Lighting Design and Economics: Where the Payback Is

The lighting system is designed twice: once in footcandles and once in dollars, and the second design has been decisively simplified by one technology. LED is the baseline; the remaining craft is fixture selection, placement against the structure, environmental discipline, and a maintenance program that keeps the calculated levels real.

Fixture physics: cutoff vs. non-cutoff. A cutoff luminaire confines its output (under 2 percent above horizontal, under 10 percent above 80 degrees), killing high-angle glare at the price of closer spacing to overlap distributions at the 5-foot visual plane; a non-cutoff fixture throws light high and wide, which is exactly what low-ceiling parking needs: distribution over and between parked vehicles, vertical illuminance on faces, ambient light on suspended signage, and uplight on the ceiling that brightens the whole room's perception. The rule set: non-cutoff below 10-foot mounting heights (which is most covered parking, at roughly 10-foot floor-to-floor), cutoff above 12 feet, calculations deciding between. On roofs and surface lots, cutoff fixtures on tall interior poles control trespass: mounting height around half the horizontal throw (a 30-foot pole for a 60-foot reach), with poles 55 to 60 feet inside the parking limits, because the modeling consistently shows tall interior poles manage trespass better than short perimeter ones, whatever intuition says. Spacing interacts with structure: fixtures centered between beams so the coffers shield the source; at typical geometry the driver sees the fixture only in the 20-to-60-foot window (vehicle roof shields closer, beams shield farther), and a 15-foot lateral offset from the drive-aisle centerline puts the source past the 10-degree offset that research says defeats most glare. The cheapest luminance upgrade in the book is paint: white or off-white ceilings, beams, and walls raise surface luminance 2-to-2.5x, cut discomfort glare by half or better, add 10-15 percent illuminance under non-cutoff fixtures, and transform perceived brightness, with the caveats that cutoff fixtures (no uplight) and unlighted tee soffits benefit far less.

The calculation discipline. Lighting software models the photometrics (inverse-square law from manufacturer test data, point grids at roughly one-third mounting height, reflected components included), and the design deliverable is the maintained level: calculated at 70 percent of initial output, verified in the field annually directly under fixtures, with sources replaced when readings fall below the calculated maintained value. Group relamping beats spot replacement on cost; annual cleaning holds the depreciation curve.

Energy code. ASHRAE 90.1's parking provisions are now a design constraint of the first order: lighting power density capped at 0.14 watts per square foot on covered floors and 0.037 on uncovered floors and lots (facade lighting at 0.10, off or cut 75 percent after hours), full occupancy and daylight control, automatic 50-percent-minimum power reduction after short vacancy intervals in zones no larger than 3,600 square feet, entry transition lighting dropped to general levels from sunset to sunrise, and daylight-responsive dimming within 20 feet of qualifying perimeter openings. The practical meaning: the code has mandated what economics already recommended, controls everywhere and LED as the only source that comfortably lives under the caps.

The economics. The manuscript's worked numbers (mid-2022 electricity prices; currency register) tell one story from three angles. Initial cost: roughly $350 per legacy HID fixture against $550 for the LED equivalent, with fixtures at $0.40-0.60 per square foot representing only 20-30 percent of life-cycle cost, so first-cost comparisons are the wrong lens. Operating cost: at a 30-by-30-foot spacing and average utility rates, a 190-watt HID fixture burns about $256 per year in 24-hour operation (~$0.28 per square foot) with energy at roughly 64 percent of its life-cycle cost, while the equivalent LED fixture draws 75 watts and about $100 per year (~$0.12 per square foot), roughly 38 percent of the HID energy bill, at better efficacy (135 lumens per watt against the legacy sources' 74-97). Maintenance: about $400 per fixture over 25 years, under 10 percent of life-cycle cost, with LED's rated life around six times legacy sources and a depreciate-don't-burn-out failure mode that rewards measurement over waiting. The composite conclusion is not close: LED's premium at purchase repays through operating cost alone in a period short enough that the analysis has stopped being interesting, and the remaining economic decisions are controls granularity and the paint bucket.

Obtrusive light. The environmental frame caps the design: light trespass limited at the off-site observer's eye by environmental zone (from 1 lux pre-curfew in intrinsically dark zones to 15 in urban centers, tightening after curfew), glare evaluated at the same observer, and skyglow governed by the IES/Dark-Sky Model Lighting Ordinance's virtual-enclosure method (a zero-reflectance calculation box 33 feet above the tallest luminaire, with lumen limits on its surfaces). Municipal ordinances increasingly enforce all three; the design that anticipates them (cutoff on the roof, interior poles, shields where required) avoids the retrofit that never looks as good as the original.