A system includes a plate configured for mounting of a reflective extreme ultra-violet (EUV) mask thereon and a zone plate configured to divide EUV light into zero-order light and first-order light and to pass the zero-order light and the first-order light to the reflective EUV mask. The system further includes a detector configured to receive EUV light reflected by the EUV mask and including a zero-order light detection region configured to generate a first image signal and a first-order light detection region configured to generate a second image signal, and a calculator configured to generate a compensated third image signal from the first image signal and the second image signal. The third image signal may be used to determine a distance between mask patterns of the EUV mask.