Lighting Assessment: reporting against ILP environmental zones

A lighting assessment planning report stands or falls on one paragraph: the justification of the environmental zone. Institution of Lighting Professionals guidance on the reduction of obtrusive light sets numerical limits for each zone, and the zones run from intrinsically dark landscapes through rural and suburban areas to urban centres with high night-time activity. Assign the site to a brighter zone than it deserves and every limit in the report is too generous. Environmental health officers and ecology consultees know this, and the zone is the first thing they check.

Everything else in the report is method, and the method is straightforward once the zone is fixed.

Establishing the environmental zone

The zones are defined by the character of the surroundings rather than by administrative boundary. At one end sit areas with intrinsically dark landscapes, including National Parks, National Landscapes and designated dark sky places, where no upward light is permitted at all. At the other sit urban areas with high levels of night-time activity, where the limits are much more permissive.

Justify the assignment with evidence rather than assertion. What supports a zone classification:

• The relevant designations covering or adjoining the site, named and referenced.
• Measured baseline light levels at the site boundary and at the nearest sensitive receptors.
• A description of existing lighting in the vicinity: street lighting type and spacing, commercial lighting, floodlit facilities.
• Any local plan policy or supplementary planning document that assigns zones for the authority.
• Whether the site sits on the edge of a zone, in which case the more sensitive classification usually applies.

Edge cases are common and they are where disputes arise. A commercial yard on the boundary of a National Landscape is assessed against the darker zone for effects falling within it, even if the yard itself sits in a brighter setting. Say so in the report rather than waiting for the consultee to say it.

Baseline light levels and existing sources

Measure the baseline, do not estimate it. Readings should be taken after dark under stable conditions, at positions that correspond to the receptors assessed later, with the instrument, its calibration date, the date and time of measurement and the weather recorded.

Two measurements matter. Horizontal illuminance at ground level gives a sense of the existing lit environment. Vertical illuminance at the plane of the nearest affected windows is what the obtrusive light limits are expressed against, so take it at the right height and orientation rather than at a convenient spot.

Where existing lighting will be removed or replaced, describe it properly: luminaire type, mounting height, orientation, optical control and operating hours. A scheme replacing unshielded floodlights with modern flat-glass luminaires can produce a net improvement, and that argument only works if the existing installation is documented.

Design illuminance and uniformity by area

The lighting design itself has to satisfy a purpose before it satisfies a limit. Set out the design criteria area by area: maintained average illuminance, uniformity ratio and, where relevant, glare rating, each referenced to the applicable standard for that task.

Then reconcile the two. Where the illuminance needed for the task cannot be delivered within the zone limits, say so explicitly and set out what has been done: lower mounting heights, tighter optical distributions, reduced lit area, part-night dimming, or a reduction in the hours the area is used. A report that quietly reports compliance without explaining the tension invites scrutiny of the calculation.

Obtrusive light limits before and after curfew

The guidance sets limits under four headings, and a complete assessment reports against all four.

1. Sky glow, expressed as the upward light ratio, the proportion of luminaire flux emitted at or above the horizontal. In the darkest zones the permitted value is zero, which rules out any luminaire with upward emission and any uplighting of a facade.
2. Light intrusion into windows, expressed as vertical illuminance at the window plane, with a lower limit applying after curfew.
3. Luminaire intensity towards the receptor, expressed in candelas, which is what controls glare from a directly visible source.
4. Building luminance, for illuminated facades and signage, expressed as an average in candelas per square metre.

Take the numerical values for each zone from the current edition of the guidance rather than from a remembered table, because the limits have been revised and the edition should be cited in the report.

Curfew is usually the decisive control in a contested case. Limits tighten sharply after the curfew hour, commonly set at 23:00 by condition, and demonstrating compliance after curfew frequently means switching off rather than dimming. Present the calculation in two sets, pre-curfew and post-curfew, at every assessed receptor.

Vertical illuminance at nearby windows

This is the calculation neighbours care about and the one most often done at the wrong place. Identify every dwelling with a window facing the development within a sensible radius, plot the assessment points at the actual window positions including upper floors, and calculate vertical illuminance perpendicular to each window plane.

Report it as a table: receptor reference, window orientation, height above ground, predicted vertical illuminance pre-curfew, predicted vertical illuminance post-curfew, and the applicable limit. An illustrative example: a service yard where the first floor bedroom window of the nearest dwelling sits above the hedge that screens the ground floor, so the upper window governs the design even though the ground floor is fully screened. That pattern recurs often enough to be worth checking on every site.

Include the isolux contour plot as well as the table. Officers read drawings faster than they read numbers.

Ecological receptors and sensitive species

Lighting assessment for planning is no longer only about human amenity. Bats are the species most often decisive, and the joint guidance on bats and artificial lighting produced by the lighting and bat conservation bodies sets out the approach: identify roosts, commuting routes and foraging habitat, then keep light off them.

The measures that work, in order of effectiveness: do not light the feature at all, then reduce the lit area, then use warmer colour temperatures with reduced short wavelength content, then add shields, louvres and baffles, then dim or switch off during sensitive periods. Mounting height and optical control matter more than lamp wattage.

Where hedgerows, watercourses, woodland edge or a known roost lie near the scheme, the lighting design and the ecological mitigation have to be the same document in substance, even if they are submitted as two. Contradictions between a lighting layout and an ecological mitigation plan are one of the easiest things for a consultee to spot. The structure of the reasoning is the same source, pathway and receptor logic used in an odour assessment, and the screening step works much as it does in health impact assessment work.

Controls, timers and the conditions expected

Finish the report with the controls that make the assessment deliverable, because that is what the condition will secure. Typical content:

• A schedule of luminaires with types, mounting heights, tilt angles and optical accessories.
• The control strategy, including timers, dimming profiles and any presence detection.
• The curfew time and what happens at it.
• A commitment to post-installation verification.

That last item is worth offering unprompted. A condition requiring a post-completion lighting survey confirming the installation matches the approved design closes the gap between a compliant model and a compliant site, which is where most lighting complaints originate.

A Lighting Assessment draft can carry the structure, the policy framework and the receptor schedule, and the calculations and the professional judgement remain the lighting designer's. Councils usually expect a suitably qualified author for this work and professional review is strongly advised. The wider report index shows the other environmental documents a scheme with external lighting commonly carries.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.