Sky glow, light spill and source intensity explained

Obtrusive light is assessed under three separate headings, and knowing which one your complaint falls under changes what you ask the council for. Sky glow is light escaping upwards and brightening the night sky. Light spill, usually called light intrusion, is light landing on someone else's window. Source intensity is how bright the luminaire itself appears when you look towards it from a particular position. A lighting scheme can pass on one and fail badly on another.

If you are objecting to a floodlit pitch, a car park or a security scheme next door, saying "it is too bright" gets you nowhere. Saying which of the three parameters is breached, and at which environmental zone, gets you a condition.

The three parameters

Sky glow is expressed as an upward light ratio: the proportion of the light emitted by a luminaire that goes above the horizontal. Flat-glass, fully cut-off luminaires emit essentially none directly, though light reflected off the ground still contributes. Tilted floodlights emit a great deal.

Light intrusion is measured as vertical illuminance in lux at the window of a nearby property, taken on the plane of the glass. This is the number that matters for anyone trying to sleep, and it is the one most often missing from a submitted lighting drawing.

Source intensity is measured in candelas, in the direction of the receptor. A small, very bright source can cause serious glare while contributing almost nothing to the illuminance figures. Anyone who has been dazzled by a neighbour's LED security light on an otherwise dark lane has met this parameter.

For floodlit buildings and signs there is a fourth: the average luminance of the illuminated surface, which is what stops a church elevation being lit like a stadium.

Environmental zones from E0 to E4

The Institution of Lighting Professionals publishes the guidance that English councils use, and it sets numerical limits for each parameter by environmental zone. The zones run from E0, protected dark areas such as dark sky reserves and observatory sites, through E1 for intrinsically dark rural areas, E2 for low district brightness such as village outskirts and rural settlements, E3 for medium district brightness in suburbs and small towns, to E4 for high district brightness in town and city centres.

The limits tighten sharply as the zone number falls. What is unremarkable in a city centre is a clear breach on the edge of a National Landscape. Check the current edition of the guidance for the figures, because the values have been revised and quoting an old table is an easy way to lose an argument.

Two practical notes. Councils do not always state the zone in their local plan, so it is often the objector or the applicant who has to propose one and justify it. And a site can straddle zones, in which case the more sensitive zone applies to the receptors within it.

Justifying a zone is easier than it sounds. Existing street lighting, the density of development, the presence of a National Landscape or National Park boundary, and any dark skies designation nearby are all evidence a council will accept. A single row of houses on an unlit lane outside a village is not E4, whatever the applicant's consultant has assumed.

Curfews and why the post-curfew figures matter

The guidance sets two sets of limits: one for the hours before a curfew and a much tighter set for the hours after. The curfew hour is commonly taken as 23:00 unless the authority sets a different time.

This is where most schemes are won or lost. A car park lit to a reasonable standard at 20:00 may be entirely acceptable. The same scheme still running at 02:00 is not, and the post-curfew limits are strict enough that the practical answer is usually to switch off or dim substantially.

So the condition to ask for is not a lower lux level. It is a curfew: lighting to be extinguished or reduced to a stated level between stated hours, other than for stated safety purposes, with the control method specified. Timeclocks fail and get overridden. A condition that names the control system and requires it to be maintained is worth more than one that names a number.

Colour temperature, wildlife and dark landscapes

Correlated colour temperature is a separate lever and an effective one. Warmer light, in the region of 2700 kelvin or below, produces less blue content, scatters less in the atmosphere and disturbs nocturnal wildlife less than the cooler 4000 kelvin sources that contractors often default to.

Bats are the usual reason this becomes a planning issue. Lighting across a commuting route, a hedgerow or a roost exit can effectively sever a flight line, and where a roost is present the works may engage licensing under the Wildlife and Countryside Act 1981 and the Habitats Regulations. A Bat Survey will identify commuting routes, and a Preliminary Ecological Appraisal should flag whether lighting is a constraint at all. Surveys and any mitigation licence require a suitably licensed ecologist.

In National Landscapes, National Parks and other intrinsically dark areas the National Planning Policy Framework (current edition published 17 August 2026) expects development to limit the impact of light pollution on the dark landscape as well as on people. That is a distinct consideration from amenity, and it is worth raising separately.

What to ask for in an objection

A well-aimed objection asks for documents rather than outcomes. Request:

1. A Lighting Assessment showing isolux contours across the site boundary and at the nearest habitable windows.
2. A luminaire schedule giving make, model, mounting height, tilt angle, output, colour temperature and light distribution.
3. Calculated vertical illuminance at named windows, pre-curfew and post-curfew.
4. Source intensity towards named receptors.
5. A statement of the environmental zone assumed, with justification.
6. The proposed hours of operation and the control method.

If the applicant cannot supply those, the council cannot properly assess the scheme, which is a stronger objection than any adjective.

Illustrative example: a village sports club applies for four 8-metre floodlight columns. The zone is arguably E2. The submitted drawing shows horizontal illuminance on the pitch and nothing else. The useful objection asks for vertical illuminance at the six houses on the lane, source intensity from each column towards those houses, and a curfew condition at 21:00 rather than 23:00 given the rural setting.

When lighting becomes a statutory nuisance

Planning conditions are one route. The other is section 79 of the Environmental Protection Act 1990, which treats artificial light emitted from premises so as to be prejudicial to health or a nuisance as a statutory nuisance, enforced by the council's environmental health team.

The exemptions matter. Certain premises are outside the regime, including airports, harbours, railway premises, bus stations, lighthouses and prisons, and outdoor sports facilities have a defence of best practicable means rather than an exemption. Street lighting on the public highway is generally not caught, since it is not emitted from premises.

Where a scheme is already built and operating badly, the nuisance route can be quicker than enforcement of a planning condition. Where a scheme is still an application, the condition is the better prize.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.