BUG Ratings for Solar Street Lights
-- B, U, G Explained
BUG Ratings for Solar Street Lights: B, U, G Explained
The BUG rating is the outdoor lighting industry's shorthand for spill. It scores a fixture on three letters — Backlight, Uplight, Glare — each on a 0 to 5 scale, where 0 is the tightest light control and 5 is the worst. A typical result reads B2 U0 G1: modest trespass behind the pole, no uplight, mild glare. For solar street lights the middle letter decides whether a project passes a dark-sky ordinance, and it is the letter most often given away by cheap optics. This page covers what each letter measures, why the scores matter at procurement, and what the rating cannot tell you.
What each letter measures
The BUG system was developed by the Illuminating Engineering Society and is published as IES TM-15-11. It grades lumens by zone around the fixture rather than by a single "brightness" number:
- Backlight (B) — light thrown behind the fixture, toward property lines, windows, and neighbors. This is the zone that generates trespass complaints.
- Uplight (U) — light emitted above horizontal. TM-15-11 splits it into two zones, 90–100° and 100–180°, because high-angle uplight scatters into sky glow while near-horizontal spill mostly lands on something.
- Glare (G) — high-angle brightness in the driver's and pedestrian's field of view. This is the zone that makes a road feel harsh even when average illumination is adequate.
The original page lists what each zone "evaluates" — light trespass, sky glow, disability glare — and all of that stands. What it does not say is the important part for a buyer: the three letters trade against each other. Push light down onto the road harder and backlight at the property line rises; flatten the beam to protect neighbors and glare rises for drivers. Optics is a budget, not a menu.
Why the U score is the hard one for solar street lights
Every lumen above the horizon is energy that left the battery and never hit the ground. On a grid fixture that is a billing question; on a solar unit it is autonomy you cannot get back. For that reason we design our street and area units to U0 — zero rated uplight — and treat a U1 result in a photometric report as a defect to correct, not a number to disclose. A wide "spill" lens that flatters a sales sheet with broad coverage usually carries U2 or worse, and in a dark-sky jurisdiction that is where the bid dies.
Where BUG shows up in procurement
- Dark-sky ordinances. Many municipalities and the Model Lighting Ordinance reference BUG ratings directly, and International Dark-Sky Association certification looks at the same zonal lumens. Check the clause before you pick a fixture.
- Light trespass disputes. The B score is the number a property-line complaint will be argued with. It is also the number you can defend with a photometric file.
- Wildlife-sensitive areas. Coastal and migratory corridors increasingly specify capped uplight; the U score is the compliance evidence.
What BUG does not tell you
BUG grades zonal lumens, not the road. A B1 U0 G1 fixture can still produce a badly lit street if aim, spacing, or pole height are wrong, and a slightly higher-rated fixture can outperform it on the same geometry. Use BUG to screen spill and compliance; use a lighting layout to confirm the road-level result. Our lighting design tool runs the layout side.
What this page does not cover
This page explains the rating system. It does not cover photometric file preparation, IES file interpretation in detail, or the installation errors that change a fixture's real-world uplight after it ships — that last topic has its own page on lamp arm angle and uplight.