A cover glass is a 100 to 200 µm thin, square (often 18 mm × 18 mm), rectangular or round glass plate. It is used in light microscopy to cover specimens. It can delay dehydration with aqueous preparations. It is important that a smooth optical surface is created through which rays of light can pass without unpredictable deflection. When using immersion oil in high-resolution microscopy, it prevents the immersion oil from mixing with the preparation.
Many microscopic preparations are made up of a one millimeter thick slide , the object to be observed itself (often in a surrounding medium such as water or glycerine) and a cover slip that covers the object. The term “cover slip” has become independent, however, as glass plates of similar thickness are also referred to as such when they are not used for covering, for example when cells grow directly on cover slips around them in a corresponding cell culture chamber from below with a microscope reversed Observe design (inverted microscope).
The cover glass is part of the optical imaging beam path and must therefore be taken into account when designing the objective . In the case of objectives that have been calculated for use with a cover glass, the image quality therefore suffers if they are used without a cover glass (and vice versa). However, up to a numerical aperture of 0.30, objectives can usually be used both with and without a cover glass; Lenses for oil immersion even up to a numerical aperture of 1.25. In the case of larger numerical apertures, an incorrect thickness or refractive index of the cover glass produces aberrations . There is increased spherical aberration . Objectives are usually made for cover glasses with a thickness of 0.17 mm and a refractive index . However, cover glasses of other thicknesses are also common. For high-resolution microscopy, either the correct cover slip for sample preparation or an objective with cover slip thickness correction ring must be used (e.g. C-Apochromat 40x / 1.2W Korr for cover slip thicknesses between 0.14 mm and 0.18 mm).
- ISO 8255-1: 2011 (E): Optics and optical instruments - Microscopes - Cover glasses - Part 1: Dimensional tolerances, thickness and optical properties