Physical Explanation Of Spread In Road Lighting

It took me a long time to understand this, though if someone had drawn me a diagram it would have been a doddle! Basically, as the book shows, spread is a value for how far across the road the road lighting fixture (road luminaire) casts its light. The usual way this is explained is to me unfathomable, but the book's diagram suddenly makes it all clear! Throw, also explained in the book, is far easier to understand.

Here are the hard (left image) and easy (right image) explanations of spread from the book, I don't know why this has never been made explicit before! Of course there is the textual explanation, but the graphical one is the one which sticks in the mind.