Apparatus selectively processes a substrate using radiant energy. The substrate can consist of any target material having a portion to be processed using the radiant energy and a larger portion to be unprocessed. The apparatus includes a source of radiant energy (preferably a quantum cascade laser) that has a customizable spectrum that can be configured to be specifically absorbed only by the portion to be processed, and a control system for targeting the radiant energy only at the portion to be processed. Specific examples of the use of the apparatus and method are in the technologies of heat-shrinking polyethylene film, fusing toner to paper in a laser printer, heating reaction vessels in DNA testing, and temperature profiling bottle pre-forms.