Lumens are the measurement for lamp light output. Fluorescent lamps are measured in initial lumens or in design lumens. Initial lumens are a measure of the amount of light that the lamp produces after about 100 hours of operation. Design lumens are a measure of the amount of light that the lamp produces after it has operated for approximately 40 percent of its rated life. Because light output from a fluorescent lamp diminishes over the life of the lamp – a process known as Lamp Lumen Depreciation – a lamp’s design lumens (or mean lumens) are always lower than its initial lumens. Lumen maintenance refers to the rate at which light output declines over time and lumen maintenance percentages are typically based on light output at 40% of a lamp’s rated life.