Headlights carry a lot of cryptic information – sometimes more visible, sometimes less. One bit says something about the power of high beam. I’m talking about the reference number.

It’s clear that high beam can dazzle. For this reason the regulations contain limits for the luminous intensity. In Europe and many other countries, a maximum of 300,000 candela is allowed per car. The US and other countries using similar regulations only allow 150,000 candela – which is something that could dampen some European tuners’ desire for “real US lights”. For people who know their physics, candela is a practical unit. And people with Latin know that it actually means “candle”. In actual fact, 1 cd – which is how candela is abbreviated – corresponds to the light of an ordinary candle. That’s why North Americans sometimes also talk about “candle power” (cp). But otherwise this unit means nothing to most people.

This realization led to the introduction of a reference number for high beam from headlights according to European rules. It can be found on the headlights near the round E symbol. The whole thing used to be cast in glass, but today the information is given somewhere on the housing. In theory and in line with the regulations, it’s supposed to catch the eye when you open the hood at the latest. Sometimes it requires a painstaking search though, which won’t be successful if you’re looking for the reference number on US headlights. Americans don’t use it.

Let’s get back to the 300,000 candela. They correspond to a reference number of 100. OK, but how is that useful?

Well, for example for retrofitting additional headlights. While these have become noticeably less popular in Germany and Central Europe over the past few years – which is also because of the design of modern cars and a lack of proper bumpers as places to fix them – additional high beam still has a lot of fans in Scandinavia and other regions with many major routes in sparsely populated areas. Truck drivers also appreciate it. Reference numbers help everyone to put as much light as possible, but not an illegal amount, onto the roads.

Most series headlights are well below 50 in reference number, which is the limit anyway for each side. If one has between 30 and 40 it’ll be pretty good. For the widely used H4 systems 17.5 is a common value. You are then allowed to “top up” the difference between the two series high-beam headlights and 100 with additional headlights. If we take H4 lights for example, you could deduct 35 – 17.5 times two – from 100, giving 65. So two additional headlights with a reference number of 30 each would be an option.

All in all, nobody should take these numbers too seriously though. They say something about high beam but not everything. You can’t make any direct conclusions as to the range. And obviously none at all as to dipped beam.

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