Complex sentence: He produced instructive demonstrations involving the incandescent conversion of infrared into visible light at the molecular level, which he called calorescence, in which he used materials that are transparent to infrared and opaque to visible light or vice versa.
Split into simpler sentences: He produced instructive demonstrations involving what he called calorescence, which is the conversion of infrared into visible light at the molecular level. Among his key laboratory tools were substances that are transparent to infrared and opaque to visible light; or vice versa.