Obtrusive light from a development and environmental zones

Obtrusive light is assessed by environmental zone, and the zone decides almost everything else. The same car park lighting scheme that passes without comment on a retail park edge will be refused beside a village green, because the limits for light intrusion, sky glow and luminaire intensity tighten by a factor of several between zones. Getting the obtrusive light planning environmental zone right at the start of the design is worth more than any amount of argument later about whether the effect is noticeable.

Councils take their limits from the Institution of Lighting Professionals guidance on reducing obtrusive light, which sets values by zone and splits them into pre-curfew and post-curfew periods. That guidance is not policy in itself. It becomes binding through a local plan lighting policy, through a condition, or through the officer's judgement on amenity.

What obtrusive light covers

Four effects, assessed separately.

Light intrusion is illumination falling where it is not wanted, measured as vertical illuminance at a window of a neighbouring property. Sky glow is upward light brightening the night sky, controlled through the proportion of a luminaire's output emitted above the horizontal. Source intensity is the glare from looking directly at a bright luminaire from a receptor position. Building luminance covers floodlit facades and illuminated signage, measured as average luminance across the lit surface.

A scheme can pass three and fail the fourth. A well-aimed floodlight with no upward spill can still put unacceptable vertical illuminance on a bedroom window 20 metres away, and a fully shielded bollard scheme can still produce a glare complaint if the source is visible at eye level along a footpath.

Environmental zones and how they are assigned

The zones run from E0 to E4 and describe the existing brightness of the area rather than its planning designation.

The assignment is a judgement, and it is the part most often contested. A developer will argue that a site on the edge of a village adjoining a lit A road sits in E3. The council's environmental health officer will say E2. The gap between those two positions can be the difference between a scheme that works and one that needs a redesign, so settle it at pre-application with a baseline measurement rather than an assertion.

Baseline measurement means going out at night, after curfew, and recording illuminance and luminance at the receptor positions you will later assess. A short survey pinning the existing condition costs a fraction of what arguing the zone on paper does.

Where a local plan policy names a zone for part of the authority's area, that policy wins. Some rural authorities assign E1 across everything outside the settlement boundaries.

Limits before and after curfew

Curfew is the point at which the stricter values apply, commonly 23:00 to 07:00 but set by the authority and sometimes varied by use. The step down at curfew is sharp. In the darkest zones the post-curfew value for light intrusion into a window is effectively zero, which in practice means the lighting has to switch off, dim to a negligible level, or be blocked from reaching that window at all.

Read the current edition of the guidance for the numerical limits, because they have been revised and older tables circulate widely. What matters for design is the shape of the requirement:

• Pre-curfew values allow normal operation of a lit space.
• Post-curfew values assume most external lighting is off or heavily dimmed.
• E0 and E1 leave almost no headroom for upward light at any time.
• Building luminance and sign brightness are usually the first things a condition restricts after curfew.

Design the control strategy around this rather than the luminaire schedule. Time clocks, dimming profiles, presence detection on low-traffic routes and separate circuits for security lighting are what let a scheme meet post-curfew values without leaving a car park unusable at 22:00.

Assessing spill onto windows

The assessment locates the worst affected windows, models vertical illuminance at the window plane, and reports the figure against the zone value for both periods. Three details decide whether the result is believed.

First, the receptor positions have to be the right ones. Ground floor rear windows of the nearest dwellings, yes, but also upper floor windows that look down into a lit yard and windows on the flank elevation that the site plan barely shows. The same discipline that applies to room use assumptions in a daylight assessment applies here: the assumption you make about which window matters shapes the answer.

Second, the model needs the real luminaire data, meaning photometric files from the manufacturer rather than generic types, with the mounting height, tilt and aiming as they will be installed.

Third, the report should show the contour plot as well as the table. A committee reading a page of lux values learns nothing; an isolux plan overlaid on the site layout shows immediately where the spill goes. The same presentation logic is set out in presenting daylight results a committee can read.

Sky glow and upward light

Upward light is controlled by specifying luminaires with zero light emitted above the horizontal when installed level, and then by making sure they are installed level. Tilted flat-glass luminaires throw light upward regardless of what the datasheet says, and tilt creeps in on site when a designer tries to reach a distant corner with fewer columns.

Column height matters more than people expect. Lower columns need more of them and produce more uniform light at lower intensity, which usually reduces both spill and glare. Taller columns reduce capital cost and increase the distance over which light travels.

Colour temperature has moved from a footnote to a condition in its own right. Warmer sources at or below 3000 kelvin, and 2700 kelvin or lower in sensitive locations, reduce the blue content that scatters in the atmosphere and disturbs wildlife. Where bats are a consideration, the joint guidance from the Institution of Lighting Professionals and the Bat Conservation Trust sets out buffer distances, warmer sources and shielding, and an ecologist should sign off the lighting drawing rather than seeing it for the first time at discharge of conditions.

Conditions commonly imposed on a lighting scheme

Expect some or all of these:

1. No external lighting to be installed other than in accordance with a scheme submitted to and approved in writing by the authority.
2. The approved scheme to include luminaire specifications, mounting heights, aiming angles, isolux contours and a control strategy.
3. Post-installation verification: a commissioning report demonstrating the installed scheme meets the modelled values, submitted within a set period after first use.
4. A curfew condition setting the hours during which lighting must be off or dimmed.
5. Retention of shielding, louvres or baffles for the life of the development.

The verification condition is the one that bites. It is also the one that catches out schemes where the lighting was value-engineered between approval and installation.

Councils can act on excessive artificial light separately through statutory nuisance powers under the Environmental Protection Act 1990, which is a reminder that a compliant planning condition does not close off every route of complaint.

Coordinating with the landscape design

Lighting and planting are designed by different people at different times, and they interact badly when nobody reconciles them. Trees intended to screen the development from the road also block the light from reaching the footway, so the lighting designer adds columns or raises output. Five years later the planting matures and the scheme is over-lit and patchy.

Resolve it by lighting the route rather than the area, positioning columns on the building side of planting lines, and specifying the lighting class from the function of each space instead of applying one level everywhere. Service yards, pedestrian links, parking bays and building entrances all need different treatment.

Where the scheme also affects daylight and sunlight to neighbours, run the two studies against the same massing model so the drawings agree. A Daylight and Sunlight Assessment and a Lighting Assessment that describe different building heights will both be queried.

Enter your postcode on PlanScope to see which environmental and amenity reports a site of this kind usually needs before you commission the lighting design.

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