Reflective tape and sheeting have been making the night time a safer place for over 80 years. During that time, reflective tapes in their various forms have saved countless lives by improving visibility both day and night. Before discussing how retro reflective tape works, it is important to make a distinction between a simple “reflective” surface that bounces light away from itself, and a “retro reflective” surface that returns light back to the source. A mirror is reflective, and a reflective road sign is retro reflective.
It is not difficult to understand that mirrors reflect our image back to us. And when a mirror is studied, it is not hard to see how this is accomplished. The more difficult concept to grasp is how a surface can reflect light only back to the source of that light, a phenomenon known as retro reflectivity. To start with, the study of how all of this works is called photo-metrics, or the science of light. This field of science deals with light intensity, geometry (bending), refraction, diffraction, color, luminescence, candelas, electromagnetic radiation, and more. As far as the mechanics of how reflective tape reflects light, it is more about geometry and angles than anything else.
As previously mentioned, reflective tape or sheeting (also known as retro-reflective tape) is unique in that it reflects light back to the light source only. Each beam of light that hits the surface is reflected back to only the source of that light, and not anywhere else, which is what makes it so bright. To clarify, a retro reflective film only appears illuminated for the person with the light source or who is in a direct line between the tape and the source. Often, the light being returned is called the cone of reflectivity and consists of an ever widening cone of light that travels towards and around the light source. As it widens, it also becomes dimmer. Anyone within that cone, facing the returning light, will see it.
As an example, consider two people walking parallel down a street with one person on each side of the street. If person “A” has a flashlight and shines it onto a square of reflective tape 200 feet away, the tape will light up for them. However, in that situation, the tape will not illuminate for person B on the other side of the street because they are not in the cone of reflectivity. If both person A and person B had flash lights and shined them onto the surface of the tape, both would see it illuminate. This occurs because the tape contains either glass spheres or man made prisms that collect light, bend it and send it back to the source. The question is, how does it do this?
Here is how it works in an oversimplified way. Imagine for a moment that you are in the very center of a round room, with a rubber ball in your hand. If you throw the ball towards the wall, it will either come back and strike you or come very close. This is because of the shape of the wall and your proximity. Reflective tape works in a similar manner by efficiently bending or refracting light. Basically, light enters a glass bead, is bent, bounces off of the back wall of the bead, and is then sent back the same way it came in. Millions of microscopic beads on the surface of a film doing this together provide substantial illumination to a viewer who is in line with the returning light. The diagram below shows how the glass beads do this. Notice how the light always leaves the way it came in.
Man made prisms return light in a similar but more efficient way. This would be like hitting a racquetball in to the corner of a court where two walls meet at 90-degree angles. The ball would in most cases strike one wall, then bounce to the adjacent wall, and then back towards you. This is a simplification of what takes place with micro prisms in prismatic reflective tapes, however, it is a good way to envision the process. Because of its efficient design, prismatic tape returns light more like a spotlight, whereas glass bead tapes return light like a flood lamp. Since less light is lost in the conversion, prismatic tapes return more light and appear brighter to the viewer. Less dispersion of light means more light returns to the light source and the viewer, provided they are within the cone of reflectivity or return of light. The diagram below shows how prismatic tapes work.
As you can see from the diagrams above, the tapes refract or bend light in such a way that it returns the direction it came in. And it does this no matter what angle a light is shined onto it. From the overhead view, you can also see that the prisms are arranged so that no matter which way the light comes in, tens of thousands of prisms in the tape will reflect that light back. As light moves and comes in from a different direction, other prisms take over and reflect. The over all effect is that you get a return of light from a variety of different positions.
This brings up another unique characteristic of retro reflective films. In the first diagram above, the example of two people walking down a street, one person had a light and the other person did not. If both had lights, both would see the tape illuminate for them. For one person, a certain set of prisms all over the tape would be sending light back to them, and for the other, it may be a different set of prisms. This is made possible because of the millions of small prisms that are in a section of reflective sheeting, all set at different angles. Remember, with light you are not just throwing one ball at a surface, you are throwing billions of light particles at it and receiving a large portion of those back. Not all, but plenty to make the surface illuminate for you. The surface lights up so that you see the red reflective background and the white letters that say stop, or the black letters on a reflective yellow background that say bridge out, in plenty of time to stop.
Also, because of the positioning of either glass beads or prisms, reflective tape does not have to be perpendicular to the viewers for it to reflect. Light beams can come in at fairly sharp angles and return to the source with no issues. Also, multiple light beams can strike the tape at the same time and all be returned to their respective sources. This is what is happening in the case of automobiles on a highway with their lights illuminating a street sign. All the drivers receive their own return of light. If one driver does not have their lights on, there will be no light returned to their headlights, and the sign will not be illuminated for them the way it is for other drivers with headlights on. The exception to this would be if they were in the cone of reflectivity from a driver in front or behind them. But in this case, the sign would be visible, and then not visible, visible, and then not. Better to have your own light.
Lastly, as previously mentioned, to see a reflective surface illuminate, a viewer’s eyes must be in line with the returning light. This return of light is often referred to as the cone of reflectivity. If a viewer is within this cone, they see the tape light up. If they are not, the tape seems dark to them. All of this has to do with entrance and observation angles. Both elements of the study of photo-metrics. We have an article on entrance and observation angles that goes into further detail about the geometry of reflectivity. It is in the “Science of Reflectivity” category.
Steven Cole (Economics, MBA – University of West Florida , Business & Innovation – Stanford University) 25 years of experience in the reflective safety business. Specializing in vehicle accident and rear end collision reduction through increased visibility.