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.