How long should a defogging film last, and what causes premature failure?

A well-specified defogging film — PET substrate, adequate wire gauge, dense wire layout — should remain fully functional for the operational lifespan of the mirror itself, typically five years or more under normal use. Premature failure is almost always attributable to one of three causes: undersized wire gauge leading to thermal fatigue and burnout; sparse wire density creating localized overheating at wire junctions; or substandard substrate material deforming under repeated thermal cycling and disrupting wire contact. These failure modes are preventable through proper component specification and incoming quality inspection at the factory level.