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Detection of the 21-cm signal coming from the epoch of reionization (EoR) ischallenging especially because, even after removing the foregrounds, theresidual Stokes $I$ maps contain leakage from polarized emission that can mimicthe signal. Here, we discuss the instrumental polarization of LOFAR and presentrealistic simulations of the leakages between Stokes parameters. From the LOFARobservations of polarized emission in the 3C196 field, we have quantified thelevel of polarization leakage caused by the nominal model beam of LOFAR, andcompared it with the EoR signal using power spectrum analysis. We found that at134--166 MHz, within the central 4$^\circ$ of the field the $(Q,U)\rightarrowI$ leakage power is lower than the EoR signal at $k<0.3$ Mpc$^{-1}$. Theleakage was found to be localized around a Faraday depth of 0, and the rms ofthe leakage as a fraction of the rms of the polarized emission was shown tovary between 0.2-0.3\%, both of which could be utilized in the removal ofleakage. Moreover, we could define an `EoR window' in terms of the polarizationleakage in the cylindrical power spectrum above the PSF-induced wedge and below$k_\parallel\sim 0.5$ Mpc$^{-1}$, and the window extended up to$k_\parallel\sim 1$ Mpc$^{-1}$ at all $k_\perp$ when 70\% of the leakage hadbeen removed. These LOFAR results show that even a modest polarimetriccalibration over a field of view of $\lesssim 4^\circ$ in the future arrayslike SKA will ensure that the polarization leakage remains well below theexpected EoR signal at the scales of 0.02-1 Mpc$^{-1}$.

Polarization leakage in Epoch of Reionization windows: I. LOFAR observations of the 3C196 field