The obtrusive light section of a Lighting Assessment

The obtrusive light section is where a Lighting Assessment stops describing the scheme and starts proving it. Obtrusive light limits in a lighting assessment come from the Institution of Lighting Professionals guidance on the reduction of obtrusive light, which sets numerical limits by environmental zone for light trespass into windows, luminaire intensity towards an observer, upward light and the luminance of lit building faces. Choose the wrong zone and every number in the section is measured against the wrong target.

Identifying the environmental zone

The guidance divides the country into five zones, from intrinsically dark landscapes through to city centres. Zone selection is a judgement about the surroundings of the receptors, not about the applicant's site.

• Intrinsically dark areas, such as dark sky reserves and the darkest parts of National Parks, where the design target is effectively zero.
• Dark areas, covering relatively uninhabited rural land, National Landscapes and National Parks generally.
• Low district brightness, meaning villages and rural settlements with sparse street lighting.
• Medium district brightness, covering suburban areas and small town centres.
• High district brightness, meaning town and city centres with high levels of existing lighting.

Justify the choice in writing with evidence rather than asserting it. A photograph of the existing night-time scene, a note of the street lighting class on the adjoining highway and the designations affecting the area do the job. Where a site sits on the edge of a settlement facing open countryside, the honest answer is often that two zones apply, one to the receptors on the settlement side and a darker one to those on the country side, and the assessment should say so.

Local authorities sometimes specify the zone in a supplementary planning document or a dark skies policy. Check before you model, because arguing about the zone after the results are calculated wastes a round of consultation.

Limits for light into windows and luminaire intensity

Two of the limits protect individual neighbours. Vertical illuminance is the light falling on the plane of a window, measured in lux, and luminaire intensity is the candela value of a fitting as seen from the receptor position, which is what makes a light source feel glaring even when the illuminance is modest.

Report both for every sensitive window, not a site average. The luminaire intensity figure is the one applicants forget, and it is the one that catches floodlights on a warehouse gable or a car park column aimed across a boundary. A scheme can comfortably meet the illuminance limit and fail badly on intensity.

Building luminance has its own limits, expressed in candelas per square metre for the average luminance of a lit facade or sign. Those apply wherever architectural lighting, fascia illumination or a lit canopy is proposed.

Curfew hours and what changes at curfew

Curfew is the point at which the tighter set of limits takes effect. It is commonly set at 23:00, and the local authority defines it, so confirm the hour with the environmental health officer rather than assuming.

At curfew the acceptable vertical illuminance and the acceptable luminaire intensity both drop sharply, and in the darker zones they drop to zero. That is a design instruction rather than a reporting detail. A scheme that relies on lighting continuing at full output through the night in a rural zone cannot comply, and no amount of aiming will fix it.

The usual responses are switching off, part-night dimming to a stated percentage, or switching to a reduced lighting scenario that leaves only what is needed for security and safe movement. Say which applies, at what time, and how the control system delivers it.

Some uses cannot switch off. A 24 hour distribution yard, a petrol filling station, a hospital entrance. Where that is the case, say so plainly and design the fittings so the post-curfew limits are met at full output, because the alternative is a condition the operator cannot comply with.

Sky glow and the upward light ratio

Upward light ratio is the proportion of a luminaire's total light output emitted above the horizontal when installed and aimed as proposed. The limits tighten from around fifteen per cent in the brightest zone down to zero in the two darkest, where no direct upward light is acceptable at all.

Modern flat glass, full cut-off luminaires deliver zero direct upward light as standard, so a compliant scheme is usually a matter of specification rather than ingenuity. Uplighting of trees, buildings and flagpoles is the exception, and in a dark zone it is generally undeliverable.

Reflected light off lit surfaces also contributes to sky glow and is not captured by the upward light ratio. Where large areas of pale hardstanding or a white clad elevation are being lit, mention it, because a reviewer in a dark skies area will.

Modelling and the calculation grid

The calculations should be produced in recognised lighting design software using the manufacturer's photometric files for the actual specified luminaires, not generic equivalents.

Set out the modelling assumptions explicitly: maintenance factor, mounting heights, tilt angles, aiming points, the reflectance values used for ground and building surfaces, and whether the model includes existing lighting on adjoining land. Present the calculation grid with its spacing, and show the vertical illuminance planes at the actual window positions of the nearest dwellings, at the correct storey heights, rather than at a notional boundary plane.

Isolux contour plots across the site and elevational plots at the receptor facades are the outputs an officer can check quickly. Include a table of results against the limits for the relevant zone, receptor by receptor, with pre-curfew and post-curfew columns. The discipline is the same one applied in odour dispersion modelling, where the credibility of the conclusion rests entirely on whether the model's assumptions were stated and defensible.

Mitigation through optics, aiming and controls

Work down the hierarchy rather than reaching for shields at the end.

1. Question the need. Lighting that serves no defined task is the easiest thing to delete, and a scheme with fewer, better placed fittings usually performs better on every measure.
2. Specify the right optic. Asymmetric distributions that throw light away from a boundary do more than any accessory bolted on afterwards.
3. Get the mounting height and aiming right. Higher columns with tighter optics can produce less spill than low, wide-beam fittings, which is counter-intuitive until it is modelled.
4. Reduce output. Lower lumen packages, lower colour temperature where ecology or amenity matters, and dimming profiles.
5. Add controls. Time switches, photocells and presence detection with a short over-run period for areas used intermittently.
6. Shield as a last resort. Louvres and shields cut spill and also cut useful light, and they get removed during maintenance.

Ecological impacts sit alongside all of this and are assessed separately. Light spill onto hedgerows, watercourses, woodland edges and known bat commuting routes is judged against ecological guidance rather than against the amenity limits, and a scheme compliant with the obtrusive light limits can still be unacceptable for bats. Keep the two analyses distinct in the report so neither gets lost.

Conditions that keep the scheme within limits

Offer conditions that are measurable. A condition requiring the lighting to be installed in accordance with the approved drawings and photometric schedule, a curfew time with a defined post-curfew scenario, a maximum upward light ratio, and a requirement that no additional external lighting is installed without approval.

Post-installation verification is worth volunteering on larger schemes. A commissioning survey confirming aiming angles and measured illuminance at the nearest windows, submitted within a stated period of first use, gives the environmental health officer something to hold and gives the operator certainty.

A Lighting Assessment is one of the report types where councils expect a suitably qualified author, and professional review is strongly advised before submission. PlanScope drafts the structure and the policy narrative from your site details and flags every input a lighting designer has to supply. Browse the report types index to see how it fits with the rest of the evidence base, or enter your postcode to see what your site is likely to need.

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.