The irradiance of visible light curing units (LCUs) has been reported to decrease over time. However, the spectral output of used LCUs has rarely been evaluated. The purpose of this study was to evaluate the irradiance and the spectral output of visible LCUs used currently in laboratories. One halogen LCU (Optilux 360, O360), two polywave light emitting-diode (LED) LCUs (Bluephase Style [ST] and Bluephase G2 [G2]), and one single-peak LED LCU (Elipar S10, S10) were activated according to the manufacturers' instructions. The irradiance and spectral output of the curing light were measured with a spectroradiometer in real time (n=10). Regarding the spectral peak and mean irradiance of the LCUs, O360 and G2 showed consistent data. However, ST showed spectral peaks at either 457 or 408 nm, and S10 at either 456 or 450 nm. The mean irradiance of ST and S10 was also inconsistent. Regarding the total energy, all LCUs surpassed the minimal energy value of 16 J/cm 2 to fully polymerize an increment of 2 mm of composite resin. However, focusing on 420-540 nm range energy where camphorquinone is activated, some data showed an energy value for ST of less than 16 J/cm 2 , when the spectral peak was observed at 408 nm. Therefore, to evaluate the performance of the LCUs, not only the irradiance but also the spectral output should be monitored, especially in the case of polywave LED LCUs, where multiple spectral peaks are present.