1 · The sky we're losing · In depth
Light pollution, in depth.
The home page covers the basics: what it is, why white LED makes it worse and what we lose with it. Here is the why behind each point: the physics of skyglow, how a sky is measured, what the law says and what each of us can do. The sky in the background is tonight's sky over Valladolid, and it changes as you read.
01 · The physics of skyglow
Light that goes up doesn't leave: it stays overhead.
Two ways up
A streetlight sends light into the sky in two ways. Directly: the part of its output emitted above the horizontal, the upward light output ratio (FHS in Spanish regulations). And by reflection: the light that reaches the ground and bounces off asphalt, pavements and façades. The regulation adds both into the total upward flux, FTA = FHS + FHR (ITC-EA-03). That is why shielding is not enough: a street with twice the light it needs still sends twice as much into the sky, even if it gets there on the rebound.
Why the sky lights up
Light heading upwards hits air molecules and suspended particles —dust, moisture, pollution— and part of it comes back down from every direction. That is skyglow: the sky turned into a lit screen. Molecules scatter short wavelengths far more, in inverse proportion to the fourth power of the wavelength (Rayleigh scattering): 450 nm blue is scattered almost three times more than the 589 nm yellow of sodium. It is why the sky is blue by day, and why at night the glow of a white LED reaches further.
And at night, the eye sees bluer
With dark-adapted vision we see with our rods, which are more sensitive to blue-green than the cones. Lumen for lumen, a 4,000 K white LED brightens the sky we perceive about 2.5 times more than the high-pressure sodium it replaces (Falchi et al., 2016). Try it: the sky in the background changes with the streetlight.
Same streetlight, same output:
- Sky brightness
- 19.00 mag/arcsec²
- Artificial glow
- 14.8 × the natural sky
- Naked-eye stars
- 529
How far it reaches
A town's glow can be seen from far away and grows with its size. Merle Walker measured it around the cities of California in 1977 and summed it up in a rule still in use: the brightness a city adds to the sky, looking towards it at 45° elevation, is proportional to its population and falls off with distance to the power of 2.5. Pick a population size:
- More artificial than natural light
- Adds more than 10%
- Adds more than 1%
- Pristine sky
Walker's law: I = 0.01 · P · d−2.5, with P the population and d the distance in km. It is an empirical rule, meant for distances larger than the town itself: it gives the order of magnitude, not the value at a particular spot.
02 · How a sky is measured
A number for darkness: magnitudes per square arcsecond.
Sky brightness is measured like a star's: in magnitudes, per patch of sky one arcsecond on a side. It is a reversed, logarithmic scale: the higher the number, the darker the sky, and each point is a factor of 2.512 —five points, a factor of 100—. A sky with no artificial light at all reads about 22.0 mag/arcsec²; a city centre, between 17 and 18: 40 to 100 times brighter.
It is measured with a photometer pointed at the zenith, on a night with no Moon or clouds. The most widespread is the SQM (Sky Quality Meter), about the size of a remote control; Astroval uses it to certify its observation sites. With the naked eye, the measure is the faintest star you can make out, and from it comes the number of stars in the sky.
The Bortle scale
In 2001 the amateur astronomer John Bortle proposed describing a sky by what you can see in it, in nine classes: from 1, the pristine sky, to 9, a city centre. Move the scale and watch the sky in the background.
Bortle described his scale by what can be seen, not by a number; the equivalence in mag/arcsec² and the star counts are those of the VillaLuz model (Schaefer's relation, 1990), the same as on the home page.
Class 1Pristine sky
The zodiacal light and the gegenschein are visible; the Milky Way is so bright that the Scorpius–Sagittarius region casts diffuse shadows, and the Triangulum Galaxy is a naked-eye object.
- Brightness
- ≥ 21.99 mag/arcsec²
- Naked-eye stars
- 4,557
03 · What the law says
The rules already ask for a better sky than the one we have.
In Castile and León two sets of rules apply at once: the national one —the Regulation on energy efficiency in outdoor lighting installations (Royal Decree 1890/2008) and its technical instructions, the ITC-EA— and the regional one, Law 15/2010 on the prevention of light pollution. Where both set a limit, the stricter one applies.
Each zone, its limit
ITC-EA-03 divides the territory by the darkness it deserves and limits, in each zone, how much light may escape upwards:
| Zone | What it is | Maximum upward light (FHS) | Maximum light trespass on a window |
|---|---|---|---|
| E1 | Dark surroundings or landscapes: astronomical observatories, national parks and protected natural areas, with unlit roads. | ≤ 1% | 2 lx |
| E2 | Low brightness: outskirts, non-developable land and rural areas with lit roads. | ≤ 5% | 5 lx |
| E3 | Medium brightness: urban residential areas, with lit roads and pavements. | ≤ 15% | 10 lx |
| E4 | High brightness: town centres and commercial and leisure areas busy at night. | ≤ 25% | 25 lx |
Values from the 2008 ITC-EA-03, tables 2 and 3. In Castile and León no luminaire may send more than 3% upwards, whatever the zone, except when lighting historic or artistic heritage (Law 15/2010, art. 13.1.a).
What Castile and León adds
- No more than 3% upwards: the law bans luminaires sending more than 3% of their output into the upper hemisphere, except those lighting heritage of special historic or artistic interest (art. 13.1.a).
- No skybeams or lasers: searchlights emitting above the horizontal and aerial night-time advertising are banned (art. 13.1.b and c).
- Decorative lighting, off at 23:00: façades, monuments and illuminated signs switch off at eleven at night; an hour later on Fridays, holidays and their eves, or at closing time (art. 12).
- Local zones: municipalities of more than 20,000 inhabitants zone their own territory and may tighten the limits by by-law, never relax them (art. 6).
Colour and quantity
Colour: national rules set no general cap on colour temperature; they only require, in E1 zones, sodium lamps or filtering out light below 440 nm (ITC-EA-03 § 1.2). The 2021 reform draft proposed a 4,000 K ± 300 ceiling, but it has not been approved; the reference recommendation is the American Medical Association's (2016): no more than 3,000 K. Quantity: lighting may not exceed the level set for each road by more than 20% (ITC-EA-02). Over-lighting is not playing it safe: it is non-compliance.
We quote the text in force: Royal Decree 1890/2008 with its ITC-EA-02 to 07 as published, and the ITC-EA-01 approved by Royal Decree-law 18/2022. In 2021 the Ministry put a stricter reform out for consultation — with a zone E0, 5% skywards in E3 and a 4,000 K cap — which circulates in much material, but it has not been approved and is not binding. In Castile and León, in any case, the regional law's 3% prevails.
04 · What a town council can do
Six decisions, from the cheapest to the finest.
The figures are those of the high street of VillaLuz, the village in our simulator, computed with its engine. We start from what still lights many streets: globe streetlights of 12,000 lm, 4,000 K white light, 22% of the output going skywards and on at 100% all night.
- Starting point
- 18.43 mag/arcsec²Bortle 83,116 kWh/year
- 01
Know what's there
An inventory of every light point: luminaire type, upward light, colour temperature, power and schedule. Without it there is no plan, and with it, lamps nobody needs tend to turn up.
- 02
No light upwards
Shielded luminaires, with 0% of the output above the horizontal. All the light that went to the sky now lands on the street… and is too much.
- Sky
- 19.15 mag/arcsec²Bortle 6-7
- Stars
- 602
- Consumption
- 3,116 kWh/year0% less
- 03
Only the light the rules require
Fewer lumens per point and better spread, up to the level of each road's class and no more. It is the step that saves the most energy.
- Sky
- 20.35 mag/arcsec²Bortle 5
- Stars
- 1,599
- Consumption
- 938 kWh/year70% less
- 04
Warm light
From 4,000 K to 2,200 K, amber. Less blue to be scattered and to be seen by the dark-adapted eye. It costs some efficacy —a warm LED gives fewer lumens per watt—, which is why consumption goes up a little.
- Sky
- 21.24 mag/arcsec²Bortle 4
- Stars
- 2,950
- Consumption
- 1,158 kWh/year63% less
- 05
Dimming after midnight
Time-based dimming: 100% until midnight and 50% for the rest of the night, when hardly anyone goes by.
- Sky
- 21.56 mag/arcsec²Bortle 3
- Stars
- 3,565
- Consumption
- 833 kWh/year73% less
- 06
Presence detection
In the small hours the lamp stays on standby and goes to full for 90 seconds when someone passes. Computed with four passers-by per hour.
- Sky
- 21.89 mag/arcsec²Bortle 3
- Stars
- 4,309
- Consumption
- 230 kWh/year93% less
05 · What you can do
At home, too, the sky goes out.
- 01
Outdoor light, pointing down
Fittings closed at the top that light the ground, not the sky or your neighbour's window. No globes, no floodlights aimed upwards.
- 02
Warm and just enough
Bulbs of 2,700 K or less, and low in lumens: seeing where you step needs far less light than it seems.
- 03
Only when needed
A timer or a motion sensor on the garden, patio or porch light. Left on all night, it lights nobody's way.
- 04
Close the blinds
Indoor light also gets out and goes up. With the blinds down or thick curtains, you also sleep better.
- 05
Measure your sky
Globe at Night is a citizen-science project: you count the stars of a constellation from home and upload it. Its data showed the sky getting 9.6% brighter every year (Kyba et al., 2023).
globeatnight.org ↗ - 06
Ask your town council
Streetlights belong to the municipality. Bring them this page, or the simulator: it shows how much they save and how much sky they get back.
Request a simulation of your town →
Sources
- 1.F. Falchi et al. (2016). «The new world atlas of artificial night sky brightness». Science Advances 2, e1600377. doi.org/10.1126/sciadv.1600377
- 2.M. F. Walker (1977). «The effects of urban lighting on the brightness of the night sky». Publications of the Astronomical Society of the Pacific 89, 405. doi.org/10.1086/130142
- 3.J. E. Bortle (2001). «Introducing the Bortle Dark-Sky Scale». Sky & Telescope 101 (2), 126. skyandtelescope.org/wp-content/uploads/BortleDarkSkyScale.pdf
- 4.B. E. Schaefer (1990). «Telescopic limiting magnitudes». Publications of the Astronomical Society of the Pacific 102, 212. doi.org/10.1086/132629
- 5.Real Decreto 1890/2008 (2008). «Reglamento de eficiencia energética en instalaciones de alumbrado exterior y sus instrucciones técnicas complementarias EA-01 a EA-07». BOE-A-2008-18634. www.boe.es/buscar/act.php?id=BOE-A-2008-18634
- 6.Real Decreto-ley 18/2022 (2022). «Disposición final: nueva ITC-EA-01, eficiencia energética, en vigor desde el 1 de enero de 2023». BOE-A-2022-17040. www.boe.es/buscar/act.php?id=BOE-A-2022-17040
- 7.Ministerio de Industria, Comercio y Turismo (2021). «Proyecto de Real Decreto por el que se aprueba el Reglamento de eficiencia energética en instalaciones de alumbrado exterior y sus ITC-EA-01 a EA-08». Información pública, 21/07–30/09/2021; no aprobado.
- 8.Ley 15/2010 de Castilla y León (2010). «Ley de prevención de la contaminación lumínica y del fomento del ahorro y eficiencia energéticos derivados de instalaciones de iluminación». BOE-A-2010-20074. www.boe.es/buscar/act.php?id=BOE-A-2010-20074
- 9.C. C. M. Kyba et al. (2023). «Citizen scientists report global rapid reductions in the visibility of stars from 2011 to 2022». Science 379, 265–268. doi.org/10.1126/science.abq7781
Titles are quoted as published. The VillaLuz figures, sky brightness and star counts are our own calculations with the simulator's engine.
And the sky over your town?
We simulate your town's lighting with its real data and tell you how much sky and how much energy can be recovered.