More often than not, a lens used in an environmental or physically harsh environment is made out of polycarbonate. Polycarbonate is derived from petroleum, and UV is causing that browning. To prolong this effect, UV stabilizers are added to the raw pellets that produce the parts for manufacturing. This UV stabilizer is not permanent and slowly goes away over time causing it to yellow. Acrylic is derived from natural gas, and is inert to UV and will not yellow.