Optical stack for privacy display

A display device comprising:

• a spatial light modulator;
• a display polariser arranged on the output side of the spatial light modulator, the display polariser being a linear polariser;
• an additional polariser arranged on the output side of display polariser, the additional polariser being a linear polariser;
• and
• plural polar control retarders arranged between the additional polariser and the display polariser, wherein an air gap divides the plural polar control retarders into at least one air gap input retarder between the display polariser and the air gap and at least one air gap output retarder between the air gap and the additional polariser,
• the at least one air gap input retarder being arranged to convert linearly polarised light passed by the display polariser into circularly polarised light, and
• the plural polar control retarders together being capable of simultaneously introducing no net, relative phase shift to orthogonal polarisation components of light passed by the display polariser along an axis along a normal to the plane of the plural polar control retarders and introducing a net, relative phase shift to orthogonal polarisation components of light passed by the display polariser along an axis inclined to a normal to the plane of the plural polar control retarders.