  It is possible to introduce external time dependent back ground fields in the
formulation of a system as fields whose dynamics can not be deduced from Euler
Lagrange equations of motion. This method leads to singular Lagrangians for
real systems. We discuss quantization of constraint systems in these cases and
introduce generalized Gupta-Bleuler quantization. In two examples we show
explicitly that this method of quantization leads to true Schr\"{o}dinger
equations.
