Complex sentence: Balsam was phased out as an optical adhesive during World War II, in favour of polyester, epoxy and urethane-based adhesives; in modern optical manufacturing UV-cured epoxies are often used to bond lens elements.
Split into simpler sentences: Balsam was phased out as an optical adhesive during World War II, in favour of polyester, epoxy and urethane-based adhesives. In modern optical manufacturing UV-cured epoxies are often used to bond lens elements.