The phrase image analysis encompasses any technology which involves the processing of captured images. For the purposes of particle characterization the major division is between static image analysis and dynamic image analysis. The distinction is whether particles are presented in a static (stationary) orientation or dynamic, flowing past the detector. Here we discuss dynamic image analysis, also known as digital image processing and the improvements implemented in the CAMSIZER technique of particle size and particle shape analysis.
The CAMSIZER measuring principle involves the projection of particle silhouettes (shadows) in transmitted light. The classic CAMSIZER presents sample particles to a single light source as the particles freely fall from a feeding chute. The CAMSIZER X2 extends this to include a second light source and multiple presentations (free fall, compressed air dispersion, liquid suspension). In each instrument two cameras record the projected shadows.
The CAMSIZER Measurement Principle.
The CAMSIZER technique consists of two cameras. The basic camera measures the big particles and ignores the small ones and the zoom camera measures the small particles. The two measuring ranges result in a very high resolution and a wide dynamic measuring range. The patented Dual Camera Capture is unique to the CAMSIZER P4 and CAMSIZER X2 particle size analyzers.
The CAMSIZER Dual Camera Technique
The CAMSIZER detectors (cameras) are very similar to those used in digital cameras. And like a digital camera, the resolution of the image is determined in part by the number and size of the pixels in the detector. A particle detection occurs when at least half of any pixel is covered by the projected shadow. So the detector doesn't actually capture a perfect image of the particle, it captures a reconstruction of the particle using square pixels (see figure below). The lower 30 µm size limit of the classic CAMSIZER is set by the requirement that at least two adjacent pixels are activated. This requirement drastically increases the likelihood that the detection event is, in fact, caused by a particle.
Resolution of the classic CAMSIZER cameras.
Now that the particle shadow is captured the question becomes: which size(s) should be calculated? For some particle size analysis techniques this is a trivial question because only one particle size may be calculated. Often the equivalent spherical diameter is reported because the spherical assumption simplifies the mathematical model used. By actually capturing an image of the particle, the CAMSIZER makes it possible to calculate various lengths, widths, equivalent diameters, etc. Shape metrics including sphericity, aspect ratio, symmetry are also calculated.
These options are available because the CAMSIZER software scans the particle shadow in 64 directions. This approach allows for a more information-rich characterization of irregularly shaped particles -- particles which constitute the majority of applications. Such understanding directly leads to process and product quality improvements. Or, where historical sieve data must be matched, an intelligent correlation can be achieved.
The CAMSIZER scans every particle in 64 directions.
View our Webinar: Size and Shape of Particles from Dynamic Image Analysis
The large field of view provided by the Dual Camera Capture ensures that every particle presented to the detectors is measured. This results in excellent repeatability and reproducibility, even with very small amounts of sample. Operator-to-operator, lab-to-lab, and instrument-to-instrument precision are outstanding because of robust construction and simple operation owing to electronic task files used to control the CAMSIZER. The result is fewer result-matching headaches, decreased labor costs, and lower re-work rates. A typical measurement finishes within three minutes and more than 500,000 particles detected and measured.
Unique and reproducible size results for four different, but related samples.
Unlike light scattering techniques, image analyzers require regular calibration to ensure correct operation. To guarantee a constant reproducibility of the CAMSIZER results, an electron-lithographic object for calibration is provided. It features an extreme small deviation of only 0.1 µm per particle. Unique to this standard: it features evaporated reference particles which cover the whole measuring range of the CAMSIZER. Calibration takes only a few minutes and is typically carried out once every month.
The CAMSIZER calibration reticle.