particles and their surrounding immersion liquid medium into a color in the When analyzing particle size distribution of colorless, translucent or transparent particles by image analysis, a major challenge is to obtain images of particles that ensure proper object detection, especially if the particles are amorphous or non-crystalline. Conventional imaging techniques, such as brightfield, darkfield, cross-polarized light, etc., might not applicable for these types of materials.

Dispersion staining (DS) is a technique that coverts the refractive index (RI) difference between visible range and renders particles optically stained with that particular color. There are two modes of dispersion staining techniques: central stop (CS) and annular Stop (AS). For image analysis, CS is preferred.

In the CS mode, the matching wavelength λm, i.e., the wavelength at which the RI of a particle equals that of liquid, is not refracted at the particle/liquid interface and therefore blocked by a 3-4 mm opaque round disk located at the center of the back focal plane of objective lens.