Is laser beam shaping necessary for optical particle counting and sizing?

The shaped beam cross section plays an important role in particle counting and sizing. The rectangular-shaped cross section, whose height is much smaller than its width, is preferred. Below, two laser beams with different cross section shapes are illustrated.

In the case of low concentration, when the particles pass through the measurement zone, the impact of different cross section shapes on counting accuracy is trivial because the interval between the particles along the sample flow is relatively large. However, if the sample concentration is high, the particles in the sample flow are closely arranged. Suppose that the height of the rectangular-shaped cross section equals or is slightly smaller than the particle size, one pulse signal will be generated when the particle is illuminated by the laser, and the particle counting and sizing are accurate. In contrast, if the height of the rectangular-shaped cross section equals, for example, two times of particle size, a counting error probably occurs when two closely arranged particles are illuminated simultaneously, resulting in one pulse signal with higher intensity. In other words, the two pulse signals that should be generated are now overlapped. The number of particles will be therefore recognized as 1. The particle size will also be calculated by mistake due to the high intensity. It is worth noting that the width of the rectangular-shaped cross section is equal to or greater than the upper measurement limit of the particle size, which makes light extinction effective for the detection of large particles.

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