Dark-sky compliance used to be a vibe. TM-15 turned it into three numbers: Backlight (light spilling behind), Uplight (light going skyward), Glare (light at high angles). Every outdoor fixture sold today carries a B0-B5 / U0-U5 / G0-G5 rating, and AlUla, NEOM, and most KSA municipalities now write LZ-class limits directly into the BOQ.
Four ideas that move dark-sky from philosophy to a spec line.
Backlight, Uplight, Glare, scored independently 0–5 each. A spec saying "U0 only" is fine for Dark-Sky compliance but says nothing about glare onto neighbours' bedroom windows.
LZ0 (intrinsic dark, AlUla observatories) caps B1/U0/G1. LZ4 (high-activity city centre) allows B5/U4/G5. The site, not the fixture, decides the budget.
Zero lumens above 90°. Requires sharp horizontal cut-off optics, not just "full cut-off" marketing. KSA observatory and protected-area projects mandate U0.
Modern IES TM-15 files include zonal lumen output by solid angle. AGi32, DIALux, and Visual all auto-compute the BUG rating, no separate report needed.
Per IES TM-15 / MLO 2011. KSA municipalities map LZ to land-use zoning.
| Zone | Description | Max B / U / G | Example KSA |
|---|---|---|---|
| LZ0 | Intrinsic dark | B1 / U0 / G1 | AlUla observatories |
| LZ1 | Rural / wilderness | B2 / U1 / G2 | Resort developments |
| LZ2 | Low-density residential | B3 / U2 / G3 | Suburban Riyadh |
| LZ3 | Medium activity / commercial | B4 / U3 / G4 | Mixed-use districts |
| LZ4 | High-activity city core | B5 / U4 / G5 | Downtown Riyadh |
| NEOM dark zone | Reserve adjacency | B1 / U0 / G1 | NEOM IDA-class areas |
Six ideas behind every "this fixture is B2-U0-G2" line on a datasheet.
TM-15 carves the sphere around the luminaire into named angular zones, FL, BL, BH, BVH, UH, UL, FVH, BVH. Each captures lumens in a specific direction (forward, back, low, high, very high). The rating is the worst zone score.
U0 = zero lumens above 90°. U1 ≤ 10 lm. U5 = no limit. Uplight values do not scale with fixture size, they are absolute, which is why a single bad uplight luminaire in a U0 project fails the entire site audit.
BL and BH zones sum the lumens going backward at low and high angles. The score is then mapped to a ratio against forward output. B0 fixtures are essentially asymmetric, throwing 95%+ forward.
G-score caps the lumens in the FVH + BVH zones (60–90° from nadir), the angles that hit people's eyes when standing or driving past. Full-cut-off fixtures land G0-G2. Old wallpacks and acorn fixtures with exposed lamps land G4-G5.
A drop-in house-side shield on a road luminaire takes BL from B3 to B1 with zero photometric redesign. KSA tenders writing "B1 max, BL house-side shield required" get the trespass control they want at modest extra cost.
The IDA Fixture Seal of Approval predates TM-15 and only checks U0. A U0 fixture can still have B5/G5 and ruin a neighbour's bedroom. Specify BUG explicitly: "U0 + B≤2 + G≤2 per IES TM-15" closes both gaps in one line.
Every NLC outdoor luminaire ships with IES TM-15 BUG scoring on its datasheet, and most carry U0 options for NEOM and AlUla protected-area work.
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