Purpose: Bow‐tie filters used in CBCT image acquisition alter photon spectra across the radiation field. Furthermore, incident spectra vary along the longitudinal direction of an X‐ray tube due to the anode‐heel effect. The goal of this study is to characterize off‐axis spectral variation of a Varian® on‐board kV imager and to investigate its impact on the quality of reconstructed images. Method and Materials: Spectra were derived by fitting the semi‐empirical Birch‐Marshall (BM) model to narrow‐beam aluminum transmission data measured with a parallel‐plate ion‐chamber at different locations along the transverse axes of the OBI field. Photon spectra with and without filtration of the full and half bow‐tie filters were measured. Simple analytical models for deriving off‐axis spectra at arbitrary detector locations from the central‐axis spectrum were developed and validated against the measured spectra. Images reconstructed from synthetic datasets with constant and spatially variant photon spectra were used to assess both bow‐tie filter and anode‐heel spectral hardening effects on image quality. Results: The derived spectra accurately reproduced the transmission measurements with mean and maximum deviations of 0.6% and 2.5%, respectively. The anode‐heel effect is insignificant at small off‐axis angles. The variation of aluminum HVLs of spectra is less than 0.5 mm within 3° off‐axis angles. Bow‐tie filters significantly alter photon off‐axis spectra. The mean photon energies at 5° off‐axis angle with the full bow‐tie and the half bow‐tie filter are 78.7 keV and 67.6 keV, respectively, compared to the centralaxis mean energy of 60.0 keV. Conclusion: Our investigation demonstrates the feasibility of using narrow‐beam transmission measurements in conjunction with the BM model to characterize relative small off‐axis spectral variations. The spectra at arbitrary off‐axis angles can be adequately characterized by hardening the central‐axis spectra. The anode‐heel effect does not significantly alter beam‐hardening artifacts in the reconstructed images. Supported by Varian Medical Systems.