Originally Posted by rickt86

They are light guns, that shoot down the runway and with a fancy computer inside they measure how far they can "see", which is converted to human talk and gives you the distance you can see from the cockpit while looking down the runway. I am trying to keep this simple. The chart put out by the FAA/airport tells you what is legal for take off at that airport/ runway from those light guns. If one light gun is broken and the report is missing you can take off, if two are broken can NOT go. However you can never ever go if they fall below what the chart says is legal. .