The quantity of light that leaves each lamp in the fixture is measured in lumens. Lamps are usually rated in both initial and mean lumens. The lumen level used in the IES files is the initial value which is the amount of light coming from the lamp after it has stabilized, typically after 100 hours of operation. Mean lumens which is less than the initial lumens value is the average lumens over the rated operational life of the lamp.

The lumen level is changed for several reasons - a new lamp model has a better lumen rating, mean lumens, or a different wattage lamp must be used. Care must be taken when using different wattage lamps as arc tube size and location can significantly change the lamp/reflector performance.

The fixture control panel allows the user to change the lamp lumen level for each fixture. By default the Lumens displayed are those embedded in the IES file. The Lumens are set to 1500 in the illustration below: