Artificial light spill into bedroom windows
Vertical illuminance at the glass, not brightness at the lamp, is what obtrusive light guidance controls. How zones, curfews and verification conditions fit together.
Light spill into a bedroom window is measured as vertical illuminance in the plane of the glass, in lux, and the limit that applies depends on how bright the area around you already is. That is the whole logic of obtrusive light assessment: a floodlit car park behind a house in a dark village is judged against a far tighter figure than the same car park in central Manchester. The Institution of Lighting Professionals guidance on obtrusive light sets those figures by environmental zone, with a much lower limit after a curfew time.
What obtrusive light means
Obtrusive light is the part of an installation's output that does nothing useful. It divides into four effects, and they are assessed separately because they have different causes.
Light intrusion into windows, which is the one that stops people sleeping. Sky glow, the upward component that washes out the night sky. Glare, the discomfort caused by looking towards a bright source. And light on buildings or surfaces where it was not wanted, such as a neighbour's garden.
A single badly aimed floodlight can produce all four. A well specified one with a flat glass lens, a shield and a correct tilt angle can light the same area and produce almost none. The physics is unforgiving in one direction only: light emitted above the horizontal is lost, and light emitted at a shallow angle towards a window arrives at close to full strength.
Environmental zones, briefly
The guidance places every location in one of five zones, running from intrinsically dark landscapes at one end to brightly lit urban centres at the other. Each zone carries its own numerical limits for window illuminance, upward light, source intensity and building luminance.
Two points matter for a resident or an officer reading an assessment.
The zone is a judgement, not a fact on a map. An assessor who places a village edge in the suburban zone rather than the rural one has quietly relaxed the limits by a factor of two, and that is the first thing to check in any lighting report. Our note on obtrusive light environmental zones goes through how each zone is described, and the practical consequences for luminaire choice are set out in the companion note on environmental zone lighting selection.
The zone applies to the receptor, not the development. A depot in an industrial area lighting the rear of houses in a quiet residential street is judged against the zone the houses sit in.
Vertical illuminance at a window
This is the number to look for. Vertical illuminance, written Ev, is measured in the plane of the window, so a lamp directly above a window that throws light downwards produces a lower figure than a lamp at the same height 30 metres away aimed horizontally.
The guidance sets a pre-curfew and a post-curfew limit for each zone. In the suburban zone the pre-curfew figure is 10 lux, falling to 2 lux after curfew. In the sparsely inhabited rural zone it is 5 lux, falling to 1 lux. Zones darker than that approach zero, which in practice means the light has to be switched off rather than dimmed.
Three things affect whether a reported figure is credible:
- Where the calculation point sits. It should be at the affected window, at the height of the window, not at the site boundary or at ground level in the garden.
- Whether existing light has been subtracted. A figure that includes a street lamp already outside the window flatters the development.
- Whether the worst case luminaire and the worst case aiming tolerance were modelled. Installers do not aim to the drawing, and a two degree error matters at distance.
Curtains and blinds are not an answer, and an assessment that relies on them has conceded the point. The relevant test is what arrives at the glass. Similar logic applies to how rooms are treated in daylight work, discussed in our note on room use assumptions in a daylight assessment.
Curfew hours
The curfew is the clock time after which the tighter limits apply, commonly taken as 23:00 to 07:00 unless the authority sets something different. It exists because the harm from light at a bedroom window is a sleep harm, and sleep harm is time dependent.
Curfew wording is where conditions most often fail. Three versions, in descending order of usefulness:
- All external lighting other than the named emergency luminaires shall be switched off between 23:00 and 07:00.
- Lighting shall be reduced to the post-curfew levels set out in the approved scheme between 23:00 and 07:00.
- Lighting shall be controlled to avoid unnecessary use at night.
The third controls nothing. If a use does run past the curfew, such as a 24 hour depot or a hospital, say so and design for the post-curfew limits rather than pretending the lights go off.
Assessing a car park or a pitch
These are the two schemes that generate most complaints, and they behave differently.
Car parks are lit to a low average level over a wide area, usually from columns of 5 to 8 metres. The spill problem comes from columns near the boundary and from the choice of optic. Moving a column three metres, changing to an asymmetric distribution and adding a rear shield usually fixes a marginal case without reducing the light on the parking bays.
Sports pitches are lit to a much higher level, from taller columns, with luminaires aimed across the pitch. The spill problem is glare and window illuminance from the far bank of floodlights, and it cannot be designed out entirely. What can be controlled is the hours of use, the switching regime, whether training levels are used rather than competition levels, and whether the top row of luminaires is needed at all outside match days.
For a pitch scheme, ask for the isolux contour plot overlaid on a plan showing residential windows, the luminaire schedule with aiming angles, and the calculated Ev at each affected window before and after curfew. A Lighting Assessment that omits the aiming angles cannot be checked by anybody.
Conditions that control spill
| Element | Wording that works |
|---|---|
| Approved scheme | Luminaire type, mounting height, position and aiming angle for every fitting, plus the isolux plot |
| Window limits | Maximum Ev at named windows, pre-curfew and post-curfew, with the zone stated |
| Curfew | Specific clock times, with any exempt emergency luminaires listed |
| Upward light | A stated maximum proportion of output above the horizontal |
| No variation | Replacement luminaires to match the approved schedule unless otherwise agreed in writing |
| Verification | Measurement after installation by a competent person, submitted within a stated period |
The verification condition is the one applicants resist and residents should press for. An approved scheme with no check simply records what was drawn.
Complaining about an installation already in place
If the lighting was approved with conditions, read the conditions first. Lights aimed differently from the approved scheme, or running past the curfew, are a breach of condition and a matter for planning enforcement.
If there were no conditions, or the installation never needed permission, the route is a statutory nuisance complaint to the council's environmental health team. Artificial light from premises can be a statutory nuisance under the Environmental Protection Act 1990, subject to exclusions for certain premises types such as railway and harbour land and a qualification for outdoor sports facilities.
Either way, keep a dated log of times and effects and take photographs from inside the affected room. Officers are persuaded by a record covering several weeks far more readily than by a single strongly worded letter.
If you are on the applicant side and expect lighting to be contested, enter your postcode on PlanScope to see which reports the site is likely to need before you commission the design.
This is general guidance, not legal advice. Numerical limits come from the current lighting guidance and the relevant British Standards, so check the current editions with a qualified lighting professional before relying on them. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How is light spill into a window measured?
As vertical illuminance, written Ev and measured in lux in the plane of the window at window height. The calculation point should be at the affected window rather than at the site boundary, existing background light should be excluded from the development's figure, and the worst case luminaire and aiming tolerance should be modelled.
What is a lighting curfew?
It is the clock time after which tighter obtrusive light limits apply, commonly taken as 23:00 to 07:00 unless the local planning authority sets different hours. The tighter post-curfew limits exist because light at a bedroom window causes sleep disturbance, which is time dependent rather than constant.
Which environmental zone applies to my house?
The zone applies to the location of the affected property rather than the development, and it is a judgement rather than a line on a map. Placing a village edge in a suburban zone instead of a rural one roughly doubles the permitted window illuminance, so the zone choice is the first thing to check in any lighting report.
Can curtains be treated as mitigation?
No. The assessment measures what arrives at the glass, and a report that relies on residents drawing curtains has effectively accepted that the limit is exceeded. Obscure glazing and blinds are sometimes conditioned for privacy reasons, but they do not answer an obtrusive light objection.
What should a floodlighting scheme submit?
A luminaire schedule with types, mounting heights, positions and aiming angles, an isolux contour plot overlaid on a plan that shows nearby residential windows, and the calculated vertical illuminance at each affected window before and after curfew. Without aiming angles the design cannot be independently checked.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.