Effects of various initial cure conditions of pulse-delay curing on contraction stress reduction were examined photoelastically. Five different initial cure conditions were considered: P-1) 200mW/cm2 for 3s, P-2) 300mW/cm2 for 5s, P-3) 300mW/cm2 for 10s, P-3′) 300mW/cm2 for 10s with no interval, P-4) 600mW/cm2 for 10s. Except for P-3′, a three-minute interval was provided between the initial and final light irradiations. All the experimental groups were subjected to the same final light exposure (600mW/cm2 for 60s). The control group was light-cured at 600mW/cm2 for 60s, with no initial cure and interval. P-2 and 3 developed significantly lower fringe order compared to the control group. P-1 and 4 failed to achieve contraction stress reduction because the light energy from the initial cure was too low and too high respectively. Comparison between P-3 and 3′ indicates that the interval between the initial and final light exposures plays an important role in contraction stress reduction as well as the reduced initial light irradiation. The results of this study indicate that pulse-delay technique can reduce contraction stress, however, both proper initial cure conditions and interval between the two light irradiations are important to attain contraction stress reduction.
