Lighting assessment competence and the ILP guidance notes

An obtrusive light assessment has to show three things: that the scheme meets the limits for the environmental zone it sits in, that the calculation producing that result used real photometric data for the specified luminaires, and that the assessment points are where the people who will be affected actually are.

Reports fail on all three, and they fail most often on the first. The environmental zone is chosen by the author, it changes every numerical limit in the assessment, and a zone assigned one step too bright makes an unacceptable scheme look compliant.

What the assessment has to demonstrate

The framework used across the industry is the Institution of Lighting Professionals guidance on the reduction of obtrusive light. It sets limits in four categories.

Sky glow, controlled by limiting the proportion of light emitted above the horizontal from each luminaire. Light intrusion into windows, expressed as vertical illuminance at the window plane in lux. Luminaire intensity towards a viewer, in candelas. And building luminance, which applies to floodlit facades.

Each limit is stated separately for the period before a curfew and the period after it, with the post-curfew figures substantially tighter. The curfew time is normally set by the local authority as a condition, commonly at 11pm, and the assessment should state which time it has assumed rather than leaving it implicit.

A Lighting Assessment that reports a single set of results with no curfew analysis has not answered the question a council needs answered, because the condition it will impose operates after the curfew.

Environmental zones and how they are assigned

Zones run from the darkest to the brightest, covering intrinsically dark landscapes such as National Parks and dark sky places, relatively dark rural areas, low district brightness such as village fringes and outer suburbs, medium district brightness such as small town centres and suburban locations, and high district brightness such as town and city centres.

The assignment is meant to reflect the actual environment of the receptor rather than the character of the applicant's own site. A distribution depot on the edge of a village does not become a bright zone because the depot itself is lit.

Three checks resolve most disputes about the zone.

• Does the council's own supplementary planning document or dark skies policy assign a zone to the area, and does the report use it?
• Are the receptors in the same zone as the site, or does the site sit at a boundary between zones, in which case the more sensitive figure applies to the receptor?
• Where the site adjoins a designated dark landscape or an ecological corridor, has that been treated separately rather than averaged in?

Where the zone is contested, the honest approach is to present the assessment against both candidate zones. A scheme that passes in either is uncontroversial. A scheme that only passes in the brighter zone tells you where the argument will be.

The calculation behind the spill diagram

An isolux contour plot looks authoritative and can be produced from almost nothing, so a reviewer should look behind it.

The inputs to check are the luminaire schedule with make, model and reference for each type, the photometric file used for each luminaire, the mounting height, tilt and aiming angle, the maintenance factor and its justification, the reflectance values assumed for surfaces, and the calculation grid spacing and height.

Two of those cause most of the trouble. Tilt is the first: a luminaire specified as flat glass and horizontally mounted produces very different spill from the same unit tilted five degrees, and drawings frequently show a tilt the calculation does not include. Maintenance factor is the second, because an optimistic factor understates the installed output and the scheme fails once it is commissioned.

Ask whether the assessment includes existing lighting on and around the site. A calculation showing only the new luminaires answers a question nobody asked, since the receptor experiences the total.

Where lighting design meets ecology

Artificial light at night affects bats, invertebrates and other nocturnal species, and the ecological side of a lighting assessment is a separate competence from the obtrusive light calculation.

The joint guidance from the Institution of Lighting Professionals and the Bat Conservation Trust on bats and artificial lighting sets out the approach: keep light off roosts, commuting routes and foraging habitat, use the lowest practicable levels, control spill with shields and accessories rather than by pointing luminaires elsewhere, use warmer colour temperatures, and switch off or dim when the space is not in use.

Where a site supports bats, the lighting design and the ecological mitigation have to be developed together. A lighting scheme signed off in isolation and then handed to the ecologist is the pattern that produces a refusal or a condition nobody can discharge. A Preliminary Ecological Appraisal that identifies dark corridors early lets the lighting designer work around them, and where a Protected Species Survey confirms roosts, a suitably licensed ecologist has to be involved in the lighting decisions rather than consulted afterwards.

Who is competent to do this work

There is no protected title and no licence. Anyone may write a lighting assessment, which is why the range in quality is so wide.

What competence looks like in practice: membership of the Institution of Lighting Professionals at an appropriate grade, working knowledge of the relevant British and European standards for the application, whether that is road lighting, outdoor workplaces or sports lighting, fluency with calculation software and photometric file formats, and enough experience of planning conditions to know what a council will actually require.

Suppliers frequently offer a free lighting design with a quotation. Those calculations are usually competent as a lighting design and are almost never structured as a planning assessment, because they are answering a different question. They will show that the car park is adequately lit. They will not show vertical illuminance at the neighbour's first floor bedroom window after the curfew.

Reviewers who are asked to sign off a lighting report they did not produce should record what they examined and what they relied on, and should decline where the photometric data behind the calculation is not available.

What to look for when reviewing, and what follows

Six questions catch most defects.

1. Which environmental zone, and on what basis?
2. Are pre-curfew and post-curfew results both presented?
3. Do the assessment points match the actual sensitive receptors, at the correct height and orientation?
4. Are the luminaires in the calculation the ones in the schedule and on the drawings?
5. Is existing and cumulative lighting included?
6. Has the ecological dimension been addressed by someone qualified to address it?

The conditions that follow a lighting assessment are fairly standard: a scheme to be submitted and approved before installation, a luminaire schedule with photometric data, isolux contours demonstrating compliance with the stated limits, a curfew regime, and often post-installation verification by measurement. Design to be able to discharge those, because a scheme that cannot demonstrate compliance once built leaves the operator with a lighting installation and no lawful way to use it.

Reviewers with the relevant competence can be found through the professional directory.

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