Retroreflection

Retroreflection is the property of a material to reflect incident light back towards the source, rather than scattering it. It is what makes a road sign or hi-vis jacket light up in headlights. It is expressed as a retroreflection coefficient (Ra) or as SIA (Specific Intensity per unit Area, in cd/lx/m²). Unlike a retroreflective sign, an LED screen emits its own light and therefore does not depend on headlights.