Rim light shader doesn't work on iPad??

Hi, I am new to Unity 3D. I used the following rim light shader code, which is almost the same with the code written in Unity document.

 Shader "Custom/glowShader" {
     Properties {
         _MainColor ("Main Color", Color) = (0.3, 0.3, 0.3, 1.0)
         _RimColor ("Rim Color", Color) = (0.6, 0.6, 0.6, 1.0)
         _RimPower ("Rim Power", Range(0.5,8.0)) = 1.0
     }
     SubShader {
         Tags { "RenderType" = "Opaque" }
         CGPROGRAM
         #pragma surface surf Lambert
 
         struct Input {
             float4 color : COLOR;
             float3 viewDir;
         };
         
         float4 _MainColor;
         float4 _RimColor;
         float _RimPower;
         
         void surf (Input IN, inout SurfaceOutput o) {
             half rim = 1.0 - saturate(dot(normalize(IN.viewDir), o.Normal));
             o.Albedo = (0,0,0,0);
             o.Emission = _RimColor.rgb * pow(rim, _RimPower);
         }
         ENDCG
     } 
     Fallback "Diffuse"
 }

As for Albedo, I set the o.Albedo black to see the effect of rim light.

This works fine on the screen view of Unity as follows.

However, the iOS build shows the whole gray circle.

Do I need some special settings for the shader working fine, like some Playersettings? Or iPad does not support some shader function?

I am using Unity 3D 4.6.1, iPad Air 2 and iOS 8.1.2.

Many thanks.