The resonance characteristics of a twisted-nematic liquid crystal Fabry-Pérot wavelength filter (TN-FP) is analysed and it is shown that it obeys some simple laws hitherto unknown. A method to calculate the transmittance and eigenstate of polarizations of the filter is first demonstrated. Next, approximate expressions of resonance wavelengths at some of the voltage regions are derived and connection formulae for two series of modes are presented. Finally, a transition of the eigen polarization in the FP etalon is investigated, and an interpretation of a paradoxical property concerning the longitudinal mode number of the filter is given.
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