People frequently ask how one headlamp bulb is able to deliver 100% more light than another although they have identical ratings for power and luminous flux. The answer – and not a value specified on any packaging – is luminance. Physicists and lighting experts define this term as quantity of light per surface area or as candela per square meter. But no lay person needs to remember these definitions. There is really only one important fact to note: when it comes to the surface area emitting the light – smaller is better. While this is certainly true for a light source such as a headlamp, we’ll be mentioning a different case later on.
A headlamp is able to illuminate the most important areas of the road in a particularly effective way when the light emanates from a small area. This works especially well in headlamps without lenses, enabling them to achieve the light increases mentioned above. The achievable increase, however, varies widely from headlamp to headlamp.
Because luminance is not specified in any licensing regulations, engineers can come up with creative tweaks. Osram’s Night Breaker is an example of this: the brightest areas of the filament are extremely small, enabling the headlamp to project the light out of the focal point extremely precisely. Due to the thermal stress it entails, this kind of fine-tuning has its limits, however. Engineers need to ensure the thin wire in the filament doesn’t melt and blow the bulb.
So is luminance good or bad?
When we’re talking about halogen lamps, a high luminance is definitely advantageous. But most drivers take a different view when they feel they’re being dazzled by the light from an oncoming car with extremely small headlights. There’s a high luminance here as well – due to the size of the emitting area. Although this luminance isn’t really dazzling, our eyes perceive it as such. Given the same brightness, and therefore a comparable performance, lens headlamps that are smaller have a higher luminance than those that are larger. Some people also feel dazzled by daytime running lights made up of individual, extremely bright LEDs, particularly in tunnels or during the twilight hours. But drivers should really be using their low-beam headlights in such situations anyway. Assuming these are halogen-powered, their luminance is unquestionably positive!