Coverglass Guide

Coverglasses are an essential tool for microscopy, it is at thin sheet of glass (or plastic) that is placed over the specimen after it has been transferred to a microscope slide. Coverglasses have multiple uses, they are used to protect specimen samples from contamination and physical damage, they provide and maintain a flat specimen and flat surface for microscopic examination eliminating refraction of light, and aberrations and lastly, they prevent curling and movement of the sample. Coverglasses also prevent the drying of liquid specimens by trapping the liquid between the slide and the cover glass, maintaining a suitable environment for observation.

There is a wide range of coverglasses to choose from, however, when selecting the right coverglass for you and your processes there are some specific characterisations you should consider that make a big difference to your results and are more appropriate depending on the specimen you are observing.

Thickness

When it comes to selecting the correct coverglass for your specimen or microscope, it is essential you understand the importance of the thickness of the coverglass you select and the wide range of thicknesses that are available.

Here’s why selecting the correct coverglass thickness is important:

1. Optical Compatibility: Many high-power objective lenses are designed to work optimally with a specific coverglass thickness. For example, standard high-resolution objective lenses such as those used for fluorescence microscopy and other high-magnification applications, are typically corrected for a No. 1.5 coverglass thickness.
2. Corrected Aberrations: Using the correct coverglass thickness ensures that spherical aberrations and other optical distortions are minimised.
3. Resolution and Clarity: The correct thickness maintains the optical path length that the microscope objective is designed for, ensuring optimal focus and sharpness. Deviations from the intended thickness can degrade image resolution and contrast. This also includes affecting the refractive index and light path, influencing image quality and brightness; providing inaccurate results.
4. Sample Protection: Thinner coverglasses are preferred for delicate samples to minimise the weight and pressure on the specimen.
5. Mounting Media: The thickness of the coverglass should be compatible with the mounting medium and the sample preparation technique used, ensuring that the sample is correctly placed between the slide and the coverglass without air bubbles or gaps.
6. Live Cell Imaging: For live cell imaging, thinner coverglasses are often used with specialised chambers or dishes to facilitate close proximity of the objective lens to the sample while maintaining the cells in a suitable environment.

When selecting the correct coverglass and choosing the right thickness for your process, coverglasses have an associated ‘number’ for the relevant thickness of the glass. Once you know the associated number, it will make searching for the right coverglasses easier for you. See below the table of ‘numbers’ with the range of glass thickness they refer to:

 

Shapes

Coverglasses come in various shapes, and depending on what speciality of Pathology or life science you work within, or the type of specimen you are examining will determine what shape of cover glass you require:

1. Square Coverglasses: Square coverglasses are versatile and widely used for general microscopy, including histology, cytology, and microbiology. They provide a broad, flat surface ideal for covering standard microscope slides and accommodating a wide range of sample types. Specific applications include:

• Tissue Sections and Histology

2. Rectangular Coverglasses: Rectangular coverglasses are often used for larger specimens or when a wider viewing area is required. They are useful in applications such as whole mount tissue sections, blood smears, and extended sample preparations. Specific applications include:

• Fluorescence Microscopy
• Tissue Sections and Histology
• Blood Smears and Cytology

3. Circular Coverglasses: Circular coverglasses are frequently used in applications requiring precise positionings and minimal movements, such as live cell imaging, immunofluorescence, and cell culture studies. They fit well into Petri dishes, multi-well plates, and specialised chambers designed for circular coverglasses. Specific applications include:

• Live cell imaging
• Fluorescence microscope
• Petri dishes
• Multi-well plates

*Bespoke sizes: Solmedia offer provides a bespoke cutting service that allows you to choose your ideal dimensions and thicknesses that is compatible with your machines and processes.

 

Other Characteristics

Coverglass have a vast amount of qualities that can influence the quality of results. The following characteristics must also be considered:

1. Transparency: High optical clarity is crucial for coverglasses to ensure minimal distortion and high-quality imaging. They are designed to be as transparent as possible, with minimal impurities and inclusions.

2. Flatness: Coverglass must be extremely flat to ensure an even surface for accurate focusing and imaging.

3. Refraction Index: The refraction index should match the imaging system (typically around 1.52 for glass) to minimise light refraction.

4. Chemical Resistance: For applications involving biological and chemical staining or fixatives, chemically inert cover glasses are required to prevent reactions that could affect the specimen or the imaging.

5. Coating: Some coverglasses are available with special coatings, such as anti-reflective coatings, hydrophobic coating (for liquid handling), or conductive coatings (for electron microscopy), depending on the specific application requirements.

 

Solmedia’s Coverglass Guide