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Direct versus Indirect Particle Size Measurement

One of the key differences between dynamic imaging particle analysis (DIPA) and other more common historical techniques (like Light Obscuration) is that when using DIPA, measurements are made directly from the particles themselves- based on digital images. Light Obscuration and the majority of commonly used automated particle analysis systems employ a technology that makes an indirect measurement; in other words they measure something proportional to particle size and convert that proportional measurement into particle size.


How Indirect Particle Size Measurments are Made Figure 1: Click to enlarge

Light Obscuration, and many automated particle analysis systems are volumetric-based in nature. They measure the volume of a particle, and assume that particle is spherical in shape in order to convert that volume measurement into a single size measurement. For this reason, the most common measurement used for particle size is Equivalent Spherical Diameter, or ESD. One of the major benefits of dynamic imaging particle analysis is that it does not have to make this assumption.  The measurements are based on an actual digital image of the particle.  Therefore unlike its volumetric cousins (Light Obscuration), it can actually make many different measurements from each particle such as length, width, area, etc.

To further complicate matters, the measurements made from the volumetric-based systems do not actually physically measure volume.  They measure the volume indirectly by measuring something that is proportional  to volume. This proportional measurement is then converted to a particle size (ESD) by comparing the measurement produced for the particle in question against measurements made when passing reference standards (calibrated spheres) through the same instrument. Figure 1 shows in a general way how this works.


How Imaging Measures Particle Size Figure 2: Click to enlarge

By contrast, a dynamic imaging particle analysis system makes a direct measurement of size from the actual image of the particle. Such a system is typically calibrated at the factory by imaging a standard reference scale (usually a traceable resolution target). Since the system creates digital images, its minimum unit of distance is the size of a single camera pixel projected from the camera onto the target. Thus the system calibration is expressed in terms of "distance per pixel", typically microns/pixel (μ/px). Once the calibration is established any measurement can be made from a particle image merely by multiplying the number of pixels across the feature to be measured by the calibration factor. Figure 2 shows how this works.

The key element to dynamic imaging particle analysis is that all measurements of the particles are made directly from the image of the object being measured, whereas in  Light Obscuration and other the volumetric systems, the measurement is being made indirectly from some other signal that is proportional to the volume of the particle. In an imaging system, once the geometry of the optical system is "locked down" (typically by the vendor at the factory), the calibration factor does not change. [Okay, before someone gets nitpicky -the calibration factor can change (not too dramatically, though) if the refractive index of the fluid the particles are being measured in is significantly different from that with which the system was calibrated].

Calibration can be verified any time by imaging calibrated beads through the system. On the other hand, systems using indirect measurement techniques (electrozone, light obscuration, laser diffraction, etc.), typically need to be recalibrated on a more frequent basis, because their measurements can fluctuate depending upon factors such as small changes in refractive index, laser power fluctuations, etc.

The fact that dynamic imaging particle analysis represents a direct measurement technique is one of its many advantages over indirect volumetric-based systems. This is before we even begin to discuss the fact that particle imagers can measure so much more than just ESD, which is the one and only measurement the volumetric system produces!

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Topics: FlowCam Technology