Imaging particle analysis
Imaging particle analysis is a technique for making particle measurements using digital imaging, one of the techniques defined by the broader term particle size analysis. The measurements that can be made include particle size, particle shape (morphology or shape analysis and grayscale or color, as well as distributions (graphs) of statistical population measurements.
Description & History:
Imaging particle analysis uses the techniques common to image analysis or image processing for the analysis of particles. Particles are defined here per particle size analysis as particulate solids, and thereby not including atomic or sub-atomic particles. Furthermore, this entry is limited to real images (optically formed), as opposed to "synthetic" (computed) images (computed tomography, confocal microscopy, SIM and other super resolution microscopy techniques, etc.).
Given the above, the primary method for imaging particle analysis is using optical microscopy. While optical microscopes have been around and used for particle analysis since the 1600's, the "analysis" in the past has been accomplished by humans using the human visual system. As such, much of this analysis is subjective, or qualitative in nature. Even when some sort of qualitative tools are available, such as a measuring reticle in the microscope, it has still required a human to determine and record those measurements.
Beginning in the early 1900's with the availability of photographic film, it became possible to capture microscope images permanently on film or paper, making measurements easier to acquire by simply using a scaled ruler on the hard copy image. While this significantly speeded up the acquisition of particle measurements, it was still a tedious, labor intensive process, which not only made it difficult to measure statistically significant particle populations, but also still introduced some degree of human error to the process.
Finally, beginning roughly in the late 1970's, CCD digital sensors for capturing images and computers which could process those images, began to revolutionize the process by using digital imaging. Although the actual algorithms for performing digital image processing had been around for some time, it was not until the significant computing power needed to perform these analyses became available at reasonable prices that digital imaging techniques could be brought to bear in the mainstream.
With these computing resources available, the task of making measurements from microscope images of particles could now be performed automatically by machine without human intervention, making it possible to measure significantly larger numbers of particles in much less time.
Imaging Particle Analysis Basic Methodology:
The basic process by which imaging particle analysis is carried out is as follows:
1.) A digital camera captures an image of the field of view in the optical system.
2.) A gray scale thresholding process is used to perform image segmentation, segregating out the particles from the background, creating a binary image of each particle.
3.) Digital image processing techniques are used to perform image analysis operations, resulting in morphological and grey-scale measurements to be stored for each particle.
4.) The measurements saved for each particle are then used to generate image population statistics, or as inputs to algorithms for filtering and sorting the particles into groups of similar types. In some systems, sophisticated pattern recognition techniques may also be employed in order to separate different particle types contained in a heterogeneous sample.
Image Acquisition Methods:
Imaging particle analyzers can be subdivided into two distinct types, static and dynamic. While the basic principles are the same, the methods of image acquisition are different in nature, and each has advantages and disadvantages.