They are acrylics like the GEs...I have held one before from a OVZ....we have a lot of those refractors in OVZs in my area.....they can yellow but not as bad as poly.......
I would of preferred them to use Glass refractors. xD But I don't see Xcel using many small fixtures with Glass refractors.. I see them from time to time, but they are mostly plastic...