how do i get shadow pixel from a texture?

I have tried with "RenderTexture" and it works fine, although it is expensive in performance, even if I only get texture it is 1x1 pixel.
is there a way to get the texture already affected by the shadow?
I don't even need to get the full texture, just a specific pixel inside the texture of "1x1 size", from which I get a gray scale to check the light intensity it receives.​