Headlight fogging occurs for exactly the same reasons as condensation on windows – everyone knows about it but no one is alarmed when that happens! Car windows mist up when the temperature of the outside air is cooler than that on the inside, and this typically happens in cooler and damper periods of the year. Headlights are no different: a film of condensation builds up on the inside of the lens due to precisely the same climatic conditions. Experts call it fogging. Unlike the condensation on car windows, however, you can’t simply switch on the fan or the air conditioning to get rid of it.
There is a similar – but unfortunately less effective – way of alleviating the headlight problem. Because the warmth emitted by the headlight lamps has a similar defogging effect to the warm air from the fan, driving with the lights on helps to avoid build-up of condensation. Modern light sources, however, generate less heat than halogen lamps and incandescent lights: Xenon headlights, for example, have a greater tendency to fog up than those using halogen lamps. Ironically, it’s the most technologically advanced headlights – those powered by LEDs – that are most impacted by this phenomenon: their modern lamps produce the least heat of all.
Technical solutions for Condensation
The problem of fogging is one that headlight designers have grappled with for years. A compounding factor is that headlights are becoming increasingly larger and therefore contain more interior air. This air heats up while the car is being driven – due to the warmth of the lamps and the heat from the engine. Almost all modern headlights have a ventilation system that prevents a build-up of pressure by displacing this hot air, and this system is carefully designed to allow no direct ingress of water into the headlight housing. However, in line with normal physical laws, air from outside is sucked back into the headlight interior when the inside air cools down again. If this outside air is humid, due to the prevailing weather conditions, condensation forms on the inside of the cover lens.
Fogging disappears of its own accord after the headlights are switched on. Designers seek to accelerate this demisting process by diverting the residual heat from lamps or control gear through the headlight housing. Despite all these efforts, not enough dry air is able to reach the farthest corners of the housing, and this is where condensation persists the longest. How long it actually takes until the lens is completely clear depends on the current weather conditions and level of humidity. When a vehicle is stationary, it takes even longer – and warmer weather – before the condensation finally disappears.
In many cases, daytime running lights have made the fogging situation even worse. Typically using LED light sources, daytime running lights generate far less heat than the low beam function – and they’re frequently installed as a separate unit. When trying to prevent fogging, it therefore makes sense to drive with your low beam headlights on instead of your daytime running lights, as more drivers tend to do in the fall and winter seasons anyway.