In this work we show that the critical exponents of the order parameter (CEOPs) of diffuse ferroelectric phase transitions (DFEPTs) occurring in lead germanate-based crystals can be determined using experimental temperature dependences of their optical rotation. We also describe the approach that suggests dividing a crystal sample into many homogeneous unit cells, each of which is characterized by a non-diffuse phase transition with a specific local Curie temperature. Using this approach, the CEOPs have been determined for the pure Pb5Ge3O11 crystals, the solid solutions Pb5(Ge1-xSix)3O11 (x = 0.03, 0.05, 0.10, 0.20, 0.40) and (Pb1-xBax)5Ge3O11 (x = 0.02, 0.05), and the doped crystals Pb5Ge3O11:Li3+ (0.005 wt.), Pb5Ge3O11:La3+ (0.02 wt.), Pb5Ge3O11:Eu3+ (0.021 wt.), Pb5Ge3O11:Li3+, Bi3+ (0.152 wt.) and Pb5Ge3O11:Cu2+ (0.14 wt.). Comparison of our approach with the other techniques used for determining the Curie temperatures and the CEOPs of DFEPTs testifies to its essential advantages.