What Is BUG Rating in Outdoor Lighting? Backlight, Uplight, and Glare Explained
Updated November 7, 2026: added BUG limits by lighting zone, a step-by-step fixture check with our IES viewer, and FAQs.
BUG rating is a key metric used in outdoor lighting design to control light pollution, glare, and unwanted spill. It helps designers, engineers, and inspectors understand how a fixture distributes light beyond the target area.
If you work with photometric plans, site lighting, or exterior fixtures, understanding BUG ratings is essential for code compliance and visual comfort.
What Does BUG Rating Mean?
BUG is an acronym that describes three types of unwanted light output from outdoor fixtures. Each component is measured separately to give a clear picture of fixture performance.
The Three BUG Components
Each BUG rating is expressed as three numbers, such as B2 / U1 / G3. Lower values mean better control of light.
- Backlight (B): Light emitted behind the fixture.
- Uplight (U): Light emitted above the horizontal plane.
- Glare (G): High-angle brightness that causes visual discomfort.
BUG ratings are defined by the IES (Illuminating Engineering Society) and are commonly used in zoning ordinances and outdoor lighting standards.
How to Read B2-U0-G1
A BUG rating lighting specification contains three independent ratings, each from 0 to 5—not a combined score or percentage. B2-U0-G1 means the fixture meets the B2 backlight, U0 uplight, and G1 glare thresholds. U0 means zero lumens in the rated upper-hemisphere zones; B2 does not mean 20% backlight.
Under IES TM-15-11, ratings are based on lumens within angular zones:
- Backlight: BL, BM, and BH are the low, medium, and high rearward zones used in Table A-1.
- Uplight: UL and UH cover light above horizontal, near the horizon and toward the sky.
- Glare: FH/BH and FVH/BVH cover high and very-high forward/rearward light. FL means forward-low; it is not a glare-rating zone.
For example, if BL and BH each qualify as B1 but BM requires B2, the resulting backlight rating is B2. The most restrictive zone determines the component rating.
How BUG Ratings Affect Outdoor Lighting Design
BUG ratings help ensure outdoor lighting provides visibility and safety without creating excessive glare or light trespass onto neighboring properties.
- Limits light pollution and sky glow
- Improves visual comfort for pedestrians and drivers
- Supports compliance with local lighting codes
Lighting Zones and BUG Limits
Outdoor sites are classified into Lighting Zones (LZ0–LZ4). Each zone sets maximum allowed BUG values based on environmental sensitivity.
- LZ0–LZ1: Very low BUG values required
- LZ2: Typical residential environments
- LZ3–LZ4: Commercial and urban areas
The existing table image shows fixture-classification thresholds by angular zone; the MLO summary below addresses allowable ratings by site lighting zone.
BUG ratings are calculated from a fixture’s photometric test data, commonly supplied in an IES file. A site photometric plan separately checks the proposed layout and applicable lighting requirements.
Which BUG Rating Does Your Lighting Zone Allow?
The joint IDA/IES Model Lighting Ordinance (MLO) is a model—not automatically your local code. Confirm the site’s assigned lighting zone and adopted requirements with the reviewing authority.
| MLO lighting zone | Maximum B (more than 2 mounting heights from property line) | Maximum U | Baseline maximum G |
|---|---|---|---|
| LZ0 | B1 | U0 | G0 |
| LZ1 | B3 | U1 | G1 |
| LZ2 | B4 | U2 | G2 |
| LZ3 | B5 | U3 | G3 |
| LZ4 | B5 | U4 | G4 |
Source: MLO Tables C-1–C-3, pages 26–28. This is a conditional summary, not a complete compliance table: closer fixtures have tighter backlight limits; glare limits tighten for fixtures not ideally oriented near a boundary. Street and area lighting must emit no light above 90° under Table C-2, even where a higher U rating is otherwise listed. Check the full tables and local rules before selecting fixtures.
Example: BUG Rating in a Real Project
In this residential exterior example, three wall-mounted fixtures were analyzed using photometric software and the manufacturer’s IES data.
- Location: Residential facade
- Lighting Zone: LZ2
- Calculated Fixture BUG Rating: B2 / U3 / G3
B2 / U3 / G3 describes this fixture’s light distribution; it does not establish site compliance. For comparison, the 2011 IDA/IES Model Lighting Ordinance lists maximum U2 and baseline G2 ratings for LZ2, so U3 / G3 exceeds those limits. Confirm the locally adopted requirements, fixture configuration, and installation before specifying it.
How to Check a Fixture’s BUG Rating
- Download the manufacturer’s IES file for the exact fixture, output, optic, and shield configuration. Check the specification sheet for a published BUG rating.
- Upload that file to our IES Viewer & Photometric Studio to inspect its BUG rating and light distribution. Use complete photometric data; missing upper-hemisphere data can prevent reliable classification.
- Compare the result with the adopted ordinance, lighting zone, property-line setback, and installation orientation. Resolve differences with the manufacturer before specifying the fixture.
Raising a pole changes site illumination and setback-to-height ratios; it does not by itself change the fixture’s tested distribution. Tilting changes where light travels relative to horizontal. Shields and optics change distribution, so use matching photometric data rather than assuming an accessory fixes compliance.
Get a Professional Photometric Plan
We create accurate photometric plans that verify BUG ratings, lighting levels, and code compliance.
When You Need a Site Photometric Plan
A fixture’s BUG rating cannot tell you the foot-candles at a neighbor’s window or property line. A site plan models fixture locations, mounting heights, aiming, and combined light levels for permit review and spill-control decisions.
See our outdoor lighting foot-candle requirements, parking lot light-spill guidance, and parking lot photometric plans.
BUG Rating FAQs
Is B0-U0-G0 required everywhere?
No. Allowable ratings depend on the adopted ordinance, lighting zone, setback, orientation, and application.
Does U0 mean no skyglow?
No. U0 addresses direct uplight from the fixture in its rated orientation. Light reflected from pavement and buildings can still reach the sky.
Does a low BUG rating guarantee dark-sky compliance?
No. Color temperature, controls, shielding, and site requirements also matter. See our dark-sky-compliant lighting guide.
Can two versions of the same fixture have different ratings?
Yes. Different lumen outputs, optics, or shields can change zonal lumens and therefore the rating. Match the exact product configuration.
Does G0 guarantee nobody will experience glare?
No. G0 is a classification, not a guarantee of visual comfort for every observer, location, or installation.
Final Takeaways
BUG ratings are a critical part of modern outdoor lighting design. They help control glare, limit light pollution, and ensure fixtures perform as intended.
If you need help verifying BUG ratings or preparing a compliant photometric plan, Stetra Lighting can support your project from fixture selection to final approval.
For the full design approach, see our guide to dark sky compliant lighting.