Why mounting height and spacing are decided together
Area lighting is one of the few product decisions in a project that cannot be finished in isolation: the luminaire, the pole height and the distance between poles form a single system. A fixture with excellent optics will still produce dark bands if the poles are pushed too far apart, and a dense layout wastes capital if the poles are too short for the mounting height the photometric file was tested at.
This workflow describes how a lighting team takes an area lighting product from "approved for the project" to "mounting height and spacing signed off", with the checkpoints, handoffs and documents at each stage. It complements the upstream release process that makes the photometric file available; here the assumption is that the file exists and is trusted.
Stage 0: Inputs to confirm before any layout work
- Approved luminaire variant — the exact model, wattage, distribution type and beam angle, with the matching IES/LDT photometric file in the same revision. A layout built on a superseded file is worthless.
- Site geometry — a dimensioned plan or survey showing kerb lines, parking bays, column bases, canopies, existing poles and any structure that can block light.
- Task requirements — target illuminance and uniformity values for each zone (driving lane, bay area, pedestrian route, entrance), plus any glare or light-trespass limits set by the client, the local authority or the project specification.
- Mounting constraints — maximum pole height permitted by planning or wind-load considerations, available pole heights from the supplier, arm lengths and tilt limits from the mounting instructions.
- Environment — wind zone, coastal or corrosive atmosphere, ambient temperature range, and whether the fixture sits under a canopy where heat and dust accumulate.
If any of these is missing, stop and request it. Layout decisions made on assumed pole heights are the most common source of later rework.
Stage 1: Choose a starting mounting height
Mounting height drives almost everything else. As a rule of thumb for pole-mounted area lighting, a higher mounting point widens the useful pool of light and reduces glare at eye level, but it also reduces the illuminance on the task surface for the same output and makes the fixture harder to aim and maintain.
- Read the photometric file's test conditions. Many files are tested at a single mounting height; if the design height differs significantly, the calculated values must be re-scaled or the file re-run rather than assumed.
- Set the pole height so the luminaire aperture sits at or near the tested height, then check the resulting glare class and light-trespass values for the neighbouring property line.
- Confirm the pole is commercially available. A 20 ft pole is standard stock in many markets; intermediate heights often require a special order, which changes the delivery date rather than the design.
- Record the chosen height as a controlling assumption in the layout file, because every later spacing calculation depends on it.
Stage 2: Derive spacing from the distribution and the uniformity target
Spacing is not chosen; it is calculated and then tested. The practical sequence:
- Compute the mounting-height-to-spacing ratio. Divide the design spacing by the mounting height. A layout with 8 m poles at 24 m spacing sits at a ratio of 3:1; the same poles at 16 m spacing sit at 2:1. The higher the ratio for a given optic, the stronger the pooling and the lower the minimum illuminance between poles.
- Check the distribution type. Type III and Type IV asymmetric distributions push light forward along a lane and tolerate wider spacing down the road than across it, which is why most area layouts use rectangular rather than square grids.
- Run the point-by-point calculation. Grid the area at a spacing appropriate to the task (fine grids for parking and pedestrian zones, coarser for storage yards), then compare the minimum-to-average uniformity and the maximum-to-minimum ratio against the project targets.
- Adjust in the cheaper direction first. Widening the aim angle, rotating the pole head or moving a pole a metre along the kerb usually costs nothing; increasing pole height or adding a pole does.
Expect two or three iterations. A first pass that meets average illuminance but fails uniformity is normal, not a sign that the wrong product was chosen.
Stage 3: Resolve the physical conflicts
The calculated ideal layout almost never survives contact with the site plan. Work through the conflicts in this order:
- Column bases versus underground services. A pole cannot stand on a water main or within a protected service corridor; moving it changes the spacing on both sides of that pole.
- Sight lines and security cameras. Poles and luminaire heads can obstruct a camera's field of view; co-ordinate the head orientation with the security designer before the base is cast.
- Tree canopies. A fixture inside a canopy produces glare into the crown and a dark area below. Either raise the head above the mature canopy height or accept a wider spacing and add a lower-level fixture.
- Maintenance access. A pole that requires a lane closure for every lamp change will be under-maintained in practice; check that a service vehicle can reach every position.
- Tilt and vibration. Tilted heads can breach the photometric assumptions used in the calculation and increase wind loading on the bracket. Any tilt beyond the manufacturer's stated range invalidates the layout and must be re-run.
Stage 4: Confirm the dependencies that can change the answer
Height and spacing remain valid only while the surrounding assumptions hold. Before sign-off, verify:
- The photometric file revision in the layout matches the revision in the product release record.
- Optics, drivers or reflectors are not going to change mid-project; any change to the light distribution requires a fresh calculation.
- Dimming strategy is defined. A layout designed for 100% output with a maintenance factor applied behaves differently if the site runs at a fixed lower level all night, because the effective uniformity at the reduced level can differ from the calculated one.
- Future site changes are known — a planned extension or re-striping of bays can invalidate uniformity margins that were accepted with little headroom.
Stage 5: The sign-off package
The output of this workflow is a small, reviewable document set rather than a single drawing:
- Layout drawing with pole positions, mounting heights, arm lengths, head orientation and tilt annotated.
- Calculation report showing the grid, the assumptions (mounting height, maintenance factor, reflectance of the surfaces) and the achieved values against each target.
- A one-page decision log recording why the chosen height and spacing were selected, which alternatives were rejected, and which inputs would reopen the decision.
- A handoff note to installation stating the tested mounting height, the permitted tilt and the aiming reference — the three things most often lost between design and site.
Who signs? Typically the lighting designer owns the calculation, the electrical or civil engineer confirms pole and base loading, and the project owner accepts the glare and trespass position for neighbouring properties. Each of those signs off on a different page of the package, so keep the pages separable.
Common failure modes
- Designing to average illuminance only. Average targets are easy to hit with fewer, brighter poles; uniformity is what users actually notice.
- Copying a spacing ratio from another site. Ratios transfer only with the same optic, the same mounting height and a similar task; otherwise treat them as a sanity check, not a design.
- Forgetting the maintenance factor. A layout that meets the target on day one but ignores lumen depreciation and dirt will fall below it before the first clean.
- Changing the pole height on site. Substituting a shorter pole to meet a delivery date silently changes the ratio and voids the photometric basis of the layout.
Treat mounting height and spacing as a designed pair with a written rationale, and the layout becomes defensible long after the installer has left the site.