Dashboards. Instrument panels. Every day we look at the arrangement of speedometer, rev counters, clock and other instruments. Sometimes they’re on, sometimes they aren’t. None of these instrument panels is the same, and especially during the past 50 years true marvels of more or less tasteful dashboard lighting and designs have emerged. In this short series we will introduce some of them to you. Let’s start off with an extremely rare specimen – high-voltage lighting in a 1966 Dodge Charger.
A greenish blue color, glare-free, dimmable and simply beautiful. The four circular dash pods in my friend Ralf’s Dodge Charger light up the dark garage like some spaceship out of Star Wars, but the car is from 1966 and definitely too old for this type of science fiction. No, they’re no bulbs, the furniture maker says. If it had been bulbs, he wouldn’t have had to spend more than 80 hours repairing the high-voltage transformer and the films behind the round instruments. What? Transformer? Film? Where on earth have I ended up?
Let me elaborate a bit. Georges Destriau actually just wanted to test the conductivity of various alloys when he applied a strong electric field to a zinc sulfide strip in the labs of nuclear physicist Marie Curie 30 years before the Charger was built. The strip had been accidentally contaminated with copper – Georges had forgotten again to wash his hands – and the strip started … to glow. Oops. More than two decades before the invention of the light emitting diode they found this material which could be lit up using high voltage. Completely and utterly without a filament.
So how does that happen? Let’s look at the physics. The electroluminescent material is zinc sulfide doped with other metals. It sits insulated between two electrodes in the form of very thin films, like in a capacitor. By applying a strong alternating field, electrons are accelerated in the zinc sulfide which reach an excited state by colliding with the metals from the doping (another word for contamination). You can probably relate. If someone forces you to brake and you crash into them you’ll also be in an excited state.
Once the electrons leave this state, energy will be released and the film lights up. If the other person involved in the accident shows you their insurance card your eyes will also light up. Got it?
Later on electroluminescence films were used in airplane cockpits and for illuminating the wings. And in cars as an experiment.
When the first Dodge Charger with this film came on the market, the light emitting diode (LED) had already been around for four years. Another one of those lamps without a filament but not very advanced yet back then. The fairly complex design of the film, its price and its sensitivity quickly snuffed out this form of speedometer illumination. Just ask Ralf; re-coiling the 200 V transformer almost drove him crazy. Electroluminescence is beautiful, produces razor-sharp contours and caresses the eye without any glare. Today you can achieve the same effects with LEDs – less effort, less sensitive and much less power-hungry. But is it only about efficiency, in old cars in particular? No, it isn’t. The legacy of Mr Destriau can today still be found in background lighting for LCD displays. Dashboards in cars received other lighting designs. See you again soon.