Eidetic Imagery is involuntary and aids perception but can occur long after stimulation.
One technique for developing after imagery is called "flashing." This involves looking at something bright for a few seconds and then quickly closing your eyes. You then concentrate on the after image you experience.
Eidetic Imagery: This is similar to after imagery in that it originates as a result intense stimulation and aids the perceptual process. It differs, however, in a number of important ways.
First, the experience from which it stems may last for a long time and may not end abruptly, although it can be the other way.
Secondly, the image may occur immediately after the experience or days, months or years later.
Thirdly, the image, in the visual modality, does not move with the eyes. Instead it stays constant and can be scanned.
NOTES: A little more information on Eidetic Imagery.