Do headlight bulbs cause the lenses to turn yellow?

A common theory among vehicle owners is that heat from the bulbs causes headlight lenses to yellow from the inside out. This belief is incorrect because the primary cause of discolouration is actually external exposure to ultraviolet radiation from the sun, not internal heat accumulation. The outer surface of the lens absorbs this solar radiation, causing the plastic material to degrade and change colour over time. MOD5 TopShield headlight protection film acts as a dedicated shield against this damaging UV exposure. If the yellowing were caused by internal heat, applying a film to the outside would serve no protective purpose, yet it effectively prevents the degradation.