air oven at 100 u00b0 c for 10 minutes and then at 120 u00b0 c for 20 minutes in addition annealing was performed with the film constrained in a pair of metal frames to prevent any further shrinkage after annealing the optical retardation of the films was measured the results are shown in table 2 cellulose triacetate films were prepared by solvent casting according to the procedures described in comparative examples 1 16 except that the annealing step was carried out in a forced air oven at 100 u00b0 c for 10 minutes and then at 120 or 130 u00b0 c for 10 minutes in addition annealing was performed with the film constrained in a pair of metal frames to prevent any further shrinkage after annealing the optical retardation of the films was measured the results are shown in table 3 the results from examples 4 and 5 are also reproduced in table 3 for convenience cellulose triacetate films were prepared by solvent casting according to the procedures described in comparative examples 1 16 except that the plasticizer loading level was varied from 5 to 15 wt and the annealing step was carried out in a forced air oven at 100 u00b0 c for 10 minutes and then at 110 120 130 or 140 u00b0 c for 10 minutes in addition annealing was performed with the film constrained in a pair of metal frames to prevent any further shrinkage after annealing the optical retardation of the films was measured the results are shown in table 4 cellulose triacetate films were prepared by solvent casting according to the procedures described.