Most objects in the universe are capable of reflecting light in some way, which is why we can see them. Light that is reflected is also why we see colors. Objects absorb some colors or wavelengths of light, and reflect others. Even a black surface that is glossy reflects. To reflect simply means that light bounces off of a surface instead of being absorbed into it. Lighter colored objects reflect the majority of light that hits them, and darker objects absorb most of the light. White reflects it all, and Black absorbs it all. As previously mentioned, this phenomenon of partial reflection and partial absorption of light is what gives the world – color, light, texture, and beauty.
There are three basic types of reflectivity. Scattered, Specular, and Retro.
“Scattered or Diffused Reflectivity”, is what makes up the majority of reflectivity in our world. We see this in both nature and in man made objects. An apple would reflect this way, and so would a car. “Specular or Mirror Reflectivity”, is when most of a beam of light reflects off of a surface and bounces back either in the direction it came from or in another direction, depending on the angle of the surface. We see this in nature and also in man made surfaces like mirrors. Finally, there is “Retro Reflectivity” which is where light reflects or bounces only back to the light source. This type of reflectivity is mostly man made, but can also be seen in the eyes of an animal such as a deer or cat.
This is the most common type of reflectivity and a characteristic that almost every object except matte black surfaces have in common. As previously stated, white reflects all colors, and black reflects none. Objects that are white reflect the most light and as they change in color towards black, they reflect less.
Note – If black reflects no light, how do we see it? The answer is that the black surface either has gloss to it and reflects light off of its surface like a mirror, or that we see it because of contrast with its surroundings. It is normally a little of both. Which is sort of how we see all colors. Contrast reveals the edges of shapes and allows us to see in three dimensions.
We refer to this type of reflectivity as diffused or scattered because when light hits this type of surface, it bounces off and goes in multiple directions. So if I shine a light on a yellow wall, I see the light and so does a person across the room from me. In other words, the wall lights up for everyone in the room. The downside to scattered reflectivity is that is not intense since the received light is being reflected back to multiple places. This is not a problem in daylight where the sun or artificial lighting indoors is the source of light. But at night, it is an issue.
We are all familiar with mirrors and how they reflect our image back to us in the morning. We also know that if we hold the mirror at an angle, we can see what’s behind us, like the mirrors on our automobile. The mirror is a very effective reflective surface in that it does not scatter light to multiple sources. Instead, it redirects almost all the light back to one particular spot. And it does this with great clarity. Lights from a city reflecting off of a calm bay are an example of a mirror in nature.
Smooth versus rough water is a good example of the difference between specular and scattered reflectivity. City lights reflecting off a rough lake would be scattered reflectivity. The same lights reflecting off of a perfectly smooth lake would be an example of specular or mirror reflectivity. One scatters the light, and one mirrors it.
Another thing mirrors can do besides change the direction of light, is intensify it. A concave mirror for example, can collect light and tighten it into a more intense beam, while at the same time sending it in a totally different direction. A convex mirror can do the opposite. With a mirror, almost all light is reflected, with very little absorbed by the surface. It is for this reason that mirrors are used in telescopes and lasers. If a mirror that was in a laser absorbed light, it would burn up in a matter of minutes. But since it reflects virtually all light, the mirror stays cool. Mirrors in lasers are also an excellent example of the redirection of light in perfect clarity with little loss of intensity. Again, mirrors can be natural or man made.
Retro comes from a latin word meaning backwards. For example, a person that is retro might want to return to the 80’s or 90’s. In the sense of reflectivity, retro means that light is bounced back to where it came from, hence the term retro-reflectivity. When this type of reflectivity was discovered, man made retro reflective surfaces were created, and night time suddenly became a much safer place.
Retro is different from mirror and diffused reflectivity because of what happens to the beam of light that strikes the surface. As stated before, diffused or scattered reflective surfaces send the light back in all directions, so although the surface can be seen, it is not necessarily bright. Mirror reflectivity sends all the light back in one direction only, just not necessarily back to the source. But retro reflectivity sends light back only to the light source.
Retro is different in that within reason, no matter what position the surface is in, and which direction the light is coming from, as long as a person is in line with the surface and the light, they see a bright return of light. A beam of light is always sent back to its source and in addition, there can be multiple beams and multiple returns. For example, 50 cars can be driving down a highway, and a retro reflective sign will reflect light back to all of them. And because automobiles send out a fairly wide beam of light, the entire retro reflective surface lights up. If the surface is the background for letters like with a road sign, all drivers will be able to clearly read it
Man made retro reflectivity is created in two ways. One is through the use of perfectly round glass spheres that take in light, bend it, and then return it. The other method is through the use of prisms that also bend light return it. Prismatic arrays are much more efficient than glass beads. Metallizing beads or prisms enhances reflectivity by giving them a mirror finish at the back.
More about Glass Bead Retro Reflectivity – History
In the early 1930’s, inventors at Potters Beads found that perfectly round glass spheres or beads had the ability to take in light, bend it, reflect it, and return it back to the source of that light. Regardless of which direction the light came from, the beads would collect it and reflect it back. Originally, Potters Beads were used to make lines on roads reflective, but later, engineers at 3m found ways to laminate the beads into flexible films, thus creating the first reflective tapes or retro reflective sign sheeting. They also found that by letting the light pass through a colored transparent sheet on its way to and from the glass bead, a color could be created. This led to the invention of multiple colors of reflective tape and would result in a huge breakthrough in traffic safety. Now signs covered with reflective film would be visible at night in color. To this day, glass beads continue to make road lines light up, and continue to make engineer grade and high intensity grade reflective tapes reflect. Glass bead films constitute Type 1, Type 2, and Type 3 retro reflective films.
More About Micro Prismatic Retro Reflectivity – History
In 1963, the Rowland Brothers, owners of Rowland Products invented what we now know as prismatic reflective sheeting. This invention consists of small micro mirrors arranged at angles to one another, in such a way so that when light enters, it is bounced and sent back to the light source. The basic process of reflecting light is very similar to glass beads, but much more efficient. Comparing the two, glass bead films return around 40% of received light to the source versus 90% for prismatic films. As a result of this increase in efficiently, micro prismatic films can be up to 10 times brighter than glass bead films and seen from much further away. So as such, prismatic films are assigned different applications that glass bead films could not fulfill. SOLAS marine reflective is one example. For ocean rescues, visibility needs to be thousands of feet instead of hundreds. A reflective tape that is type 4 or above would be prismatic by design.
In any event, the invention of prismatic reflective films vastly improved night time safety for drivers. Signs sheeted with a prismatic reflective tape could be seen from many times the distance away, providing much more response time for drivers. Prismatic reflective tapes on vehicles themselves prevent collisions by making the vehicles visible for thousands of feet way. Far in excess of the distance needed to avoid an accident. In any event, Rowland Products became Reflexite, and later Reflexite became Orafol Americas. Orafol continues to be a worldwide leader in reflective sheeting and markets their tapes under the Oralite brand. The company carries both vehicle reflective films, and reflective sign sheeting.
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.