Synthetic optical gels fill in the gaps in photonic devices, offering a no-leak alternative to optical liquids and a viscous alternative to traditional epoxies. Since the mid-1980s, manufacturers have been filling their fiber optic splices with a special class of crystal clear optical coupling gels. Now, synthetic optical gels are finding their way into medical sensors and instruments, photodiode arrays, industrial borescopes, laser transceivers and more familiar applications that nondisclosure agreements prevent me from specifying. The reasons for the recent rush to gels are clear and compelling. Hardly the oil-bleedy, yellowing, petroleum-jelly-like gels of old, today's synthetic index-matching gels are precisely engineered materials. And the good news is that they can be precisely engineered according to your specifications.