Wednesday, Oct 14, 2009 at 21:54
Robin, as I understand, the difference is in the design of the lamp for different purposes - this relates to the shape of the reflector and the sheilding of the globe/bulb.
There are strict standards that headlights have to comply with so that the low beam won't shine into the eyes of other drivers - they should point downwards (you'll see a defined line above which they don't shine) and the light will 'kick-up' to the left (for RH drive vehicles, opposite side for LHD vehicles - this is to throw light onto the shoulder of the road to watch for wildlife, pedestrians etc.)
Unfortunately, the way a low beam lamp is focussed away from the eyes of other drivers obviously makes them less visible hence DRL's which are focussed straight ahead so as to be seen by other drivers but because of this, they are a lower wattage/power than standard headlights. (Having said that, I'd expect most motorbikes would just run the headlight whenever the engine is running - unless it's a european bike).
As far as I understand, there are no standards governing fog lights (hence the laws about when to/not to use them) - they just throw light ahead (like a high-beam or driving lamp) but are mounted low so as to penetrate beneath the fog. Additionally, fog lights are usually the same wattage as standard headlights, but due to the lack of focussing and sheilding of the light they often cause dazzling (or at least irritation!) to other drivers.
That's my understanding of the differences.
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