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What is Retroreflectivity? - THE BASICS
1. How we see things - Diffuse Reflection
2. Retroreflection
3. Retroreflective Optical Systems – Beads and Prisms
4. The “Cone of Retroreflection”
5. Observation Angle
6. Entrance Angle
7. Units of Retroreflectivity Measurement - RA
8. Headlight Illumination
9. Want to Know More?
10. Retroreflectivity 101 – Did You Get It?

What is Retroreflectivity? - THE BASICS
1. How we see things - Diffuse Reflection
Everything we see in our everyday lives is seen by reflected light. The surface of virtually every material (except mirrors) is such that light is reflected from it in all directions (“diffusely”) and therefore, in typical circumstances, the brightness of surfaces seems to us to be about the same no matter from what direction we look at them. We’re also accustomed to having bright light available (the bright sky outdoors by day or lighting fixtures indoors) so that things can be easily seen. With enough light available, our eyes are sufficiently sensitive that diffuse reflection works well to see our way.

When driving at night, however, a motorist usually has only the light from his vehicle’s headlights to enable him to see the road ahead and be guided by its surface and its edges. It’s also the only light he has to see the road markings (centerlines, lane lines, etc.) and the roadsigns alongside and over the road. The diffuse reflection from the road surface directly ahead (strongly illuminated by his headlights which are intentionally aimed downward) is sufficient to see the road itself for a reasonable distance ahead.

However, road signs and pavement markings need to be seen and read at a much longer distance ahead to be effective. At long distances ahead of the vehicle, objects receive very little light from the headlights (again, the brightest part of that beam is aimed downward) so that when that little light is reflected diffusely in all directions, as by ordinary objects including painted signs – they can not be seen by the driver. The far field of view is black to the driver. Only when the vehicle and its headlights come very close to a sign can he read it but probably too late to read and react to it. Similarly, road construction personnel working on roads at night could not be seen until the vehicle may be too close to avoid hitting them and road markings on the road would not give the far-ahead guidance which is their purpose.
2. Retroreflection
To meet the need for these objects to be bright to the nighttime driver, a special material with unique optical properties is used; this material is able to reflect light in a very special way: It reflects almost all of the light striking it from the headlight (or from any source) not diffusely but directly back toward the headlight (reversing the direction from which it came) and contained in only a very, very narrow cone, spreading out just enough to include the driver (almost directly behind the headlights.) That special type of reflection (“back to the source”) is called “retroreflection.”

Returning that light only within an extremely narrow cone (instead of in every direction as does diffuse reflection) is what makes the sign bright to an observer who views it within that narrow cone. Such retroreflective materials are used to make signs visible far away, markings on road personnel bright and markings on the surface of the road seen far enough ahead to be useful – all using only the very small amount of light available at that far distance from the vehicle’s headlights.


Figure 1. Retroreflection Basics
Figure 1 illustrates the example of retroreflection for a roadsign. While the actual headlight “beam” (not indicated here) spreads out over a broad area ahead of the vehicle, only that light that directly reaches the sign results in its brightness. Only that part of the light from the headlight is considered in this diagram and it is represented very simply by a line from the headlight to the sign. (Later, in the “Advanced” section, we’ll give this line a name: It’s the “illumination axis”). But carefully note that this line has nothing whatever to do with where the headlight “beam” is “aimed” (and thus it is not the “headlight-beam axis”). Since every element of retroreflective geometry develops from this imaginary line it will be useful to remember it.

Contrary to the belief of some, retroreflective materials do not actually reflect more light overall than many other surfaces; they appear bright only to a viewer located right behind a light source (including headlights) simply by confining all of the reflected light into that extremely narrow cone. If the viewer isn’t near a light source (ahead or behind), many diffuse-reflecting materials will usually be brighter to him than efficiently retroreflective materials.
3. Retroreflective Optical Systems – Beads and Prisms
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.

Glass beads (large beads – up to a centimeter diameter or more, were called “cats-eyes”) 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 ‘50s. Small glass beads also provide retroreflection for pavement markings including both paint (they are spread onto the paint before curing) and in manufactured markings where they become exposed and functional as the material wears away through usage on the roadway.

Reflective prisms (“cube-corner” prisms) have been incorporated into both highway and vehicle markings since the 1920s. Prismatic sheetings, using very tiny “micro-prisms” have been commercially available since about 1990, and typically have a higher efficiency overall than beaded sheetings and thus can be significantly brighter. Most Raised Pavement Markers incorporate prisms (either “large” or microprisms) to provide a bright retroreflective signal.
4. The “Cone of Retroreflection”
The retroreflected light comes directly back to the headlight, only spreading in a very narrow cone. This cone is centered on that line from the headlight to the sign. In Figure 1, this cone is drawn at roughly about 15º so that it can be seen as a cone in the diagram but on the road the actual effective cone is sometimes as little as 0.2º for a sign read far away. Note: It’s hard to show the exact cone in a diagram since a cone of 0.2º spreads only four-hundredths of an inch in a distance of a foot! (3½ millimeters in a meter distance).

Despite the fact that nearly all the retroreflected light is contained within this very narrow cone, the “cone of retroreflection” doesn’t have a definite limit; there is no “edge” to the cone beyond which there is no reflected light at all. The retroreflected light is brightest near the center of the cone and becomes far less bright at larger cones. Thus reference can be made to the reflectivity at the “1º cone” or the “2º cone.” (The angles used here to measure the “Cone of Retroreflection” are “half-angle” values and are for the angle from the cone axis – that line from the headlight to the sign - to a line in the cone).

The angular “size” of the retroreflected cone of light is important because it determines when that material will be “bright” to the driver of an oncoming vehicle. The driver, while quite close to being directly behind his headlight, is actually separated a short distance from the line from the headlight to the sign and therefore from the center of the cone of retroreflected light.

At longer distances on the road, the driver of a small sports car is only slightly displaced from his headlight and sees signs as bright because he’s close to the center of the cone. The driver of a very large truck sits well above his headlights and therefore is further away from the center of the cone. He sees the same sign as less bright. This is shown by the diagram in Figure Two.
Figure 2. Cone of Retroreflection 
5. Observation Angle
The most important angle in the geometry of Retroreflection is the “Observation Angle”. It’s really quite simple to learn and essential to an understanding of how retroreflection works. It can be described in two somewhat different ways that actually do refer to essentially the same thing.

Consider the Cone of Retroreflection - the retroreflected light coming back to the headlight. The portion of this cone of retroreflection which is seen by an observer at any one time is measured by the angular value of the half-cone. This angle is commonly known as the “Observation Angle.” In terms of the Cone of Retroreflection, the Observation Angle relates to the angle of the cone at which an “observer” (the driver) sees the sign. (It may be useful to note that the light itself is being reflected in this cone whether there is an observer or not).

The more commonly used definition of “Observation Angle” (used in the ASTM specifications, for example) relates it to the geometry of measurement, without any reference to the cone of Retroreflection. (See Figure 3.) This definition of “Observation Angle” says that the line from the headlight to the retroreflective material (sign) forms an angle with a line from the sign to the observer’s eye (or, in a photometer, the detector). This is specific for the laboratory measurement but if you remember the cone of Retroreflection defined by the observation angle you’re more likely to understand how the changes in observation angle affects the reflective efficiency of the retroreflective material and, ultimately, the brightness of the sign.

Figure 3. Simple Observation Angle

The retroreflected light is strongest at the center of the cone (smallest Observation Angles) and continues to drop lower in value at the wider parts of the cone (equal to larger Observation Angles). Thus a “curve” of retroreflectivity values may be created for a given retroreflective sheeting, by laboratory measurements of that sheeting from small to large observation angles; this very useful data is an Observation Angle “curve.”

On the road, the driver is separated from the headlight (i.e., from the “cone axis”) by a relatively “fixed” amount. Therefore, as the vehicle approaches a sign at a long distance the Observation Angle at which the driver views the sign is initially small and then becomes larger and larger at an increasing rate of change when closer to the sign. As previously described, the driver of a large truck, sitting well above his headlights will see the sign at correspondingly larger Observation Angles at each distance than drivers of cars.

How retroreflectivity changes with changes in Observation Angle, and thus with changes in approach distance, is critical to understand how retroreflectivity works on the road in a practical sense together with the changing illumination from the headlights to produce sign brightness. [This is discussed in much more detail in: Retroreflectivity- Advanced.] 
6.Entrance Angle
Another concept important to the understanding of retroreflection in the roadway applications considered here, and a part of the “geometry” of retroreflection is the angle called “Entrance Angle.” This is the angle at which the light from the light source (headlight) enters the surface of the retroreflective material, particularly in the case of a sign. This concept also has nothing to do with where the headlight beam is actually directed or what portion of the headlight beam strikes the sign; it is simply the angle that the light which comes from the headlight strikes the surface of the sign.

Entrance angle is the angle between that line from the light source to the sign (we’ve referred to this line before) and an imaginary line exactly perpendicular to the sign. If those two lines are superimposed (i.e., the same line) the light is “head-on” to the material and it is defined as being “0º Entrance Angle”. This is shown in Figure 4.

Figure 4. Simple Entrance Angle
All retroreflective sheeting materials have substantially lower retroreflectivity at higher entrance angles but at those larger angles some materials retain somewhat higher efficiencies than others; these materials are described as having more “Angularity.”

Typically, signs are viewed at a very small entrance angle which increases only slightly as the vehicle approaches the sign. (See Figure 5.) Almost all properly mounted roadsigns are read at entrance angles of 10 degrees or less throughout their approach.

Large entrance angles can be created by an accidental twist or tilt of the sign, such as might result from an impact by a vehicle. They can also result when a sign is mounted far off the roadway and the vehicle is quite near, but this circumstance also generally results in the light from the headlights missing the sign, which becomes the predominant factor in determining sign brightness.

Figure 5. Entrance Angle Change with Distance 
7. Units of Retroreflectivity Measurement - RA
Performance values for retroreflective sheeting are an expression of the efficiency of that material to retroreflect the light at a particular “geometry”; i.e., at one set of observation and entrance angles. (One “geometry” often used in specifications is: 0.2 degrees Observation angle and 4 or 5 degrees Entrance angle.) The number given tells how much light is retroreflected at that “geometry” for a given unit of light falling on a given area of the material. Technically, it is termed the “Coefficient of Retroreflection,” designated by the symbol RA. (Here we’ll sometimes refer to it as the “retroreflectivity value or simply, “retroreflectivity”).

“RA” essentially expresses the relationship: 

Light OUT (Retro)
________________ = “Efficiency” or RA       
   Light IN

It is not necessary to know the exact technical meaning of the factors comprising that term (but they are: candela per incident lux per square meter, often abbreviated to “cd/lx/m2”); the important concept to understand is that the retroreflectivity value RA is a ratio. It’s similar to “per cent” and does not tell how bright the sheeting will be on a given sign (even at the distance corresponding to the “geometry” for that particular value). It is simply its “efficiency” at returning light to the source at that particular geometry (the set of observation and entrance angles).

[Note: The units and “standard geometry” used in the specifications for the retroreflectivity of road markings is slightly different due to the particular way that virtually all road markings are illuminated and viewed, but the principle is the same].

The efficiency of a retroreflective material varies with different observation and entrance angles and the various materials vary in different ways. The practical effect of these differences is what influences the choice among different retroreflective materials for a particular situation, taking into account headlight illumination, practical viewing requirements and distances, and the viewing needs of the driver. These factors and considerations are discussed in Advanced Retroreflectivity.
8. Headlight Illumination
While different reflectivity values are important and, for example on signs, a higher value generally means a brighter sign, the brightness of a sign is much more dependent upon the level of the light reaching it from the headlight. For example, in recent years automobile headlights project far less light upward toward signs – and thus overhead signs in particular, are seen as less-bright by drivers of new and recent vehicles. Also, where signs are displaced far off the roadway or, as on a curve, the headlight beam is aimed far away from the sign, the sign is far less bright. The most efficient retroreflective material cannot be bright if the light level is very low.

Such circumstances also often exist when the sign is far off to the side, far out of the main headlight “beam” and no material properties can compensate for the absence of light. This is also important to consider when selecting locations for signs. An easily understood example is the driver who, at night, pulls up alongside a corner street name sign to read it out his side-window - the sign’s retroreflectivity is useless to him (unless he holds up a flashlight by his eye!)

The effect of headlight illumination upon road markings differs from signs primarily as a result of the fact that the geometry of the road marking ahead of the car is essentially a “constant” and therefore, higher retroreflectivity values do directly result in brighter markings as seen by the driver and equally efficient markings will generally be seen as equally bright.
9. Want to know more?
10. “Retroreflectivity 101”
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Maintaining Traffic Sign Retroreflectivity: Impacts on State and Local Agencies) is a new publication issued April 2007 by the Federal Highway Administration. This report updates a 1998 report on the national impact of minimum maintained traffic sign retroreflectivity levels and addresses concerns expressed in four FHWA-sponsored workshops that were held in 2002.
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