OPOD - Prismatic Road Signs

Last updated on September 16, 2023

Prismatic Road Signs: Unveiling the Science Behind the Colors

When traveling on a sunny day as a passenger, take a moment to observe the road signs around you. You may notice something intriguing - strong prismatic colors emanating from these signs. These vibrant hues only appear at specific angles between your eye, the sign, and the sun, making them a fascinating optical phenomenon that requires some searching.

The older generation of road signs is constructed using glass beads, which give rise to these colors. These beads act as tiny prisms that refract sunlight, splitting it into its component colors and creating sections of rainbows on the sign's surface. It's a charming reminder of the simple yet effective technology used in the past.

In contrast, modern road signs are far more complex. They incorporate aligned arrays of small prisms and lenses, often embedded within a plastic sheet applied to the sign support. These intricate designs ensure that colors appear when sunlight passes between specific prism faces, resulting in a spectrum of colors rather than a traditional rainbow.

But why make road signs so complicated? The answer lies in their functionality. Road signs not only rely on back illumination but also need to reflect headlamp light towards the driver's eyes, especially during power outages or low light conditions. As the angles between a vehicle and the sign constantly change, optical elements are necessary to redirect headlamp light back to the driver, regardless of the sign's position.

To achieve this, road signs utilize a fascinating optical principle known as a "corner cube." A corner cube is essentially a tetrahedron formed by slicing a cube. By placing three mutually perpendicular mirrors within this tetrahedron, light entering the sliced face reflects internally and totally off all three original cube faces, exiting in precisely the opposite direction to the incident ray.

In large-scale optical systems, corner cubes are usually ground to have circular slice faces. However, in road signs, the goal is to maximize reflectance by packing tiny corner cube elements in arrays with minimal truncation and dead space between them. This ensures that the reflective properties of the sign are optimized, enhancing visibility for drivers.

However, the science behind road sign design is not as straightforward as it may seem. The ultimate goal is not to reflect light exactly back to its source but rather to ensure that it reaches the driver's eyes effectively. This requires careful consideration of the angles between the headlamp, the sign, and the driver's eye, which vary depending on the vehicle's distance and position. Road signs placed at the side of a road face additional challenges due to the multitude of angles they need to accommodate.

In conclusion, prismatic road signs offer us a glimpse into the fascinating world of atmospheric optics. These signs, whether made of glass beads or modern prism arrays, harness the power of light refraction and reflection to enhance visibility for drivers. Understanding the science behind their design helps us appreciate the complexities involved in creating effective road signage systems. So, the next time you're on the road, take a moment to marvel at these colorful beacons and the ingenious science that makes them possible.

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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Reference Atmospheric Optics

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