Film supertwisted nematic liquid crystal displays (F-STN-LCDs) with a single birefringent plastic film can be used for monochromatic black-and-white (B/W) LCDs, but their contrast ratio is inferior to that of Double layered (D-)STN-LCDs. By measuring the wavelength dependence of the ellipticity and azimuth in STN-LCDs, the F-STN cell conditions which increase the contrast ratio are obtained. The incident linearly polarized light needs to pass through the STN-LCDs without changing the state of its polarized light in all wavelength regions. Accordingly an optical compensating layer which has the same wavelength dependence for the ellipticity and azimuth as the driving STN cell, must be positioned between the polarizer and the driving STN cell. By laminating two birefringent films with different optical axes, the optical activity caused by the liquid crystal's twist structure is canceled, and the contrast ratio of the F-STN-LCDs increases from 10:1 to 20:1.

Film supertwisted nematic liquid crystal displays (F-STN-LCDs) with a single birefringent plastic film can be used for monochromatic black and white (B/W) LCDs, but their contrast ratio is inferior to that of double-layered (D-) STN-LCDs. We optimized the wavelength characteristics of the birefringent film refractive index anisotropy (Δn) through calculations and experiments to increase the contrast ratio of F-STN-LCDs. It was found that the wavelength characteristics of the Δn should be large at shorter wavelength regions and small at longer wavelengths. Moreover, the main axis in exit elliptical polarized light of the film must be crossed perpendicular to that of the STN-LC cell in all wavelength regions to obtain a high contrast ratio of the B/W F-STN-LCD.