BRDF & Integral

[sick rivet]

Hi, I am now reading the PBS chapter of RTR. But I'm confused about how to integrate BRDFs. The book says that I need to multiply them by Pi before usage, but where's the Pi from?

And are formulas in these images correct?

It's using lambertian diffuse as the entire BRDF

[potent ginkgo]

I'm on the same chapter. These equations might help
I think c == L_i

Why pi is involved I'm not certain but I imagine it has to do with the hemisphere shape the BRDF uses

[sick rivet]

Ok, thank you