cutoff is an older IES classification: no light at or above the horizontal, and only a small share just below it. A full cutoff luminaire will usually rate U0.
BUG goes further. Cutoff looked mainly at light near and above the horizontal, as a share of output; BUG looks in all three directions and counts actual lumens, so it also captures backlight and glare. Many specifications and model lighting ordinances now ask for BUG ratings instead of cutoff classes.
How specifiers and bylaws use it
Outdoor lighting ordinances often divide an area into lighting zones, from dark natural areas to busy urban centres, and set a maximum BUG rating for each. A park edge or residential street may call for low backlight and U0, while a commercial district may allow more.
For a specifier, BUG is a quick way to screen luminaires before running a photometric design. The design then shows the actual light at the property line and on the task surface.
What dark-sky friendly lighting means
Dark-sky friendly lighting follows a few simple principles, promoted by organizations such as DarkSky International.
Some programs also approve individual fixtures. That approval is separate from a BUG rating, and whether a product holds it should be checked with the program itself.
- Useful: light only what needs to be lit
- Targeted: shield light and aim it down, where it is needed
- Low level: no brighter than the task requires
- Controlled: dim or switch off when areas are not in use
- Warm colour: favour warmer colour temperatures, generally 3000 K or lower