Obtrusive light calculations in a lighting assessment

Obtrusive light calculations answer four questions, and a lighting assessment that answers three of them is incomplete. How much light falls on the windows of nearby homes. How bright each luminaire appears when you look at it from those windows. How much of the installation's output goes upwards. And, where a facade or structure is being lit, how bright that surface reads against its surroundings. Each has a numerical limit that depends on the environmental zone and tightens after the curfew hour.

The zone is where most reviews start, because an over-generous zone makes everything else comply. A reviewer who accepts the zone without checking the surroundings has accepted the conclusion.

What the calculations are measuring

Four quantities, with the units that matter.

Vertical illuminance at the plane of a window, in lux, which is the light trespass figure. It is calculated on the glass of the most affected window serving a habitable room, not on the ground or at the boundary.

Luminous intensity of each luminaire in the direction of the observer, in candelas, which stands in for glare. A low illuminance figure can coexist with an unacceptable intensity figure, which is why both are needed.

Upward light ratio, as a percentage of the luminaire's total output emitted at or above the horizontal. Modern flat-glass fittings aimed below the horizontal achieve zero, and a scheme that cannot achieve a low figure usually has the wrong fittings rather than the wrong aiming.

Luminance of an illuminated surface, in candelas per square metre, used for floodlit buildings, signs and structures.

Environmental zones and how one is assigned

The institution guidance on obtrusive light sets limits across zones running from intrinsically dark areas, through dark rural and low-brightness rural or village settings, to suburban and then urban centre locations. The limits differ by a large factor across that range, so the assignment does real work.

Assignment is a judgement about the character of the area around the receptors, not around the development. Points a reviewer should test:

• Whether the receptor sits in a different zone from the application site, in which case the more sensitive zone governs for that receptor
• Whether the local plan or a supplementary document assigns zones for the district, because some do
• Whether a National Park, National Landscape or designated dark sky area covers or adjoins the site
• Whether the existing lighting character has been described from observation, with a night-time site visit recorded
• Whether the assessment has assumed a brighter zone because an adjacent main road is lit

That last point is the common weakness. A lit distributor road does not convert a village edge into a suburban zone for a dwelling 80 metres away behind a hedge.

Vertical illuminance at windows

The calculation plane is the window glass, oriented as the window is, which means a window facing away from the source receives much less than the same window facing it. Assessments that calculate on a horizontal plane at ground level, or on a vertical plane facing the source regardless of the actual orientation, overstate or understate the result depending on which error was made.

Deciduous screening is the argument worth having. A hedge that blocks the view in July does nothing in January, and an assessment relying on it should say what happens when the leaves are off.

Luminaire intensity and glare

Intensity is directional, so the figure depends entirely on the aiming angle, and aiming tolerance in the field is wider than the model assumes. A scheme that complies at the designed aim and fails two degrees either side is a scheme that will fail on commissioning.

Ask whether the assessment reports sensitivity to aiming, whether the fittings have internal louvres or shields, and whether shields are shown on the schedule rather than mentioned in the text. Accessories that are not on the order are not installed.

Upward light ratio and sky glow

Upward light ratio is the proxy for sky glow at the scheme level, and it is the easiest limit to meet with current equipment. Where it is not met, the causes are predictable: fittings with a bowl or drop lens below the horizontal, luminaires tilted upward to reach a target at distance, or floodlights aimed at a surface from below so that spill passes over the top of it.

Reflected light is not captured by the ratio. A large area of wet paving or a light-coloured facade lit to a high level sends plenty of light upward without a single luminaire emitting above the horizontal, which is a limitation of the metric worth stating in the report rather than exploiting.

Curfew hours and the stricter limits

Limits fall sharply after the curfew hour, which the guidance sets and local policy sometimes varies. In the darkest zones the post-curfew figure for window illuminance is close to zero, which in practice means lights off rather than lights dimmed.

So the assessment needs two sets of results, and the control strategy has to be capable of delivering the second. A dimming regime that relies on a time clock needs a clock that cannot be overridden by a key holder, and a condition requiring switch-off is only as good as the commissioning record. Where security lighting is said to remain on after curfew, the assessment should treat it as part of the scheme rather than as an exception.

Reporting against the relevant guidance notes

A reviewable report states its inputs. The list is short and the omissions are consistent:

1. The guidance version applied, and the environmental zone assumed with its justification.
2. The luminaire schedule with photometric file references, mounting heights, orientations, tilt angles and any accessories.
3. The maintenance factor used, and why.
4. Calculation grids and receptor positions shown on a drawing, not described.
5. Surface reflectances assumed for any illuminated facade.
6. Results against each limit, pre-curfew and post-curfew, with pass or fail stated rather than implied.
7. The control strategy, including dimming, switching and the commissioning check proposed.

Ecology runs on different criteria. Bats commuting along a hedge, treeline or watercourse are assessed against guidance on bats and artificial lighting published jointly by the lighting institution and the bat conservation body, and a scheme that complies with obtrusive light limits can still be unacceptable for a commuting route. Where bats are a possibility, a Preliminary Ecological Appraisal and, if roosts are suspected, a Bat Survey prepared by a licensed ecologist come before the lighting design is fixed, not after.

A Lighting Assessment that sets out these inputs can be checked by a reviewer in an afternoon. One that gives results without inputs cannot be checked at all, and should be returned rather than argued with. Where several lit schemes sit close together, the cumulative question arises, and there is a related piece on cumulative effects in an environmental statement.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.