A design method is proposed for low-cross-talk twisted nematic liquid-crystal optical switches. In this method the transmission spectrum of a liquid-crystal cell is simulated by using parameters such as wavelength, refractive index of the liquid crystal, cell thickness, and spacer density. With this simulation, it becomes possible to define the parameters that give the lowest cross talk at a given wavelength. Liquid-crystal cells are fabricated by using the simulation results, and the transmission spectra are characterized. A very low cross talk of -32 dB is achieved for a wavelength of 633 nm.