All light sources experience a reduction in light output over time known as lumen depreciation. This depreciation begins right away and continues over the life of the light source. In general, high power surface mount LEDs degrade much more slowly than other common light sources such as incandescent, halogen, metal halide, or CFL bulbs. The human eye generally can’t detect a change in light output until there has been 30% depreciation, so this threshold is often used by light source manufacturers for determining the “useful life” of their products. With high power surface mount LEDs, it generally takes 50,000 hours or more for 50% of the population to lose 30% of the original brightness.