Lux contour plots and what they should show

A lux contour plot in a lighting assessment exists to answer one question an environmental health officer will ask: how much light from your scheme lands where it should not. The plot is an isolux map, usually overlaid on the site plan, showing calculated illuminance at a defined plane. Done properly it has two versions, one at ground level for the lighting you need and one at the windows of neighbouring properties for the light you are spilling. Done badly it has one plot, no calculation grid, no maintenance factor and no stated height, and it tells the officer nothing.

What the plots demonstrate

The lighting assessment is making two separate cases at once. The first is that the lit area works: a car park, a loading yard, a footpath or a forecourt reaches the illuminance and uniformity the task needs, referenced to BS EN 12464-2 for outdoor work places or BS 5489-1 for roads and public areas. The second is that light does not escape to cause a nuisance, which is judged against the obtrusive light limits in the Institution of Lighting Professionals Guidance Note 1.

Those cases pull in opposite directions, which is the whole point. A design that hits 50 lux across a yard with wide-beam floodlights on tall columns will fail the trespass test at a boundary 8m away, and the contour plot is where that conflict becomes visible.

A Lighting Assessment that shows only the lit area, with the plot cropped at the red line, is the most common reason an officer asks for more information. Extend the calculation beyond the boundary. If the contours are clean, the plot is your best argument.

Horizontal against vertical illuminance

Horizontal illuminance is lux measured on a flat plane, normally the ground, and it describes whether a space is usable. Vertical illuminance, written Ev, is lux measured on a vertical plane, and it is the figure that matters for light intruding into a habitable room.

The two are not interchangeable and they are not proportional. A downward-aimed luminaire can produce a high ground-level figure directly beneath it and almost nothing at a window 15m away. A poorly aimed floodlight with upward tilt can give a mediocre ground figure and a serious vertical reading at a first-floor bedroom.

So a lighting assessment needs both, and they must be labelled. State the calculation plane for every plot: ground level at 0m for horizontal, and the vertical plane at the window with its height above ground given.

The grid and the maintenance factor

Two inputs determine whether the numbers mean anything. The calculation grid spacing has to be fine enough to pick up peaks rather than averaging them away, and a grid that is coarse near the boundary will hide the worst reading. The maintenance factor accounts for lumen depreciation and dirt on the optic over the cleaning cycle, and a design that complies only at a maintenance factor of 1.0 is a design that complies on day one and nowhere else.

Both should appear in the calculation output header. If they do not, the plot has not been checked.

Receptor window positions and heights

This is where most assessments are weakest. A plot that shows contours across the neighbouring garden but no readings at the windows has measured the wrong thing, because the obtrusive light criteria are written for the window of a relevant property.

Identify the receptors on a drawing and number them. For each one, give the room use where it is known from the planning history or a site visit, the orientation of the window, the sill and head heights, and the horizontal distance from the nearest luminaire. Ground floor and first floor behave differently: a 2m close-boarded fence screens a ground-floor window and does nothing for a bedroom above it.

Where a neighbouring property is itself under construction or has consent, assess the approved window positions rather than the existing wall. Officers do check.

Limits by environmental zone and curfew

Guidance Note 1 sorts locations into environmental zones running from E0, intrinsically dark areas such as recognised dark sky sites, through E1 for relatively uninhabited rural areas, E2 and E3 for low and medium district brightness, to E4 for town and city centres with high night-time activity. The obtrusive light limits tighten sharply as you move down that scale, and the figures are set out in the current edition of the guidance, which is worth checking because the numbers have been revised.

Each zone carries limits for vertical illuminance at windows, luminaire intensity in candelas, upward light and, for some uses, the luminance of an illuminated building face. Most have a stricter post-curfew value. The curfew hour is not fixed nationally and the council sets it by condition, commonly 11pm, so the assessment should state the assumed curfew and model both periods.

Picking the zone is a judgement, and it is the judgement most often disputed. A site on the edge of a market town bordering open countryside is not E3 at the boundary that faces the fields. Where the classification is arguable, assess against the stricter zone for the sensitive boundary and say so, because the alternative is a second round of comments.

Spill beyond the site boundary

Boundary spill is assessed as a line, not a point. Run a calculation along the full site boundary at a sensible spacing and report the maximum, not the average, because a single overrun at the gate is what generates the complaint.

Three design moves usually fix a failing plot without losing the lit area:

1. Reduce column height and increase the number of luminaires, which cuts the required intensity and the throw.
2. Specify asymmetric or flat-glass optics aimed at or below the horizontal, with zero upward light from the luminaire itself.
3. Add rear shields or louvres to the units nearest the boundary, and show the shielded photometry in the calculation rather than the unshielded file.

Lighting also matters for ecology. Bats commuting along a hedgerow or using a roost access point are sensitive to even low light levels, and a scheme that passes the amenity test can still fail on ecology. If the site has a hedge, a mature tree line or an old building, the lighting plot should be read alongside the ecologist's findings, and a bat survey may set a dark corridor that the lighting design has to respect. Bat Conservation Trust guidance on bats and artificial lighting is the usual reference.

Luminaire schedules that support the plots

A contour plot is only as good as the schedule behind it. The schedule should list, for every unit, the manufacturer reference, the lumen output and circuit wattage, the correlated colour temperature, the optic or distribution code, the mounting height, the tilt angle and the aiming point, and the control regime.

Colour temperature is a planning issue in its own right. Warmer sources, typically 3000K or below, are increasingly expected in rural and ecologically sensitive locations, and the National Planning Policy Framework (August 2026 edition) chapter on conserving and enhancing the natural environment asks authorities to limit the impact of light pollution on intrinsically dark landscapes and nature conservation. Planning Practice Guidance on light pollution sets out what a council should consider.

Controls change the answer. Dimming to a lower output after curfew, presence detection on a service yard and a timed shutdown of display lighting are all conditionable, and each lets a scheme pass the post-curfew limits without redesigning the installation. Model the dimmed scenario as a separate plot rather than asserting it in text.

For a commercial unit, the lighting evidence usually arrives alongside the plant noise case, since both are environmental health matters and both get conditioned together. A Noise Impact Assessment and the lighting assessment should use the same receptor numbering, which sounds obvious and is routinely not done.

PlanScope drafts a lighting assessment structured to the sections above and flags the inputs it cannot know with a "TO BE CONFIRMED" marker, so the photometric calculation itself still comes from a lighting designer. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Browse the full set of report types on the planning documents index to see where a lighting assessment sits in the bundle.