Optical stack having birefringent layer of optically symmetrical crystallites

An optical stack comprising:

• a plurality of alternating birefringent and isotropic layers, the birefringent layers comprising semi-crystalline polymer having optically symmetrical crystallites, wherein:
• x- and y-directions define a plane parallel to the layers and a z-direction normal to the plane, the x-, y- and z-directions being orthogonal to each other;
• the birefringent layers have refractive indices n1x, n1y and n1z in the x-, y- and z-directions, respectively;
• and
• the isotropic layers have refractive indices n2x, n2y and n2z in the x-, y- and z-directions, respectively;
• the optically symmetrical crystallites being substantially aligned such that:
• |n1x−n2x|>5|n1y−n2y|.

An optical stack comprising:

• a plurality of alternating birefringent and isotropic layers, the birefringent layers comprising semi-crystalline polymer having optically symmetrical crystallites, wherein:
• x- and y-directions define a plane parallel to the layers and a z-direction normal to the plane, the x-, y- and z-directions being orthogonal to each other;
• the birefringent layers have refractive indices n1x, n1y and n1z in the x-, y- and z-directions, respectively;
• and
• the isotropic layers have refractive indices n2x, n2y and n2z in the x-, y- and z-directions, respectively;
• the optically symmetrical crystallites being substantially aligned such that
• |n1x−n2x|>5|n1z−n2z|.

A method of making an optical stack, comprising:

• providing a film comprising a plurality of alternating first and second layers, x- and y-directions defining a plane parallel to the layers and a z-direction normal to the plane, the x-, y- and z-directions being orthogonal to each other;
• conveying the film within a stretcher;
• and
• stretching the film in the x-direction within the stretcher, such that the first layer comprises a first birefringent layer comprising semi-crystalline polymer having first optically symmetrical crystallites.