It depends on the fiber material. Polymer fibers will generally attenuate UV very dramatically, even at somewhat longer wavelengths - say, the 350-400nm region that is of particular interest with alt. process printing. Glass fibers will also attenuate these wavelengths, but much less so, with generally acceptable losses if the thickness of the material (in this case the length of the fibers in the faceplate) remains below a few mm or so. For both materials, attenuation increases as wavelength decreases, with the implication that anything below ca. 350nm gets attenuated dramatically. But since this part of the spectrum is usually a little less relevant for alt. processes (the light sources we use generally emit mostly in the 350-400 part of the spectrum), it's of lesser concern.