Sky glow, light trespass and luminaire intensity limits

Obtrusive light limits in planning are not one number. The Institution of Lighting Professionals guidance on reducing obtrusive light sets separate controls for light thrown upwards into the sky, light falling on the windows of nearby properties, and the intensity of the source as seen by a person looking at it, with a fourth control on the luminance of lit building surfaces. Each has its own unit, its own measurement position and its own pre-curfew and post-curfew value. An assessment that reports a single lux figure and declares compliance has not addressed three of the four.

Reviewers see that failure constantly. The design may be perfectly good and the report still cannot be relied on, because it does not demonstrate the things the conditions will be written around.

The three measures and what each controls

All four tighten as the environmental zone gets darker, which is why zone selection has to be settled before any calculation is run. In the darkest zones upward light is expected to be nil and post-curfew values fall away almost entirely.

Upward light and how it is calculated

Upward light ratio is the proportion of a luminaire's total light output emitted at or above the horizontal, in its installed position. That last phrase carries the weight.

A luminaire with a flat glass and full cut-off has an upward light ratio of zero when mounted horizontally. Tilt the same unit by ten degrees to reach further across a car park and the figure is no longer zero, because part of the beam now crosses the horizontal. The photometric data file describes the luminaire, and the installed geometry describes what it actually does.

Two consequences for a reviewer. Check whether the reported ratio was calculated for the installed tilt or taken from a manufacturer's datasheet. And check whether the model includes upward reflected light from paving and building surfaces, which the ratio itself does not cover and which matters on a light coloured hard surface.

The practical design move is a flat glazed, fully shielded luminaire mounted level, with the throw achieved by optics rather than by tilt. Where a tilt is unavoidable, say so and quantify what it does.

Vertical illuminance at a window

This is the light trespass measure, and it is assessed in the vertical plane of the window of the property affected rather than at the site boundary.

Three details decide whether the figure is meaningful:

• The receptor position, which should be at the window of a habitable room, at the relevant floor level, not at ground level below it
• Whether every relevant window has been assessed, including upper floors that see over a boundary fence
• Whether the calculation includes all light sources, including lit signage, security lighting and headlights where these form part of the proposal

Assessing at the boundary rather than the window is the most common error, and it usually understates the result on a site where the nearest house sits close to the fence. Upper floor windows are the second, because a screen or wall that protects a ground floor room does nothing for the bedroom above it.

Where an existing condition matters, a baseline measurement taken with the site as it is gives the reviewer something to weigh the increase against. Where the affected property is not yet built, use the consented drawings.

Intensity towards an observer

The intensity limit controls glare rather than the amount of light landing on anything. It is expressed in candelas towards the observer, and both the observer position and the angle have to be stated for the figure to mean anything.

Choose observer positions honestly. A single position at a first floor window of the nearest dwelling is rarely enough on a site with several boundaries. Footpaths, cycle routes and roads matter too, because glare towards a cyclist at a junction is a safety issue as well as an amenity one.

High mounting with good shielding usually beats low mounting with a wide beam on this measure. That trade sometimes conflicts with the upward light figure and with the visual effect of tall columns in a rural setting, and the assessment should show that the conflict was considered rather than resolved silently.

Pre-curfew and post-curfew values

The guidance sets two values for each measure, and the tighter set applies after a curfew time. The curfew time is not set by the guidance. It is set by the local planning authority, commonly at 11pm where residential amenity is the concern.

Post-curfew compliance usually cannot be achieved by the same installation running at full output. It needs a control strategy, and the strategy has to be described in the assessment rather than assumed:

1. Switch off non-essential lighting entirely after the curfew time, including signage and feature lighting.
2. Dim the remainder to a stated percentage output, with the calculation repeated at the dimmed level rather than scaled by hand.
3. Use presence detection for areas that need light only when someone is there, with a stated hold time.
4. Keep any lighting required for safety or security at the minimum that serves that purpose, and identify it specifically.

Where a site operates at night, say so plainly and make the case for the hours needed. An honest late finish with a tight specification is easier to condition than a claim of curfew compliance that the operation will breach in its first week.

Modelling outputs an assessment should contain

A reviewer should be able to reproduce the result. That means the report needs the luminaire schedule with manufacturer, model, photometric file reference, output, mounting height, tilt and any accessories, and a layout drawing keyed to it.

Beyond that: calculation grids with spacing stated, isolux contours across the site and over the boundary, receptor points shown on a plan with their heights, the environmental zone and the limits applied, results tabulated against those limits for both curfew conditions, and the assumptions on reflectance and maintenance factor.

Ecology often sits alongside this. Where bats use a hedgerow or watercourse, the lighting design has to keep those features dark whatever the obtrusive light figures say, and the constraints come from the bat survey and the wider Preliminary Ecological Appraisal. In a sensitive landscape the night-time visual effect belongs in the Landscape and Visual Impact Assessment as well.

Conditions that hold the design in place

Lighting is easy to change after approval, so conditions need to bite on the equipment rather than on the outcome alone.

The wording that works names the approved luminaire schedule and layout drawing, prohibits any increase in output or change of luminaire type without consent, prohibits upward tilt, secures the curfew regime and its control method, and requires a post-installation verification report of measured vertical illuminance at the agreed receptor positions. A verification requirement with no agreed receptor positions produces an argument instead of a measurement.

One honest uncertainty is worth acknowledging. Measured illuminance on a real site includes light from streetlights, neighbouring premises and passing traffic, so a verification measurement rarely matches the model exactly. Agree the method and the tolerance when the condition is drafted rather than on the night of the survey.

A Lighting Assessment drafted with those conditions in mind saves a discharge application later. Councils usually expect a suitably qualified author for lighting assessments, and professional review is strongly advised.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.