Introduction: The purpose of this study was to compare the percentage degree of cure (%DC) of orthodontic adhesive resins irradiated with 3 types of light sources of various intensities: plasma arc, halogen, and light-emitting diode (LED). Methods: Twenty maxillary incisor stainless steel brackets (0.018-in slot) were divided into 4 groups of 5 brackets each. A standardized amount of composite resin was applied to each bracket base, simulating its clinical application. The brackets were light-cured with a halogen light for 20 seconds (10 seconds each from the incisal and cervical bracket edges), an LED light for 20 seconds (10 seconds per edge), a plasma light for 10 seconds (5 seconds per edge), and an LED light for 10 seconds (5 seconds per edge). The %DC was assessed by using micro-multiple internal reflectance Fourier transform infrared spectroscopy, and the results were analyzed with 1-way ANOVA. Results: Overall, the LED and the halogen lights, with the 20-second regimen, produced higher %DC. No significant difference was found in %DC between the plasma light and the 10-second LED light, whereas both showed lower %DC compared with 20 seconds of halogen or LED light. Conclusions: Significant variations in %DC are associated with the various light-curing units, which must be considered in selecting the light source and the irradiation time.