02 Aug 2006
Brazil is the place to visit if you want to enjoy the best blue sky.
A 27-year-old TV researcher who won a competition to travel the globe in search of the world's "bluest" sky has found that Rio de Janeiro in Brazil comes top of the list. Anya Hohnbaum, who visited 20 different destinations on a 72-day round-the-world trip organized by on-line travel agents Expedia, was equipped with a special portable spectrometer adapted for her by scientists at the UK's National Physical Laboratory (NPL).
The challenge for the NPL scientists, led by physicist Nigel Fox of the lab's optical-radiation group, was to adapt a typical spectrometer so that it would be robust and simple enough to be carried in a small suitcase around the world. The equipment consisted of a Hamamatsu spectrometer, a fibre-optic cable to feed light into the device, and a tripod. Light striking the spectrometer's grating was focused by a mirror onto a sensor.
At each destination on her trip, Hohnbaum pointed the portable spectrometer at the sky to measure the spectrum of light at that location. All measurements were taken at 10 a.m. local time and in the same direction relative to the Sun. She also had to ensure that the spectrometer was properly callibrated at each destination by shining a special LED torch onto the device from a fixed distance. Hohnbaum then sent the data to the NPL via e-mail before heading off to her next destination.
Back at the NPL, Fox and co-workers analysed the spectral power distribution at each destination. This analysis yielded a set of three numbers known as colorimetric co-ordinates, which show how much red, green and blue light would have to be mixed to recreate a particular overall colour. Two of these co-ordinates - by convention, these are always the red and green - were then plotted on a standard chromaticity diagram.
The data from Rio de Janeiro were found to be closest to the bluest part of this diagram - hence Brazil was deemed to have the bluest sky. Bay of Islands in New Zealand came in second, followed by Ayers Rock in Australia, while San Francisco came bottom. NPL says that its measurements, which can be traced to the SI system of units, could provide a new standard for measuring the colour of the sky.