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Important note at the beginning: I practice mainly vipassana, if samatha then I do metta meditation. Therefore I don't have a huge amount of knowledge on this subject.
What I do know is this:
Anyhow. Here two site with more info. Maybe it can help you further. http://rc.leighb.com/more/Nimitta.htm http://realitysandwich.com/27172/jhanas_meditative_absorptions/