A survey of output intensity and potential for depth of cure among light-curing units in clinical use

Objectives: The light intensity of curing lights used in private dental offices was measured using commercial curing and heat radiometers and related to uniformity of cure depth of standardized composite specimens. Methods: The intensity of 130 curing lights from 107 dental offices was measured with curing and heat radiometers. Due to analogue readings, results were recorded in steps of 25 mW cm−2 and assigned a category number. A total of 50 lights were randomly selected to polymerize standardized 3 mm thick composite cylinders. The composite was irradiated for 50 s according to the manufacturerʹs instructions. The Knoop hardness value was measured at the top and bottom surfaces and the uniformity of cure depth was calculated from the ratio of these two values. Results: Light intensity measured by the curing and heat radiometers was in the range of 25–725 and 0–325 mW cm−2, respectively. Functions of maximum likelihood estimation of the top and bottom surface hardness were 57 N/N+1.3 and 80 N/N+17.7, respectively (N=light intensity category number). The relationship between the logarithmic transformation of the hardness ratio and light intensity was linear (R2=0.84 p<0.001). Conclusion: According to the manufacturer, a curing light is considered as unsuitable for use with a reading of <200 mW cm−2 by the curing radiometer and >50 mW cm−2 by the heat radiometer. Applying these criteria to the present study, 46% of the lights (without repetitions) required repair or replacement. The strong correlation found between the hardness ratio and light intensity verifies the usefulness of the curing radiometer in predicting the polymerization ability of the light activation units.