Gerry T:
Does anybody here have experience with perceiving a Nimitta during meditation?
Yes. But don't let these terms (like
nimitta) become exotic or mysterious in your mind. They're not. A
nimitta is simply a sign of some kind that helps the meditator confirm having established something (some ground or ability). It could be any phenomenon (visual or tactile/sensual) that arises that the individual meditator relates to which helps establish concentration (
samadhi). And that could easily well vary from person to person. Some people perceive light and use that, others sense a sensation and use that to help them establish concentration.
Personally, I when I was first learning about this I didn't trust visual signs (like light of some kind) because they are often created by the mind and auto-suggestion. Hence, in my way of thinking, they weren't very substantial, and therefore unreliable as a sign for me. A physical sensation was a different story for me. It just seemed more substantial in my mind. Besides that, I had already established an idea of what the sensation of concentration "felt" like when I was growing up.
So, when I first experienced a pressure in the center of my forehead (what one could rightly term a
nimitta) I knew when that sensation arose that my concentration was becoming well established on whatever object I had taken as my object of concentration (like the breath, for instance). From that point, I would watch to see the establishment of the second jhana (a kind of effortless feedback loop where the mind becomes established on an object like the breath). Once the second jhana became established, I knew I was on my way to achieving 3rd and 4th jhana by
just calming the mind even more while performing
samatha meditation.
Gerry T:
The Nimitta seems critical as well as reaching Jhana prior to vipassana practice.
It's only critical if one proceeds according to certain instructions and invests one's ideas and perception of success into that instruction. Otherwise, it's not as critical it is sometimes made out to be.
Gerry T:
When I am meditating I spend most of the time looking at the back of my eye lids. Some light shows through. I'm either looking at that or at times a "mental" image will appear and I note it and go back to the breath. Same with the light behind the eyes.
Is there any relationship between the lights behind the eyes and a Nimitta?
There could be; then again it may mean nothing at all. It all depends on how you perceive, use or not use, and react to it. In other words, only the individual meditator can answer that question for himself.