Lighting Assessment outline to the ILP guidance

A lighting assessment outline that a planning officer can check has seven parts, and the first one decides everything else: the environmental zone. The Institution of Lighting Professionals guidance on the reduction of obtrusive light sets limits for sky glow, light intrusion into windows, luminaire intensity, building luminance and threshold increment, and each limit changes with the zone you are in. Fix the zone wrongly and every figure in the report is measured against the wrong benchmark.

What follows is the structure I would expect from a competent author, written for the reviewer who has to sign it off.

Environmental zone and its consequences

Zones run from E0, dark landscapes such as national parks and areas protected for their night skies, through E1 intrinsically dark rural areas, E2 sparsely inhabited rural areas, E3 suburban and small town centres, to E4 town and city centres with high levels of night-time activity. The report should name the zone, explain the reasoning from the site's actual surroundings, and say whether the local authority or a supplementary planning document assigns one. Where a site sits on a boundary between zones, the lower-brightness zone applies to the receptors in it.

Quote the current edition of the guidance for the numerical limits rather than reproducing a table from memory, because the figures have been revised and readers hold older copies. State the parameters you are testing against by name: upward light ratio for sky glow, vertical illuminance at windows for light intrusion, luminaire intensity in candelas towards the receptor, average luminance of a lit building facade, and threshold increment where road users are affected.

Then say plainly which limits are pre-curfew and which are post-curfew. Half the weak assessments I see test only the pre-curfew case.

Baseline light levels and the receptors that matter

A baseline survey gives the reviewer something to hold the prediction against. Measure horizontal and vertical illuminance at the notional receptor positions on a dry night with the existing lighting operating, record the instrument, its calibration date and the measurement height, and note what else was contributing: street lighting, a neighbouring car park, security floodlights, vehicle headlights.

The receptor schedule should be explicit. For each one give the address or reference, the use, the window orientation and height, the distance and bearing to the nearest proposed luminaire, and the zone. Bedroom windows, not front doors, are the sensitive point for residential intrusion. Where ecology is engaged, add roost access points, commuting routes, hedgerows, watercourses and dark corridors as receptors in their own right, cross-referenced to the Preliminary Ecological Appraisal rather than duplicating it.

Bats are the usual reason a lighting scheme is redesigned. The joint guidance from the Institution of Lighting Professionals and the Bat Conservation Trust sets out the approach, and the practical answers are warm colour temperature, tight cut-off optics, low mounting heights, part-night operation and leaving hedge lines unlit.

The luminaire schedule and the data behind it

Every luminaire in the scheme needs a row: reference, manufacturer type, mounting height, tilt, orientation, lumen output, correlated colour temperature, optical distribution code and the photometric file used in the model. Without the photometric file the modelling cannot be audited, so state the file name and version.

But tilt is the item that fails most often. An amenity floodlight specified flat and installed at 15 degrees produces an entirely different spill pattern, which is why the schedule should record the design tilt and the report should recommend a condition fixing it.

Colour temperature deserves a line of its own. Warmer sources, commonly 2700K or below in sensitive settings, reduce the blue content that carries furthest in the atmosphere and disturbs nocturnal species most.

Modelling spill light and glare

Set out the software, the version, the calculation grid spacing, the maintenance factor and the reflectance values assumed for ground and building surfaces. State whether obstructions such as boundary walls, fences and existing buildings are modelled, because a scheme that only passes with a 2 m fence in the model must have that fence secured by condition.

Glare is assessed separately from spill. Report luminaire intensity in candelas in the direction of each receptor, and for road users report threshold increment against the relevant road lighting standard. A car park that meets its vertical illuminance limit at a window can still put a bare optic in a driver's sight line.

An illustrative example. A 40-space customer car park backs onto the rear gardens of six houses in an E3 suburban setting, with 6 m columns proposed at 4 m from the boundary. The initial model shows vertical illuminance at first-floor bedroom windows above the pre-curfew limit, and luminaire intensity towards them well above the post-curfew figure. Rotating the optics through 180 degrees, dropping columns to 5 m, specifying an asymmetric distribution with a house-side shield and switching to part-night operation from 23:00 brings all five parameters inside the zone limits. The numbers here are illustrative and each scheme needs its own model.

Presenting contour plots a reviewer can check

Contours are where reports become unreadable. Give one plan per scenario at a stated scale, with the isolux intervals labelled, the receptor positions marked and the curfew condition named in the title block. Include a vertical illuminance plot on the elevation of the nearest dwelling, rather than a horizontal plan alone, because the limit is a vertical one.

Three plots normally do the work: horizontal illuminance across the site for the operational case, vertical illuminance at the most affected facade pre-curfew, and the same post-curfew with the control regime applied. Add a spill contour at the site boundary showing the 0.5 lux and 1 lux lines where a dark landscape is nearby.

Tabulate the results against the limits alongside the plots. A reviewer reads the table first and uses the plots to test whether the table is honest.

Curfew hours, controls and what can be conditioned

Controls are part of the design, so describe them as such: time clocks, photocells, dimming profiles, presence detection on rear service areas, and manual override arrangements with a named responsible person. If the assessment relies on a 23:00 curfew then the report must say what happens between 23:00 and opening, and how a late delivery or a cleaning shift is handled without defeating the control.

Say who holds the commissioning duty. A post-installation verification survey, carried out at the receptor positions in the report and submitted to the authority, is the only way anyone finds out whether the model was right. Offer it.

The compliance conclusion and the conditions to offer

The conclusion should be a short statement per parameter, per receptor, per scenario, each marked as within or outside the limit, with the mitigation that achieves it. Avoid a general assurance. A reviewer wants to see that the author knows which receptor is closest to failing and by how much.

Close with drafted conditions rather than leaving them to the officer. In practice these cover the approved luminaire schedule and tilt, mounting heights, colour temperature, the curfew and control regime, the boundary treatment relied on in the model, and a verification survey within a set period of first use. A Lighting Assessment that arrives with workable conditions attached is far more likely to be accepted without a further round of questions.

One honest uncertainty. The guidance limits are design targets rather than statutory standards, and an authority can take a stricter line where its own development plan policy or a dark skies designation demands it. Check the local policy before you set the benchmark, and if a refusal follows, the parameter-by-parameter table is what an appeal statement of case will be built on.

Browse all 66 report types on PlanScope if you want the narrative sections drafted while the lighting designer completes the modelling.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.

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