The invention comprises methods for the photolithographic patterning of features in a photo-curable polymer composition coated onto a plastic substrate. In one embodiment of this invention, the plastic substrate is coated with a reflective film such as a metallic barrier. In another embodiment, the plastic substrate is coated or co-extruded with a polymer barrier layer containing an additive that absorbs the photo-curing radiation. In yet another embodiment the plastic substrate contains an intrinsic additive that absorbs the photo-curing radiation. Combinations of these embodiments are also within the scope of this invention.The methods of the present invention may be advantageously applied to the fabrication of optical waveguides comprising a photo-curable polymer supported on a plastic substrate, but are applicable to the fabrication of any device or object comprising a photo-curable polymer supported on a plastic substrate.