LIGHTING AND DARK SKIES

For Cranborne Chase AONB

Reaching for the Stars Seminar 6/11/14

Howard Lawrence

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Campaign for Dark Skies

Content• Observation – ideal conditions and eye response• Causes of sky glow• Luminaire design• Blue-rich white light• Highways agency specification changes• Domestic / commercial lighting• Checklist for good lighting

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Desirable Observing Conditions

• Astronomers seek high contrast – very low sky-glow due to (mostly street) lights

• Clear skies – no residual vapour trails• Steady seeing – low wind and thermal currents• Good observing site – elevated, accessible, screened

from parking / road / stray lights, good amenities • Potential to use dark adapted (& averted) vision

Visual and imaging observers have different perspectives. Sky gazers want it all, imagers less concerned with contrast / sky-glow and unlikely to need to become dark adapted

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Visual Observation

Photopic

colour – daylight, rods and cones Mesoptic

colour - twilight, rods and cones illuminated condition – luminance 0.001 to 3 cd/m2

Scotopic monochrome - green/blue sensitive rods mostly

levels correspond to moonless night

Dark adaption

20 -30 mins, slower for old people

Human eye spectral sensitivity

Eye registers full dark to bright sun, luminance range of 1 billion, contrast range only 1000

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Scotopic / Photopic (S/P) ratio is measure of blueness in white light

Sky-glow caused by atmospheric scattering

• Mie scattering• All colours• Particles / vapour• Local• Forward & side

scattering

• Rayleigh• Blue dominant

• function is inverse to power of 4• 16 times more for blue than red

• Molecules• Distant / overhead• All round scattering

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Poorly aimed street lighting

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Sky glow Vs cut-off angle –results from Dr C Baddiley research

A 180-100° Critical area for sky glow from within urban areas but proportionally less impact to rural areas.

B 100-95° Significant contributor to sky glow, especially in rural areas where it is most aerosol dependent. Less likely to be obstructed.

C 99-90° Critical zone for sky glow and obtrusion seen at 10s of km (in rural areas) where it is strongly dependent on aerosol scattering.

D 90-70° Significant contributor to sky glow seen at a distance through reflection but reflected light more likely to be obstructed by buildings, trees and topography.

E 70-0° Ideal light distribution.

A

B

C

D

E

Gamma cut-off angle for globe light

65 70 75 80 85 90 95 100 105 110 115 120 125 130% contibution to max sky glow Average over

all elevs

1.3 1.4 1.8 2.2 3.0 5.6 27.5 44.9 58.9 69.5 78.5 84.1 88.2 91.9

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Good street lighting

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Blue-rich white light• The proportion of blue in the white

light is quantified by the Colour Correction Temperature CCT

• Many recent white light sources colour temperatures > 5500K

• Reflection from grass scatters light into atmosphere

• 5 to 8 times more sky glow than orange sodium equivalent

• Models predict more sky glow in countryside due to spill from distant cities with new white lights

• IDA and CfDS recommends CCT < 3000K

• (incandescent domestic bulbs circa 2700K so not proposing mono-chrome orange pink)Extracted from IDA White Paper on BRWL

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Highways Agency Draft TD 34/07DESIGN OF ROAD LIGHTING FOR THE STRATEGIC MOTORWAY AND ALL PURPOSE TRUNK ROAD NETWORK

• New version nearing publication• Environmental Impact Assessment

• Comprises Energy and Carbon Consumption and Dark Sky Preservation assessment factors

• Uses results from Dr C Baddiley sky-glow modelling research

• Stresses that clients need to supply their own weighting factors for balancing:• Economic• Environmental

• zones • Scotopic/Photopic ratio (BS5489-1)

• Societal

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Highways Agency Draft TD 34/07‘Dark Sky Preservation Indicator’ •Calculation starts at 100% with points subtracted for unfriendly features / specs.•Geometry - less than 70° is ‘safe’ otherwise negative•Spectral weighting – points dropped x S/P ratio•Environmental Area – weighted according to GN01:2011 zones E0 – E4•Luminaire lens profile - rating doubled if not flat glass

Fisheye all-sky photo showing milky way over Gloucestershire – C.Baddiley

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Highways Agency Draft TD 34/07 Extract

‘The client supplied weighting are designed to reflect society’s values. However, schemes will by their very nature often impact the local stakeholders in a different way to that of the wider society, and, local stakeholders will place different priorities on some aspects of the scheme. Designers need to manage the risk to the client of local stakeholders objecting to the scheme due to light pollution issues, either obtrusive light or sky glow, regardless of the environmental zone as the impact of light pollution can carry for many miles *. While these aspects may often not carry much overall weighting within the assessment, they can be easily and cost effectively managed. The designer should be encouraged to not only to provide excellence of design in terms of the sustainability assessment , but directed to minimise delivery risk by ensuring the scheme is not identified or perceived by local stakeholders as unnecessarily impacting their environment or quality of life.’

*CfDS proposed wordingExtracted table from TD34/07

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Commercial and Domestic Lighting

• Measure can be sold as good practice for both local sky glow reduction and wildlife protection, e.g. moth disorientation, bird migration disruption

• Churches could lead by example• Good lighting guide leaflets are provided by some

vendors for new installations.• Would retrofit / modification guide be useful?

Consistent with ecological principles.

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Good Lighting ChecklistLuminaire - modern full cut-off, energy efficient lights, fitted

with shields where light trespass is an issue Colour - warm white or orange colour lights with CCT of less

than 3000KControl - remotely controllable lighting technology allowing

lights to be dimmed or turned off completely if local conditions and policies permit

Brightness - the minimum luminous intensity consistent with relevant codes of practice taking care not to over-specify; for example, due to maintenance and colour rendering factors

Design - lighting schemes should be designed by lighting professionals given the brief to minimise sky-glow both locally and into the surrounding country

Community - householders and businesses - guidance to achieve star-quality results by taking extra care when installing flood and other lighting

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