A system and method for measuring small particles suspended in a fluid are
disclosed. The system employs optical imaging using diffraction
enlargement. A sample of small particles illuminated by a light source is
imaged onto a pixel array of detector elements using an imaging optical
system having a reduced magnification not sufficient for forming a large
enough image of a smallest particle of interest. A low-aperture imaging
optics with NA<0.05 is used to add diffraction enlargement to the
image corresponding to at least 5 pixels to enable accurate measurement
of images of smallest particles of interest, and to increase an optical
sampling volume. Suitably programmed processor is used for determining at
least a pixel count for each of the diffraction-enlarged images, and for
generating a number, size or distribution of particles accounting for
pre-determined diffraction enlargement of particle images of different
sizes. The method enables analysis of large samples of small particles in
one measurement.