Coverglass

Coverglass, commonly called a coverslip, is a small, thin piece of glass placed over a specimen mounted on a microscope slide. Its primary functions are to protect the specimen and mounting medium, provide a flat, optically clear surface for the objective, and help seal the specimen against evaporation and contamination. In fluorescence and high-resolution imaging, the coverglass also influences optical quality and refractive index matching.

Coverglasses are made from soda-lime glass, borosilicate glass, or, for UV applications, quartz. Thickness is standardized to ensure compatibility with objective lenses: typical grades are No. 1 (about 0.13–0.16 mm), No. 1.5 (about 0.17 mm, the most common for high-NA objectives), and No. 2 (about 0.2 mm). The most common size is 18×18 mm, with other sizes such as 22×22 mm available for particular mounting formats.

In routine light microscopy, coverglasses are used with mounting media to preserve specimens; in fluorescence and immunostaining work, they must be clean and free of autofluorescence and scratches. Special low-autofluorescence or high-NA-compatible coverglasses are chosen to optimize imaging performance. The glass should be handled with forceps and cleaned and stored in dust-free containers to prevent contamination or scratches that can compromise image quality.