UNDERSTANDING THE RETROREFLECTIVE PROPERTIES OF PAVEMENT MARKINGS
Retroreflective lane markers use special kinds of paints and materials. Most retroreflective paints and other pavement marking materials (PMM) contain many thousands of glass beads per square foot that are bonded to the highway with a strong binder. Instead of scattering light, as normal paints do, retroreflective paints containing glass beads turn the light around and send it back in the direction of your headlights. For beads to retroreflect light, two properties are necessary: transparency and roundness. Beads made of glass have both of these properties. The need for transparency and roundness can be seen to be important if you follow the path of light as it enters a bead embedded in an applied roadway marking. The glass bead must be transparent so that light can pass into and out of the sphere. As the light ray enters the bead it is bent (refracted) downward by the rounded surface of the bead to a point below where the bead is embedded in the paint or PMM. Light striking the back of the paint-coated bead surface is reflected from the paint surface, with only a small fraction of the light going back toward the illumination source.
The glass beads are applied to pavement marking materials in one of three ways. They can be premixed in marking materials before application, or they can be dropped or sprayed into the wet paint directly behind the paint sprayer, or a portion can be dropped onto premixed two-part epoxy or thermoplastic materials. The top surface of beads is enveloped by the paint, with the wicking action of the paint rising up to above the midpoint of the bead. This provides two actions. It locks the glass beads into the paint and allows the paint to act as a diffuse reflecting surface for retroreflection, with the paint color affecting the color of the retroreflected light. The light entering the glass bead is bent and focused towards the back of the bead and reflected back out towards the headlights and driver. A good application of beads results in the top layer of glass beads being embedded to about 60% of the diameter of the bead. There should be consistent quality of both glass beads and paint so that the paint thickness and bead coverage promotes even retroreflectivity across both directions of road travel. Too little paint results in under-embedded beads. This will result in improperly anchored beads that will fall out prematurely, and thus will not be effective retroreflectors. Under-embedded beads cause a large percentage of the light that enters them to exit out the back. Too much paint results in over-embedded beads. While over-embedded beads may remain in the binder, light cannot enter them and thus no retroreflection can occur.
Quality Assurance of Retroreflectors
One of the problems with pavement markings is their inconsistency. Because of this inconsistency, highway agencies cannot reliably predict the performance of pavement markings and must test pavement markings regularly with a retroreflectometer.
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