Size Resolution of Laser Optical Particle Counters

The size-resolving capabilities of laser optical particle counters have been explored. The ideal size resolution displays a singular behavior (i.e., the leading edge of the pulse height spectrum observed when calibrating with strict monodisperse aerosol increases, whereas the trailing edge falls off infinitely). When a real test aerosol is used the singularity is suppressed; as a consequence, it has become common practice to take the observed regular pulse height spectrum as “the” resolution.

The tendency to judge the alignment of laser optical particle counters from the symmetry of the observed pulse height spectrum results in definite alignment errors, with a serious deterioration in functioning. The ambient Junge size distribution will not be distorted when sizing it by means of a laser optical particle counter. Essentially, the particle size-dependent number concentration will be underestimated up to some 15% for large particles.

When interpreting measured size distributions using a commonly assumed Gaussian resolution, differences of up to some 25% could result, particularly in the large size regime.