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The most important criterion for choosing a system is the size of the smallest particle. For particles of 5 micrometers and more, for example, a materials microscope with fixed optics is recommended. For particles of 20 to 50 micrometers you can use a macroscope with zoom optics.
The calibration value indicates the size of a pixel in the digital image (micrometer/pixel). It is calculated from the pixel size of the camera chip and the total magnification of the optical system. The higher the optical magnification or the smaller the pixel size of the camera, the smaller the calibration value. It‘s important to remember that optical resolution should be in adequate proportion to the calibration value!
Optical resolving power determines whether two objects, for instance particles, can be imaged as separate entities. The smaller the optical resolution ‘d’, the smaller the distance between two points.
With many products, for example moving components such as turbochargers or cylinders, the height of a particle is a critical parameter. Particle height can be measured by field depth. To do this, you need an objective with a high magnification and a low depth of field (e.g. 20x).
To measure the height you need optics with low field depth. The necessary axial resolution is only provided by light microscopes with fixed optics. Stereomicroscopes or macroscopes have high field depth and are therefore unsuitable for measuring z height.
At least 10 pixels are needed to render an image of the particle length (VDA19/ISO16232). 1 pixel corresponds to the calibration value of the system, e.g. 5 micrometers/pixel.
In image analysis, the length of a particle is given as feretmax or feret length. This is determined by plotting tangents at various angles around the particle in a similar way to using a caliper rule. The largest distance is called feretmax or feret length. Similarly, feretmin is the smallest measured tangent distance.  
Oblong particles are aligned in a flow pattern. The length of a particle alone (feret max) would not give an adequate representation of the damage potential in products such as jets. In this case, the breadth is a critical parameter.
The maximum inner circle diameter is another measurement parameter for the particle breadth. To take this measurement, various circle diameters are laid in the particle. The maximum inner circle diameter is a better indicator of the damage potential of bent particles or fibers; feretmin values are too high. In the case of compact particles, on the other hand, feretmin is usually a more accurate indicator of damage potential. If there is any doubt, the maximum damage potential of a particle is always indicated by the feretmin value.
The feret length is too short in the case of bent fibers. Therefore, the so-called fiber length is used as a measure of fibers. This value is obtained by stretching the fiber mathematically.
Automatic differentiation of fibers and particles can be achieved by using the length-to-breadth ratio. The fiber length is calculated by mathematical stretching of the fiber. The maximum inner circle diameter is taken as the breadth. If the fiber is long in proportion to the maximum inner circle diameter, the object is classified as a fiber.
Reflections are visible in incident light brightfield or darkfield.
Using crossed polarizers eliminates reflections on the particles – they appear homogeneously black or gray. This method is therefore suitable for determining such measurement parameters as length, breadth and area.