Parking Garage Retrofit: Uniformity, CCTV and Dimming
A three-level, 1,050 m²-per-level employee parking garage with 2.4 m clear height is being converted from 100 W high-pressure sodium to LED. The owner's standard calls for 50 lx maintained horizontal on drive aisles, at least 25 lx vertical at 1.5 m for the security cameras and facial identification of patrons, and max/min uniformity no worse than 4:1. The contractor proposes 40 ceiling-mounted luminaires per level: 80 W, 12,000 lm, 3000 K, 70 CRI, symmetric 90° optics, at 2.3 m above the floor on 6 m centers in a 3 × 40 layout. Commissioning reads 62 lx directly under the fixtures and 9 lx midway between them. The security staff report noisy, grainy camera images in the aisle and washed-out faces at the elevator lobby, and a driver has complained about glare through the windshield at the ramp. Total lighting power is within the garage allowance.
1. The measured 7:1 uniformity fails the 4:1 requirement. Diagnose the cause and specify the change, keeping total power inside the garage allowance.
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The problem is geometry, not wattage. With 2.3 m mounting height and 6 m spacing the spacing-to-mounting-height ratio is 2.6. A symmetric 90° downlight with a typical spacing criterion near 1.0–1.4 cannot produce 4:1 at S/MH 2.6 - the cones barely overlap, so you get a bright pool under each fixture and a dark band halfway between.
To hold 4:1 you need spacing no more than about 3 m at this mounting height, i.e. roughly four times the fixture count at about one quarter the output each - for example 160 luminaires per level at 20 W, 3,000 lm, which keeps connected load essentially where it was. The alternative is to change the distribution rather than the count: a wide asymmetric aisle optic aimed down the drive lane, or mounting at 2.1 m on columns along the aisle with a grazing distribution, which effectively lowers the mounting height and increases overlap.
The cheapest first step is to ask the manufacturer for the luminaire's spacing criterion at the required uniformity and lay out to that number rather than to a round figure like 6 m. Also confirm the mounting detail - a fixture hung below the beams on the aisle centerline performs differently from one tucked between beams. And check the measurement grid: a two-point max/min along an aisle is not a uniformity measurement; use a proper grid over the task area.
2. Security reports grainy camera images and washed-out faces. What is wrong and what do you specify?
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Two separate issues.
The metric. The design target is 25 lx vertical at 1.5 m, but the contractor has been checking 50 lx horizontal. Vertical illuminance at face height comes from light reaching the subject from the camera side, not from a downlight directly overhead, which lights the top of the head and leaves eye sockets and nose shadowed. Measure the vertical plane facing the camera at the aisle and the elevator lobby; if it is below 25 lx, add or re-aim fixtures toward vertical surfaces, use wall-washer distributions at the lobby, and define the plane using the camera's mounting height.
Spectrum and contrast. 3000 K at Ra 70 is a poor match for most security cameras: auto white balance drifts, faces go yellow or magenta, and low light plus low CRI forces the camera to raise gain, which produces the grain. Specify 4000 K with Ra 80+ and require the same CCT on all three levels so cameras do not need a different balance per floor. Keep luminaires out of the top of the camera's field of view or use shielded optics to prevent blooming. Ask for the camera's minimum illumination specification and verify the designed vertical illuminance against it - the camera, not the light meter, is the final judge.
3. The owner proposes occupancy-based dimming to 30% to save energy. What has to be true before that is acceptable?
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Three conditions.
Egress. Drive aisles and pedestrian routes are part of the means of egress and must stay illuminated whenever the garage is occupied; a sensor dropping the aisle to 30% can take the floor below the required minimum. Keep the marked egress route, stairs and lobbies on a separate unswitched or relay-controlled circuit with the emergency fixtures, and dim only stalls and non-path areas.
Detection and coverage. Dimming only saves energy if the sensor actually sees occupants. In a 2.4 m garage, structural beams and columns cut a ceiling PIR sensor's coverage badly; microwave or a networked per-fixture system covers stalls better. Set hold and ramp times so lights do not pop on and off in front of a moving driver.
Security cameras. Dropping to 30% may push vertical illuminance below what the cameras need; either hold the camera-relevant zones at full output overnight or verify image quality with the security consultant at the dimmed level before commissioning. Measure the egress and camera illuminance in the dimmed state, not the design state, and record the settings in the commissioning report.