BUG rating and obtrusive-light checker

What is a BUG rating?

A BUG rating is a three-part classification of an outdoor luminaire's light distribution defined in IES TM-15 (Luminaire Classification System for Outdoor Luminaires). B rates backlight behind the pole, U rates uplight above the horizontal, and G rates the high-angle forward and back light that causes glare. Each runs from 0 (least) to 5 (most), for example B2-U0-G1.

TM-15 splits the space around the luminaire into angular zones (forward and back light in 0-30°, 30-60°, 60-80° and 80-90° bands; uplight in 90-100° and 100-180° bands) and compares the lumens in each zone with threshold tables, so a bright luminaire with a tidy optic can still score higher than a dim one with a poor optic. U0 means no lumens at or above 90°. Manufacturers derive the rating from the photometric file and print it on the datasheet.

How to use the BUG rating checker

1. 01Choose the MLO lighting zone, from LZ0 (no ambient light, protected areas) to LZ4 (high ambient light, city-centre districts); LZ2 suits most edge-of-town retail and commercial sites.
2. 02Set the setback to the nearest property line in mounting heights (more than 2, 1-2, 0.5-1 or less than 0.5 MH) and, within 2 MH, whether the luminaire is ideally oriented with its backlight facing that line.
3. 03Enter the luminaire reference, description and lumens for the record, then select its B, U and G ratings (0-5 each) from the manufacturer's datasheet.
4. 04Click Check Compliance for a PASS or FAIL verdict overall and for each of B, U and G against its allowed maximum. The check runs on NR Lighting's servers and nothing you enter is published.
5. 05Pro subscribers can add project details, list the luminaires assessed, record a curfew dimming period and export a PDF report with recommendations for failed checks.

The method: MLO Tables C-1 to C-3

Pass when B ≤ Bmax (zone, setback) and U ≤ Umax (zone) and G ≤ Gmax (zone, orientation within 2 MH). All three checks must pass.

The limits are the maximum allowed ratings in the Joint IDA-IES Model Lighting Ordinance (MLO, 2011), Tables C-1 (backlight), C-2 (uplight) and C-3 (glare). Uplight and glare limits equal the zone number; backlight limits tighten as the luminaire nears the property line, because backlight is what spills over a boundary.

A luminaire within 2 mounting heights of the boundary that is not ideally oriented gets reduced glare limits (G1 rather than G2 in LZ2 at 1-2 MH, for example), and Table C-2 allows street and area lighting no emission above 90° in any zone, whatever its U rating.

Worked example

A car park on an edge-of-town retail park is lit from 6 m columns; the nearest stands 8 m from a boundary with housing, so the setback is 8 ÷ 6 = 1.33 mounting heights (the 1-2 MH row). The site is LZ2, moderate ambient light, and the luminaire is a 9,000 lm area light rated B3-U0-G2 with its backlight towards the houses (ideally oriented).

The LZ2 limits at 1-2 MH are B3, U2 and G2, so B3-U0-G2 passes. Rotate the column so the backlight faces the car park and it is no longer ideally oriented: the glare limit drops to G1 and G2 fails. Move it to 4 m from the boundary (0.67 MH) and the backlight limit drops to B2, so B3 fails; the fixes are a house-side shield, a B2 optic or a different column position.

UK context: ILP GN01/21 environmental zones and upward light ratio

UK planning authorities assess obtrusive light against ILP guidance note GN01/21, which uses environmental zones E0 to E4 and limits, per zone, the installed upward light ratio (ULR), vertical illuminance at neighbouring windows before and after curfew, luminaire intensity towards dwellings and building luminance.

This checker does not compute ILP GN01/21 zone compliance: it does not calculate ULR, window illuminance or source intensity, and the E zones are different tables from the MLO zones, even though both match the lighting to the ambient brightness of the area. Use the MLO check to sieve luminaires and a GN01/21 assessment for the planning submission.

Limitations, and when to involve a lighting designer

The verdict uses only the B, U and G ratings, zone, setback and orientation. Lumens are recorded for the report but do not affect the result, and the MLO's site lumen allowances are not implemented. Ratings are not derived from photometric files, so use the rating for the exact product code and lumen package; tilting a luminaire adds uplight and invalidates a level-mounted U0.

The MLO is a US model ordinance and this check gives indicative design values, not compliance sign-off; UK planning conditions usually call for an ILP GN01/21 assessment, with the environmental zone set by the local authority.

I use this check as a first sieve when choosing exterior luminaires for showroom forecourts, retail car parks and hospitality terraces. Where a site borders housing or a dark landscape, or a planning condition requires an obtrusive light report, I model the scheme, calculate the GN01/21 metrics at the affected windows and produce the report the authority expects.