Vertical illuminance at a window and the curfew

Vertical illuminance at a neighbouring window is the number that decides most light nuisance objections, and it is assessed twice: once for the evening and once for the period after the curfew hour, when the limit drops sharply. In a low district brightness area the pre-curfew figure in the Institution of Lighting Professionals guidance is 5 lux at the window and the post-curfew figure is 1 lux. Hitting the first is usually a matter of aiming. Hitting the second normally means switching something off.

Get the environmental zone and the curfew hour agreed with the environmental health officer before the lighting designer starts, because those two choices set the target for everything that follows.

What vertical illuminance measures

Vertical illuminance, written Ev, is the illuminance falling on a vertical surface, measured in lux. For obtrusive light assessment the vertical surface of interest is the window of a property that could be affected, and the quantity being controlled is the light arriving at that glazing from the proposed installation.

It is a different measure from the horizontal illuminance used to design a car park or a footway, and a different measure again from the luminance of a surface. A scheme can meet its horizontal design criteria across the site and still fail at a first-floor bedroom window on the boundary, because the geometry that lights the ground at 20 lux also throws light upward and sideways.

Two related quantities appear in the same assessment. Luminaire intensity in the direction of the receptor controls the glare experienced by someone looking at the fitting, and the upward light ratio controls sky glow, which the note on sky glow and upward light ratio covers in its own right. Ev deals with light trespass into a dwelling, nothing else.

Where the calculation plane sits

The convention is a vertical plane coincident with the glazing, with the assessment taking the maximum value over the window. In modelling practice that means placing a calculation surface on the facade at the position of each relevant window, with a grid fine enough to catch the peak rather than an average across the elevation.

Three details decide whether the result is defensible.

• Which windows count. Habitable room windows of existing dwellings are the standard receptors. Bathroom and landing windows are usually excluded, and the position of windows in a consented but unbuilt scheme next door is a judgement to agree with the officer rather than assume.
• Whether the plane sits at the glass or at the building line. Where a window is deeply reveal-set, the glass plane gives a lower figure, and that difference needs stating in the report.
• Whether existing lighting is included. The guidance addresses the proposed installation, so most assessments report the proposed scheme alone. Where an officer asks for the cumulative figure, model both and present them separately rather than merging them.

Write the convention you adopted into the methodology section. Reviewers cannot check a number they cannot reproduce.

Limits by environmental zone, before and after curfew

The environmental zone classification runs from an intrinsically dark area through to a high district brightness area, and the limits tighten as the zone darkens. The vertical illuminance values practitioners work to, from the ILP guidance on the reduction of obtrusive light, are:

An intrinsically dark zone, covering National Parks, National Landscapes and designated dark sky areas, is treated as admitting no trespass at all. Take the working figures from the current edition of the guidance, because the document has been revised and some authorities still quote an earlier version in their supplementary planning documents.

The curfew is the hour after which the lower limits apply. It is set locally and is commonly 23:00, running to 07:00, but some authorities adopt a different hour and some vary it between weekdays and weekends. Ask. A design optimised for a 23:00 curfew is not necessarily compliant against a 22:00 one, and the post-curfew figure of 0 lux in a dark zone means the installation goes off.

Luminaire intensity as a separate limit

Meeting the Ev figure does not dispose of glare. The guidance sets a separate limit on luminaire intensity in candelas, measured in the direction of the potentially affected view, with the same pre-curfew and post-curfew structure and the same zone bands. A narrow-beam floodlight aimed below the horizontal can satisfy the trespass limit at the window while still being painful to look at from a garden.

The two limits respond to different changes. Trespass is reduced by aiming, shielding and lowering output. Intensity in a given direction is reduced by optic selection and by physical shielding at the fitting, and sometimes by moving the fitting rather than adjusting it. Report both, with the receptor positions marked on a plan, and say which receptor drives the design.

Modelling against measurement

Almost all planning-stage assessment is modelled, using photometric files from the specified luminaires in a lighting design package. That is the right approach for a proposal, and it is only as good as the assumptions: the correct photometric file, the aiming angles and tilt as they will be installed, the mounting heights as built, the maintenance factor, and the reflectances of nearby surfaces.

Measurement has a place at two points. Where an existing installation is the subject of a complaint, a measured survey with a calibrated illuminance sensor at the window establishes the baseline. And after installation, a commissioning check confirms that the aiming matches the model, which is where schemes most often drift. The same tension between a modelled prediction and a measured value runs through air quality work, and the discussion of diffusion tubes against continuous analysers is a useful parallel on when each tool earns its place.

Measure after dark with the installation at full output, record the instrument, its calibration date, the weather and the position of every reading, and photograph the sensor in position. Officers reject measured evidence for missing metadata more often than for the readings themselves.

Mitigation that actually moves the number

In rough order of effect for a given scheme:

1. Reduce the mounting height and increase the number of columns. Trespass at a window falls faster with height reduction than with almost anything else, because the geometry improves.
2. Change the optic to a tighter distribution with a flat glass and a cut-off above the horizontal.
3. Add physical shielding on the house side: cowls, side shields, internal louvres. Specify the part number, because a shield fitted on site from offcuts does not perform as modelled.
4. Reduce output and accept the lower horizontal illuminance where the task permits it. A delivery yard lit for occasional reversing does not need the level a public car park needs.
5. Use a central management system to dim to a defined percentage at the curfew hour, and switch off anything that is not needed for safety.
6. Reposition or screen. A 2 metre acoustic fence already proposed for plant noise may double as a light screen, and planting does not count as mitigation in winter.

The sixth point is worth repeating. Deciduous planting between a luminaire and a window is not mitigation for a scheme that operates in December.

Securing the result by condition

A lighting condition that works names the document, the limits and the verification. Ask for all four of these:

• Approval of a lighting design drawing showing luminaire types, mounting heights, aiming angles, tilt and isolux contours at the relevant windows.
• The Ev and luminaire intensity limits to be met, with the environmental zone and the curfew hours stated in the condition itself rather than left to the guidance.
• A commissioning report, produced after installation, confirming the as-built aiming and a measured check at the nearest affected windows.
• No alteration to luminaire type, position, height or aiming without further approval.

Without the fourth limb the condition is unenforceable in practice, because the installation that gets built is the one the electrical contractor could source. A Lighting Assessment submitted with the application, setting out the zone, the curfew, the receptors and the predicted figures, gives the officer the material to write that condition properly.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Lighting assessment against the ILP guidance is work councils expect from a suitably qualified author, and professional review is strongly advised.

To scope the environmental reports a scheme needs, enter your postcode on PlanScope and work through the constraints it detects. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.