Inadequate intensity of the polymerization light source can compromise the quality and longevity of dental composite restorations. In order to maintain optimal strength of polymerization devices, regular control of polymerization units is necessary. The aim of this study was to compare the accuracy of two radiometers in the measurement of light intensity of photopolymerization devices concerning the time point of measurement. Light intensity measurements of 16 halogen and 8 LED curing lights were performed using three different devices at the beginning as well as 10 and 40 seconds after the start of illumination. Two were handheld radiometers: Bluephase meter (BM) and Cure Rite (CR), while an integrating sphere (IS) represented the reference device. Data were statistically analyzed using Friedman's test and Wilcoxon signed-rank test (p < 0.05). The values at the beginning and after 10 seconds measured by BM were significantly higher than the measurements by IS, whereas CR showed higher values after 10 and 40 seconds. Both commercial radiometers tended to overestimate the light intensity of LED and halogen curing units when compared to the reference device. The time point of measurement influences the output value. The heating of radiometers was proposed as a possible explanation for the inaccuracy.