Imaging particle analysis: Difference between revisions

Revision as of 10:22, 16 October 2013

Revision as of 16:33, 18 October 2013

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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 article is limited to [[real image|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 microscope|optical microscopes]] have been around and used for particle analysis since the 1600's<ref name="Hogg1854">{{cite book|author=Jabez Hogg|title=The Microscope: Its History, Construction, and Applications|url=http://books.google.com/books?id=AAc9AAAAYAAJ&pg=PA8|year=1854|publisher=Illustrated London Libr.|pages=8–}}</ref>, 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 earlylate 19001800's<ref name="Hogg1887">{{cite book|author=Jabez Hogg|title=The Microscope: Its History, Construction, and Application: Being a Familiar Introduction to the Use of the Instrument, and the Study of Microscopical Science|url=http://books.google.com/books?id=wzM5AAAAMAAJ&pg=PA157|year=1887|publisher=G. Routledge and Sons|pages=157–}}</ref> 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, [[Charge-coupled device|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. The first dynamic imaging particle analysis system was patented in 1982 <ref>{{US patent|4338024}}</ref>.

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[[File:Flow cell Cross Section.png|thumb|Flow cell Cross Section|Diagram showing the flow cell cross-section perpendicular to the optical axis in a dynamic imaging particle analyzer.]]The flow cell is characterized by its depth perpendicular to the optical axis. In order to keep the particles in focus, the flow depth is restricted so that the particles remain in a plane of best focus perpendicular to the optical axis. This is similar in concept to the effect of the microscope slide plus cover slip in a static imaging system. Since depth of field decreases exponentially with increasing magnification, the depth of the flow cell must be narrowed significantly with higher magnifications.

The major drawback to dynamic image acquisition is that the flow cell depth must be limited as described above. This means that, in general, particles larger in size than the flow cell depth can not be allowed in the sample being processed, because they will probably clog the system. So the sample will typically have to be filtered to remove particle larger than the flow cell depth prior to being evaluated. If it is desired to look at a very wide range of particle size, this may mean that the sample would have to be fractionated into smaller size range components and run with different magnification/flow cell combinations.

The major advantage to dynamic image acquisition is that it enables acquiring and measuring particles at significantly higher rates of speed, typically on the order of 10,000 particles/minute or greater. This means that statistically significant populations can be analyzed in far shorter time periods than previously possible with manual microscopy or even static imaging particle analysis.

The major advantage to dynamic image acquisition is that it enables acquiring and measuring particles at significantly higher rates of speed, typically on the order of 10,000 particles/minute or greater. This means that statistically significant populations can be analyzed in far shorter time periods than previously possible with manual microscopy or even static imaging particle analysis. In this sense, dynamic imaging particle analysis systems combine the speed typical of [[particle counter|particle counters]] with the discriminatory capabilities of microscopy.

== References ==