Impurities such as Fe and Mg are intentionally incorporated into Ga2O3 to control the intrinsic n-type conductivity. This work examines the defect level of the intentional and unintentional impurities in β-Ga2O3 substrates grown for electronic applications. First, several dominant impurities, Fe3+, Mg0, and Ir4+, are identified using electron paramagnetic resonance spectroscopy. Then, by illuminating the crystals with selected wavelengths between 1500 and 300 nm, the photothreshold for excitation or quenching of each impurity is examined. Defect levels are estimated from the photothreshold after considering interdefect transitions and lattice relaxation. Thresholds for Ir4+ (2.0-2.3 eV) and Fe3+ (1.2-1.5 eV) support the reported values of the Ir3+/4+ and Fe2+/3+ defect levels, but the photothreshold of 1.6-1.9 eV observed for the quenching of neutral Mg requires further consideration.