This looks like an XY problem.
What you've asked for is a material which changes opacity based on lighting. This is doable, but somewhat needlessly complex to get working well:

This hacked example is accomplished with this surface shader:
Shader "Custom/LightToAlpha" {
Properties {
_Color ("Color", Color) = (1,1,1,1)
_MainTex ("Albedo (RGB)", 2D) = "white" {}
_Glossiness ("Smoothness", Range(0,1)) = 0.5
_Metallic ("Metallic", Range(0,1)) = 0.0
_ALightThreshold("Light Threshold", float) = 0.0
_ALightContrast("Light Contrast", float) = 1.0
}
SubShader {
Tags { "RenderType"="Transparent" "Queue"="Transparent" }
LOD 200
CGPROGRAM
// Physically based Standard lighting model, and enable shadows on all light types
#pragma surface surf Standard fullforwardshadows finalcolor:alphaLight alpha:fade
// Use shader model 3.0 target, to get nicer looking lighting
#pragma target 3.0
sampler2D _MainTex;
struct Input {
float2 uv_MainTex;
};
half _Glossiness;
half _Metallic;
fixed4 _Color;
half _ALightThreshold;
half _ALightContrast;
void surf (Input IN, inout SurfaceOutputStandard o) {
// Albedo comes from a texture tinted by color
fixed4 c = (fixed4)1.0f;
o.Albedo = c.rgb;
// Metallic and smoothness come from slider variables
o.Metallic = _Metallic;
o.Smoothness = _Glossiness;
o.Alpha = c.a;
}
void alphaLight(Input IN, SurfaceOutputStandard o, inout fixed4 color)
{
fixed4 c = tex2D(_MainTex, IN.uv_MainTex) * _Color;
color.a = 1.0f - saturate(c.a * saturate(dot(color.rgb, fixed3(1.0f, 1.0f, 1.0f) / 3.0f) - _ALightThreshold) * _ALightContrast);
color.rgb = c.rgb;
}
ENDCG
}
FallBack "Diffuse"
}
But from your clarification, what it seems like you really want is to be able to make a movable circular hole in an image. This doesn't require all the complexities of lighting at all, and can be accomplished much more simply & robustly:

This shader exposes a vector parameter whose x, y, z correspond to the worldspace position of the hole, and whose w is the worldspace size of the hole. You can rig up a script to set these parameters from the transform of a hole object for ease of editing/animating.
Shader "Unlit/TextureHole"
{
Properties
{
_MainTex ("Texture", 2D) = "white" {}
_HolePos ("Hole Position & Size", vector) = (0, 0, 0, 1)
}
SubShader
{
Tags { "RenderType"="Transparent" "Queue"="Transparent" }
LOD 100
Blend srcAlpha oneMinusSrcAlpha
ZWrite Off
Pass
{
CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"
struct appdata
{
float4 vertex : POSITION;
float2 uv : TEXCOORD0;
};
struct v2f
{
float2 uv : TEXCOORD0;
float4 vertex : SV_POSITION;
float3 worldPos : TEXCOORD1;
};
sampler2D _MainTex;
float4 _MainTex_ST;
float4 _HolePos;
v2f vert (appdata v)
{
v2f o;
o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
o.uv = TRANSFORM_TEX(v.uv, _MainTex);
o.worldPos = mul(_Object2World, v.vertex).xyz;
return o;
}
fixed4 frag (v2f i) : SV_Target
{
// sample the texture
fixed4 col = tex2D(_MainTex, i.uv);
float3 offset = _HolePos.xyz - i.worldPos;
float range = dot(offset, offset) / (_HolePos.w * _HolePos.w);
range = 1.0f - saturate(range);
range *= range * 5.0f; // this multiplier controls the sharpness of the fade around the hole.
col.a *= saturate(1.0f - range);
return col;
}
ENDCG
}
}
}