Thank you very much!!!!!!!!!!! When I'm studying this case I even forgot the most most basic knowledge.......................... :( "The only way we can see something is that light must be able to enter our eye. And our mind will try to interpreted where is the image. " Let me conclude somthing We can see the image without screen in this case because the system reflect light rays in almost all directions then enter our eyes. For the other case(e.g real image by concave mirror/convex lens) light rays are going in the same direction, in the condition that screen is absent, we can't see the image unless we sit on the area that dirtributing light rays or the image point. Right?? One more question, we're going to apply this technique on projector to form a real 3D image but it seems to be somebody done that already http://www.io2technology.com/index http://www.fogscreen.com/ Although those products are projecting 2D aerial image, I think the theory used in those products are close to my case. We must make our project faster..................