(a) Comparison of photon fluxes from the theoretical (dashed line) and practical (solid line) undulators. The acceptance angles are 0.6 mrad in both the vertical and horizontal directions. The inset shows the photon fluxes calculated using equation (6) (dots) and SPECTRA under the condition of zero energy spread (solid line). (b) The corresponding linear polarization curves. Plus and minus signs of the polarization axis correspond to polarizations in the X and Y directions.