Spectral Compatibility Between Photoinitiators and LED Curing Lights: Photographic and Spectroscopic Evidence from Model Resin Systems

Polymerization of resin-based composites depends on an overlap between photoinitiator absorption and light-curing unit (LCU) emission, yet resins that fail to cure cannot be measured and go unreported. This study tested how photoinitiator chemistry and concentration interact with light-emitting diode (LED) emission spectrum, documenting outcomes photographically. Sixteen unfilled bisphenol A glycerolate dimethacrylate/triethylene glycol dimethacrylate (Bis-GMA/TEGDMA; 1:1 wt/wt premixed resin blend) formulations containing camphorquinone (CQ) combined with ethyl 4-(dimethylamino)benzoate (EDAB), 1-phenyl-1,2-propanedione (PPD), diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), or phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO) at four concentrations were cured for 20 s with a single-emission-peak (SLED) or multiple-emission-peak (MLED) LED unit (n = 3; 96 specimens). Specimens were photographed and classified as fully polymerized, partially polymerized, or unpolymerized; degree of conversion (DC) was measured by attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) and analyzed by two-way analysis of variance (ANOVA) and Tukey’s honestly significant difference (HSD) tests (α = 0.05). Formulation, LCU, and their interaction significantly affected DC (p < 0.001), and these effects remained significant when specimens assigned 0% DC were excluded. CQ+EDAB was the only system that polymerized fully with both units (58.6–76.5%). CQ+TPO achieved a maximum DC of 68.5% with the MLED, which was not statistically significantly different from the highest value obtained with CQ+EDAB. However, when polymerized with the SLED, CQ+TPO produced unpolymerized specimens that could not be measured at any of the tested concentrations. CQ+BAPO responded to both units, more strongly to the MLED (61.2% vs. 48.1%). CQ+PPD, formulated without an amine co-initiator, failed with both units (≤6.7%). Photographic classification agreed with ATR-FTIR and recorded the outcome where spectroscopy was impossible. Higher radiant exposure outside the photoinitiator absorption band did not compensate for spectral mismatch, whereas within the band, conversion also depended on irradiance and photoinitiator concentration. Spectral compatibility should therefore be verified rather than presumed.