In the past, headlight lenses were made from glass and were easily broken. Manufacturers were limited in the size and shape they could produce. Most vehicles built since 1990 have polycarbonate lenses. This modification mainly occurred due to the change in the design of cars to become lighter and more aerodynamic. Plastic was by far less expensive to produce than glass and can be transformed into unlimited sizes and shapes to satisfy new car designs. The problem with polycarbonate is its tendency to yellow and cloud over time as a result of UV exposure, which transforms new, clear, bright, shiny plastic to a dull yellow tint that appears to be foggy, hazy or cloudy. Fortunately a number of products have been formulated to keep your lenses crystal clear. Regular use and proper maintenance keeps them looking great for years for a fraction of the cost of replacement.