Best way to make an unlit Cg shader

What’s the best way to write an unlit Cg fragment shader? We’re writing a very computationally intensive sky shader so I’d like to eliminate unnecessary overhead to save time — particularly light calculations since it’s an unlit object.

Adding “Lighting Off” doesn’t seem to have any effect. (Why is that?)

I think you need a Custom Lighting model.
this may help

vertex/fragment (not surface) shaders are unlit by default - there’s no lighting calculations, so shader writed as vertex/fragment does not support lighting

so answer - link below is the best way to write an unlit Cg fragment shader.