Project Y

42 // TECHNOLOGY
Glass beads for safety
CAT’S EYE PRINCIPLE It seems paradox – glass beads (as perfectly round as possible)
fixed to the road can increase road safety. This is because of an optical characteristic which
also is believed to make the eyes of cats glow.
// iNeS ScHMieDMAier
Two different
technologies
are typically
employed in pro-
ducing reflecting
material – the
glass-bead meth-
od and applying
retro-reflecting
micro prisms.
M
any might have had their
adventure of a ‘special
kind’ in the dark or at
night: Two points a few millimetres
apart mysteriously glow in the dark
and disappear as quickly as they
showed up. Once breathing is back
to normal it is usually discovered
that it only was our favourite pet.
Its eyes sometimes give us spooky
fantasies. The so-called ‘glowing
carpet’ (Latin: Tapetum lucidum) is
the culprit. It is a strongly reflective
layer of cells behind the eye’s
retina. It reflects back the light that
has already passed the retina. This
layer gives a cat better night vision
and also reflects the light that hits
its eyes in the dark - and makes
them ‘glow’.
retro-reflection as
safetY factor
In physics this characteristic is
called retro-reflection – this is
when catadioptric (mirroring and
refractive) elements reverse the
direction of incoming light back to
its source point. The production of
retro-reflective materials utilizes
this characteristic. It is applied to
safety clothing and surface markings
to increase visibility. These
materials have an important advantage:
they reflect back the light in
exactly that direction where the
light came from.
aPPlication in safetY
clothing
If the headlights of a car hit the
reflecting material of the clothing
of a construction worker the
light is reflected back exactly to
the approaching car. A person at
the side of a road is therefore visible
to the driver sooner and from
a greater distance. This is of great
advantage at construction sites,
motorways and state roads where
vehicles travel at great speeds. This
increased safety factor also applies
to surface markings as the use of
glass beads makes them glow when
hit by a beam of light. Glass beads
sticking out of the surface markings
far enough reliably reflect incoming
light and provide good visibility of
the markings even at bad visibility,
in the dark, during rain or fog.
glass Beads Provide 100
Percent reflection
Reflective material is used with
safety clothing, traffic signs, street
markings, promotion and bicycle
reflectors.
Even though a cat’s eye on bicycle
has little in common with the eye of
a ‘real’ cat they yet follow the same
principle of retro-reflection. //