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This is the cross-section aspect ratio of the hair and is usually between 0.85 and 1.0 for realistic hair. This is most often used when rendering curly hair. (Note this depends on the hair normals, you should supply user normals through a PrimVar other than specifying N.)

Glow

Glow can be used to create hair fibers that appear to emit light or "glow" in the scene. 

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