Spacing from the distribution ratio
S = ratio x MH
The spacing-to-mounting-height ratio for the photometric distribution, multiplied by the pole height above grade.
Guide
Outdoor and Parking Lot Lighting: Pole Spacing, Uniformity, and Light Trespass
How to space poles so a lot reads as one lit surface, and how to keep the light off the neighbours and the sky.
Set the target from the activity and the risk
General parking sits around 10 to 30 lux, security lighting and high activity areas 50 lux, walkways and plazas 10 to 30, and residential streets around 10. Loading aprons and industrial yards run 30 to 50. These are maintained values, so the installation has to be brighter at day one by the maintenance factor.
Spacing comes from height and optics
Area lights and street lights throw their light across the ground, so the spacing-to-height ratio is much larger than indoors: three to four times the mounting height for forward throw optics, four to five times for square distributions. A 25 foot pole with a Type III optic therefore covers a spacing near 75 to 100 feet, subject to the illuminance target.
Uniformity is the complaint generator
An average illuminance can be met by a bright pool under each pole and darkness between them. Ratios of maximum to minimum illuminance of 10 to 1 read as uneven and unsafe, so the spacing chosen in the previous step has to be checked against the uniformity criterion as well as the average.
Trespass, glare, and skyglow
Full cut-off optics, back shields at property lines, and a mounting height that keeps the beam on your own land are the three practical controls. Height above about nine metres starts to spread light far beyond the intended area, and it also raises maintenance cost, which is why tall poles are often an economy that does not hold.
Control the schedule before adding more poles
A dusk-to-dawn photocell on the full output is the most expensive default in outdoor lighting. Two-level dimming with a late-night reduction of thirty to fifty percent, or partial shutoff on low-activity circuits, is usually available at little extra cost and often removes the need for the last pole or two.
Formulas used
Spacing from the illuminance target
S = sqrt(Phi x UF x MF / E)
A uniform-illuminance approximation. If this comes out lower than the ratio-based spacing, the illuminance target governs and the poles must move closer together.
Fixtures for a density
Fixtures per 1,000 m2 = 1,000 / S^2
A quick way to price a lot once the governing spacing is known, before a point-by-point model exists.
Worked examples
A 25 foot installation with forward throw optics
Inputs: 20,000 delivered lumens, Type III optic, utilization factor 0.6, exterior maintenance factor 0.7, target 20 lux.
The ratio rule allows 4 x 25 = 100 feet between poles. The illuminance rule gives a spacing of sqrt(20,000 x 0.6 x 0.7 / 20) = 20.5 metres, or 67 feet. The illuminance target governs, so the poles go at about 67 feet, not 100.
The same lot lit to security level
Inputs: The same fixtures, but a 50 lux target for a high activity area.
The illuminance spacing drops to sqrt(8,400 / 50) = 13 metres, about 42 feet. Roughly two and a half times as many poles as the 20 lux case, which is why the target belongs to the client decision, not to the contractor.
Questions readers ask
How many lux does a parking lot actually need?
General parking and circulation work at 10 to 30 lux, and areas used for loading, queuing, or night-time pedestrian activity at about 50. The higher figure is a safety and liability decision, and it doubles or triples the pole count, so agree it in writing before the layout is priced.
Do I need shielding against light going off my property?
Most jurisdictions regulate light trespass at the property line, and back shields or a different optic are the usual fix. Even where it is not regulated, a complaint from a neighbouring property usually costs more than the shield.
Is a taller pole cheaper?
Taller poles cover more ground, so the count falls, but the beam spreads further, uniformity worsens, and maintenance gets harder and more expensive. Above about nine metres the economics usually stop improving for a parking lot.
Work out the pole spacing for your site
Sources
- Standards and recommended practice Illuminating Engineering Society · 2026-10-15
- Qualified Products List for commercial lighting DesignLights Consortium · 2026-10-15
- Lighting and energy saving resources U.S. Department of Energy, Energy Saver · 2026-10-15