Near-infrared spectroscopy (NIR) is an established method for the rapid and non-destructive identification of various types of polymers.

In this process, the material is irradiated with near-infrared light. Characteristic molecular bonds—specifically C–H, O–H, and N–H groups—absorb specific wavelengths of the radiation, producing a material-specific absorption spectrum. Since every polymer has a unique chemical structure, a characteristic spectrum is generated that serves as a chemical fingerprint for unambiguous material identification.

This rapid and precise analysis is a crucial prerequisite for efficient sorting and high-quality plastic recycling. A well-known limitation of NIR spectroscopy concerns black plastics. If they contain carbon black as a pigment, near-infrared radiation is almost completely absorbed. Consequently, no analyzable reflection signal is generated, meaning that these materials can be identified only to a limited extent—or not at all—using NIR spectroscopy.