These uniform microspheres are NIST-traceable by calibration methods such as optical microscopy. Products from 1 to 160µm in diameter are packaged as aqueous suspensions of polystyrene spheres in dropper-tipped bottles. Diameters of 200µm and larger are packaged as dry spheres in screw-capped bottles. These larger particles are composed of polystyrene DVB (divinylbenzene). The spheres have a density of 1.05g/cm3 and an index of refraction of 1.59 @ 589nm.