Lumen maintenance describes how much luminous flux a light source retains as it ages, usually expressed as a percentage of its initial output after a stated number of operating hours. It must always be read together with the stated time, test conditions, and failure criterion. Common lifetime notation such as L70 refers to the point at which maintained light output reaches 70% of its initial value under the specified method and conditions. The result may describe a population or statistical projection rather than the exact failure time of one lamp. Temperature, drive current, sealing, and thermal management influence real service performance.