Optical stack for imaging directional backlights

A backlight device comprising:

• a first array of plural light sources and a second array of plural light sources;
• a first waveguide comprising:
• a first input end along a first side of the first waveguide;
• and
• opposed first and second guide surfaces extending across the first waveguide from the first input end, the opposed first and second guide surfaces operable to guide light input at the first input end along the first waveguide and to extract input light through the first guide surface;
• a second waveguide comprising:
• a second input end along a second side of the second waveguide;
• and
• opposed third and fourth guide surfaces extending across the second waveguide from the second input end, the opposed third and fourth guide surfaces operable to guide light input at the second input end along the second waveguide and to extract input light through the third guide surface;
• wherein the first array of plural light sources is disposed along the first input end and arranged to input light into the first waveguide,
• wherein the second array of plural light sources is disposed along the second input end and arranged to input light into the second waveguide;
• a prismatic input layer arranged to receive the light extracted from the first and second waveguides and to direct the received light towards a normal to a plane of the prismatic input layer;
• a switchable liquid crystal retarder, wherein the backlight device is arranged so that the light from the first and second waveguides is directed to provide first and second viewing windows;
• and
• a control system arranged to control the first and second arrays of plural light sources, wherein the control system is arranged to provide switching between:
• in a first mode of operation, the first array of plural light sources being operated along the first input end, and a first applied voltage across the switchable liquid crystal retarder;
• and
• in a second mode of operation, the second array of plural light sources being operated along the second input end, and a second applied voltage across the switchable liquid crystal retarder, wherein the second applied voltage is different to the first applied voltage.