The Past, Current and Future Colour of Outdoor Lighting of Japan-Standing out from the Rest of the World

Looking at the photos of cities at night taken by NOAA’s DMSP satellite, one can only be overwhelmed by the accidental beauty of the lit urban areas that signify the human presence and progress1). Indeed in the eyes of an observer in space, the light from cities at night is the only evidence of our existence and the lighting technologies are the only visible human products. A global picture of these lights, as composed by the satellite reveals the most developed places on the planet in terms of economic and industrial activity. It is also true that light reaching the skies signifies waste and a kind of pollution that affects the lives of all living things so the pressure is on by various groups to minimise this waste for both economic and environmental reasons. Perhaps in future decades this will be achieved and light will be directed towards our activities and living spaces but not upwards. But what another fascinating thought is triggered by this about the time we live in. A couple of hundred years ago and a couple of hundred years from now the planet did not and will not look like that at night. However this image that most people are familiar with is panchromatic so individual colours (and lighting technologies) cannot be distinguished. Now, thanks to photographs taken by the astronauts of the International Space Station2)3) over the past decade we can see the lit cities at night in colour. The colour they emit is due to the dominant lighting technology that is employed by the city for outdoor lighting. The technologies have certainly changed over time. From burning oil and gases we moved to the incandescent lamp when electricity penetrated our lives. The next technology that was adopted for outdoor lighting was the one based on mercury (low pressure fluorescent in the 30 s and high pressure in the 50 s). But the light sources that dominated outdoor lighting due to their record high efficacy values were the sodium vapour discharge lamps. The low pressure version reaches 200 lm/W as its monochromatic yellow emission at 589 nm is near the peak of the human eye photopic sensitivity curve4)5) and high pressure versions can reach more than a 100 lm/W. On the other hand most mercury vapour lamps reach values well below 100 lm/W. A look at all these photos2)3) from space verifies the known fact that sodium is the undisputed king of outdoor lighting and that the light source technologies based on this element dominate the adorned face of the