By Thilo Haiss, Product Line Manager Automotive Lighting, W.L. Gore & Associates
Headlamps have to cope with environmental conditions that change every day. What's more, they have to be protected against dust, dirt, deposits, and condensation if they are to work effectively and guarantee drivers a clear view at all times. Various options are available to automobile manufacturers and suppliers for removing moisture from headlamps. When selecting a venting membrane as an option, the solution must be able to equalize pressure within the headlamp housing, form an effective barrier against dirt and water ingress, and be able to reduce condensation.
Moisture in headlamps comes from three main sources.
Three causes of moisture in vehicle headlamps
• Desorption, which is triggered by differences in temperature. When the light source is switched off, the temperature drops and the plastic that the headlamps are made of soaks up moisture like a sponge. When the light source is switched on again, the temperature rises, releasing this accumulated moisture from the material. At the same time, the dew point rises, which can cause condensation to form at the coldest place in the headlamp. The next time the light source is switched off, the temperature drops and the plastic absorbs moisture. This process accounts for roughly 80% of the moisture in headlamps;
• Permeation, by which water vapor from the outside continuously diffuses through the plastic into the housing interior over a longer period, and
• The vents and drains, through which moisture can get in and out of the headlamp.
Essentially, there are two methods of removing moisture and ventilating headlamps: convection and diffusion. Convection involves open ventilation with at least two venting tubes which conduct moisture outside by circulating air through the headlamp. This process is triggered by pressure differentials produced when the temperature rises inside the headlamp, or as a result of drafts induced by vehicle motion through the air. The disadvantage of an open venting solution is that dust, dirt, insects, and other foreign matter gets into the headlamp along with the air. Also, convection works only when the vehicle is in motion or the headlamp is switched on. Also, there is often the problem that air cannot circulate to the required extent around headlamps due to the large number of components crammed together in the engine compartment. Diffusion causes water vapour to move from regions of high concentration to regions of low concentration in accord with this, the equation of the law of diffusion: