The amount of luminous flux (lumens) still being emitted at a given time (in hours) during the life of LEDs. This varies between different type and quality of LEDs and the characteristics of the luminaire, particularly the temperature at which the LEDs are operated. Expressed as L and B percentages at a specific number of running hours. E.g. L80B50 50,000 hrs means that after 50,000 hours of operation the LEDs are on average emitting 80% of their initial lumens (L80), of which 50% of LEDs are emitting less than 80% (B50).