Sky glow, dark skies and upward light ratio

Upward light ratio is the proportion of a luminaire's output emitted at or above the horizontal, with the fitting in its installed position. It is the parameter most rural lighting conditions are written around, and it is a useful measure. It is also incomplete, because sky glow upward light ratio figures capture only the light a fitting sends up directly, and say nothing about the light that goes down, hits a pale wet surface and comes straight back.

On a dark rural site, that second path often matters more than the first.

What upward light ratio measures

The figure comes from the luminaire's photometric data and depends on two things: the optical design, and how the fitting is mounted.

A full cut-off luminaire with flat glass, mounted horizontally, emits effectively nothing above the horizontal. The same luminaire on a bracket with a five degree tilt does, and the figure rises quickly with angle. Decorative lanterns with exposed lamps or curved refractors emit above the horizontal by design.

So the parameter is a property of the installation, not of the product. A specification that names a fitting with zero upward light and then shows it on a tilted bracket has not delivered zero upward light, and a competent reviewer will spot it on the elevation.

Direct upward light against reflected light

Divide the problem in two.

Direct upward light is what upward light ratio controls. It is straightforward to design out with the right optics and the right mounting angle, and there is rarely a good reason not to.

Reflected light is the part that escapes the parameter. Light delivered to a surface is partly absorbed and partly reflected, and the proportion reflected depends on the surface. Wet tarmac after rain, pale concrete, light gravel, white render, polytunnel film and glasshouse roofs all return a substantial share of what lands on them.

That leads to a design rule that gets missed: the most effective way to reduce sky glow from reflection is to put less light on the ground in the first place. Lower illuminance with good uniformity, delivered only where it is needed and only when it is needed, beats a high output scheme with immaculate optics.

Dark sky designations and their policies

A number of areas in the United Kingdom hold dark sky designations, and many National Parks and National Landscapes have lighting policies in their management plans and in local plan policy.

Where a site sits within or adjacent to one, expect the darkest environmental zone to apply, expect zero direct upward light to be a starting point rather than a target, and expect curfews to be sought. Expect also that the authority will have a consultee who reads lighting reports properly, which is not true everywhere.

Adjacency matters as much as being inside the boundary. A scheme half a mile outside a designated area, on rising ground, can be more visible from within it than something inside the boundary in a valley.

Worth understanding too that designations are held by the bodies that manage the areas rather than granted by the planning system. They carry weight through local plan policy and through the management plan, and an officer will expect an applicant to have read both rather than to have noticed the designation on a map.

Luminaire selection for zero upward light

The specification that works in a rural setting is short and unglamorous.

• Full cut-off optics with flat glass, mounted horizontally, with the mounting angle stated on the drawing.
• The lowest column or bracket height that achieves the required uniformity, since lower mounting reduces spill.
• Warm correlated colour temperature, specified as a figure, with amber sources considered in the most sensitive settings.
• Output matched to the task rather than to the largest fitting in the range, with dimming available.
• Accessories such as louvres or shields where a specific view has to be protected.
• Controls: time clocks, photocells and motion detection, with the strategy described rather than implied.

Avoid uplighting of buildings and trees entirely in these settings. It is the single clearest signal to a consultee that the scheme has not engaged with the policy, and it is almost never necessary.

Bollard lighting is the usual alternative offered for paths, and it is better than a column but not automatically good. A bollard with a clear diffuser throws light horizontally into eyes and hedgerows; one with a shielded downward distribution does not. Read the photometric data rather than the product photograph.

Surface reflectance and unintended sky glow

Surfaces are part of the lighting design, and treating them as a landscape matter is how reflected light gets ignored.

Darker surfacing reflects less. A permeable gravel yard in a mid tone returns less than pale concrete. Matt finishes return less than polished ones. Where a scheme involves glasshouses or polytunnels, the film and glass are effectively mirrors at night and internal lighting behind them becomes external lighting, which is why blackout screening is routinely conditioned on horticultural schemes.

Say something about this in the assessment even where it is not asked for. A report that identifies reflected light as a contributor, and proposes surface and screening measures alongside the luminaire specification, reads as though a person thought about the site.

Curfews in rural environmental zones

Curfews are the most common condition and the most commonly breached.

Design them against the operation. A farm yard needs light at milking, a livery yard needs it at evening turnout, and a rural business needs it at the end of a shift. A blanket switch-off at 11pm may suit a car park and be unworkable on a working holding.

Seasonal variation is the part conditions usually miss. A yard that needs no lighting in June needs it from mid afternoon in December, and a condition written in summer around observed practice will fail its first winter.

Propose the curfew yourself, with the hours you can actually keep, and offer motion detection outside those hours for security. That is a far stronger position than accepting a condition you will breach. The same reasoning applies to car park lighting, illuminance and curfews.

Evidencing compliance in a rural application

Written assurances do not survive consultation. Submit the evidence.

1. A lighting layout showing every luminaire, its position, mounting height and mounting angle.
2. Photometric data sheets for each luminaire type, with the upward light ratio stated for the installed position.
3. Correlated colour temperature for each source.
4. Modelled illuminance contours across the site and at the boundary, with vertical illuminance at any nearby dwelling.
5. The environmental zone assumed, with the justification for it.
6. The control and curfew strategy, described in operational terms.
7. Surface finishes and any screening proposed, cross-referred to the landscape drawings.

A Lighting Assessment pulls that together. PlanScope can produce an AI-assisted draft structured to those sections, with every photometric and modelled input marked as to be confirmed until the lighting designer supplies it. Councils normally expect a suitably qualified author for this work, and professional review before submission is strongly advised.

On a prominent rural site the lighting case usually sits alongside a Landscape and Visual Impact Assessment, and night-time views from public vantage points are worth including in both. Browse the 66 report types for the rest of the pack.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports the site needs.

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