Sky glow and upward light calculations
A flat-glass floodlight with no upward output on the datasheet emits light to the sky as soon as it is tilted. How the upward light ratio is calculated, and the evidence dark area councils want.
The upward light ratio calculation measures the proportion of a luminaire's total light output that is emitted above the horizontal, as installed, and it is the figure a council in a dark area will look for first. It is reported as a percentage, and for the darkest environmental zones the published maximum is zero. That sounds easy to meet with a flat-glass downlighter until you tilt it, at which point a luminaire with no upward output on the datasheet starts throwing light at the sky.
What upward light measures
Sky glow is the brightening of the night sky caused by light scattering in the atmosphere, and upward light is the part of it a scheme can be held responsible for. The Institution of Lighting Professionals Guidance Note 1 on the reduction of obtrusive light treats it as one of four obtrusive light effects, alongside light intrusion into windows, luminaire source intensity and building luminance.
Two measures get confused. A luminaire's upward light output ratio is a property of the product measured in the laboratory at zero tilt. The upward light ratio for a scheme is a property of the installation, counting the actual mounting angle, any tilt applied to reach the far corner of a yard, and the orientation of each unit. Only the second one answers the planning question.
Reflected light is the gap in both. Light that lands on a pale concrete yard and bounces upward contributes to sky glow and appears in neither number, so a scheme with a large area of light-coloured hard surfacing generates more sky glow than its calculated ratio suggests. Say so in the assessment rather than letting an objector point it out.
Calculating the ratio for a scheme
The arithmetic is simple and the inputs are where the work is.
- List every external luminaire with its total luminaire flux in lumens, taken from the photometric file rather than the lamp rating.
- For each one, apply the installed tilt and orientation and calculate the flux emitted above the horizontal plane.
- Sum the upward flux across the whole installation.
- Sum the total flux across the whole installation.
- Divide the first by the second and express it as a percentage.
Report the total installed upward flux in lumens as well as the ratio. A development with a 1 per cent ratio and 200,000 lumens of total output emits more light to the sky than one at 2 per cent with 20,000 lumens, and a ratio on its own hides that. Officers in dark areas have learned to ask for both.
Where existing lighting is being replaced, calculate the baseline the same way and present the comparison. Replacing twelve tilted 400W floodlights with sixteen flat-glass LED units at zero tilt can cut upward flux substantially while increasing the number of fittings, and the comparison is the strongest argument you have.
A Lighting Assessment should set the calculation out as a table, one row per luminaire type, with the quantity, flux, tilt and upward flux, and a total line. Not a single summary figure.
Dark landscapes and stricter limits
Guidance Note 1 sorts locations into environmental zones, from E0 for intrinsically dark areas such as designated dark sky sites, through E1 for relatively uninhabited rural areas and E2 and E3 for low and medium district brightness, to E4 for busy centres. The maximum upward light ratio falls to zero in the darkest zones and the published figures should be taken from the current edition.
Several National Parks and National Landscapes hold dark sky designations, and Exmoor, Northumberland and the South Downs are among the areas with adopted dark skies policies or supplementary planning documents. In those places the policy expectation goes beyond the national guidance: a presumption against new external lighting at all, a requirement to justify each luminaire against a specific need, and limits on correlated colour temperature as well as on output.
The National Planning Policy Framework (August 2026 edition) chapter on conserving and enhancing the natural environment asks authorities to limit the impact of light pollution from artificial light on local amenity, intrinsically dark landscapes and nature conservation. Planning Practice Guidance on light pollution sets out the factors a council should weigh.
Picking the zone is the judgement that decides the assessment. A farmyard 2km from a village, with no street lighting on the lane, is E1 whatever the applicant would prefer, and assessing it as E2 will be corrected at consultation.
Luminaire selection, tilt and shielding
Four specification decisions do nearly all the work.
Mount the luminaire with zero tilt and use an asymmetric optic to throw the light where it is needed. Tilt is the single largest cause of upward light, and a 15 degree tilt on an otherwise compliant floodlight can put the unit outside the limit on its own. Where a tilted unit is unavoidable, a top shield or a cowl brings the upward component back down, and the shielded photometric file must be the one used in the calculation.
Specify full cut-off or flat-glass optics rather than a bowl or a drop lens. Reduce the mounting height and increase the number of units, because lower columns need less intensity to reach the same ground illuminance. And keep the correlated colour temperature at or below 3000K in rural areas, with 2700K or amber sources where ecology is engaged, since shorter wavelength light scatters more in the atmosphere and is more disruptive to wildlife.
Bats are a planning issue in their own right here. Light levels that pass every amenity test can still sever a commuting route along a hedgerow or deter bats from a roost exit, and the joint Bat Conservation Trust and Institution of Lighting Professionals guidance on bats and artificial lighting sets out dark corridor principles. Where the site has hedgerows, mature trees or older buildings, a Preliminary Ecological Appraisal should inform the lighting layout rather than be written after it. Councils expect a suitably qualified author for ecological work and professional review is strongly advised.
Curfew hours and part night operation
A scheme that cannot meet the limits all night can often meet them for the hours that matter. Councils in dark areas routinely condition a curfew, and the mechanisms are well understood.
Full switch-off after a stated hour is the cleanest and suits yards, arenas and car parks that are unused overnight. Part-night operation, typically off between midnight and 5am, suits security lighting on an operational holding. Dimming to a lower output after curfew works where a minimum level is needed for safety. Presence detection with a short run-on time removes the baseline entirely for infrequently used areas, and it is the measure most often resisted and most often effective.
Equestrian arenas are the common rural case, and the usual condition package is a stated finish time, no use of the floodlights on specified days, columns no higher than a given height, zero tilt, and a requirement that the approved luminaires are not substituted. Offer that package yourself. An applicant who proposes a 9pm finish will usually get one, and an applicant who proposes nothing will get 8pm.
Agricultural buildings are the quiet offender. A livestock building lit internally with open sides or an open gable emits more light than any external fitting on the holding, and the fix is operational: doors and roller blinds closed after dark, lighting set below the eaves line, and opaque rather than translucent cladding on the elevations facing open country.
Evidence to submit in a dark area
A council in an E0 or E1 location will expect a lighting layout drawing keyed to a luminaire schedule, photometric datasheets for every unit with the upward light output ratio stated, the installed tilt and aiming point for each one, the upward light ratio and total upward flux calculation as a table, an isolux spill plot extending beyond the site boundary, and the proposed control and curfew regime.
Two additions make a real difference. Baseline night-time photographs taken from the nearest public viewpoint show what is actually there now, and they are more persuasive than any description of existing conditions. And a short lighting management plan, capable of being secured by condition, covers the matters a drawing cannot: who switches what off, how substitutions are controlled, and what happens when a unit fails.
Where the site sits in a prominent landscape, the lighting evidence should be read alongside a Landscape and Visual Impact Assessment prepared to the third edition of the landscape and visual impact assessment guidelines, with night-time receptors identified rather than daytime ones only.
PlanScope drafts a lighting assessment structured to these sections and marks missing site data as "TO BE CONFIRMED", so the photometric calculation still comes from a lighting designer. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. Browse the full set of report types on the planning documents index.
Frequently asked questions
What is the upward light ratio?
The proportion of a scheme's total installed luminaire flux that is emitted above the horizontal plane, expressed as a percentage. It is calculated for the installation as built, counting mounting angle, tilt and orientation, rather than taken from a product datasheet measured at zero tilt. For the darkest environmental zones the published maximum is zero.
Why report total upward flux as well as the ratio?
Because a percentage hides scale. A scheme at 1 per cent of 200,000 lumens sends more light skyward than one at 2 per cent of 20,000 lumens. Reporting the total installed upward flux in lumens alongside the ratio lets an officer compare the proposal with the existing situation and with alternatives on a fair basis.
Does tilting a floodlight breach the upward light limit?
It can, on its own. Tilt is the largest single cause of upward light, and a 15 degree angle applied to an otherwise compliant flat-glass unit may put it outside the limit. Use an asymmetric optic at zero tilt instead, and where a tilted unit is unavoidable fit a top shield and use the shielded photometric file in the calculation.
What counts as a dark sky area for planning purposes?
Zones E0 and E1 in the Institution of Lighting Professionals guidance cover intrinsically dark and relatively uninhabited rural areas. Several National Parks and National Landscapes hold dark sky designations, and areas such as Exmoor, Northumberland and the South Downs have adopted dark skies policies that go beyond the national guidance.
Will a council accept a curfew instead of a redesign?
Often, yes. Full switch-off after a stated hour, part-night operation between midnight and 5am, dimming to a lower output, or presence detection with a short run-on time are all conditionable. Propose the regime yourself with a finish time that matches how the site is actually used, because an applicant who proposes nothing gets the council's figure.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.