Prismatic Road Signs
Denis Joye spotted these coloured reflections from road signs near Paris.
©Denis Joye, shown with permission

On a sunny day – and when a passenger! – look for strong prismatic colours on road signs. They appear only at certain angles between your eye, the sign and the sun and sometimes need to be searched for.

Old signs are made of glass beads and the colours are sections of rainbows.  

Modern signs are complex aligned arrays of small prisms and even lenses often within a plastic sheet applied to the sign support.   Colours appear when sunlight is dispersed into its components by passing between certain of the tiny prism faces.   The result is a colour spectrum rather than a rainbow.

Why make signs so complicated?  
Signs can be back illuminated but they also must rely (especially during power outages) on reflection of headlamp light towards the driver’s eyes.    The angles between a vehicle and the sign are constantly changing and optics are therefore needed that reflect headlamp light back to the driver regardless of the position.  

Three mutually perpendicular mirrors will reflect light exactly backwards regardless of the angle of incidence.  The mirrors are most conveniently held within a single optical block - a "corner cube".  A corner cube is actually a tetrahedron (but not a regular one) formed conceptually by slicing a cube.  One of the tetrahedral vertices is the original cube corner.     Rays entering the sliced face reflect internally and totally off all three of the original cube faces and leave in exactly the opposite direction to the incident ray.

Corner cubes in large-scale optical systems are usually ground so that the slice face is circular.   In road signs it is desirous to maximise reflectance by packing the tiny corner cube elements in arrays with minimum truncation and dead space between them.

As usual, it is not that simple.   We do not actually want light to be exactly reflected back to its source. We want it to reach the driver’s eyes.  

As at right, the angles between the headlamp and sign, between the sign and eye depend very much on the vehicle and its distance.   Signs placed at the side of a road have yet more angles to cope with.  

Successful road sign design has some optical challenges.

Thanks to Denis Joye for unearthing sign design literature.

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A corner cube reflects light exactly backwards.

Road signs ideally need to cope with a range of headlamp - eye angles.