The Initial Lumens rating of a light bulb is the measure of how much light that bulb will produce, in lumens, when it is brand new. The light bulb manufacturers have started to note the rated lumens for their products that way because all light bulbs grow dimmer with age. There’s not much they can do about that, and they don’t want to be held responsible when one of their older bulbs isn’t putting out as much light as it did when it was new.

The fading happens with all light bulb types, but it is particularly noticeable in vapor-arc lamps, which include fluorescent tubes and compact fluorescent lamps, or CFLs.

Initial Lumens is not, as the term might imply, a measure of the amount of light a light bulb will give out when it is first switched on. Waiting the minute or two it takes some CFL bulbs to warm up to their full brightness is one example of the difference.