  Tight frames can be characterized as those frames which possess optimal
numerical stability properties. In this paper, we consider the question of
modifying a general frame to generate a tight frame by rescaling its frame
vectors; a process which can also be regarded as perfect preconditioning of a
frame by a diagonal operator. A frame is called scalable, if such a diagonal
operator exists. We derive various characterizations of scalable frames,
thereby including the infinite-dimensional situation. Finally, we provide a
geometric interpretation of scalability in terms of conical surfaces.
