This heading functions as a residual: if a mounted optical element is a lens, it goes to the appropriate lens heading; if it is a filter, it goes to the filter heading; if it is part of a complete instrument presented as such, it may be classified with that instrument. Articles that do not fit those more specific descriptions—prisms, mirrors, diffraction gratings, waveplates, beam splitters—belong here when mounted. The classification analysis should start with the more specific headings and arrive here only after ruling them out.

Detailed technical specifications—including the element type (prism, mirror, grating), substrate material, coating specifications, wavelength range, and intended application—should accompany the commercial invoice. For high-value precision optics, a manufacturer's datasheet or certificate of conformance is advisable. This documentation supports both correct classification and accurate customs valuation, which can be scrutinized for high-unit-value optical components.