L70 (the L is shorthand for lumen depreciation) means the point at which an individual LEDs light
output has decreased to 70% of its initial output. B50 indicates the sample size, in this case 50%.
So a rating of L70 /B50 greater than 50,000 hours means that 50% of the LEDs in a product will fall
to 70% of their initial power after a period of 50,000 hours. The overall lumen output of the
luminaire will still exceed 70%.
70% is significant because it is based on research into human perception. The human eye does
generally not detect a decrease in light until the decrease exceeds 30%.