public Ray ScreenPointToRay (Vector3 pos);



Resulting ray is in world space, starting on the near plane of the camera and going through position's (x,y) pixel coordinates on the screen (position.z is ignored).

Screenspace is defined in pixels. The bottom-left of the screen is (0,0); the right-top is (pixelWidth -1,pixelHeight -1).

//Attach this script to your Camera
//This draws a line in the scene view going through a point 200 pixels from the lower-left corner of the screen
//To see this, enter Play Mode and switch to the Scene tab. Zoom into your Camera's position.
using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour { Camera cam;

void Start() { cam = GetComponent<Camera>(); }

void Update() { Ray ray = cam.ScreenPointToRay(new Vector3(200, 200, 0)); Debug.DrawRay(ray.origin, ray.direction * 10, Color.yellow); } }