How to Enable a Light When a Camera Enters a Collider.

Use Activate Trigger script and put it on collider that you are going to trigger with camera. Camera must have enabled collider that is not trigger. When you make script put it on collider set source with gameobject you put script on, and in target select gameobject that you want to activate.

 using System;
 using UnityEngine;
 using Object = UnityEngine.Object;
 
 namespace UnityStandardAssets.Utility
 {
     public class ActivateTrigger : MonoBehaviour
     {
         // A multi-purpose script which causes an action to occur when
         // a trigger collider is entered.
         public enum Mode
         {
             Trigger = 0,    // Just broadcast the action on to the target
             Replace = 1,    // replace target with source
             Activate = 2,   // Activate the target GameObject
             Enable = 3,     // Enable a component
             Animate = 4,    // Start animation on target
             Deactivate = 5  // Decativate target GameObject
         }
 
         public Mode action = Mode.Activate;         // The action to accomplish
         public Object target;                       // The game object to affect. If none, the trigger work on this game object
         public GameObject source;
         public int triggerCount = 1;
         public bool repeatTrigger = false;
 
 
         private void DoActivateTrigger()
         {
             triggerCount--;
 
             if (triggerCount == 0 || repeatTrigger)
             {
                 Object currentTarget = target ?? gameObject;
                 Behaviour targetBehaviour = currentTarget as Behaviour;
                 GameObject targetGameObject = currentTarget as GameObject;
                 if (targetBehaviour != null)
                 {
                     targetGameObject = targetBehaviour.gameObject;
                 }
 
                 switch (action)
                 {
                     case Mode.Trigger:
                         if (targetGameObject != null)
                         {
                             targetGameObject.BroadcastMessage("DoActivateTrigger");
                         }
                         break;
                     case Mode.Replace:
                         if (source != null)
                         {
                             if (targetGameObject != null)
                             {
                                 Instantiate(source, targetGameObject.transform.position,
                                             targetGameObject.transform.rotation);
                                 DestroyObject(targetGameObject);
                             }
                         }
                         break;
                     case Mode.Activate:
                         if (targetGameObject != null)
                         {
                             targetGameObject.SetActive(true);
                         }
                         break;
                     case Mode.Enable:
                         if (targetBehaviour != null)
                         {
                             targetBehaviour.enabled = true;
                         }
                         break;
                     case Mode.Animate:
                         if (targetGameObject != null)
                         {
                             targetGameObject.GetComponent<Animation>().Play();
                         }
                         break;
                     case Mode.Deactivate:
                         if (targetGameObject != null)
                         {
                             targetGameObject.SetActive(false);
                         }
                         break;
                 }
             }
         }
 
 
         private void OnTriggerEnter(Collider other)
         {
             DoActivateTrigger();
         }
     }
 }