Luminance to Alpha

Hello! I've been fiddling around with surface shaders and trying to make luminance affect alpha, but it doesn't seem to be working properly. The Luminance (rgb) function, from what I can see, keeps returning a constant value for the entire object. Am I doing something very wrong, if so, what's the solution?

 Shader "Luminance"
 {
     Properties
     {
         _Color ("Main Color", Color) = (1,1,1,1)
     }
 
     SubShader
     {
         Tags
         { 
             "RenderType" = "Opaque" 
             "Queue" = "Transparent"
          }
         
         CGPROGRAM
         #pragma surface surf Lambert alpha
  
         float4 _Color;
 
         struct Input
         {
             float2 uv_MainTex;
         };
           
         void surf (Input IN, inout SurfaceOutput o) 
         {
             half4 c = _Color;
             o.Albedo = c.rgb;
             o.Alpha = Luminance (c.rgb) * c.a;
         }
  
         ENDCG
     }
  
     Fallback "Diffuse"
 }