Understand the BUG rating

Decode B-U-G ratings (Backlight, Uplight, Glare) used by IES TM-15-11 and many outdoor lighting ordinances.

4 steps · 3 min read

BUG ratings are the standard shorthand for "how well-behaved is this fixture in dark-sky terms?". They're used in permitting, code compliance, and increasingly in client requirements.

  1. Where to find it

    Open any luminaire's Photometric tab and scroll down to the BUG Rating card. You'll see three letters — B, U, G — each followed by a number from 0 (best / least spill) to 5 (worst).

  2. What the three letters mean

    • B — Backlight: light that goes behind the fixture (away from the area you're trying to illuminate). High B means light trespass onto the property behind the pole.
    • U — Uplight: light that goes above horizontal. High U means contribution to sky glow / wasted light.
    • G — Glare: light at high vertical angles in the forward direction. High G means uncomfortable glare for drivers and neighbours.

  3. How the rating is calculated

    OpenLumen integrates lumens in each TM-15 zone (defined by vertical and horizontal angle ranges) and looks up the resulting lumen total in the IES TM-15-11 BUG tables. The card shows the per-zone lumen totals so you can see *why* the rating is what it is.

  4. Use it as a design constraint

    Many outdoor lighting ordinances cap the allowed BUG values for a given lighting zone (LZ0 wilderness through LZ4 urban core). Match the rating against the local code and pick a fixture that fits — or aim it differently to drop the G or U numbers.

    A lower BUG isn't always better: a tightly controlled fixture might leave dark spots. Trade BUG against your uniformity / target footcandle goals.