FAQ - General

Resolution Graph

Question
What is the meaning of particle counter resolution and why is it important?

Answer

A particle counter's resolution is its ability to resolve small differences in particle size. A number of factors combine to cause the resolution of a particle counter to be other than perfect. These include the uniformity of illumination of the sampling volume, the quality of the optical system, the quality of the electronics in the Pulse Height Analyzer, and noise due to photon statistics.

A low signal to noise ratio (S/N) can also produce an apparent degradation of the resolution by allowing artifacts (noise pulses) to be counted as particles. For purposes of this discussion, we will assume that the S/N ratio is adequate to allow us to ignore this factor.

If it were possible to introduce particles all exactly the same size to a real-world particle counter, the factors above would cause the reported distribution to be the familiar "bell curve" (normal or Gaussian) shape. The figure here displays the reported distributions which would result from introducing a group of particles all exactly the same size to particle counters with perfect, good and poor resolution.

Note that with perfect resolution, the particle counter would always put each of the particles in the same size class regardless of the width of the size class.

The resolution described up to this point could be called fundamental resolution because it is the best resolution of which the instrument is capable.

Note that the minimum possible width of the particle counter size classes or channels tends to be determined by the fundamental resolution. Thus a particle counter with good resolution can have more size classes across the range of the instrument than a particle counter with poor resolution.

The term instrument resolution relates to the number of particle size classes selected by the manufacturer to span the size range of the instrument. The minimum possible width of the size classes is limited by the fundamental resolution. The instrument resolution selected for Particle Measuring Systems' optical particle counters is much more coarse than the fundamental resolution. This is done to simplify use of the instrument and because these instruments are used in applications where high instrument resolution is not required.

Particle Measuring Systems also manufactures a number of particle spectrometers with the instrument resolution selected to be near the fundamental resolution. The LAS-X II has 100 size classes over the range from 0.09 microns to 7.5 microns. The HSLAS II also has up to 100 size channels over the range from 0.06 microns to 1.0 micron.

Optical Particle Spectrometers are used by Particle Measuring Systems to calibrate the in-house standard counters that are used to check calibration on production particle counters. Commercially available monosized standard particles are used with the particle counters and display a narrow distribution from all not being exactly the same size. The distribution reported by the particle counter is the combination of the distribution contributed by the particle counter's resolution (or lack thereof) and the distribution contributed by the standard particles used to calibrate or check resolution. Thus, the effect of imperfect resolution is to broaden the reported distribution.

Sensor resolution may be quantified by introducing a sample of standard particles to the particle counter. If the variance of the standard particles distribution is VarStd, the variance of the distribution reported by the particle counter is VarRpt, and the % resolution = (100/D)SQRT(VarRpt - VarStd)

While % resolution is a term used in some specifications, you may be more familiar with the synonymous terms Coefficient of Variation and Relative Standard Deviation.

A useful indication of particle counter resolution may be provided by specifying the point where counting efficiency reaches 100%. Since the stated sensitivity defines the particle size at 50% counting efficiency, with the size corresponding to 100% counting efficiency, one can calculate the approximate resolution. For example, Particle Measuring Systems specifies the 100% counting efficiency size at 0.14 microns for the LASAIR® II - 110. This corresponds to a resolution of about 10%. The particle size corresponding to 0% and 100% counting efficiency is annotated on curves A, B, and C of Figure 1.

There are two factors that affect the apparent sensitivity and resolution of a particle counter, but are beyond the control of the designer. The light scattered from a particle follows Mie theory and is a function of the ratio of the refractive index of the particle to the refractive index of the transport media and the size and shape of the particle. Most particle counters are calibrated using polystyrene latex spheres in a transport media of air or water, although some are calibrated in oil. A real-world distribution of particle refractive indices will produce an apparent degradation of resolution, while a change in the refractive index of the transport media will produce an apparent change in sensitivity. Although special calibrations are available for certain combinations of particle and media refractive indices, it is generally considered to be impractical to calibrate each OPC for a specific combination.

For more information, see our paper Particle Counting Efficiency and Resolution in Particle Counters

To better understand how to interpret data, purchase Understanding Liquid Particle Counters.