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The present study, divided into two parts, analyses the stability and reproducibility of the spectral and energy emission of the present light-curing units (LCUs), quartz tungsten halogen (QTH) and light-emitting diodes (LEDs). In part I, QTH LCUs were studied. The results showed that the QTH LCUs studied presented high stability and reproducibility in terms of their spectral emission with VAF (variance accounting for) values from the Cauchy-Schwarz inequality, all close to 100%. With respect to the energy stability, the QTH LCUs studied can be considered stable under practical clinical conditions, although for some devices the initial irradiance value is critical. This result should be taken into account in those works which is researched in polymerization kinetics of dental materials as well as in clinical practice.