The purpose of this work was to characterize the stem-effect signal and the Cerenkov light ratio (CLR) in various light guides as functions of measurement geometry and magnetic field strength. Two PMMA-, two silica-, and one polystyrene-based light guides were considered in this work. Spectra measurements were performed as functions of depth, fiber-beam angle, and magnetic field strength using an optical spectrometer. All measurements were performed using a clinical linear accelerator at a nominal photon beam energy of 6 MV. Depths ranging from 1 cm to 10 cm, fiber-beam angles ranging from 90 degrees to 30 degrees, and magnetic field strengths ranging from 0 T to ± 1.40 T were investigated. The CLR was calculated from each spectrum by taking the ratio of the integral signal between 400 nm and 500 nm to the integral signal between 500 nm and 600 nm. A maximum increase of 80.5% in the stem-effect signal was observed in the magnetic field. Variations in spectral shape and, consequently, the CLR were observed for all of the fibers as functions of magnetic field strength and measurement geometry, particularly for wavelengths less than 400 nm. The plastic fibers exhibited decreases in the CLR as a function of magnetic field strength at all depths investigated, whereas the silica fibers exhibited increases in the CLR with decreasing magnetic field strength. A maximum variation of 11.1% in the CLR was observed for the polystyrene fiber due to the magnetic field. The sensitivity of the CLR to the magnetic field decreased as the fiber-beam angle decreased. The measured spectral response, shape, and CLR were found to be sensitive to the applied magnetic field strength and polarity where the variations in response were unique to each fiber.