How far do your car lights shine? 25 meters? Or 50? Maybe even 100 or further? All these answers are correct. But without an indication of how bright the light still is at these distances, that information is pretty useless.
Even an only average car headlight will cover many hundred meters. Anyone can see that. After all, the lights of an oncoming car are already visible from such distances. Reflective signs and marker posts also reflect the light back from two or three hundred meters away. But an obstacle or even a person at the same height would have no chance to be seen without a reflector. Not enough light would get there over such great distances.
The closer to the headlight, the more the illumination increases.
The full specification for range information has to be something like this: 80 meters at three lux (lx). And that’s the sort of information that reputable lamp manufacturers such as OSRAM provide. So the distance or range information refers to the point at which illuminance has dropped to this value. The closer to the headlight, the more the illumination increases. Beyond 80 meters it quickly drops off, but there’s still some light of course.
Choosing three lux was not random. Drivers can still make good use of these lighting conditions and illuminated obstacles are visible. Now, the eye can also cope with significantly lower illuminance levels. 0.25 lux is enough to read the newspaper when you’re young. At 0.1 lux you can still walk without breaking your neck. But the major difference to driving lies in what is called adaptation. The eye needs several minutes to get used to the dark and become sensitized. When driving, there is no complete darkness to adapt to though – for one thing because of your own headlights.
A dipped beam range of 80 meters – to stick with the example above – doesn’t mean that there is three lux in all areas on the road.
A dipped beam range of 80 meters – to stick with the example above – doesn’t mean that there is three lux in all areas on the road. The information always refers to the outermost tip or finger of the light distribution, which is asymmetrical in dipped beam. Your own side of the road – the right one in right-hand traffic – is illuminated much stronger than parts further to the left. For left-hand traffic it’s the other way round of course. In other words, the point the range information refers to is located on the side of the road. In Europe, for example, as near as 25 meters away and 50 centimeters high only one lux is allowed to reach the side of the oncoming traffic to prevent glare. In the US higher values are allowed.
Measuring the light that reaches this point requires expensive, high-quality measuring equipment. Just any old exposure meter won’t do. And the professionals actually don’t measure what gets onto the road in lux, but what the headlight gives off in candela. However, the difference between illuminance in lux and luminous intensity in candela is a topic for a physics lesson. The only important thing to know is that you need one of the two for reliable range information.