Homogeneity of dental curing unit beam profile and its effect on microhardness of dental composites with varying thicknesses

Objectives: The dental curing unit (DCU) has been reported to emit inhomogeneous light. However, there are no studies on which elements could affect the inhomogeneity of DCUs. This study aimed to analyze the effect of attenuating factors such as the aperture of the lens or neutral density (ND) filters on the DCU's beam profile and evaluate the effect of DCU's inhomogeneous beam profile on the microhardness distribution on composite resin specimens with different thicknesses. Methods: Radiant emittance and spectrum of eight DCUs were investigated with and without ND filters using different optical density (OD) values. Beam profiles of eight DCUs were photographed while increasing the OD values of the ND filter. The change in the beam profile while changing the f-number of the aperture and the OD value of the ND filters were recorded. The Vickers microhardness of Filtek Z350XT and SDR cured by Bluephase Style 20i with 1, 2, 3, and 4 mm specimens of nine points on each surface was measured. Results: Irradiance and spectrum of DCUs uniformly decreased after attenuation by the ND filters. The beam profile of the DCUs blurred when the f-number of the aperture was higher. The microhardness of Filtek Z350XT showed differences between the central and peripheral areas, and between the points under violet LED and the center on the bottom surface of the 4 mm specimen. The microhardness of the SDR did not show any differences. Significance: The inhomogeneity of the beam profile can be affected by attenuation conditions. DCU's inhomogeneous beam profile may have different effects on the composites depending on the thicknesses and types of composite resin used.