| http://www.w3.org/ns/prov#value | - In the film making art, a material such as PET is said to be positively birefringent because the index increases in the stretch direction, but in the art of optics, the same material, after biaxially stretching to film, is said to have uniaxial negative birefringence because the z-index is lower than the in-plane indices which are substantially equal.
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