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); 
    		}
    	}
    }

How about changing this:

function OnTriggerEnter (other : Collider) {
     if(lightObject.enabled == true){ 
                if (other.CompareTag ("Respawn")) { 

to this:

function OnTriggerStay (other : Collider) {
  if(lightObject.enabled == true && other.tag == "Respawn") {