Observation of pore structures in thin section by traditional impregnation with blue-dyed epoxy becomes difficult when the pores are smaller than about 1 micrometer and/or linear in shape. These types of pores are common in many low-permeability gas reservoirs, such as coal, shale, and tight sandstone.

Incident-light fluorescence microscopy can be used to observe small, narrow pore structures in tight rocks. This common medical technique can be adapted to petrography by staining the impregnation epoxy with fluorescent rhodamine B dye along with the usual blue dye, and using an incident-light microscope equipped for epifluorescence to observe the prepared thin section.

Under excitation of green light at a wavelength of 5,400 Angstroms, rhodamine fluoresces a brilliant reddish orange, clearly showing impregnated pore spaces. A dichromatic interference filter in the light path passes the reddish orange light and blocks nonfluorescing mineral grains so that only the pore structure is visible. Fluorescing epoxy behaves as an emission source, and even submicron-size pores, which may be narrower than the wavelengths of visible light, become visible if they are impregnated. The incident-light configuration of the microscope also allows the observation of impregnated pore spaces in opaque materials such as coal. Fluorescence microscopy not only has potential for studying pore structures in tight sedimentary rocks, but also can be applied to crystalline rock and other materials with narrow, linear, or small pores.