Retroreflectivity is a special kind of light reflection. “Retro” was originally a latin word meaning “backwards” until is was adopted by the English and French as standing for “the past”, or being reminiscent of its origin. There’s even a retroreflective MUTCD standards entry, you can find the retroreflective MUTCD standards here
How Does Retroreflectivity Work?
Retro-reflective materials reflect light at the angle as close as it can to the angle at which the light was received. So if the light enters retroreflective tape at a 40° angle, then the tape will reflect the light as close to a 40° as it can. When retroreflective surface is designed properly, this property exists at most viewing angles. Often, microspheres of plastic or glass, or cubed corners are used to give reflective signs this retro reflective property.
First thing you need to make sure is that your eye’s are “calibrated” to the different intensities of retroreflective surfaces. There are minimum-reflectivity calibration signs you can buy that inspectors use to compare the retroreflectivity of different signs and stickers and different locations. Contact the Federal Highway Administration to learn where to get your calibration signs.
Measure Sign Retroreflectivity
After visually checking retroreflectivity with a calibration sign, the next step is to mechanically record the retroreflectivity with a special tool that measures the intensity of the light over a distance, even taking into account variable factors like viewing angle and time of day.
How Retroreflection is Measured
Retroreflectivity is measured by evaluating the amount of a premeasured value of light that returns to the source over a set measured distance; cd / lx / m2.
Coefficient of Retroreflection (or RA) RA = cd / lx / m2.
How does this translate?
Intensity: candela or cdI Illuminance: the light falling on a unit area or lx Lumiance: Intensity / unit area or cd / m2.
Measuring distances is also a common way retroreflective materials are used. There are handheld devices that can record and characterize the retroreflection of reflective vinyls. These devices will tell you very detailed information about the light being reflected off the retroreflective surface. You can test at different viewing angles and distances with these types of devices to see how the reflected light behaves.
Retroreflective Material Management
From the FHWA website: “Management methods provide an agency with the ability to maintain sign retroreflectivity without having to physically inspect each individual sign. While it is not required by the MUTCD, some agencies have chosen to determine the sheeting type and age or retroreflectivity levels of existing signs before using a management method. This is done by those agencies to prevent signs currently near or below minimum levels from being left in place several additional years.”
1) Expected Vinyl Lifetime
In this method, business can monitor the age of individual signs and replaces them before the expiration date. This is just like throwing out food before it starts to smell. You’ll want to replace your reflective decals, labels, or signs before their expiration date. This expiration date can vary depending on ink color, substrate, where material is applied, and what type of material is being used (engineer grade, high intensity prismatic, etc).
2) Blanket Replacement Method
The business can manage reflective materials in groups rather than individually. Common groupings include install method, location, type of material, or color. A common variable for life expectancy is used to judge when retroreflective vinyl should be replaced. These variables can be things be vinyl warranties, weathering deck results, or measurement of field signs.
3) Control Sample Method
This method involved monitoring of a control sample of individual vinyls in a lab setting. These individual vinyls represent a larger family of vinyls signs, decals, or labels. The shortest lifetime in the group should be used to ensure safety and quality.