OnTriggerEnter forcefield help!

I'm making game that will create a force field around an area if you turn on a certain light, I want this force field to kill all things tagged respawn that touch it. I have all the code working except that when you turn on the light, if a monster is already inside the forcefield the monster does not die unless it goes outside and then back in. I think this is because I use an OnTriggerEnter function, so it doesn't run again if the if conditions are fulfilled. It only runs when they enter. Is there a different way I could do this?

just incase it is hard to grasp my question here's my code, it works fine.`

     var Monster : GameObject; 
     var lightObject : Light; 
     function OnTriggerEnter (other : Collider) {
            if(lightObject.enabled == true){ 
                if (other.CompareTag ("Respawn")) { 
                        Destroy (other.gameObject); 
                        Instantiate (Monster, Vector3( -60,-13,70), Quaternion.identity); 
             }
         }
     }