Two Kinds of Vision
1. Acquired images (uggaha nimitta): Sometimes when the mind settles down, a vision of one sort or another may appear — a lump or a cloud of black, red, or white, etc.; a vision of one's own body or of a person acting in one way or another; a vision of the Buddha or of one of the Noble Disciples, or of heaven or hell — there's no end to what may appear. In short, when we sit with our eyes closed meditating, whatever images arise in the mind are classed as acquired images. If we see a good one, we tend to assume that it's a sign that we've attained a good level, and so we fasten onto it. If we see an unpleasant one, we tend to become fearful or upset.
So we should make ourselves wise to the fact that there is no truth to these visions. They're simply illusions, deceiving the heart. They come under the laws of all that is inconstant, stressful, and not-self. Their nature is to arise and then pass away. To latch onto them and take them seriously is a form of defilement and attachment, called nimittupadana, clinging to signs. So if a vision arises, you should leave it alone. Keep conscious of your own body and mind.
Actually, these visions don't come from anywhere other than your own heart. To fall for them is like being duped by your own reflection. Just as when a bird is eating food and we show it its reflection in a mirror, it'll open its beak — out of greed or envy — and try to steal the food in its reflection's beak, dropping the food in its own beak, so it is with acquired images: If we latch onto them and take them seriously, right concentration and discernment will drop from our grasp.
This being the case, we should leave these visions alone. If we start making assumptions based on them, they will turn into a form of attachment and so become our enemies. If an ugly or frightening image arises, we may get unnerved. So no matter what sort of image arises, don't get involved in it. Remind yourself that there's nothing constant or dependable about it, that it's simply a camp-follower of defilement, attachment, and unawareness. Visions of this sort have also been termed kilesa-mara, the demons of defilement, tempting the mind to become fixated on their contents.
The important point is not to bring them into the mind, because our purpose in meditating is to train the mind to be pure. We're not trying to "get" anything at all. Focus on the body and mind, know your own body and mind, until you know that you're free from defilement, suffering and stress: Once you truly know this, you've reached what you're here to know. Everything else, you should let pass. Don't fasten or dwell on it.
2. Divided images (patibhaga nimitta): This means that you separate the image from the mind and the mind from the image so as to see the true nature of the image as inconstant, stressful, and not-self. If you can't separate things in this way and instead get caught up in playing along with the vision, your mind will go astray from right concentration.
If you really want to know the mind, you have to get the mind out of the vision and the vision out of the mind. And before you can do this, you have to consider the vision from the standpoint of its three inherent characteristics, as inconstant, stressful, and not-self. For instance, the various visions that appear can be small, large, broad, narrow, bright, murky, near or far. This shows that they're inconstant. So separate the mind from them. The mind will then be freed from them, and you should then return your attention exclusively to the body and mind as before. As your powers of mindfulness become firmer and stronger, mindfulness will turn into fixed penetration. And when fixed penetration acquires enough power, you will be ready for the exercises of insight meditation.
Not everyone experiences visions of this sort. Some people have a lot of them; others never have any at all, or at most only rarely, because they're things that are inconstant and undependable. If the power of your tranquillity is strong, there tend to be a lot of them. If the power of your insight is strong, they most likely won't appear. At any rate, the important point is that if you're constantly aware of your body and mind, you're on the right track. If you can be aware to the point where you know that your mind is released from its mass of defilements, so much the better.
Even if you don't experience visions, concentration still has its rewards. Even the lower levels of concentration — momentary concentration and threshold concentration — are enough to provide a basis for the arising insight.