Retro-Reflection happens when light enters the bead finds the edge and is returned directly back to the source of that light only. Retro-reflection is directional. You only see the reflective glass beads reflect when your eyes are at the same level or in between the light source and the beads. Factors that affect the brightness (refractive index) are clarity, roundness and inclusions/air bubbles. The size of the reflective glass bead affects the angles that the bead can reflect.
COLE SAFETY PRODUCTS https://colesafety.com has several types of reflective glass beads, retro-reflective tapes and products to choose from.