Friday, 18 December 2015

klaxon or blues and twos

I vaguely recall learning that emergency response vehicles—ambulances, fire-trucks, police cruisers—in the States at least bore complementary flashing lights in red and blue to shine at the most visible spectrum both during the day and at night—though I could not remember which colour was best for either condition.
In Germany (and for the UK as well), those same beacons are just blue—arrayed for each kind of dispatch a bit differently with distinctive sirens but only on the one wave-length. If the two-coloured light system had a higher visibility profile—I wondered, why it had not been adopted everywhere. It turns out that the insight into the discernability of different spectra—which figures in traffic lights as well, and the reason for Germany’s blue lights date to 1938, like much of the German infrastructure—the Autobahn and the people’s coach. It does not have anything to do with psychological colour associations or some Doppler effect, but rather anticipating the possibility of air-raids as Europe once again began to take a belligerent posture, field-engineers experimented with different colours and discovered that while red lights sent up a beacon high into the atmosphere, attracting the attention of bomber-pilots, blue light dissipated at a much lower altitude. The convention, like equipping windows on homes with Rollladen (roller shutters) to effectively black out the lights, has endured.