Piece made only for study and portfolio purpose. Reference found on pinterest (can't find it again :/ )
I started to study light in portraits a couple of months ago. The book "Light for Visual Artists" by Richard Yot https://www.amazon.com/Light-Visual-Artists-Understanding-Design/dp/185669660X/ref=sr_1_1?ie=UTF8&qid=1473559412&sr=8-1&keywords=light+for+visual+artists is very helpful, if anyone is interested.

Cheers.