Lamp
Life
During the initial burn-in phase of gas-discharge lamps, light output varies significantly from rated light output levels (often exceeding it). After the initial burn-in, the lamp's output begins to decrease. That is why most manufacturers rate their lamp's light output after 100 hours of operation. Different light sources have a quicker lumen decrease than others. As lamps become older, they steadily become less efficient. Ultimately, it becomes economic to replace lamps before they burn out. These figures illustrate typical lamp lumen decrease, called depreciation.
and
Typical Fluorescent Bulb Light Depreciation