Microscopy inspection of specimens

About Microscopy

Inspection of specimens

Microscopy is used for the inspection of specimens by using artificial magnification in order to enhance the appearance of small features. This is in contrast to macroscopy, which involves inspection of a specimen using the naked eye.

Optical microscopy is used for the inspection at magnifications of up to 1,000x microstructures. Electron microscopy with magnifications of up to 500,000x is typically used for failure analysis, in R&D labs and at educational institutions.

Types of microscopy

Four types of microscopy are used in materialographic testing, depending on the nature of the workpiece and the object of investigation, and they are described below.

Optical microscopy
In optical microscopy different filters are used to improve contrast and emphasize specific features based on material properties. This can be achieved with magnifications typically ranging from 2.5x up to 1000x. In materialography, reflected light is the most commonly used type of Light Optical Microscopy. Transmitted Optical Microscopy is also used, but mainly for mineralogy specimens.

Stereo Optical Microscopy
The stereo microscope is an optical microscope variant designed for low magnification observation of a specimen, using the light reflected from the specimen surface.

Scanning Electron Microscopy
A scanning electron microscope (SEM) is a type of electron microscope that produces images of a specimen by scanning the specimen surface with a focused beam of electrons. The electrons interact with the atoms in the specimen, producing various signals that can be translated into information about the surface topography and the composition of the specimen.

Transmission Electron Microscopy
Transmission electron microscopy (TEM) uses a beam of electrons transmitted through an ultra-thin specimen and that interacts with the specimen as it passes through. Generated signals can be translated into various types of information including information on the type and orientation of individual crystals.

How to do light optical microscopy

Light optical microscopy specimen preparation

1. Specimen preparation

The surface condition of the specimen will affect the light during reflection or transmission. The acceptable level of this effect is defined by the type and size of feature relevant for the examination.

Correct specimen preparation is essential to achieve the required surface quality and contrast.

Light optical microscopy light source

2. Light Source

The image of the surface is based on the interaction between light and the surface. Different light sources such as LED, halogen or mercury, together with different illumination types such as coaxial, ring light or point light will cover a broad variety of surfaces to be inspected in terms of surface characteristics, e.g. roughness, colour and alignment.

Correct illumination is essential to investigating complex topography.

3. FIlters

Brightfield (BF) contrast is the most common contrasting technique. Only details with difference in reflectivity will be distinguished from each other.

Contrasting techniques like Darkfield (DF), Differential Interference Contrast (DIC) and Polarized light (POL) makes it possible to observe details which differ from those seen in BF.

The choice of filters in optical microscopy depends on the nature of the surface and the characteristics and details to be investigated.
Contrasting technique Darkfield plastic layers

DarkField - Plastic layers

Plastic layers in different colours which can be seen in their original colours in Darkfield.

Microscopy Darkfield Semi Opaque phases

DarkField - Quality of a polish

Fine scratches, pores and pull-outs can be distinguished better in DF than in BF. Irregularities like pores or cracks reflect the light into the lens while all the well-polished areas are dark. This technique permits easy differentiation of pores and inclusions, very fine crack propagation, and evaluation of polish quality.

Microscopy DarkField Semi opaque phases

DarkField - semi-opaque phases

Semi-opaque phases can be identified by their inherent colour, e.g. copper oxide inclusions (Cu2O) which are grey in the Bright field, but can be determined in copper matrix by their garnet-red colour in Darkfield.

Microscopy Polarised light

Polarized Light

Is used:
- for contrasting the structure of optical anisotropic metals which are difficult to etch, such as some titanium- and tin alloys, beryllium or uranium.
- to identify many intermetallic compounds and slag inclusions by their characteristic anisotropic effects.
- to differentiate between optical anisotropic phases and optical isotropic phases.
- to examine optical isotropic metals if their surface can be rendered optically active by etching (e.g. anodizing)

Microscopy Differential Interference Contrast

Differential Interference Contrast

Using DIC relief is made visible and, for example, special types of intermetallic phases can be detected by referring to their morphology.

Microscopy Flouresence


The parts of the specimen that is not flourescent remain dark and it is easy to see e.g. the cracks or pores by using florescent dye in the mounting material

4. Image aquisition

In addition to the above, capturing the true microstructure depends on factors which are important during image acquisition. The two most predominant factors are exposure and white balance.

Application specialists

The fast track to expert knowledge…