A Lumen is a unit of measurement of light intensity falling on a surface. A Lumensecond refers to a light of 1 Lumen intensity for a duration of one second, or the equivalent, such as 2 Lumens for half a second. The absolute amount of light emitted each time a flash system is fired is correctly specified in lumenseconds. The number of lumenseconds produced by a particular flash system depends on the efficacy, how effectively the system turns electrical energy into light energy, or wattseconds into lumenseconds. The efficacies of commercial photoflash systems typically fall within the range of from 15 to 50 lumenseconds per wattsecond. What this implies is quite simple: a highly efficient 300ws system may produce as much actual light energy as an inefficient system rated at 1000ws.