In the past, automotive headlight lenses were made of glass and came in a limited number of sizes and shapes that fit all automotive applications. With the move toward lighter materials and more aerodynamic vehicle designs, automakers began using predominantly polycarbonate as the headlight lens material of choice over the last 20 years. Vehicles with polycarbonate headlight lenses now make up the vast majority of the vehicles on the road. This plastic material is very light and can be molded into an unlimited number of shapes and sizes to satisfy today's vehicle designers.
Polycarbonate, unlike glass, has a slightly porous surface, with these pores opening and closing with changing temperatures. The pores open when the headlight is on, or on sunny days when the lens is hot, and close when the headlights are turned off or on cloudy days. As a result, various types of debris from the road become trapped in the pores over time, leading to discolouration of the lens. In addition, as polycarbonate is a hydrocarbon, it oxidizes over time with exposure to heat and ultraviolet (UV) rays from sunlight. This oxidation results in the lens discolouring to a dull brownish/yellow tint, appearing fogged or faded and losing much of its optical clarity. This condition can be found on headlights as little as 2-3 years old and is very common on older vehicles.
As an overall result of the degradation processes described above, the headlight not only becomes unsightly, but also unsafe. Less light passes through the degraded headlight lens, resulting in diminished vision during night driving. The only initial solution to the appearance and safety issues of degraded headlights was to purchase replacements, which could run well over $500 for a pair of replacement headlights for many vehicles.
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