Environmental zones and obtrusive light limits explained

Every number in a lighting assessment follows from one decision: which environmental zone the site sits in. Get the zone wrong and the limits are wrong, the luminaire schedule is wrong and the assessment is worthless. Get it right and the rest of the document is arithmetic.

The zones run from E0, intrinsically dark areas where the night sky itself is the receptor, to E4, town and city centres with high levels of night-time activity. The obtrusive light limits tighten sharply as the number falls.

The zone sets every limit

Obtrusive light is controlled against a small set of parameters, and each one has a different permitted value in each zone. The framework used across England is the Institution of Lighting Professionals guidance on the reduction of obtrusive light, which is what councils, their environmental health teams and inspectors refer to.

The parameters are sky glow expressed as an upward light ratio, light intrusion measured as vertical illuminance at a window, luminaire intensity towards a receptor measured in candelas, and the average luminance of a lit building facade. Road lighting adds a threshold increment for glare.

In a dark rural zone the permitted vertical illuminance at a residential window before curfew is a low single figure in lux, and after curfew it falls close to zero. In a city centre zone it is several times that. Take the actual figures from the current edition of the guidance rather than from a spreadsheet inherited from a previous job, because the guidance has been revised.

How a zone gets assigned

Some councils publish a zone map, usually in a lighting or design supplementary planning document. Where one exists, use it and say so.

Where none exists, the assessment has to justify the zone from the character of the area, and this is the part an environmental health officer will test. Relevant evidence includes the existing street lighting class and whether the road is lit at all, the density and type of surrounding development, proximity to a National Park or National Landscape, the presence of any designated dark sky area, and measured background sky brightness where the case is marginal.

Two errors are common. Assigning E3 to a village because there happen to be a few street lights, when the surrounding parish is unlit and reads as E2. And assigning a zone to the development rather than to the receptor: if the lit scheme sits in a commercial estate but the affected windows are in a rural lane, the limits that matter are the ones for the receptor's zone.

Where a site straddles a boundary, assess against the more restrictive zone for the receptors in it rather than averaging.

Sky glow, intrusion and intensity

Sky glow is controlled by limiting the proportion of a luminaire's output emitted above the horizontal. Modern flat glass optics achieve zero upward light in the as-installed position, so the usual failure is not the luminaire but the tilt. A flat glass unit tilted 15 degrees to reach a far corner of a yard throws light into the sky and into the next field.

Light intrusion is measured as vertical illuminance in the plane of the window, taken at the centre of the glazing, and it is the parameter neighbours complain about. It is also the one most easily fixed at design stage by moving a column, lowering it, or fitting a rear shield.

Luminaire intensity towards the receptor is a glare control. A low vertical illuminance can still coexist with a painfully bright source in view, which is why both are assessed. For sites where the lit development will be seen across open country, the night-time visual effect should also be described, and our note on viewpoint selection covers how to choose the positions that matter.

Curfew hours and part-night operation

The guidance applies tighter limits after a curfew hour, which the local authority sets and which is commonly 23:00. Pre-curfew and post-curfew values are different numbers and both belong in the results table.

Part-night operation is the mitigation that makes many schemes acceptable. Options in ascending order of sophistication: switch off entirely outside operating hours, dim to a set percentage at a fixed time, dim on a profile matched to the actual activity, and control by presence detection for infrequently used areas such as a rear service yard.

Security is the usual objection to a curfew. Answer it with design rather than with lux: controlled access, natural surveillance, a lower uniform illuminance across the whole area rather than bright pools, and detection-controlled lighting where movement is rare. A scheme that floods a car park all night is often less secure than one that lights it to a lower level with better uniformity.

Say in the assessment exactly what happens at curfew, which circuits change, and how the control is set. Conditions get drafted from that paragraph.

Ecology and dark corridors

Lighting limits for human amenity and lighting limits for wildlife are different tests with different units, and satisfying one does not satisfy the other.

Bats are the usual driver. Illuminating a hedgerow, treeline, watercourse or building used for commuting, foraging or roosting can sever a route or delay emergence. The mitigation is spatial before it is technical: keep the light away from the feature, then shield, then aim, then reduce output, then change the spectrum. Warmer colour temperatures with reduced short-wavelength content are generally preferred.

Where a bat survey or a Preliminary Ecological Appraisal identifies a route or a roost, the lighting design has to respond to it, and the ecologist should agree the target illuminance at the feature. Surveys and any mitigation licence require a suitably licensed ecologist under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017.

Do not leave the ecological lighting drawing to be produced after permission. It changes column positions.

Measuring and verifying after installation

A lighting assessment predicts. Verification proves. For anything beyond a small scheme, the assessment should propose a post-installation survey measuring vertical illuminance at the identified receptors, pre-curfew and post-curfew, with the instrument, its calibration date, the weather and the measurement heights recorded.

Take the readings with the development lighting on and again with it off, so the contribution of the scheme can be separated from existing street lighting. Without that pair of readings, a complaint about a bright street lantern becomes a complaint about your car park.

Aiming diagrams and a commissioning record matter as much as the numbers. Most non-compliance after installation is a luminaire aimed differently from the design, and a signed commissioning record is what lets the applicant prove the scheme was installed as approved.

Conditions that reference the right guidance

A condition requiring a lighting scheme "in accordance with ILP guidance" without naming the edition, the zone and the parameters is a condition nobody can discharge cleanly.

Useful wording names the environmental zone, the parameters assessed, the curfew hour, the requirement for a post-installation verification survey within a stated period of first use, and the requirement that the scheme is maintained and not altered without further approval. Where ecological mitigation is involved, the condition should also fix the target illuminance at the protected feature.

Councils usually expect a Lighting Assessment to be prepared by a suitably qualified lighting professional, and professional review is strongly advised before submission. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

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