Distribution Center Exterior: Light Trespass, BUG Ratings and Photocell Control
A 28,000 m² distribution center has 60 perimeter LED floodlights, 250 W, 33,000 lm, 5000 K, Ra 80, on 12 m poles, plus 20 yard poles serving the trailer area. The local ordinance limits illuminance at the property line to 0.5 fc, requires full cutoff and a BUG rating no worse than U0/G2/B3. A homeowner 25 m west of the west pole line complains that the lights shine into second-floor bedrooms. A contractor measures 0.4 fc at the property line and declares compliance. The owner's security standard for the trailer yard is 1 fc maintained average with 4:1 max/min, and drivers report dark patches between poles. The site controller is a photocell set to switch at 10 fc; the lights cycle at dawn. After 1 a.m. a time clock turns off everything except the yard poles. The west poles are aimed horizontally outward toward the fence line and have Type IV optics; the aiming schedule is not documented.
1. The meter reads 0.4 fc at the property line, under the 0.5 fc limit. Why is that not a resolution, and what do you measure and do?
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The ordinance limit is on illuminance at the line, but the neighbor's complaint is about glare - the brightness of the source as seen from a window. Those are different quantities. A luminaire 12 m up and 25 m from the window can be perfectly compliant at the ground line and still put a very bright image into a second-floor bedroom, especially aimed horizontally toward the fence with a Type IV optic that throws light laterally.
The relevant measurements are: vertical illuminance on the neighbor's facade at window height (many ordinances cap this too), and the luminance of the luminaire as seen from that window, with the meter aimed at the fixture using the correct acceptance angle, plus the as-installed aiming angle and tilt.
What I would do: pull the aiming schedule - the setup says there isn't one, which is the root cause - and re-aim the west poles down and inward so the beam's backlight edge falls inside the property. Add backlight shields or specify a Type III instead of Type IV optic on the west side; reduce west-pole output after 10 p.m. if the yard does not need it; and consider lowering the west poles, since height increases the distance at which the source is visible over the fence. Then re-measure line illuminance and facade vertical illuminance and document both. Compliance at the ground line with no aiming control is not compliance in practice.
2. Explain what the B3 backlight rating permits and why it may not protect the neighbor. What would you specify instead?
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In the BUG system, backlight (B) rates how much light the luminaire sends behind its mounting position - the zone that matters to the property line and the neighbor. B3 is a moderately high backlight rating, and on a pole set 1–2 m inside the fence with the optic facing the yard, the backlight zone still points at the neighbor. It is entirely possible to be U0, G2 and B3 and still put objectionable light over the fence, because the rating classifies the distribution; it is not an absolute limit for a specific geometry. The ordinance's B3 ceiling is a maximum, not a target.
Specify a luminaire with a lower backlight rating - B1 or B2 - or keep the fixture and add a field-installed backlight/house-side shield, often the least-cost fix on an existing pole. Confirm the rating from the manufacturer's BUG report, not the catalogue description, and confirm the report matches the optic and aiming actually installed; a changed pole tilt changes the effective distribution. Check glare separately: G2 limits high-angle intensity in the forward direction, but the neighbor may be looking into the backlight zone, which is a different number. If the poles must stay at 12 m near the fence, the practical combination is a lower-output, narrower-distribution fixture with a shield, plus a late-night dimming schedule.
3. The yard needs 1 fc maintained average with 4:1 uniformity, but drivers report dark patches; the photocell is set at 10 fc and the lights cycle at dawn. Address both, and comment on the 1 a.m. shutdown.
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Dark patches are a uniformity problem, not an average problem. Measure a proper grid over the trailer yard and compute max/min and max/avg; a 4:1 target with 12 m poles at long spacing usually fails because the spacing-to-height ratio is too large - the same geometry problem as an interior layout. Fix the layout first: closer spacing, wider asymmetric yard optics aimed along the rows, or a lower mounting height with more poles. Raising output to chase the average makes the dark patches look darker by comparison.
Photocell cycling at dawn is a setpoint and deadband problem. A 10 fc setpoint is far too high for site lighting; at that level it is still daylight, so the lights come on, the sensor sees the light it produced, and the controller oscillates. Set the photocell to about 1–2 fc with a time delay of a minute or more, or use an astronomical time clock with photocell override, and make sure the sensor is not inside a floodlight's beam.
The 1 a.m. shutdown. If the owner's security standard requires 1 fc over the trailer yard all night, switching off the non-yard poles is fine, but reducing the yard below 1 fc is not - either hold the yard at design level or reduce it only with the security manager's written acceptance, and verify the dimmed state still meets the cameras' needs.