Quantifying Retroreflective Materials Using Digital Imagery

Retroreflection occurs when a light ray incident on a surface is reflected back towards the light source. The performance of a retroreflective material is of interest to accident reconstructionist, human factors professionals, lighting professionals, and roadway design professionals. The retroreflective effect of a material can be defined by the coefficient of retroreflection, which is a function of the light’s entrance angle and the viewer’s observation angle. The coefficient of retroreflection of a material is typically measured in a laboratory environment or in the field with a retroreflectometer. Often the material in question cannot be taken to a laboratory for testing and commercially available portable retroreflectometers are limited to entrance angles of 45 degrees or less and may be cost prohibitive in some cases. This paper presents a methodology to capture images of a retroreflective material at entrance angles between -4 degrees and 75 degrees and observations angles between 0.2 degrees and 1 degree. The process of calibrating the camera and the light source is presented and the coefficient of retroreflection and coefficient of retroreflected luminance are calculated from the images and presented as a function of entrance angle and observation angle.