Sky glow, light trespass and glare as separate metrics

Obtrusive light is controlled through four separate measurements, each with its own unit, its own limit by environmental zone and its own mitigation. Sky glow is measured as upward light ratio, expressed as a percentage of luminaire flux emitted above the horizontal. Light intrusion is measured as vertical illuminance in lux at the plane of a window. Glare is measured as luminous intensity in candelas towards the receptor. And a lit facade is measured as average luminance in candelas per square metre. A lighting report that gives one number and calls it compliant has not done the assessment.

The limits sit in the lighting industry guidance on reducing obtrusive light, tabulated against environmental zones E0 to E4. Take the figures from the current edition, because they have been revised and the zone descriptions with them.

Why one number cannot describe light pollution

The four metrics describe different harms that happen to travel together.

A floodlight aimed at the ground can produce almost no upward light and still shine straight into a bedroom window. A well shielded luminaire can keep illuminance at the window low and still be unbearable to look at from a footpath, because the intensity in that direction is high. A facade wash can score well on every receptor measure and still be the brightest object on the skyline.

So the mitigation for one metric can make another worse. Raising a column to reduce glare at eye level increases the spill onto upper floor windows. Tilting a luminaire to keep light off a neighbour pushes flux above the horizontal. This is the reason lighting design is iterative rather than a single calculation.

Environmental zone selection sits underneath all four. E0 covers intrinsically dark areas such as National Parks and dark sky places, E1 relatively uninhabited rural areas, E2 sparsely inhabited rural, E3 well inhabited urban and E4 high district brightness city centres. State the zone, state who designated it, and where the local plan or a supplementary document assigns zones, use that rather than your own judgement.

Upward light and sky glow

Upward light ratio is the proportion of a luminaire's flux emitted at or above the horizontal, measured for the luminaire as installed rather than as catalogued.

The control is straightforward and the compliance test is unforgiving in the darkest zones, where the permitted ratio is zero. That means flat glass optics, no tilt, and no uplighting at all. In brighter zones a limited percentage is allowed, which permits some tilt and some architectural uplight.

Two errors recur in submitted reports. Quoting the manufacturer's upward light figure for a luminaire mounted horizontally when the design tilts it, which converts a compliant luminaire into a non-compliant installation. And omitting the reflected component from the discussion, because upward light ratio measures direct flux only and a light coloured hard surface under a floodlight contributes to sky glow that no luminaire specification will fix.

Where the scheme includes an extensive area of pale paving or a large glazed roof, say so and describe how the surface reflectance was considered.

Vertical illuminance at a window

Light intrusion is measured as illuminance on a vertical plane at the window of a habitable room, in lux, with separate pre-curfew and post-curfew limits by zone.

Getting this right depends on the receptor selection more than the calculation. Identify every window of a habitable room with a line of sight to the installation, at every floor, and calculate at each one rather than at the worst case only. Neighbours object at the window that was not assessed.

Two points to state explicitly. The curfew time you have used, and its source, because a condition or a local policy may set 23:00 while the guidance discusses curfews generally. And whether the figures are maintained values, after allowing for lamp depreciation and dirt, or initial values, because an initial value that just complies will fail once the installation is new no longer.

Post-curfew limits are far tighter than pre-curfew ones, which is why a dimming or part-night regime usually does more for compliance than any change of luminaire. How those regimes are secured is covered in curfew hours and part-night lighting conditions.

Luminous intensity and glare

Glare is assessed as the luminous intensity, in candelas, emitted from the luminaire in the direction of the receptor, with limits by zone and separate treatment before and after curfew.

Intensity is a property of the luminaire and its aiming, so the mitigations are optical rather than positional: a different beam distribution, an internal baffle, an external visor or shield, or a lower output with more luminaires. Screening with planting works for a fixed viewpoint and fails for a moving one, so it is more useful for a garden than for a footpath.

Road lighting uses a different glare measure, threshold increment, expressed as a percentage increase in the contrast threshold, which belongs in the highway lighting design rather than in the obtrusive light assessment. Sports lighting brings its own criteria again. Say which framework each part of the scheme was assessed under, because a mixed scheme with a car park, a pitch and a footway needs three sets of criteria.

Building luminance for lit facades

Average luminance of a lit facade, in candelas per square metre, is the metric that controls architectural lighting, signage illumination and feature lighting on a building.

It is the measure most often missing from submitted reports, partly because facade lighting is frequently added after the lighting assessment was produced. Limits by zone are low in dark areas and permit almost nothing in the darkest. Uniformity matters as much as the average, since a facade with bright patches reads as glare even where the average complies.

Practical responses are dimming, reducing the lit area rather than the intensity, lighting from close range with tight optics rather than from a distance with wide ones, and switching feature lighting off at curfew. Where the client wants a landmark effect, the conversation to have early is about hours of operation rather than brightness.

Which metric drives which mitigation

Ecology runs alongside these four and is not covered by them. Bats are affected at illuminance levels well below any obtrusive light limit, and the relevant guidance on bats and artificial lighting works in lux at the roost, the access point and the commuting route rather than at a window. Where a Preliminary Ecological Appraisal identifies bat activity, the lighting design has to satisfy both frameworks, and designing dark corridors for bats usually constrains the scheme more tightly than the neighbour metrics do.

Reporting all of them consistently

1. State the environmental zone, its source, and the reason if you have departed from a designated zone.
2. State the curfew time used and where it comes from.
3. List every receptor with a reference, a location on a plan, the floor level and the window it stands for.
4. Report all four metrics for every relevant receptor and facade, pre-curfew and post-curfew.
5. Say whether values are initial or maintained, and give the maintenance factor and surface reflectances used.
6. Provide isolux contours and a luminaire schedule with make, model, output, optic, mounting height and tilt.
7. Name the guidance edition and the calculation software.
8. Set out the control strategy: dimming profile, switching times and how it will be commissioned.

An illustrative example: a rural business park extension in zone E2 with a service yard, a car park and an illuminated entrance sign. The first design passes on upward light and fails on vertical illuminance at two first floor bedroom windows 40 metres away, and on building luminance for the sign. The revision lowers the yard luminaires, fits external shields on the two nearest, drops output by a third with an additional column, dims the car park to 30 per cent after 23:00 and switches the sign off at the same time. All four metrics then comply at every receptor, and the report shows both versions so the officer can see the reasoning.

PlanScope drafts a Lighting Assessment from your site details and answers, structured to UK planning policy, flagging the photometric inputs a qualified lighting designer has to supply. Councils usually expect a lighting report from a suitably qualified author, and professional review is strongly advised before submission.

This is general guidance, not legal advice; take the numerical limits from the current edition of the relevant guidance and check the zone with your local planning authority. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.