4.5 Light Pollution Reduction

The final SS credit, Light Pollution Reduction, often feels esoteric until candidates see the photographs: poorly designed parking-lot lighting that turns a suburban street into perpetual twilight, or a building facade visible from miles away on Google Earth at 2 a.m. This credit limits four kinds of light pollution: sky glow, light trespass onto adjacent property, glare, and interior light escaping through windows after curfew.

SS Credit: Light Pollution Reduction (1 point)

Intent

To increase night sky access, improve nighttime visibility, and reduce the consequences of development for wildlife and people.

Lighting Zone Classification

Before any calculation, the project team classifies the site into one of five lighting zones defined by the IES (Illuminating Engineering Society) and the IDA (International Dark-Sky Association):

The zone determines all subsequent BUG and trespass limits. Misclassifying a site into a more permissive zone is one of the most common reasons this credit is denied at review.

The BUG Rating System (IES TM-15-11)

Every outdoor luminaire is assigned three letter-and-number ratings under IES TM-15-11:

• B (Backlight) — light cast behind the luminaire toward adjacent properties; rated B0 (lowest) to B5 (highest)
• U (Uplight) — light cast above the horizontal; rated U0 to U5
• G (Glare) — light at high angles toward observers; rated G0 to G5

The credit specifies maximum allowable B, U, and G ratings by lighting zone:

Exact values are taken from the LEED v4.1 reference guide; the pattern above shows the progressive relaxation as zones become more developed.

Uplight Limit

A stricter independent limit applies: the percentage of total initial fixture lumens emitted above 90° must not exceed a zone-specific cap (e.g., 0% for LZ0, up to 6% for LZ4). U0 fixtures emit virtually zero uplight and are required in LZ0 and LZ1.

Indoor Light Trespass (IES LM-83 Method)

Light escaping interior fixtures through windows after curfew is also regulated. The compliance options are:

1. Shielded fixtures that direct interior light away from windows after curfew
2. Automated controls that reduce interior lighting power by at least 50% after curfew
3. Building automation system that turns off non-essential interior lighting after curfew
4. IES LM-83 calculation method demonstrating compliance through whole-building modeling

Curfew Definition

For LEED purposes the standard curfew is 10:00 p.m. (sometimes the project may use the locally adopted curfew if different). After curfew, both exterior trespass and interior light escape must be reduced.

Special Restrictions

• Sports field lighting — must be turned off within one hour of the event ending
• Signage — illuminated signs have separate luminance limits and curfew shutoff
• Facade and architectural lighting — must be shut off at curfew except in LZ3/LZ4 entertainment districts (which still have limits)
• Emergency and security lighting — generally exempt but must use motion sensors where feasible

Documentation

• Site plan showing every exterior luminaire with its BUG rating, mounting height, and aiming
• Photometric calculations at the property line showing trespass values
• Lighting zone determination memo with supporting context
• Controls narrative for indoor and outdoor curfew compliance
• Cut sheets for each fixture confirming TM-15-11 BUG values

Common Exam Traps

• The credit is 1 point, not stackable.
• BUG ratings come from IES TM-15-11; interior trespass uses IES LM-83. Mixing these up is common.
• The five zones are LZ0–LZ4, with LZ0 the darkest. LZ5 does not exist in the LEED system.
• The default curfew is 10pm, not midnight.
• Uplight is the single most heavily restricted component — even a small amount of U2 or higher fixtures can fail a project in LZ0 or LZ1.
• A vegetated roof or PV array does not exempt a project from this credit if exterior site lighting is present.