Daylight model of a Solatube

Hello! just reaching out again about this. I was able to model the Solatubes using this older post in this forum. something that is not clear though is if we have to model the Soletubes as round surfaces or half spheres? do we have to model the tube?
Please help, the results I am obtaining look too dark, and I am not sure they are reliable.
@chris @mostapha