Clarity and roundness are the two essential properties that all beads must possess to retroreflect light. The need for transparency and roundness can be explained by examining the path of light as it enters a bead embedded in a marking. The glass bead must be transparent so that light passes through the sphere. Clarity is strongly affected by the type of raw material used. Beads that are less than transparent block a portion of the light from entering.

The rounded surface of the bead causes the light ray to bend downward to a point below where the bead is embedded in the paint. Light striking the back of the embedded portion of the bead is reflected back to the path of entry. Roundness is greatly influenced by the properties of the blast furnace. Beads that are less than perfectly round have diminished retroreflective properties. Roundness is tested in the lab by Test Method .