Retroreflectivity

Retroreflectivity is the property of a material or surface to reflect light back toward the direction from which it came. It is used to improve nighttime visibility for drivers and pedestrians. Retroreflective materials incorporate microstructures that send light back to the observer rather than spreading it sideways. The three main retroreflection mechanisms are glass-bead systems, corner-cube prism systems, and microprismatic laminates.

Glass-bead systems use transparent spheres embedded in a binder. Light enters a bead, is internally reflected and re-emerges through a near-normal path. Corner-cube systems use three orthogonal faces that reflect light back toward the source, regardless of small deviations in the incident angle. Microprismatic systems use small prisms etched into a surface that redirect light toward the source. The effectiveness depends on viewing angle, incidence angle, surface cleanliness, and aging.

Retroreflective materials are widely used on road signs, highway barriers, lane markings, and safety apparel. They are categorized by performance (engineering grade, high-intensity, very-high-intensity) and by construction (sheeting, tapes). Measurements report a retroreflectivity value, commonly expressed as R or by units such as cd/lux/m²; standards and required minimums vary by country and application.

Environmental factors such as dirt, moisture, snow, wear, and UV exposure reduce retroreflectivity over time; maintenance and cleaning can restore some brightness. New technologies pursue higher R at wide observation angles and under wet or dirty conditions.