The so called Glass Cap test is a method to investigate the chemical compatibility of materials to LEDs in an accelerated lifetime environment. High power LEDs are prone to failure mechanisms called browning (discoloration of silicon lens near conversion layer, causing drop of luminous flux) and corrosion (discoloring of lead frame silver coating, also causing lumen flux drop). Both failures are caused by volatile compounds that migrate into the lens and the coating, respectively.

The glass cap test operates high power LEDs together with material samples under accelerated lifetime conditions in a sealed environment. Thus it can predict long-term interactions of materials like glues, sealing, pottings, etc. with LED performance.