Lumen maintenance is the measurement of how well a lighting product retains its initial lumen output over time. It is a factor in evaluating the performance and longevity of light sources and luminaires. Lumen maintenance is typically expressed as a numerical value followed by the letter L or LM, which stands for lumen maintenance. This value represents the percentage of light output that remains after a certain number of hours of operation. Commonly used values include L50, L70, L80, and L90, or their LM equivalents.