Fluorescent microsphere

A fluorescent microsphere is a small spherical particle containing or carrying fluorescent material. Excitation light produces a detectable emission signal, allowing the particle to serve as a tracer, positional marker or calibration object.

For imaging resolution measurements, the bead should be small relative to the instrument's point spread function; otherwise the recorded image includes the bead's finite size as well as the optical blur. In flow experiments, its motion tracks the fluid only when buoyancy, inertia and surface interactions do not materially bias the trajectory. The same bright signal can therefore support different measurements with different assumptions.