What is Retroreflection?

Drivers and highway department personnel have long recognized that good pavement markings and legible road signs are essential for efficient traffic flow, driving comfort, and highway safety. This is especially true at night and during inclement weather.

A technology that has contributed significantly to improving the visibility of pavement marking and road signs is retroreflectivity. Many highway signs and pavement markings use special sign sheeting and pavement marking materials that send a large portion of the light from a car’s headlights straight back along the same path from which it came. This is what retroreflection is all about. Retroreflection makes objects shine much brighter than those without the retroreflective surface.

This excerpt is taken from the RoadVista Guide to Retroreflectivity, for additional information download the free RoadVista Retroreflectivity Guidebook

Retroreflection Returns Light to its Source

The three types of reflection (mirror reflection, diffuse reflection and retroreflection) are illustrated in Figure 1. Retroreflection, as can be seen in the bottom illustration, is the phenomenon of light rays striking a surface and being redirected back to the source of light. By definition, light sources emit some amount of their energy in the form of visible light. An ideal point light source directs its light equally in all directions. If a perfect sphere light source were enclosed in a perfect sphere, every point on the sphere would be illuminated by an equal amount of brightness or light intensity.

A directed light source, such as a car’s headlights, directs its light in a cone around the direction that it is pointed. If one of your headlights put out a total amount of light energy equal to the point source, and it was enclosed in the same perfect sphere, the points on which your headlight shines would be brighter than each point illuminated by the point source.

This brings us to the discussion of light flux and brightness or light intensity. Light flux is a flow rate of light energy. Light flux can be compared to the flow rate of water; it describes how much light is flowing per unit of time. Brightness or light intensity is like the velocity of water flow. If there are two pipes that discharge equal amounts of water every second and the diameter of one pipe is half that of the other pipe, then the velocity of water in the smaller pipe must be twice as great as the water flowing through the larger pipe.

The same is true for light. If there are two directed light sources that release the same total light flux in the same angular distribution pattern and the first source radiating area is twice the area of the second, then the intensity of the second source will be twice that of the first. The radiating area of the second source will appear brighter, just as the water in the smaller pipe will have a higher velocity, thus flowing with more water per unit of area. This is the concept of radiance or luminance.

Now, if you move away from these two light sources to a distance over ten times the size of the source and look back at them, the two sources will be the same brightness. This is because the total amount of light filling the cone angle defined by your eye pupil and the apex at the surface of the light (now a point source for both lights) is the same for both lights. This is the concept of radiant intensity or luminous intensity.

General Principle of Retroreflection Use

Understanding these concepts will help you understand the phenomenon of retroreflectivity. It was stated that the point light source would have a uniform distribution of light flux in all directions around it. A perfect retroreflector, as shown in the bottom of Figure 1, would simply reverse the direction of the light that fell upon it. In this idealized case, the intensity of the light emitted from the reflector would be zero in all directions except that of the source.

A perfect retroreflector would not be useful for highway signs and lane markings, since all reflected light would be returned directly back to the headlights. The reality is that retroreflectors are not perfect. The distribution of this retroreflected light depends on the type of retroreflective material. In the case of retroreflectors that use beads, some light is absorbed by the reflector, and more importantly, light is scattered in the general direction of the light source. In the case of prismatic and microprismatic retroreflectors, light is reflected back toward the headlights in a pattern determined by the design and orientation of the microprismatic corner cubes and scattered from imperfections. This occurrence is depicted in Figure 2. It is this imperfectly retroreflected light that is useful for highway signage and stripes (delineation).

What does this mean to the driver? It means that signs can be clearly seen from greater distances at night, giving drivers more time to plan lane changes and prepare for exits. It also means that road markings appear brighter and can be seen more clearly. All of this is making our highways much easier and safer to navigate at night.

For additional information on retroreflectivity and highway safety download the RoadVista Retroreflectivity Guidebook