Dynamic image analysis is a perfect method, if the particle size of powders and information about their shape needs to be determined.
For this purpose, the FRITSCH GmbH’s Particle Sizer ANALYSETTE 28 ImageSizer is a functional, low-cost instrument that allows taking dry measurements of bulk solids and free-flowing powders, and supports measurements with a wet dispersion unit.
Measuring principle Particle Sizer ANALYSETTE 28 ImageSizer
The measuring principle of the ANALYSETTE 28 ImageSizer is explained below:
Particles are passed in front of a large scale LED flash, and a digital camera then photographs the particles in back light in fast sequences.
This optical set-up is comparable with that of transmitted light microscopy, where high contrast is obtained between the homogeneously-lit, light background and the particles shadowing the light. All of the images are analyzed with specialized software, and after the measurements are completed, the corresponding selected data is displayed.
The magnification performance of the utilized lens regulates the size of the generated image on the camera sensor or retina, as with a standard camera or microscope.
For a certain combination of lens and camera, the yield from the sensor parameters (overall size and pixel size) and the magnification limit the obtainable size range. A mega pixel camera with a 2/3 inch CMOS sensor is present within the ANALYSETTE 28 ImageSizer.
A lens size of 10 µm per pixel is obtained with a pixel size of 3.45 µm when the lens magnification is 0.35 times. If, for the lower measuring range of the system, images of particles of at least 8 x 8 pixels are required, then a lower measuring range of 80 µm can be obtained.
Similarly, the upper measuring range of 10 mm can be achieved with a combination of camera and lens when the particle size ranges from 80 to 10000 µm.
All of this is possible under the assumption of a constant magnification factor, if the distance of the camera to the particle is always constant when regular camera lenses are used.
If the distance changes, the detected particle size is falsified on the same scale. FRITSCH uses tele-centric lenses, because it is not possible to guide the particles exactly on a plane when they are made to pass the camera. The distance between camera and object does not regulate the size of the image generated on the sensor, as in the case of conventional camera lenses.
The ANALYSETTE 28 ImageSizer has four different lenses to cover varying measuring ranges. It is simple to manually change the lenses rapidly.
Various tele-centric lenses for different measuring ranges
The software recognizes lighter areas as background and dark ones as particles. However, there are many gradations between dark and light: the amount of available levels of gray of the camera is 28 = 256 (i.e. the dynamic circumference of the camera is 8 bit).
A value of 255 denotes complete white, and a 0 represents black. A threshold is specified in the software to determine whether a pixel belongs to a particle or to the background. This threshold can be easily and individually adapted for transparent glass spheres and other such very specialized sample systems.
Depth of Focus
The depth of focus is another parameter of the optical system that has to be taken into account. The depth of focus represents the distance area within which a particle appears sufficiently clear. When magnification increases, the depth of focus of a lens decreases. In microscopy, when magnification is increased, it is more difficult to produce a focused image.
A transition from black to white is gradually shown for the edges of particles that do not exactly pass the focal plane of the camera. Based on this transition, the software determines which particles are still adequately visualized in order to be considered for evaluation.
Image Acquisition Speed
In addition to the sensor, pixel size, and depth of focus, the image acquisition speed, usually expressed in frames per second (fps), is an important factor, although it has no central role for most applications. The camera of the ImageSizer obtains up to 75 fps.
Such high image rates generate enormously large volumes of data quickly, and this requires corresponding specifications from the computer hardware to handle the measurements. For instance, data management for large data becomes difficult if dry measurement has to be completely taken for a large sample amount, and all images need to be saved.
By not permanently saving all images used for result analysis or obtained during the measurement, such difficulties can be easily reduced. Using this method, each and every individual particle cannot be viewed later, although it is not necessary for routine measurements.
The question “how much have actually be measured” depends on the respective sample and the question linked to the measurement. However, for most tasks, several ten to hundred thousand particles are enough. This may be even less for large particles in the high millimeter range. The quantity of sample required is different in dry and wet measurements.
In a dry measurements, the sample material is continuously fed to the measuring process via a funnel and a vibratory feeder, where the sample feed speed (how much material is fed through per minute) can only be increased within fixed margins.
The overlapping of two particle images, which pass the same visual axis of the camera must be kept as low as possible.
Using this strategy, a sample particle is available only once for analysis. In order to avoid possible segregation tendencies influencing the result, the entire sample amount which was added to the feed system may need to be processed, particularly for samples having a broad particle size range. Particles of sizes from approximately 20 µm up to approximately 20 mm can be measured in dry measuring.
To provide statistically dependable measurements, sufficient amounts of material must be used, but care must be taken to avoid unnecessary wastage of storage space and time. A good sample division needs to be performed to avoid problems with analysis amounts that are too large.
For instance, a FRITSCH Rotary Cone Sample Divider LABORETTE 27 can divide a large overall sample amount into many small individual samples with identical representative particle size spectrums.
Good sample division is more important for a wet measurement than for a dry one. When a closed liquid circuit with a total volume of approximately 500 ml, is pumped continuously through a measuring cell, the necessary sample amount will be less when compared to that obtained with a dry measurement.
However, the upper measuring limit is determined by the geometry of the measuring cell during the wet measurement. Particles of approximately 20 µm up to 2.8 mm can be measured with the wet measuring unit of the ANALYSETE 28 ImageSizer.
Reliable reproducibility through pixel exact evaluation
The obtained images can be used initially to determine the particle size. Although an imaging system offers different possibilities to define the diameter of an irregularly-shaped particle, during static light scattering only one value is given for the particle diameter.
Examples of this are the diameter calculated from the particle circumference, the surface equivalent diameter (a sphere’s diameter where the cross-sectional area of the sphere is the same as that of the evaluated particle), or the Feret diameter, where two parallel lines on opposing sides of a particle are arranged so that they touch the particle, but do not intersect the particle edge.
The Dynamic Image Analysis is advantageous, as the basic determination of the diameter and information about the geometry of the particles can be obtained. The aspect ratio is one of the simplest shape parameters, and is the difference from minimum to maximum Feret diameter.
It is possible to easily and quickly generate distributions and correlations in any random combination of particle sizes by using the ImageSizingSoftware ISS of the ANALYSETTE 28. This can be done as a simple size distribution or the connection between the aspect ratio and the particle size.
Such correlations can be easily and quickly displayed in a Cloud presentation. Each analyzed particle is shown as a point, and its coordinates in the Cloud depend on the values of each selected parameter. One of the features of the cloud is particularly helpful with new sample materials and the analysis of problematic cases: by clicking on the point of a selected particle, the corresponding image opens.
Minimum Feret - Diameter in [µm]
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.