Standards labs like the NPL are interested in colorimetry -- the science of measuring colour -- because of its many useful applications. Brewers, for example, can judge the quality of their beer from its colour, while environmental scientists can monitor climate change by looking at the amount of green in satellite pictures. NPL decided to get involved to ensure that Expedia's competition made use of, and adhered to, international standards of colour measurement. The collaboration has also helped to make the public more aware of what the laboratory does.

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 enough 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 (figure 1).

At each destination on her trip, Hohnbaum was asked to point the portable spectrometer at the sky to measure the spectrum of light at that location (figure 2). All measurements were taken at 10 a.m. local time and in the same direction relative to the Sun. She also had to ensure 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, which is essentially of a plot of intensity versus wavelength. This spectrum was then converted into 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" (figure 3).

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 Cornwall in England came bottom (figure 4). 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.