According to an aspect, there is provided a display unit for generating a display output, comprising a first light source; a first back polarizer arranged to polarize light from the first light source in a first polarization direction; a second light source; a second back polarizer arranged to polarize light from the second light source in a second polarization direction that is orthogonal to the first polarization direction; a first substrate; a second substrate; a liquid crystal layer positioned between the first substrate and the second substrate, wherein the first substrate, second substrate and liquid crystal layer are arranged to receive light from the first light source that has been polarized by the first back polarizer and receive light from the second light source that has been polarized by the second back polarizer; and a front polarizer arranged to polarize light, the front polarizer being for polarizing light that has passed through the liquid crystal layer; wherein operating the first light source to generate light generates a positive display output, and operating the second light source to generate light generates a negative display output.