Drivers and highway departments have long recognised that good road markings and legible road signs are essential for efficient traffic flow and highway safety. This is especially true at night and during inclement weather. Something that has contributed significantly to improving the visibility of road markings is retroreflectivity. Retroreflection makes objects shine much brighter than those without a retroreflective surface and in the case of road markings allows the light from a vehicle's headlights to bounce back towards the vehicle and the driver, making the markings easier to see.
Today there are a wide variety of materials available for such markings and these include paints, cold applied plastics, thermoplastic and modified epoxy resins. The majority of retroreflective road marking materials contain many thousands of glass beads that are bonded to the highway.
The retroreflected light from glass beads is a function of three variables;
Bead shape, size, and surface characteristics
For beads to retroreflect light, two properties are necessary: transparency and roundness. Beads made of glass have both of these properties. 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. Light striking the back of the paint-coated bead surface is then reflected from the paint surface.
The glass beads are applied to road marking materials in one of three ways. They can be premixed in marking materials before application, dropped or sprayed onto the wet paint, 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 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 with the paint colour affecting the colour of the retroreflected light.
Reflective Glass Beads Refractive Index (RI)
The bead’s Refractive Index (RI) is an important physical parameter. The higher the RI of the bead and the fewer impurities in the glass material, the costlier it is to manufacture. The RI is a function of the chemical makeup of the beads. The higher the RI, the more light is retroreflected. Beads used in traffic paint commonly have an RI of 1.5. There are some 1.65 RI beads used with thermoplastics, and 1.9 RI beads are often used in retroreflective airport markings.
Number of beads and light exposure
Beads size distribution refers to the mix of various sizes of glass beads used. This is relevant for both premix beads and drop-on beads. The size will typically vary between 800 to 150 microns. Big bead gradations typically vary between 1600 to 600 microns. The optimal size distribution depends on the type of road marking system, application thickness and the kind of application equipment being used. In some applications, smaller beads may become heavily embedded allowing only the larger beads to protrude above the surface. If the gradation of the beads is too large, wear from the traffic may push the beads out of the binder system. A surface application rate of 400 g/m2 is recommended for maximum retro-reflectivity and durability. Too many beads applied to the surface can result in less effective retroflectivity due to the beads being too close together.
A good application of beads results in the 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 promote 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, that while they may remain fixed in the binder, light cannot enter them and thus no retroreflection can occur.
For more information click Here: http://adbruf.com/pdf/retroflectivity-diagram.pdf