English (United Kingdom)
Particles are detected using a threshold measurement and then checked for being in focus before any measurements are made. Measuring out of focus particles would reduce the accuracy of the data produced.
The ViPA measures the Feret diameters of all 'in-focus' objects at four fixed angular intervals. This is because not all particles are spherical and the only particle that can be accurately described by a single size measurement is a sphere.
The ViPA directly measures the projected perimeter of each particle by first counting the number of pixels that make up the particle boundary and then using an 'length per pixel' conversion factor to determine the overall length of the particle perimeter in microns.
The ViPA directly measures the area of each particle by first counting the number of pixels in a particle and then using an 'area per pixel' conversion factor to determine the overall area of the particle in square microns.
Using these and other measurements it is possible to calculate some classical measurements related to particle shape that will enable ViPA to differentiate between the specific types of particle being measured, for example, oil droplets and gas bubbles, sand grains and bio-growth and so on.
Each particle, droplet or bubble that is analysed within an image captured by the ViPA video microscope is analysed in many different ways. Among the fundamental measurements made are size, area and perimeter; size is measured in four orientations as not all particles are round. From these measurements we can determine aspect ratio, critical in specifying and understanding filter performance, and shape factor, a measure of the roundness of a particle which is very useful in distinguishing between different types of particles. In fact, each particle is measured and assessed in 18 different ways, enabling the ViPA software to accurately classify different materials types and report on each particle population separately; and it does this at a rate of around 20 images every second.
