RoadVista 922 Handhled Retroreflectometer

Laboratory measurement of retroreflective materials is accomplished using tightly controlled procedures. These procedures are defined and controlled by the American Society for Testing of Materials (ASTM). The ASTM procedures for laboratory measurements require the use of a tungsten lamp operated at a correlated color temperature of 2855.6 degrees Kelvin. The spectral power distribution of a tungsten lamp operated at this color temperature approaches the ideal CIE (the international standards generating organization for illumination and color) Illuminant A, which is an internationally agreed upon standard type of illumination used for comparison and specification of colors. The photoreceptor used to measure the illuminated retroreflective surfaces must match the CIE 1931 human eye response function within a tolerance of 3% defined by the f₁ analysis method in CIE publication 69.

This excerpt is taken from the RoadVista Guide to Retroreflectivity, for additional information download the free RoadVista Retroreflectivity Guidebook

The CIE 1931 human eye response function is also called the photopic response, photometric response function or the Y-bar function. This level of precision in the measurement instruments yields a precision and bias of about 6% between well-maintained and well-staffed laboratories. It is not practical to apply these very rigorous standards in the field. ASTM has developed standard procedures that provide for the use of portable instrumentation for measurement of retroreflective materials in the field. These standard procedures allow the use of illumination sources quite different than those used in the laboratory.

The procedures allow compensating the detector response by the amount the light source deviates from 2855.6 degrees Kelvin. Field measurements and laboratory measurements taken over many years have shown that the combination of the spectral power distribution of a light source and the detector spectral sensitivity must match the combination of the CIE Illuminant and photometric response functions. Filters can be used to compensate for temperature differences and to provide the required match to the CIE photometric response function.

For pavement markings, the Federal Highway Administration has adopted the 30-meter observation distance geometry used in Europe, defined in ASTM E1710-95, Figure 8. It represents what a driver in an average height U.S. automobile would see during inclement weather conditions at night. ASTM has recently adopted E1710, which establishes the parameters for 30-meter geometry retroreflectometers.

The international standard geometry for measurement of retroreflective materials is shown in Figure 9. Given that the characteristics of retroreflectors change so quickly according to variations in lighting and viewing angles, one of the most important factors affecting retroreflector measurements is the accurate setting of angles.

Laboratory measurement equipment can simulate the exact angles at which drivers view safety marking materials. The angle marked α (alpha) represents the angle between the vehicle headlamps and the driver's eyes. Similarly, the ε (epsilon), β₁ (beta one) and β₂ (beta two) angles simulate the angles that the sign or other retroreflective element make with respect to the illumination source.

Four Important Measurement Attributes

• Accuracy - this is the bottom line for measurements. If they aren't accurate, they tell nothing, so don't take them. Figure 8 - The ASTM E1710 geometry is required for all hand held retroreflectometers to be used on the public roads Figure 9 – Laboratory measurement setup duplicates the vehicle headlamp and driver position geometry.
• Reproducibility/Repeatability - instruments can drift as a function of time and use. Keep batteries charged, instruments clean, calibrate frequently, service regularly, and operate within specified limits.
• Reliability - an instrument that fails in the field wastes time and money. The actions taken to keep an instrument reproducible will help keep the instrument reliable.
• Traceability - before any instrument can be used in the field it must be calibrated. Accuracy and reliability revolve around the traceability of the calibration standards and robust calibration procedures.

Field Instruments

• Pavement Marking Retroreflectometers - This tool is used for measuring pavement marking retroreflectivity. It measures or determines how bright the markings appear at night to motorists.
• Handheld Pavement Retroreflectometers- Employed widely in field to spot-check the condition of selected retroreflective pavement markings.
• Mobile Pavement Marking Retroreflectometers - Takes continuous retroreflective readings while driving down the road at highway speeds
• Sign Retroreflectometers - This tool is used for measuring sign retroreflectivity. It determines if the sign meets nighttime retroreflectivity requirements.
• Handheld Sign Retroreflectometers - This is an instrument capable of accurately and reliably measuring the retroreflection properties of road signs and retroreflective sheeting materials.
• Mobile Sign Retroreflectometers- Mobile sign retroreflectometers are in the development stage and are not yet commercialized.
• Raised Pavement Marker (RPM) Retroreflectometers - This tool measures the retroreflectance of these important roadway delineators in their regular and snow-plowable mounting configurations.

For additional information on retroreflectivity and highway safety download the RoadVista Retroreflectivity Guidebook

Other Posts on Retroreflection and Retroreflectivity

• Introduction to Retroreflectivity and Highway Safety
• What is Retroreflection?
• Reflective Glass Beads
• Retroreflection Measurement
• How Pavement Markings are Measured