I am struggling about this problem, I have this geometry setup
According to the book
$$ dh = D \sec^2\theta \ d\theta $$
So, the illumination reaching point $p$ from this narrow surface is
$$ dE_p = \frac{Lw}{\pi r^2}\cos\theta \sin\theta \ dh $$
I can't figure out how $dh$ is calculated
You can read the book here: https://archive.org/details/in.ernet.dli.2015.284487/page/n271/mode/2up
Any help is highly appreciated.