[over sounds of crickets and frogs] 
Have you ever thought about light and where it goes?
Don’t worry, there’s nothing wrong with
your device, I’m standing in the middle
of a country road with no illumination at
night and thus…
there’s nothing to see.
Well, that’s not quite true, but there’s
not enough light for the sensor in the camera
to make any use of and so we’re left with
nothing but darkness.
And in fact, my eyes can’t see much either.
It’s real dark!
I’m using my phone for this, and like many
phones I can turn on a weedy little LED
which will emit a smattering of photons to get reflected
off of things and hopefully make it back to
the camera where it will detect them and form an image.
But, because that LED is so dainty...
well it’s only really helpful at close range.
Looking down the road reveals…
not a lot.
But now I’ve turned around, and watch what
happens when I turn on the LED.
Woah!
Look at those bright chevrons!
The road curves here rather suddenly, prompting
the local road authority to erect these warning signs.
And even though they’re lit by a tiny LED
that’s about 20 meters away from them,
they appear incredibly bright and the camera can easily
see them.
In a weird twist, though, the signs don’t appear
that bright to me.
I have to have the camera right in front of
me, basically in my line-of-sight,
in order for the signs to appear brightly.
And if I turn off the camera’s own light source
and switch to a flashlight,
well now the signs don't appear that bright even though they are illuminated with a much brighter light source.
In order for them to shine brightly, I need
to move the flashlight right next to the camera's lens
to basically the same position of its own LED in the beginning.
What’s going on here?
Well, those signs are what is called retroreflective.
Retro, as in old schoo -
I mean, back or backwards,
and reflective as in reflects light.
In common usage, this might simply be called
reflective but, that’s not really an adequate description.
Everything’s reflective to some degree,
even Vantablack!
This vinyl record is reflective,
this toaster is very reflective,
I’m reflective,
you’re reflective,
some people are even deflective!
Ehh, I’m sorry, and I'm gonna need a yes or no answer here,
are you able to explain what you mean by "deflective"?
Our vision works because things are reflective.
So long as there’s some source of light
which can land on things to be reflected back
and make it into your eyes, you’ll be able
to see them.
But when your only light source is a tiny
little LED or, perhaps a pair of car headlights,
your everyday light reflection technique becomes
limited with distance.
See, when light hits most objects it scatters
in all directions.
Even if you shine a bright spotlight at something,
only a small fraction of that light ends up
coming back into your eyes.
The rest goes in all sorts of directions,
allowing anyone to see it
from virtually any vantage point.
Making matters worse is the inverse-square
law.
The farther away something is from a light
source, the less light that hits it because
it spreads out and thus the same amount of
light covers a broader surface.
You can help combat that by focusing your
light source into a tight beam for a longer throw,
but you can’t fix the return trip.
Or can you?
That’s what these signs do.
They’re covered in a coating that forces
light to be reflected back to its original source
and virtually nowhere else.
How can they do that?
Geometry.
Consider the humble mirror.
It may not seem like it has much control over
how light gets reflected but in fact it does.
A lot, really.
The mere fact that you can see a reflection
in a mirror is astonishing when you think about it.
Again, most objects just reflect light all
scattered-like,
allowing you to see *it* but not much else.
But mirrors let you see an image of other
things in the environment,
and that’s only possible if we can get light to follow some rules.
And that rule is;
The Angle of Incidence is
equal to the Angle of Reflection.
The path that any light ray hitting a mirror
will take after it hits it is entirely predictable.
Say you shine a laser pointer onto a mirror.
Well, whatever angle it hits the mirror at will be the same that it leaves the mirror.
So, a thirty-degree angle on the way in equals
a thirty-degree angle on the way out.
That’s the entire reason mirrors allow you
to see an image at all -
light that hits them simply changes direction, so it has whatever structure it did in the first place
creating what looks to be a window into another world....
Anway, knowing that angle in equals angle
out, you can put two mirrors at a 90 degree
angle to one another and end up with a mirror
that always sends light in the direction it came from.
See, no matter what this angle is, you’ll
get the same angle coming out here.
Then it hits the other mirror, and we know
this is a right angle
so this angle is just 90 minus whatever this one was, (since we're dealing with a triangle)
and since the exit angle is the same as that result,
the sum of these angles is always 180 degrees.
So, no matter how light enters this little
corner, it will go right back to its source.
Take a look with the laser pointer.
The return line is always parallel to the
incoming line no matter where I shine the laser from.
Another fun way to visualize this is simply
to look into the corner.
You’ll see your reflection no matter how
you move, which is pretty neaahhohhhh
right.
There’s a third dimension.
I’m always forgetting about that third one.
Ugh, no matter, just add another mirror.
Three mirrors arranged in this shape, an inside
corner of a cube, is all you need to create
a retroreflector.
Now we really can move around however we like
and still see our reflection,
within a limited range, though.
And now the return beam from the laser stays
parallel even when the height changes.
You don’t even need to use mirrors to do
this, instead you can create prisms from a
single piece of glass or plastic.
You can see some of these in use in this video
in which Geoff Marshall and Steve Mould explore
their use in surveying the London Underground.
And you can also see many of the same demos
I just did in Steve’s video ostensibly about
stealth aeroplanes.
But what’s the point of this?
Well, it’s to combat the inverse-square
law by cutting its influence in half.
When driving at night, no matter how good you think
your car’s headlights might be,
they don’t have that fantastic a range.
As things get farther away, the beam gets
more spread,
and so objects farther away from you are hit with less light.
The same thing happens on the return trip
to your eyes, so you really can’t see things
much more than about a hundred meters away.
At least, not well.
But a retroreflector will be visible from
many hundreds of meters away,
and if conditions are right kilometers away.
How?
Well, whatever light that hits them from your
headlights will be directed in nearly-parallel
lines right back to them.
It won’t spread out like the light reflecting
from virtually all other objects
because we’ve mastered geometry.
Using a corner-cube prism or other methods
virtually eliminates the spread of light
and nearly all of it gets returned to sender.
You might have noticed, though, that your
headlights and your eyes
aren’t exactly in the same place.
That’s no matter, the lines are *nearly*
parallel, so there’s margin for error.
Plus, at any distance more than a few meters,
the angular distance between your headlights
and your eyes is mostly negligible.
It does however mean that the effectiveness of
retroreflectors is influenced to an extent
by the design of your car.
If the headlights are mounted in an odd location where they're far away from your eyes,
you might not be able to see reflective markings quite as well.
In fact, I’ve often wondered if truck drivers,
as in big rig over-the-road truckers,
have a harder time seeing reflective markings because
of how high up they sit.
Let me know in the comments.
Still, though, these reflectors are remarkably
effective at limiting the spread of reflected light.
That’s why I couldn’t see the chevron
signs with my naked eye, and why they didn’t
show on the camera when lit by the flashlight.
The phone’s own LED is right next to the
camera, so even though it’s really not a
bright light at all, whatever light does manage
to make it to the sign will be sent back
bright and clear.
And this is why using a camera flash when
there’s anything retroreflective in a scene tends to…
not go well.
Just as a demonstration of how remarkably
effective these are, I’ve placed a few around the set.
To the camera, they don't seem to be all that bright.
That’s because none of the lighting illuminating
me or the set is in-line with the camera's lens.
But watch what happens when I shine a flashlight
at them right next to the camera.
Even though the flashlight adds so little
light to the scene that it might as well not be there,
whatever manages to land on the
reflectors goes right back to it,
and into the lens.
And of course if I step up to something a
little brighter, the effect is dramatic.
Anyway, the use of retroreflecting things
for roads and associated markings
is in my opinion one of the most clever things we’ve
done for road safety and underappreciated.
Though it’s a simple thing, it dramatically
increases safety when driving at night
in countless ways.
Making things more visible is always a good
idea, and using your car’s own headlights
as an incredibly effective means of illumination
is so clever it’s almost like a hack.
Road signage can appear as though it is lit
which can dramatically increase safety.
Barriers and other warning devices become
glaringly obvious when they’re retroreflective.
Even something like a railroad crossing can
become instantly identifiable from a mile away
when the crossing gates and crossbuck
are retroreflective.
And of course, all this is done without any
electrical infrastructure
or the use of any energy at all of their own.
Retroreflectors are able to take miniscule
fractions of your headlights’ output and
direct it so effectively that signs can appear
brighter than if they were conventionally lit
even when you’re well outside the effective
range of your headlights.
It also eliminates light pollution associated
with lighting signage
because it’s your car’s own lights providing the illumination.
But personally, signs aren’t my favorite
use of retroreflectors.
Instead it’s simple markings.
Placing basic reflectors like this in strategic
places can dramatically increase safety
particularly in inclement weather.
In situations like heavy rain, having embedded
markers between the lanes like these
makes it so much easier to stay in your lane, and
also makes it easier for those around you
which is important!
It can even help during the day, one of many
reasons that your headlights should
(and are usually required by law to) be on when it's raining.
Here in the US it’s common to see white
markings at regular intervals on guardrails
to make them more visible, and it has the
lovely effect of guiding you on the road
almost like runway lighting.
I think this is not only beautiful on a curvy
stretch of rural road, but helps you guide
your car in a nearly subconscious way and
put more attention towards avoiding other hazards.
And of course, putting reflectors on those
hazards themselves is also a great idea.
This is why *every car* has red reflectors
somewhere on its rear.
While your taillights do of course light up
at night, it would be nice to see a parked car, too.
Or the rear-end of the startlingly large number
of moving cars out there these days
with drivers who somehow don’t know when and how to use their headlights.
Yes, I’m talking to you, person who thinks
their headlights are automatic
when in fact they’re just daytime running lights and
thus your tail lights aren’t lit.
Sure, automakers deserve some blame for making
that mistake possible,
but you are piloting a two ton machine capable of killing people.
Learn how it works and how to use it properly!
Cars also have reflectors on their sides,
too, for the same reasons.
But of course the most important hazard to
avoid when operating a car is a person.
And retroreflectors can help with that!
Hi-visibility safety vests almost always contain
retroreflective stripes in addition to being
a ridiculously vibrant color to make a human
body as noticeable as possible.
If you’re ever working somewhere that vehicle
traffic is a hazard, it's imperative that
you wear one of these for your own safety
because drivers are careless idiots.
Because of that fact, bicycles have reflectors
on them to help make them more visible regardless
of what their rider is wearing, but I would
suggest that you go a little above and beyond
and invest in a vest, lights, and whatever
else you can because again,
drivers are careless idiots.
Really, if you're ever spending time as a pedestrian
or cyclist around motor vehicle traffic at night,
reflectors are your friend!
Sure, you might not want to go all out with
a neon vest, but there are plenty of backpacks,
shoes, and other accessories donned with highly
reflective markings.
I suggest you seek them out for your own safety.
Anyway, let’s talk a little bit about the
reflectors themselves.
There are actually many types out there, and
they’re finding their ways into more and more places.
Many commercial retroreflectors simply mold
a pattern of corner cubes into plastic,
like these red ones.
It looks like a sea of honeycombs but look
inside and you can see the distinctive shape
of a corner cube.
It's even more obvious from the back.
An interesting thing about these is the argyle-esque
pattern you can faintly see.
This helps them have a wider effective angle
because, well, a corner cube as you can see
here does in fact have a limit to its retroreflectiveness.
Having those cubes in two slightly different
orientations means that the reflector as a
whole is effective over a wider range of angles.
But the corner-cube isn’t the only method
of retroreflectifying.
Spherical shapes are effective, too.
One of the earliest examples of retroreflectors
seeing road use is the so-called cat’s eye.
These reflectors were inspired by the eyes
of a cat.
Let’s take a look at one.
This is a cat.
The reason the eyes of a cat and in fact those
of many vertebrates are retroreflective is
because their eyes contain a tapetum lucidum.
This structure behind the retina serves to
reflect some light that passes through the
retina back onto it in order to improve night
vision.
A fortuitous side-effect of this is that many
animals near a roadway at night are often
very visible so long as they’re looking
in your general direction.
The eyeshine as its called of a cat inspired
the cat’s eye reflector, invented by
Percy Shaw of England in 1934.
This was the first raised roadway marker with
many copy cats appearing over the years.
No, I will not apologize.
You can also just use tiny glass or plastic
beads.
A lot of reflective fabrics use these, as
well as some reflective tapes like this.
They are quite effective and have a more forgiving
range of angles than corner-cube based designs,
however it’s not quite as bright.
It’s not far off though, and the ability
to use this over a wide range of materials
and applications makes for an extremely versatile
technique.
Here’s a fun product, raw reflective beads designed for you to make your anything reflective!
Just figure out how to get them to stay on
something,
I suppose a glue and maybe a varnish on top, and voila!
Reflective miscellanea.
You could even write secret messages with
these, visible only to those in a car at night!
Or with a flashlight on their head.
Or with a camera phone.
Or with discerning eyesight - it’s really
not that invisible
but I do like the idea of writing something naughty with this on the back of your car.
In fact, products similar to this are often
added to roadway paint to make lane markings
themselves retroreflective, at least to some
extent.
3M calls this Connected (nice) Roads All Weather
Elements,
and while there’s no doubt in my mind they make lane markings much easier to see at night,
I can’t say I’ve encountered anything quite as effective
as good ol' embedded roadway markers like you can see here.
Oh, fun fact!
Many of these embedded road markers are white on one
side and red on the other.
On limited access highways or other divided
roads where there’s the potential to be
going the wrong way, they’ll be placed so
that the white side faces oncoming traffic
(so, you)
and the red side faces forward.
If somehow you ended up facing the wrong way,
all of the roadway markers would appear red
to hopefully signal to you that something
is very wrong.
I say hopefully because I feel like few people actually 
know this and don’t have a lot of faith
that people who might end up facing the wrong way would even recognize the novelty of red lane markers
let alone what they’re intended
to communicate.
Anyway, the last kind of reflector I’d like
to show you here is
this kind.
This is a different style of reflective tape,
and it appears to use very tiny corner-cube reflectors.
I’m very curious about how this is manufactured.
You can see that different squares reflect
differently, and there appear to be four groups.
Undoubtedly this is to provide the same widening
of effective angle that we see in the conventional
plastic reflector.
The tessellated hexagons you see appear to be there
for… style?
They actually break up the reflective portion
so… yeah this is a bit of a mystery.
My guess is they are designed to allude to
the honeycomb structure you see in a traditional
prismatic reflector, which ya know now that
I think about it
it’s gonna make people more likely to buy it.
It just seems like it would definitely be more effective
than this plain-looking tape.
It actually is, so I guess that’s good,
but it would be even MORE effective if that
pattern weren’t there.
Oh, and while this is admittedly a complete shot in the
dark, I think that a lot of newer road signage
uses this sort of reflector as a coating and
perhaps that’s why they can sometimes be obscured
by things like condensation.
You might have noticed a blotchy-looking sign
at some point, often uneven condensation patterns
are the culprit, or other partial obstructions
like light snowfall.
Because the individual prisms are so small,
perhaps they’re more susceptible to things
like dew droplets bending light off the course
we want it to go.
It’s probably for situations like these
that some critical signage
(and other warning devices like road construction barriers)
get their own illumination despite being retroreflective.
And while we’re on the subject of signage, I do think
it’s possible for some signs
to be a little *too* reflective.
In a setting like this, where one is using
their high-beams, a lot of newer signs are
downright dazzling and… well maybe NHTSA
should investigate different reflectivity
techniques and standards depending on the
setting.
A really bright sign can wreck your night
vision, and when one of the hazards you’re
looking out for is deer or moose, that’s
a bit problematic.
And finally, this video doesn't really have
much of a structure, does it?
Let’s talk about some of the other things we use retroreflectors for.
They’re not just useful for making things
visible at night.
Corner cube reflectors work thanks to geometry,
so they work on pretty much anything from
radio waves to light to sound.
Things like buoys will have corner reflectors
in them to make them appear more clearly on
a boat’s radar display, and so will boats
themselves.
Unless of course it’s a stealthy boat.
You’ll sometimes see a retroreflector just
like these ones used in simple electronic trip wires,
either for burglar alarms or motion
sensors.
A simple device with an infrared LED and photodiode
can be pointed somewhere at a reflector's
general direction and because the output of the LED gets reflected right back to it,
the photodiode can see it easily.
Just modulate the light so it knows that
it's its own signal, and if anything crosses
the path between the emitter and the reflector,
that signal will be lost and
woop goes the burglar alarm.
Interestingly you tend not to see these in
use for garage door openers safety beams.
Instead you get a pair of these doohickeys
and it can be a pain to line them up.
The nice thing about a retroreflector would
be that alignment is pretty easy because the
return beam is self-aligning.
I’m presuming there’s a reason we don’t
use these in garage door openers, but you’d
think you could get away with just one device
and a reflector on the other side.
Corner reflectors are also often used in surveying.
We can measure distances quite precisely by
timing how fast it takes a laser pulse to
return to its source, and when you place a
retroreflector some place as a target,
well you can use angles and junk combined with
measured distances to triangulate the coagulants
and quantify the hypotenuses and survey says
building.
Continuing on those lines, one thing I’m
surprised doesn’t get much cover is the
use of retroreflectors to help automated systems
find their bearings.
Maybe you saw that thing about road trains
of self-driving tractor-trailers following
one another with a human pilot at the front,
and they used computer vision systems combined with
a pattern on the back of each truck to work
out their relative position.
Hi, quick editor’s note, this is a brain-worm
of mine that I cannot find the source for.
I know I saw something which included a picture
of that pattern, but I can’t seem to find it.
Best I could come up with were some scholarly
articles comparing different autonomous truck
platooning (as this is apparently called)
systems.
If any of you out there know what I’m talking
about and can share a link, please do.
Otherwise, well, hopefully you understand what
I’m getting at.
It seems to me like perhaps we should start
working on standardizing roadside markings
so that self-driving cars or even just driver-assistance
technologies can have an accurate understanding
of where they are in space.
Just a thought.
And of course, let’s not forget the most
famous retroreflectors of all.
The ones we left on the moon.
Astronauts on Apollos 11, 14, and 15 left
some reflectors up there so we could go all
zippy zappy with a laser, time how long it
took to get back to us, and know precisely
how far away the moon is at any given time.
There were also retroreflectors placed on
 two rovers sent to the moon by the USSR
to allow for similar experiments.
We’ve tracked that data over time and learned
all sorts of cool things
about the nature of the universe.
Science is pretty neat.
Remember that?
Remember science, the process?
Ah, good times.
Anyway, the next time you go for a drive at
night, well first of all be careful.
Please.
But also take a moment to appreciate the beauty
of retroreflection.
Though we may take it for granted today, it
took the combined efforts of mathematicians,
materials scientists, highway engineers, curious
biologists, and above all,
people wishing to make the world a safer place.
♫ glaringly smooth jazz ♫
Light that hints it...
hints?
And I've already ruined it!
And that rule is -
I regret writing it that way!
And we know this is a right angle, so this angle is just 90 minus whatever this one since it's a triangle.
Daaahhh!
Whatever this one?
One more try...
But what...
But what's the p...
[clears throat and makes very silly noise]
But, a retroreflector will be visible from many hundreds of meters away, and if conditions are right
kilometers away
[laughs]
my brain broke
[clears throat]
Any - oo
Anyway, the last...
Anyway, the last kind of reflector I'd like to show you here is...
this... that's so dumb
Reflectors are pretty snazzy, huh?
Think about the fact that every single one out there was planned out and spaced precisely for your benefit.
We really ought to appreciate those sorts of things a little more, wouldn't ya say?
REFLECTORZ RULE
