We propose a determination method of a cell thickness d and a time-dependent twist angle Φt in a twisted-nematic liquid-crystal (TN-LC) cell using circularly and homogeneously aligned LC cells and multiple wavelengths of incident light. Influences of a spectrum distribution on determining Φt and d are theoretically discussed, compared with a monochromatic laser light source. It is found that the influence of the spectrum distribution becomes stronger with an increase in the thickness of TN-LC cells. We propose a compensation method of the influence of the spectrum width in order to accurately determine Φt and d of TN-LC cells. Results of the determination of Φt and d of two TN-LC cells are presented, and the availability of the proposed determination method using three interference filters is confirmed experimentally.