Retroreflective Microspheres - Principles and Operation
What is retroreflectivity and how does it work?
Almost all surfaces reflect some light. Night time visibility depends on an object’s retro reflectivity—the direct return of light straight back to the light source with a minimum scattering of light. The greater the retro reflectivity, the stronger the visibility of objects in dark or night time conditions.
Retroreflectivity is a term used to describe how light is reflected off of a surface and returns to its original source (“retro”-reflector). Retro-reflection is a type of reflection that redirects incident light from the surface back to the source, for example to the vehicle headlights. Retroreflection is achieved through either Glass Beads or microprisms imbedded in the sheeting. Traffic sign sheeting materials now use technology with small Glass Beads or prismatic reflectors that allow light from vehicle headlights to be reflected by to the vehicle and the driver’s eyes, thus making the sign appear more bright and visible to the driver.
The visibility of a pavement marking is determined by the amount of light reflected off the marking’s surface to a driver’s eye. During daylight hours, marking visibility is achieved through light from the sun striking the marking surface and scattering in all directions, some of which reaches the driver’s eyes. However, in dark environments at night (without roadway lighting), vehicle headlamps produce most of the light striking a pavement surface, and therefore the retroreflective properties of the pavement marking govern the amount of light that reaches the driver’s eyes.
Retroreflectivity is a measure of how efficiently the pavement marking returns light from the headlamps back to the driver. In mathematical terms it is a ratio of the reflected luminance to the headlamp illuminance at a certain viewing geometry. Retroreflective materials fall into one of two categories: those that derive their retroreflective properties from incorporating spherical Glass Beads into its surface and those that incorporate the shape of “cube-corner” microprisms. Pavement-marking retroreflectorization is accomplished through the use of retroreflective Glass Beads partially embedded on the surface of the marking binder material. Glass Beads play the most important role in pavement-marking retroreflectivity. The glass bead returns light from a headlamp back to a driver. Bead coatings are available that assist applicators in achieving proper bead embedment depths.
Glass Beads, also know as glass spheres, microspheres, or microparticles, have been used for 80 years in signage legend and markings. Much smaller beads were spread onto the surface of painted signs to produce a degree of retroreflectivity before the first manufactured glass bead sheeting for signs was produced in the 1950s. Small glass beads also provide retroreflection for pavement markings including both paint and in manufactured markings where they become exposed and functional as the material wears away through usage on the roadway. Glass beads greater than 100 microns in diameter have long been used to enhance visibility in road lining paints.
Retro-reflective metallized spheres technology is based on hemispherically aluminum-coated barium titanate glass beads. The hemispheric aluminum coating creates the mechanism for retro-reflectivity since light passing through the clear half of the glass bead “bounces” off the reflective aluminum-coated back, directing the light back to the source. Proprietary coatings create tension between the microsphere and the surrounding resin that repels the sphere to the surface. This “pop up” effect positions the sphere to maximize the light transmission and enhance the retro-reflectivity. Fluorochemical and silane-based coatings can be applied to any of glass microspheres but their application is particularly important for its metallized spheres: because of the high relative density of barium titanate glass, these spheres will typically sink in any resin system in which they are incorporated. Unless metallized spheres are at the surface of a formulation, light cannot travel through the clear hemisphere to bounce off the metallized shell and create a retro reflective effect. Metallized glass beads dispersed throughout a resin provide little benefit in terms of retro-reflectivity. Placement at the surface allows for a reduction in the amount of microspheres needed to achieve the desired level of retro reflectivity.
Specially formulated coatings for adhering pigments or dyes to glass spheres promote truer color matching when metallized spheres, in particular, are incorporated into pigmented formulas. Colored clear spheres can be substituted for more expensive pigments and still maintain the targeted color intensity and the desired level of retro-reflectivity. Proprietary coatings that expand the effectiveness of its glass microspheres to enhance visibility in: decorative and safety inks, road lining and guard rail paints, and retro reflective powder coatings.
Bead properties that are controlled during the manufacturing process include those that are chemical and physical in nature. The chemical and physical properties of beads have a major influence on how well the beads reflect light. These properties include: bead size, refractive index, clarity, roundness.
The size of a glass bead can affect retroreflectivity, especially under wet conditions. Larger beads have slightly higher retroreflectivity than standard beads under dry conditions. Note that bead size has no effect on refractive index.When markings are wet, beads are often rendered useless because the film of water that covers the marking causes light to scatter before it can enter the bead. This causes the wet markings to be nearly invisible at night. Large beads may be more effective when roads are slightly wet because their higher profile protrudes through the film of water better than small beads. Note that as the thickness of the water film increases, large beads begin to lose their effectiveness as well.
The refractive index (RI) is a function of the chemical makeup of the beads, which is determined by the raw material used to make the bead. The higher the refractive index, the more light is reflected. Most beads used in roadway striping nationwide have a refractive index of 1.50. A bead with a 1.50 refractive index is made from recycled windowpane glass. Beads with higher refractive indices, including 1.65 and 1.90, are made from virgin glass and have a different chemical makeup. Higher refractive index beads are not frequently used because they are more expensive and may be slightly less durable than 1.50 RI beads. Some agencies use a blend of higher and lower refractive index beads.
Clarity and Roundness 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.
Factors Influencing Retroreflectivity
While glass beads are responsible for most of the retroreflectivity in pavement markings, retroreflectivity is influenced by numerous characteristics of the marking, including properties of the glass beads themselves. Table below shows some of the major factors that influence the amount of retroreflectivity that a marking produces. Retroreflectivity is a complex phenomenon that is influenced by many factors.
· Amount and Dispersion
· Embedment Depth
· Refractive Index
· Pavement Surface Roughness
· Dirt or Other “Blinding” Material
· Type of Retroreflectometer
· Used for Measurements