Here is an interesting article about Nimittas. It goes into some great details and some interesting directions...
THE MYSTERY OF THE BREATH NIMITTA
OR
THE CASE OF THE MISSING SIMILE"preparatory image or sign" (parikamma-nimitta)
"acquired sign" (uggaha-nimitta)
"counter-image" or "counter-sign" (the patibhaga-nimitta)
This has been discussed before
HERE but it was immediately hijacked and the article was never really discussed.
I thought is worthy of pointing out again and possibly discussing.
~D