  In the framework of the canonical quantization of the electromagnetic field,
we impose as primary condition on the dynamics the positive definiteness of the
energy spectrum. This implies that (Glauber) coherent states have to be
considered for the longitudinal and the scalar photon fields. As a result we
obtain that the relation holds which in the traditional approach is called the
Gupta-Bleuler condition. Gauge invariance emerges as a property of the physical
states. The group structure of the theory is recognized to be the one of $SU(2)
\otimes SU(1,1)$.
