Question on nomenclature of ... mappings? I would like to understand "in english", what this sentence is saying here:

I understand what $R^3$ means, but I am not sure I understand the rest...
Thanks! 
EDIT:
The image is from this paper. 

The pre-print is really badly written, not stating what $\Omega$ is. Anyway it's easy to get the idea. $\Omega$ represent a surface which is supposedly the camera receiver. The projection is a map that goes from the 3D space to the camera receiver. The back-projection is aimed to reconstruct the projected 3D-space from the received image, so it is a map from $\Omega \times R$ (here every point of the surface $\Omega$ is linked to a one-dimensional ray) to the ordinary 3D.
What probably is disturbing is $\Omega \times R$. Probably in the author's mind this notation is a way of saying that for every point of the camera receiver $\Omega$ you have a line or ray of possible position $R$ that could have been the source of the received pixel.
This it would not be what I would have done, but if the authors choosed this set up they probably have a reason for that.
