How to go about optimising this class?

I'm rather new to actionscript, but I'm figuring it out as I go.

I am making a game. One of the primary features of this game is darkness. I chose to use tiles to represent areas outside the players sight range, this works great on a small scale. The problem is that once there's lots of darkness tiles it gets a bit laggy.

This is the class for light sources, currently its a proof of concept.. I plan on having in betweens later so its not just a true or false.

package
{
    import flash.display.MovieClip;
    import flash.events.Event;    
    public class LightEmitter extends MovieClip
    {
        private var i:int = 0;
        private var closestH = 0;
        private var lightResistance = 0;
        private var brightness = 1;
        private var targetTile;
        private var baseSightLevel = 400;
        public function LightEmitter() 
        {
            this.parent.setChildIndex(this, 0)
            stage.addEventListener(Event.ENTER_FRAME, onJoinFrame);
        }
        public function onJoinFrame(e:Event) 
        {    
            //Hide darkness
            for (i = 0; i < MainDocument.darkTiles.length; i ++)
              {
                targetTile = MainDocument.darkTiles[0]
                lightResistance = MainDocument.darkTiles[1];
                   if (Math.sqrt(Math.pow(targetTile.x-this.x, 2) + Math.pow(targetTile.y-this.y, 2)) < (baseSightLevel*brightness)/lightResistance)
                {
                    targetTile.alpha = 0;
                }
                else
                {
                    targetTile.alpha = 0.6;
                }
            }
            this.x = parent.getChildByName("player").x
            this.y = parent.getChildByName("player").y
        }
    }
}

This is the class for a tile of darkness.

package
{
    import flash.display.MovieClip;
    public class Fog extends MovieClip
    {
        var aDarkTile: Array = new Array();
        public function Fog() 
        {
            aDarkTile[0] = this;
            aDarkTile[1] = 1;
            MainDocument.darkTiles[MainDocument.darkTiles.length] = aDarkTile;
        }
    }
}

I need to load lots of tiles at a time in addition to any other elements of the game. (but I can unload, or at least hide, tiles that are off screen)

Any suggestions/tips/alternatives would be greatly appreciated.