Birefringence simulations use a spectrum correction factor that modifies the influence of viscoelastic deformations on the birefringence results.
The default value for the spectrum correction factor is 1, but you may adjust it to suit your situation. Typically, the value should be a positive number between 0.2 and 2.
Autodesk Simulation Moldflow have performed experimental observations of birefringence using specific materials, and as a result have adjusted the spectrum correction factor for those materials to provide more accurate birefringence results. If the default value of the spectrum correction factor is not 1, this indicates that the material you have selected is one of the materials that Autodesk Simulation Moldflow have adjusted.