51 Image Acquisition Speed
Another important factor besides the depth of focus, sensor and pixel size is the image acquisition speed, but for most applications it does not play a central role though. The image acquisition speed is usually given in frames per second ‘fps’. The CCD camera of the ImageSizer obtains up to 30 fps. With such high image rates enormously large volumes of data are generated within a short time period of time, demanding the corresponding specifications from the computer hardware in order to handle the measuring task. Furthermore if a large sample amount is to be completely measured, this easily leads to only hard to handle amounts of data and enormous measuring times, since the sample feed speed – how much material per minute can be conveyed - can only be increased in limits. The overlap of two particle images which coincidentally pass on the same visual axis of the camera shall be kept as low as possible.
An important criterion for the exactness of the measurement is the total number of the obtained and evaluated particles. If this is too low, the statistical significance of the data decreases especially for particle sizes which do not occur too often.
In Practice
In practice it is to be deliberated that sufficiently enough material has to be used to allow dependable measurements. But not too much, in order not to waste unnecessary time, computing capacity and memory space. It is also important to know that samples with a wider particle size range easily segregate on the feeder during the conveyance, so it is recommended to always measure the complete sample volume. In order to avoid the problems of a too large analysis amount, a very good sample division should be conducted. For this, a Fritsch Rotary Cone Sample Divider LABORETTE 27 can be used, which divides a larger total amount into sufficiently small individual samples with a correspondingly identical, representative particle spectrum.
Evaluation
Now what can you do with all these obtained images? Initially of course the particle size can be determined. However here begins the agony of choice: for example, during static light scattering only one value is given for the particle diameter. An imaging system offers different possibilities to define the diameter of a mostly irregularly shaped particle. Examples for this can be the surface equivalent diameter (the diameter of a sphere, which its cross section displays the same area as the evaluated particle), the diameter calculated from the particle circumference, or the so called Feret diameter, where two parallel lines on opposing sides of a particle are arranged so that they meet the particle, but on no area intersect the particle border.
Single image analysis from the image gallery
A decisive advantage of the Dynamic Image Analysis lies of course in the possibility besides the basic determination of the diameter, also obtaining information about the geometry of the particles. As one of the simplest shape parameter, the aspect ratio shall be mentioned here, which is yielded as the quotient from minimum to maximum Feret diameter.
With the ANALYSETTE 28 ImageSizing-Software ISS it is possible to generate distributions and correlations in any random combination of particle data clusters. Whether this is only a simple size distribution or the connection between the particle size and the aspect ratio. Such correlations can be displayed simple and fast in a Cloud presentation. Each analysed particle is shown here as a point, its coordinates in the Cloud depend on values of the correspondingly selected parameters.
Minimum Feret - Diameter in [µm]
Application
And where can all this be applied at now? Generally always when particles are measured which initially can be obtained with the utilised measuring range and which do not agglomerate or stick to the feeder, i.e. all free-flowing, dry and fat-free powers.
Reliable reproducibility through pixel exact evaluation
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