National Physical Laboratory

Blue Sky Science

Blue Sky Science

Best Blue Banner

Scientists of NPL's Optical radiation measurement team, led by Dr Nigel Fox, have ensured that Expedia's blue sky explorer, Anya, was able to observe the world's best blue sky and unequivocally assign a set of internationally accepted colorimetric coordinates so that it can be fully defined.

Summary Report (PDF File PDF 425 KB)   |   Full Report (PDF File PDF 2.28 MB)

Anya took a spectrometer (an instrument used to measure the full spectrum of light and analysing its shape compared to standardised values) with her on her travels so that she could make measurements of the sky around the world! NPL ensured that the spectrometer was calibrated, thereby providing reliable measurements at each location. All data from the measurements made with the spectrometer, were sent back to NPL via email for processing and analysis.

and the winner is…                Brazil

The sky colour can be specified as: x = 0.2775 y = 0.2842 and is equivalent to a 'colour temperature' of 10,637 K (10,910 °C). A tungsten lamp has a colour temperature of around 3000 K. The test pictures from Brazil are shown below.

Best Blue Winner

Test pictures from Brazil

Full Results

The summary of the spectrometer based results, and the Best Blue Sky Ranking, is shown below. The centre of the chart shows each destination's chromaticity coordinates, on an extract of the chromaticity diagram. The pictures around the outside show the fisheye photographs of each destination in order of which has the Best Blue sky, running clockwise around the chromaticity diagram, starting with Brazil with the Best Blue Sky and San Francisco with cloudy hazy sky!

Best Blue Skies
Summary of the spectrometer results

The National Physical Laboratory (NPL)

NPL is the UK's national measurement institute and is responsible for the establishment and maintenance of all physical measurement quantities e.g. the kilogram, second and the metre. Any measurement made in the UK is ultimately traceable to NPL including light and colour.

The concept of colour and the ability to describe it in standardised terms dates back to 1931 and is based on work carried out by scientists at NPL in conjunction with Imperial College London during the 1920s. This colour system was adopted internationally and although refined, still remains the basis of modern colorimetry. (Colorimetry is the science that describes colours using numbers, or provides a physical colour match using a variety of measurement instruments).

If you require further information (beyond that contained in the reports above) please contact us


Further information on the NPL website
Beginners Guide to Colour
Beginners Guide to Light

About NPL
Find out more about NPL and its history.

Last Updated: 15 Dec 2015
Created: 26 Oct 2010


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