A complete objective lens assembly — glass elements housed in a mechanical barrel — classifies as a mounted optical element under this heading rather than as a mechanical part. The optical function governs, and the barrel is considered part of the mounted assembly. If the barrel is imported separately without the optical elements, it may classify as a part under 900211 or 900219 depending on the intended instrument.

Optical filters designed and presented as optical elements — even when destined for laser or industrial systems — generally classify under 900220 as mounted optical filters. However, if a filter is an integral, non-separable component of a complete machine presented together, the complete machine's heading may govern. A binding ruling from CBP is advisable for integrated assemblies.