Specifying luminaires to control sky glow

Upward light ratio luminaire specification is the part of a lighting condition that decides whether a scheme actually controls sky glow, and it is where most schemes quietly fail. Upward light ratio is the proportion of a luminaire's output emitted above the horizontal, measured in the attitude the luminaire is installed in. A flat glass unit with a published figure of zero can emit five or ten per cent once it is tilted five degrees to reach across a carriageway. The number on the datasheet is the number at zero tilt, and the number in the condition has to be the number on site.

What upward light ratio measures

Guidance Note 1 from the Institution of Lighting Professionals sets limits on obtrusive light by environmental zone, running from E0 for intrinsically dark areas such as designated dark sky places, through E1 for rural areas with no district brightness, to E4 for city centres with high ambient brightness.

Upward light ratio limits tighten sharply as the zone darkens. In the darkest zones the expectation is effectively no direct upward light at all. In an urban zone a modest percentage is tolerated because the background is already bright.

Two things follow. First, you cannot specify luminaires until you know which zone the site sits in, and that is a judgement the local authority makes, sometimes with a published position in a local plan or supplementary planning document. Second, upward light ratio is only one of the limits in the guidance. Vertical illuminance at windows, luminous intensity towards receptors and building luminance are assessed separately, and a scheme can satisfy the upward light limit and still cause a nuisance to a bedroom window.

Flat glass against bowl optics

A flat glass luminaire with a controlled optic places the light where it is wanted and emits nothing above the horizontal when mounted level. That is the starting point for any scheme in a dark zone.

A bowl or drop lens optic emits above the horizontal by design, because the refractor sits below the lamp and spreads light widely. It spreads light onto the wall of a building, which can be useful for perceived safety, and it spreads light into the sky, which is not.

The trade is real rather than one-sided. A full cut-off scheme in a residential street puts almost all the light on the road surface and very little on faces, which some residents dislike. Say that in the lighting report rather than pretending the choice is free.

Mounting height, spacing and tilt

These three move together, and the relationship is the heart of a sensible specification.

Raise the mounting height and each luminaire covers more ground, so spacing increases and the number of columns falls. Raising the height also means the optic can point further down, so tilt reduces, so upward light falls. Against that, a higher column is more visible in the landscape and throws light over a wider area including places you do not want lit.

Lower the mounting height and you need more columns and more tilt to make the spacing work, and tilt is what destroys the upward light figure.

1. Fix the environmental zone and the upward light limit first.
2. Set the maximum tilt you are prepared to accept, which in a dark zone is usually zero.
3. Choose the mounting height and optic that meet the lighting class at that tilt.
4. Work out the spacing from the photometric calculation rather than from a rule of thumb.
5. Only then check the column count against the budget, and go back to step three if it does not work.

Doing it in the opposite order, which is what happens when a contractor values engineers the scheme, produces a design that meets the lighting class and breaches the condition.

Colour temperature and how policy treats it

Correlated colour temperature is now as contentious as upward light, and for two separate reasons.

Blue-rich light scatters more in the atmosphere, so a 4000 kelvin installation produces more visible sky glow than a 3000 kelvin one of the same output. And blue-rich light has a stronger effect on nocturnal wildlife, particularly bats and night-flying insects, which is why ecological guidance on lighting and bats points towards warm sources at 2700 kelvin or below, with amber sources near sensitive corridors.

The figures above reflect what authorities commonly ask for rather than a national standard, so check the local plan policy and any ecological report for the site before you specify.

Controls that cut hours as well as lumens

A luminaire that is switched off is the only one with no upward light at all, which makes control strategy part of the specification rather than an afterthought.

Part night operation suits footpaths and car parks that empty at a known time. Dimming profiles suit adopted highways, where the lighting class can be relaxed at low traffic flows. Presence detection suits service yards and rear access ways, provided the sensor is set with a sensible hold time so it does not strobe all night.

Write the regime into the schedule with times and levels. A condition that says lighting will be controlled, with no detail, is unenforceable and both sides know it.

Writing the luminaire schedule for a condition

A dischargeable lighting condition needs a schedule that a planning officer can check against an installation. Give, for each luminaire type: the manufacturer reference, the lumen output and circuit wattage, the optic or distribution type, the correlated colour temperature, the upward light ratio in the installed attitude including tilt, the mounting height and the control regime.

Add the drawings: a layout showing every luminaire position and column height, an isolux plot across the site and over the boundary, and a vertical illuminance check at the nearest sensitive windows.

Scale this to the scheme. A domestic driveway does not need a photometric model. A commercial yard adjoining houses, a sports pitch or a scheme beside a dark landscape does, and that work belongs in a lighting assessment. Where the lighting is also a landscape issue, because columns and glow are visible from a valley or a ridge, it feeds the landscape and visual impact assessment as well. The mechanics of sky glow itself are covered in sky glow, dark skies and upward light ratio.

Verification after installation

Conditions are discharged on evidence, and the evidence has to come from the finished installation.

Commission the scheme and record the as-built tilt angle of each luminaire, because site conditions move brackets. Take vertical illuminance readings at the agreed receptor positions after dark, with the readings logged with date, time, instrument and weather. Photograph the installation from the agreed viewpoints.

One honest point to put in the report. Measured obtrusive light depends on atmospheric conditions and on what else is lit nearby, so a single set of readings is a snapshot. Offer a repeat survey after a season rather than claiming the first set settles the question.

Draft the lighting section of your application on PlanScope and send it to a qualified reviewer before submission.

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

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.