##### It seems intuitively correct to measure retroreflection as a ratio of the intensity of light returned in the direction of the driver to the intensity of their car’s headlights. This ratio would give a scale for retroreflection that consisted of a similitude (dimensionless number) between 0 and 1. Unfortunately there are practical problems with this approach. In addition, there must be a system of units to define light flux, intensity and other optical quantities.

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