Unity Greyscale image effect

Greyscale.cs

using UnityEngine;
using System.Collections;

public class Greyscale : MonoBehaviour {
    public Material mat;

    void Start() {
        mat.SetFloat( "_Power", 0.0f );
    }

    void OnRenderImage( RenderTexture source, RenderTexture destination ) {
        Graphics.Blit( source, destination, mat );
    }
}

Greyscale.shader

Shader "Custom/Greyscale" {

	Properties {
		_MainTex ("", 2D) = "white" {}
		_Power ("", Float) = 1.0
	}
 
	SubShader {
 
		ZTest Always Cull Off ZWrite Off Fog { Mode Off } //Rendering settings
 
		Pass {
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#include "UnityCG.cginc" 
			//we include "UnityCG.cginc" to use the appdata_img struct
    
			struct v2f {
				float4 pos : POSITION;
				half2 uv : TEXCOORD0;
			};
   
			//Our Vertex Shader 
			v2f vert (appdata_img v){
				v2f o;
				o.pos = mul (UNITY_MATRIX_MVP, v.vertex);
				o.uv = MultiplyUV (UNITY_MATRIX_TEXTURE0, v.texcoord.xy);
				return o; 
			}
    
			sampler2D _MainTex; //Reference in Pass is necessary to let us use this variable in shaders
			float _Power;

			//Our Fragment Shader
			fixed4 frag (v2f i) : COLOR{
				fixed4 orgCol = tex2D(_MainTex, i.uv); //Get the orginal rendered color 
     
				//Make changes on the color
				float avg = (orgCol.r + orgCol.g + orgCol.b)/3;
				fixed4 col = lerp( orgCol, fixed4(avg, avg, avg, 1), _Power );
     
				return col;
			}
			ENDCG
		}
	} 
	FallBack "Diffuse"
}