Specification sheets on reflective material often reference the terms “Observation Angle” and “Entrance Angle”. You often see metrics like the following – (Observation Angle : .20 / Entrance Angle : -4) or (Observation Angle – .50 / Entrance Angle : 30), where the observation metric is normally very small, and the entrance metric is much larger. Many readers look past these metrics without understanding what they are, or their importance as it pertains to reflective performance. The purpose of this article is to explain what these two terms mean, and to explain why they are important when it comes to photo-metrics and light return.
Entrance Angles for Reflective Sheeting
An Entrance Angle is the angle at which a beam of light enters a sheet of retro reflective sheeting. Light hitting a section of sheeting at a perfect perpendicular angle is considered a “0” degree entrance. As an example, light from car headlights entering a sign at the same height as the lights, directly in front of the car, would be hitting at a perfect right angle and would be a “0” degree entrance angle. In regular traffic conditions with signs along the side of the road, from a long distance away, the headlights of an automobile would strike a post-mounted sign at a fairly low entrance angle. Then, when the vehicle got closer to the sign, and it was off to one side, the angle of entry would increase. As you can see from the diagram below, an entrance angle is measured by the number of degrees from the perpendicular to the beam of light.
Typical retro reflective sheeting can reflect up to an approximate 60 degree entrance angle from the perpendicular. If the angle is above that, the glass beads or micro prisms in the sheeting begin to be ineffective. To test this, walk around a sign, continuously shining a light on the surface. When your beam is hitting the sign squarely, it will be bright. However, as you get closer to your beam becoming parallel (off to one side) to the reflective sheeting, you will see less return of light and then no reflection. That is due to the entrance angle being too sharp. Lower entrance angles result in better reflectivity. However, observation angles are much more important.
Observation Angles for Reflective Sheeting
An Observation Angle is always quoted along with the entrance angle when describing the reflective performance of a retro reflective film. It is generally a very small number. It refers to the difference between the entrance angle and the eyes of the person viewing the sign or retro reflective sheeting. So basically, it is the difference in angle between the beam of light going from headlights into the film and the beam of light returning to the eyes of the driver. To be concise, an observation angle is NOT measured from the perpendicular to the film, instead it is from the entrance angle. So, because of this, observation angles change for different vehicles. As an example, the observation angle for a person in a compact car would be narrow. However, for a person in a tractor-trailer rig, the observation would be much greater. This is because the eyes of the truck driver are a greater distance from their headlights than the eyes of the compact car driver. So, the closer a person’s eyes are to the source of light going to the reflective film, the lower the observation angle will be, and the more the tape will illuminate for them.
The diagram below shows how observation angles differ for automobiles versus trucks. As stated previously, observation angle changes depending on how far the viewer’s or driver’s eyes are from the headlights of the car, truck, or whatever the light source may be. Look carefully at the diagram below, and you can see that for trucks, the observation angle is generally much more, which creates a reduction in the amount of returned light they can see, especially close in. This is because the greater distance is placing their eyes at the outer perimeter of the cone of reflectivity.
Wider observation angles present a challenge for traffic departments responsible for road signage, because in traffic, every vehicle must be able to clearly see signage. Several reflective tape manufacturers, in an effort to remedy this issue, have developed wide observation retro reflective sheeting. Wide observation sheeting allows for observation angles to be wider, thus keeping signs bright for more traffic. Oralite 7900 Wide Observation and Oralite 9900 All Observation films are two examples. These reflective tapes are brighter to begin with, and spread out light a little more so that viewers at higher angles can see the film illuminate. With this technology, the cone of reflectivity is broader, yet still bright.
Example of wide observation technology – imagine you have a flashlight with a pencil beam. When you shine that light, it only hits a small area. If you shine it into a crowd of people, because it is a pencil beam, only a few will see the beam. Now suppose you brighten that beam and adjust it, so the light is spread out. Sort of like turning a spot light into a flood light. Now the light hits more people. And more people see the light.
So the question is, why don’t all tapes spread out the light like wide observation films. The reason is that for some applications, tapes need to be seen from a very long way away, like in marine environments where search and rescue are actively searching for a person. For that situation, tapes with tighter observation angles and longer sight distance are used. Oralite M82 FD1403 or 1404 SOLAS tapes would be an example of this. These types of films are generally not used for sign sheeting.
So, in summary, the entrance angle is the angle that light strikes the surface of the sign, with zero being perpendicular or straight on. The observation angle is the angle that the viewer’s line of sight is from the entrance angle, or beam of car light. The farther the viewer’s eyes are from the light source, the greater the observation angle. If a person held a light on their nose, the observation angle would effectively be zero.
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.