The convention in science wasn't established and there were two major problems with red one.

There wasn't yet a way to make red pigment that wouldn't fade and to read is way harder to see.

At night.

You see your eye has three types of cones that detect color red, green and blue cones.

But at night there isn't enough light to activate these cones in your retina.

So your eyes rely on rods which are much better at perceiving low light.

And if we look at the wavelength sensitivity of rods, you'll notice they actually don't even pick up red wavelengths when we see red at night, it can only be picked up by cones and as a result red is the hardest color to see in low light.

So while a traffic light shining red is one thing a lowly lit red sign is completely different on top of this, even in daylight, our eyes are actual Most sensitive to a yellow greenish color.

This is because it's right in the overlap between our red and green cone sensitivity, peaking at about 550 nm making it a perfect option for visibility, but not exactly one for consistency with red meaning stop.

So when did the switch to red signs happen and how do we overcome this hurdle.

It's actually one of the most ingenious and coolest inventions to ever come about retro reflection.

I'm here at three M.

Innovation theater, the home of the retro reflector where it was invented, created and first implemented and we're gonna talk about what they are and how they work.

I have a little laser light here but to not blind you, we understand if a light comes into the mirror it's gonna reflect back into this lens or your eye.

But if you point a laser at an angle to a mirror it exits at the equal angle.

The other way you can see over there, I have that laser pointer on the shelf and that's not very useful if you're trying to get light reflecting back to someone on a sign.

So we need a better solution and three M.

Came up with that many decades ago.

They first used these glass beads.

Now when light enters glass is a different refractive index so it actually bends the light.

We put this down here at the white surface that's reflective behind it.

And I shine this laser through it.

You can see that laser comes back to me now instead of bouncing out the other direction, it's actually shooting back at my stomach a little faint but still there but three M.

Took that principle even further with a full cube retro reflector if we take this and just talk about these first two mirrors here, if I shine a laser in one side of that mirror it will bounce out at the equal and opposite angle and then do it again and you can see on me this laser looks basically just as strong as it did going in as it does on me.

So that's really useful that it's reflecting back to the source where it's coming from.

And when you think about signage, you want the light to hit it even if it's at a weird angle and come back to you.

So you can see the thing by adding a third mirror down here.

We can now basically point anywhere on here and we're gonna have reflection that ends up back towards me.

You can see it here as well even though I'm pointing on the bottom and that is the basic 101 of how a retro reflector works.

Now, the question is how do we go from the principle of retro reflection to getting it on a huge variety of surfaces, like not only signs, but multiple plastics.

The lines on the road and even clothing.

Walk me through what the heck you are moving around here, These are our reflective sheeting, What we have here.

Different type of reflective sheeting.

So these are all basically full of retro reflectors correct.

This is full of retro reflectors.

Basically each of these Little squares has 6000 retro factors per square centimeter.

Yes, you did their homework.

Pretty good.

This though is like a metal backing of which these retro reflective sheets are put onto totally correct.

The most important sign of the stop signs with our materials.

I wasn't about to leave without a sign of my own.

So I made sure to ask specifically how the signs get put to.

We're gonna start with the retro reflective sheeting so we'll cut it to size And our retro reflective sheeting is basically a big sticker.

So there's a liner and adhesive on the back.

So once you apply it on here, you get kind of this lip this overhang.

So we have to trim that off and then separately we'll take the electoral cut overlay film or some people call it and we'll put it through a friction fed plotter we wrote Science to keep this, we just kind of grab the edge and just push it and then you can just lift the letter off.

Oh you can't.

Okay, there we go.

Alright.

Some letters like a will actually have a little floating piece right in the middle.

And so applying this pre space allows you to keep everything in place and remove the liner essentially you end up with something like this.

So you remove the liner, the green is stuck to the white retro reflective sheeting.

It's on aluminum.

And the last thing to do is to remove this pre space which you want to give it.

I would love to are we ready.

Oops, Wow, look at that Asap science.

Almost every street name sign is made like this before digital printing.

That's crazy.

And another way to manufacture science is using digital printing.

So digital printing has been around for about 10.

So this is printing on retro reflective sheeting.

Exactly this specific printer uses HP latex inks.

So it's water based.

So it's similar to like your printer at home, we're just laying down all right, can we see a little something?

Something?

It's more a sub science science.

That's so cool.

Okay, we're outside in the dark and we're going to do a little experiment with the sign now to see it in action.

So, if I shine this flashlight on the ground, my immediate surroundings are pretty visible but become increasingly less visible as you go further out.

And that's because light scatters in all directions and less photons are actually making it back to the camera lens.

But if I shine this light now over towards Greg holding our fancy retro reflecting sign and put the light right beside the camera lens, you'll see just how much brighter the sign is than almost anything else the light is touching, even though it's all the way over there.

Now, of course, if I move the light away from the lens, the sign becomes less bright.

Even here, I'm about six inches go a little further about a foot, you can see how much darker the sign is and that's because the camera lens or you are no longer the source and instead the majority of the light is bouncing back over here.

Over here, where I'm holding the light, I see the sun just as bright, but you don't and that's because the light is always reflecting back to the source.

You've likely seen this while driving at night when there are minimal overhead lights signs super far in the distance are extremely bright while things closer are completely dark, even the metal post seems invisible here.

Now.

What about when it's raining outside?

Sure, in the broad daylight you can still see the lines on the road.

But I think we're all familiar with that scary problem of lines disappearing at night.

The problem here is that the water actually changes the refraction of light.

So where the retro reflectors act ideally under dry conditions as soon as water is on top of them, their refractive index is lowered by a ratio of 1.33 and so the light leaves at a different angle sending less back to the source.

And this is incredibly important because while only 25% of driving occurs at night, 55% of deaths from car accidents happen when it's dark with rain being disk proportionately dangerous at low light conditions compared to the day to combat this rain visibility problem three M actually developed these elements which are retro reflectors that have different refractive indices.

So on this side we have elements that have a refractive index of 1.9 and they work well in dry conditions and on this side we have elements that have a 2.4 refractive index and work better in wet conditions and on top they're both in air and on the bottom they're both submerged in water.

And I'm gonna show you what happens when we're in the dark and shine a light on both of these.

You can see that the one on the left at the top is optimized for dry conditions because it's flashing back more light and then the one at the bottom on the right is optimized for water and is sending more light back to us even though it's submerged in water.

And of course the best solution is to include a mix of these elements in the product, whether it's the layer put on top of painted lines or directly into taped lines so that people can see them in either weather condition.

If it's dry, the one 0.9 will send the light back while the 2.4 won't.

And when it's wet, the opposite occurs.

So behind me we have a variety of painted lines.

Some have the regular glass beads on them and other ones have the three M.

Elements on them with the 1.9 refractive index for the 2.4 refractive index or a combination of those and we're gonna see what happens in the dark when they get rained on here's the setup in the dark before the rain machine has been turned on and you can see all the lines are equally visible.

But as the rain begins to run, some of the lines slowly start to become obscured and harder and harder to see.

It's only the few stripes on this right hand side that remain visible as they have varying amounts of the water reflective elements.

And here's the difference in the real world.

On the left is a wet line without water reflective elements.

And on the right you can see it with these elements and as a result you can actually see that line even in the rain.

So there you have it.

Road signs are officially way cooler than you probably or at least are a lot cooler than I ever realized they were before making this video and hopefully I've convinced you of that too.

I want to send a huge thank you for having me at the facility and to JC and gus for showing me around and teaching me and especially making me multiple signs that I can bring home back here to the ASAP science headquarters.

I will cherish it forever.

Thank you so much for watching.

Make sure you like the video.