Light pollution reduction in central Slovenia (around Ljubljana) – important results in just 2 years!

Light pollution reduction in central Slovenia (around Ljubljana) – important results in just 2 years!

As you already know Ljubljana has changed a lot of public lighting. Most of the main roads (avenues) are already replaced by fully shielded luminaires. Power reduction by 40% was done just on that main roads (from 250W to 150W), on other roads we do not see significant power reduction.

It is hard to say how many luminaires in Ljubljana are fully shielded right now, we guess that 40% of total energy of public lighting in Ljubljana is now emitted by fully shielded luminaires.

Measurements of light pollution: summer 2009 vs. summer 2007 (before Slovene Light Pollution Law):

1. Zaplana astronomical observatory, 22 km from line distance from center of Ljubljana ( Now sky over Zaplana is about 0.15 magnitude darker, Milky way is brighter, all members of astronomical club UAD claims that the sky is better now. Light dome of Ljubljana is about 30% smaller than 2 years ago. Limiting magnitude at Zaplana observatory is now about 21.20.

2. Črni Vrh astronomical observatory, 37 km from Ljubljana in similar direction as Zaplana observatory. Herman Mikuž reports that the sky ( is now 0.10 magnitude darker than 2 years ago. Limiting magnitude at Črni Vrh observatory is now about 21.25.

In both cases uncertainty of estimation is +-0.05 magnitude, measurements were done by Sky Quality Meter without lens. Unfortunately we cannot provide reliable photos of light domes, because we didn’t have the same camera & lenses all the time. But on all-sky photos light pollution reduction is visible. We are in process to develop special software for reduction of all-sky photos.

We think that this is a rare example of documented light pollution reduction on large scale (Ljubljana has about 270.000 inhabitants). Ljubljana is exception, because they changed to many luminaire in a very short time. Other municipalities in Slovenia are more slow, according to Slovene Light Pollution Law municipalities must change all public lighting into fully shielded until December 31, 2016. We got many reports that in municipalities power reduction of -60% is very common.

The main reason of light pollution reduction in central Slovenia are retrofitted fully shielded luminaires. Power reduction of few roads is second, less important reason.

We do not see many changes of other outdoor lighting in Ljubljana (churches, private buildings, shopping centers, commercial buildings). Several billboards were switched off, many of them replaced illumination from bottom to top.

This result in Slovenia is a clear confirmation that so called “box theory” of light distribution promoted by a few lighting experts does not work at all! Results in Slovenia confirm that it is very important that luminaires are full shielded.

We are happy right now with the results and we are sure that our sky will be darker in next years – if new LED and metalhalogen white light will not spoil the complete process!!!