Dynamic Image Analysis of Particle Size and Shape

Particle size as and information about the shape of powders can be determined using Dynamic Image Analysis. FRITSCH’s Particle Sizer ANALYSETTE 28 ImageSizer can be used to determine the particle size and shape of dry, free-flowing bulk solids.

Measuring Principle of Particle Sizer ANALYSETTE 28 ImageSizer

In the Particle Sizer ANALYSETTE 28 ImageSizer, the sample material is directly introduced into a falling chute through a controlled feeder. From the falling chute, the sample reaches a collection container (Figure 1). A large-area LED array flashes the particle flow on the way down and a CCD camera located on the opposite side captures the images in fast sequences. The software is used to analyze all images and display the respective selected data after completing the measurement.

Diagram of the particle sizer ANALYSETTE 28

Figure 1. Measuring principle of Particle Sizer ANALYSETTE 28 ImageSizer

Optical Set-Up

The Particle Sizer ANALYSETTE 28 ImageSizer features a 5MP camera with a 2/3 inch CCD sensor. With a combination of a lens magnification of 0.184 times and pixel size of 3.45 x 3.45µm, the system has a lens size of 18.75 x 18.75µm per pixel. Its lower measuring range is 150µm and upper measuring range achievable is 20mm, thus covering a particle size range of 150 - 20000µm with single lens.

With four different lenses, the Particle Sizer ANALYSETTE 28 ImageSizer can cover varying measuring ranges. As a result, a lower measuring range of 20µm can be achieved with the lens having the optimum magnification. Standard lens is presented in Figure 2.

Figure 2. Standard lens

Particle Characterization

In the Particle Sizer ANALYSETTE 28 ImageSizer, the number of shades of grey of the camera is 28 = 256 as the camera’s dynamic range is 8 bit. If black is 0, the corresponding white value is 255. A threshold is set in the software to decide whether a pixel corresponds to a particle or background. This threshold is not significant until the appearance of the particle via the optical system creates images with a sharp contrast between white and black. However, this parameter influences the results if a higher number of particles are out of the focus area of the lens used.

Depth of Focus

The depth of focus of the optical system is the distance area within which particles appear clearly in the images. This parameter decreases when the lens size increases, causing particles that are not exactly passing the camera’s focus area to appear as blurred smudges in the images. The software now makes a decision which particles need to be used for the assessment and where the edge of the particle is exactly positioned. This can cause issues in the case of using lenses of varying magnifications.

Image Acquisition Speed

Image acquisition speed is another factor and is expressed in frames per second or fps. The image acquisition speed of the ImageSizer’s CCD camera is up to 30fps, thus generating large volumes of data in a short time. As a result, it requires the computer hardware to have the corresponding specifications to deal with the measuring task.

It is advised to keep overlapping of two particle images passing on the same visual axis of the camera to a minimum. The total count of the obtained and measured particles is essential for the accuracy of the measurement. If this value is very low, the data’s statistical significance lowers particularly for particle sizes that do not come up too often.

Measurement Process

It is necessary to use adequate material to get reliable measurements. However, computing capacity, memory space and time should not be wasted unnecessarily. Samples having a broader particle size range tend to segregate while passing on to the feeder. Hence, it is advised to quantify the entire sample volume. It is essential to conduct a very good sample division to eliminate the issues associated with large sample volumes. For this purpose, a FRITSCH Rotary Cone Sample Divider LABORETTE 27 can be employed to divide a large sample volume into adequately small individual samples with a correspondingly matching, representative particle spectrum.

The acquired images are used to determine the particle size. However, only one value can be obtained for the particle diameter during static light scattering, thus offering an imaging system with different options to describe the diameter of an irregularly shaped particle, such as the surface equivalent diameter and Feret diameter.

The Dynamic Image Analysis not only determines the particle diameter but also acquires some information about the particle geometry (Figure 3). For instance, the aspect ratio, the simplest shape parameter, is obtained as the quotient from minimum to maximum Feret diameter.

Figure 3. Reliable reproducibility through pixel exact evaluation

The ImageSizing-Software ISS of the ANALYSETTE 28 can yield correlations and distributions in any random combination of particle data clusters. These correlations can be shown rapidly and easily in a cloud presentation. Each analyzed particle is depicted here as a point, its coordinates in the cloud based on the values of the correspondingly selected parameters (Figures 4 and 5).

Figure 4. Minimum Feret - Diameter in [µm]

Figure 5. Single image analysis from the image gallery

This information has been sourced, reviewed and adapted from materials provided by FRITSCH GMBH - Milling and Sizing.

For more information on this source, please visit FRITSCH GMBH - Milling and Sizing.

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