  The underlying mathematics of the wavelet formalism is a representation of
the inhomogeneous Lorentz group or the affine group. Within the framework of
wavelets, it is possible to define the ``window'' which allows us to introduce
a Lorentz-covariant cut-off procedure. The window plays the central role in
tackling the problem of photon localization. It is possible to make a
transition from light waves to photons through the window. On the other hand,
the windowed wave function loses analyticity. This loss of analyticity can be
measured in terms of entropy difference. It is shown that this entropy
difference can be defined in a Lorentz-invariant manner within the framework of
the wavelet formalism.
