Due to various industry standards, such as those for highway safety or automotive LiDAR applications, it can be important for designers to evaluate the retroreflectivity of materials in their optical systems by including light measurements of these materials in design simulations.
Retroreflection represents the hemispheric portion of light that strikes a surface and scatters back toward its light source. It is important to measure retroflection effects in many types of optical systems, but it can be challenging to accomplish. For example, a classic goniometer measurement can’t capture retroreflection because the detector hides the light source (or the source hides the detector, depending on the configuration).
If determining the retroreflection of your material is a challenge, the Synopsys Mini-Diff V2 instrument could be the solution you need. The Mini-Diff V2 measures scattered light on materials, including the effects of retroreflection and how it interacts with the material.
Traffic signs use retroreflective paint to reflect the headlights from an approaching vehicle to make signs easily visible to drivers at night.
For this example, we measure a bidirectional reflectance and distribution function (BRDF) traffic sign directly on the material with the portable Mini-Diff V2 instrument. The following figures show that the measurement results correspond to the angle of incidence at 20° for the red area (RGB measurement at top) and for the white area (green light measurement at bottom).
BRDF measurement for red area of the sign under RGB light (top); BRDF measurement for the white area of the sign under green light (bottom)
The Mini-Diff V2 allows you to measure and quantify, via a bidirectional scattering distribution function (BSDF), the retroreflection of a luminous road sign and determine whether the sign will be sufficiently visible at night. You can then export measurements for use in software such as LightTools to perform an illumination simulation.
LightTools simulation results obtained from the stop sign measurements are shown in the following figure. The simulation results provide a luminance map from the driver’s perspective.
Compare the stop sign on the left, which has material properties extracted from the measurement of a retroreflective paint, to the sign on the right, which has standard scattering paint. Both signs are illuminated by a car’s headlights at a distance of 50 meters. The retroreflectivity level is clearly higher for the sign on the left. In addition, you can distinguish the retroreflective lob effect (left side of the road sign) compared to the regular diffused component of the paint BRDF.
Using the portable Mini-Diff V2, it is easy to gauge the visibility of traffic signs from different angles and distances. This can be useful for assessing the deterioration of an existing sign, choosing the best location for a sign, or evaluating the quality of retroreflective paint during manufacturing.