Illuminance: Footcandles and Lux in Parking Lighting

Illuminance is the amount of light falling on a surface, measured in footcandles (lumens per square foot) or lux (lumens per square meter).

Illuminance is the amount of light actually reaching a surface, measured in footcandles or lux at the floor, not the brightness of the fixture. Even coverage matters as much as the average: it is the dark gaps between pools, not the headline number, that make a deck feel unsafe. Illustrative.

One footcandle equals about 10.76 lux, so designers often use a rough 1:10 conversion. Illuminance is what a light meter reads and what lighting guidance for parking facilities specifies. It is different from luminance, which is how bright a surface looks to the eye.

Horizontal and vertical

Horizontal illuminance is light on the floor or pavement, which lets people see the driving and walking surface.
Vertical illuminance is light on an imaginary vertical plane, about face height, which lets people recognize each other and lets cameras capture faces and plates.
Good parking lighting specifies both, plus limits on how uneven the light may be. See ambient lighting.

Using it in practice

Lighting guidance from the Illuminating Engineering Society sets recommended minimums for garages and lots, including higher daytime levels at garage entrances.
Light-colored ceilings and walls raise illuminance with the same fixtures.
Controls can lower illuminance in unoccupied zones as long as minimums are met. See adaptive lighting.

The Parkonomics view

A light meter reading is only half the story. People judge a garage by its faces and corners, which is why vertical illuminance and uniformity matter as much as the floor number.