focus on Laboratory Products
Particle Analysis with a CCD Camera Dr Günther Crolly, Product-Manager Particle Sizers, Fritsch GmbH, Milling and Sizing, Email:
crolly@fritsch.de
Dynamic Image Analysis is a perfect method if not only the particle size of powders is to be determined, but if you also want to ascertain something about their shape. For this purpose the Fritsch GmbH provides the Particle Sizer ANALYSETTE 28 ImageSizer, a functional, economically priced instrument, which is utilised for dry, well free-fl owing bulk solids.
Measuring Principle
The underlying measuring principle is quickly explained: Via a controlled feeder the sample material is directly fed into a falling chute from which it falls down into a collection container.
On the way down the particle fl ow is fl ashed by a large-area LED array and photographed in fast sequences by a CCD camera arranged on the opposite side. Therefore, the optical set-up is comparable with transmitted light microscopy, where a high contrast is obtained between the homogeneously lit, light background and the particles shadowing the light.
All images are then analysed via software and the respective selected data is displayed after the conclusion of the measurement.
Optical Set-up
As with every microscope or common camera, the size of the generated image on the retina or camera sensor depends on the magnifi cation of the utilised lens. In addition, the parameters of the camera sensor are important for the quality of attainable results. Substantial parameters are the complete size of the used sensor, the number of pixels and the maximum possible image acquisition rate.
Inside the ANALYSETTE 28, runs a 5 mega pixel camera with a 2/3 inch CCD sensor. The pixel size is at 3.45 x 3.45 µm, which in combination with a lens magnifi cation of 0.184 times, results in a lens size of 18.75 x 18.75 µm per pixel. Now if for the lower measuring range of the system images of at least 8 x 8 pixels are required, a lower measuring range of 150 µm is obtained. Through similar deliberations in this combination, the upper measuring range of 20 mm is achieved, i.e. the particle size range of 150 - 20000 µm is covered with one lens.
Altogether four different lenses are available for the ANALYSETTE 28 ImageSizer with which varying measuring ranges can be covered. The lens with the maximum magnifi cation then achieves a lower measuring range of 20 µm.
Particle Characterisation
How does the instrument recognise the particles? The number of available shades of grey of the camera is 28
= 256 (i.e. the dynamic
Measuring principle of the Particle Sizer ANALYSETTE 28 ImageSizer
range of the camera is 8 bit). Complete white corresponds with a value of 255, black is 0. In the software a threshold is set where it is decided whether a pixel belongs to the background or to a particle. As long as the appearance of the particle through the optical system generates images with a sharp contrast between black and white, the choice of this threshold is not very critical. But, if an increasingly large number of particles are outside of the focus area of the utilised lens, a clear infl uence of this parameter on the results can be observed.
Standard lens Depth of Focus
An additional parameter of the optical system comes into play now, the depth of focus. It describes the distance area within which a particle appears suffi ciently clear. Basically, the depth of focus decreases with increasing lens size. Maybe this is familiar from personal experiences in microscopy where with large magnifi cations it becomes increasingly diffi cult to create a well- focused image. This causes particles which do not exactly pass the focus area of the camera to appear as more or less blurred smudges in the images. Now the software has to decide which particles appear suffi ciently enough to be considered for the evaluation and where exactly the edge of the particle lies. This can lead to problems if lenses with varying magnifi cations are used.
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