An additive manufacturing device includes a container for containing feed stock material and having an optically transparent bottom wall. The additive manufacturing device further includes a radiation source configured to emit radiation in an optical path incident on the bottom wall and a sensor positioned in the optical path of the radiation above the bottom wall. The sensor is configured to sense a temperature of feedstock material at a reaction interface as the radiation polymerizes at least a portion of the feedstock material. The sensor is constructed to meet a transparency threshold and a flexibility threshold.