A fluorescence microscope is an optical microscope that uses specific excitation wavelengths to stimulate fluorescent labels; it does not have a dedicated SITC subheading and therefore classifies under the general optical microscope heading (SITC 87145). The same applies to phase-contrast and darkfield configurations — these are optical techniques applied to a standard compound microscope platform.

Some HTS subheadings for optical microscopes may qualify for duty-free entry under scientific instrument provisions if the importer is a qualifying nonprofit research or educational institution. This typically requires a formal application to US Customs and Border Protection demonstrating institutional eligibility and that no commercially equivalent domestic instrument is available. Importers should consult a licensed customs broker to determine whether their specific instrument and institution qualify.