Welcome back to Smarter Every Day.

You won't believe your eyes.

You've heard this before right?

It's usually like a click bait title to get you to watch an internet video

or read a stupid article.

But are there cases when you actually can't believe your eyes?

Make this video as large as you can on the screen that you're watching

and we're gonna do an experiment.

Put your head at a set distance from the screen and look at this photo.

It's a lighthouse on top of an island but I've inverted the colors.

I want you to focus your eyes right on the tip of that lighthouse and don't move them.

I'm gonna be quiet now and I'm gonna let you transport

your mind to this island off the coast of Tasmania.

[ seagulls ]

OK I'm about to invert the picture back but I want you to stay focused on that lighthouse.

Are you ready?

Here we go.

Out of the corner of your eye you should see a pale blue sky and a deep royal blue sea with little light green grass spots on the island.

Now, take your eyes and move it to the edge of the island.

You see that? There is no color in this picture.

It's a black and white image.

Your brain just made up color that wasn't there.

Can you believe your eyes?

By the way, just in case you're curious

this is what the original image looked like.

OK, so I just showed you a black and white image

and for some reason your brain saw something like this.

What is happening?

Is that something going on with your eye?

Or is that your brain?

For all we know it might be the optic nerve connecting the two.

Let's investigate a little bit further.

A couple of years ago I saw this video on the internet

and at first it just looks like a neat little light toy.

But my mind saw something way different.

I realized for the first time that I literally could not believe my eyes.

Not that I didn't understand what was happening,

but I knew that what I was seeing didn't actually exist.

As soon as I saw this video it was very clear to me that the dude that made this

was a genius.

The bad news is though,

he lives hundreds of miles away in the desert.

So I'm in a kind of a sleazy hotel

and uh, I don't want to say sleazy.

Have you ever seen somebody do something really cool on the internet

and you wanted to meet them?

Well that's kinda what happened here.

This is Greg.

- Hello

- And Greg has a pretty interesting gadget that you made?

Is that what we're gonna call it?

- That's what we're gonna call it. The device.

- Did you design this PCB?

- I did.

- Did you populate it?

- I did.

That's a lot of small chips on there, but..

- You're a geek man.

- Yeah I know. [laughs]

It gets easier.

- How long did it take to populate the board?

- It takes about an hour and a half.

- You did that whole thing in an hour and a half?

- Yeah.

- Yeah but can you change a water pump on a 1990 Chevrolet pickup?

- [laughs]

- So those are LEDs right?

- Yeah they're red green blue RGB LEDs.

- That's pretty cool.

Now how are you controlling that?

- So these LEDs are mounted on a circuit board that's mounted on a DC motor.

And if I apply power to the DC motor, it spins.

And if I time it just right, I can essentially light up any LED

anywhere on the circle that I want, as we can see here.

- So you just created these bitmaps and then uploaded them

through some software that you wrote?

- Yep, so this is a 63 by 63 pixel bitmap,

and essentially I take that and there's actually an

infrared sensor on the display that can receive data.

- Oh look at that.

Smarter Every Day.

Little homage.

So how did you do that?

Did you upload that, or..

You did that today didn't you.

- Yes actually I just created that image

right before driving out to meet you.

- This is pretty amazing.

So ok, here's the deal.

I asked you about this because I wanted to do this.

I wanted to use this high speed camera

to look at what you've got here.

This is a Phantom Miro that we're using here.

This is a Miro 320S, and we're gonna setup..

What's your update rate on the microcontroller?

- Um it's pretty fast. It's spinning at about 25 revolutions every second.

- Ahuh.

- And within those it updates 256 times.

So we're looking at about a 200 microsecond rate that the LEDs get updated.

- OK so a thousand frames per second is not fast enough, is what you're telling me.

- No.

- OK.

Alright, well let's figure out what frame rate we need to hit in order to understand what's going on here.

- Alright.

- What do we call these? Are these pixels?

Cause it's not really like a square thing like cartesians,

it's kinda like an arc..

- Right, it's kind of like an arc pixel.

- OK, so what do you call that?

- An arxel.

- Arxel.. like it. We're gonna go with that.

How far does the LED bar travel for each individual arxel as you call it?

- It's about one and a half degrees.

- If it's only going one to two degrees, then why is my brain still seeing that light

the whole time it's around?

Because it's only one 360th of the sweep, but the rest of it's dark.

According to the high speed camera,

which I had to crank up to 5500 frames per second,

this is what's actually happening.

Check it out.

Those's dots, or the arxels as Greg likes to call them

are kind of flipping around all over the place.

Most of the image is dead space.

So why is your brain making that image?

To understand why the brain sees something that's not there

I found a guy that studies this sort of thing

that's published over 130 different papers on similar topics.

OK I'm on a pretty bad Skype connection with Dr Stuart Anstis

who is a genius at the University of California San Diego

and what do you study Dr Anstis?

- [british accent] I study visual perception, in particular visual illusions

which tell us about the normal processes of vision,

how the eyes send information to the brain.

- That's fantastic. And obviously your accent makes it very clear

that you know exactly what you're talking about because I would

expect nothing less from a person who studies visual perception.

So I want to understand why my eyeball is seeing something where I know there is not light.

Why am I seeing that?

- It's because of persistence of vision, which means the eye averages what it sees over a short period of time.

It's analogous to a camera where you have a long exposure time,

and this will give you more light coming in,

greater sensitivity, but you have a more sluggish response.

So anything moving gets blurred out.

- What is the difference in time from the moment the LED is illuminated

until my eye registers that the light is there?

There has to be a delay time there.

What is that?

- That delay time varies enormously, over a ten-fold range,

anything from 10 to 100 milliseconds.

- According to Dr Anstis there's two things going on,

and let's look at it.

Let's pretend that we have a flashing LED

and we want to look at the brain's response to that LED.

First of all he said there's a delay,

so when the LED first comes on, our brain's not going to

immediately see vision, it's gonna take some finite amount of time later.

Secondly he said that the eye averages what it sees over a short period of time.

Think about that.

If we have a moving average, that means that our vision has some sort of inertia to it.

It works like this.

As the average comes along and is exposed to that LED flash, it starts to ramp up.

As the light goes away, that moving average starts to ramp back down.

As the light comes back that average starts to go up again,

and instead of having gaps that are complete darkness, we have this nice trough in the bottom.

Therefore we have a persistence of vision even though there is no light to see at that point in time.

Let's look back at that slo mo image from before with all the blinking LEDs.

Now, let's add this time average of light and see what the image looks like.

Check it out.

How cool is that?

It looks just like what our eyes see.

At what rate would you expect that I would quit seeing a uniform image

but I would start to see like a tail dragging across the screen?

- Well if the average time is less than one revolution then you're going to see a gap.

But supposing as you say, the propellor goes around in 1/25th of a second, that's 40 milliseconds.

- I get it now.

Do you understand?

Think about it.

Greg's wheel is rotating at 25 frames per second,

and that has to do with the moving time average of the human eye.

That's why this video is at least 25,

it's actually 30 frames per second.

If it wasn't, you would see flickering of the image.

So what seems like an imperfection in our eye

is actually what smooths things out and makes the world work smoothly for us.

That's pretty awesome.

If the people watching this video were students in your class

what would you want them to know about Greg's wheel and the persistence of vision?

- [laughs] Well I would say every system has got a limited time resolution

The eye has an engineering problem of trading time resolution against sensitivity to light.

And in fact it's got sort of knobs inside which can change that trade-off relationship automatically.

The eye is in many ways much cleverer than the camera.

It's a beautiful piece of engineering.

- Well thank you very much sir, I really appreciate your time.

- Thank you.

- Alright I hope you enjoyed this episode of Smarter Every Day.

It was sponsored by lynda.com which helps pay for people like Micah who's a video editor.

- Hello.

- Who happens to actually have a certification from Lynda.

Lynda is like a Smarter Every Day but on very technical topics.

So in this particular case, after effects.

I had no idea how to do this.

So Micah came along and helped me.

He actually took a bunch of classes on Lynda and we found one together right?

- That's right.

- And what was it called?

- Echo.

- This was the tutorial that taught me to model persistence of vision using After Effects.

But you can learn almost any topic you want.

They've got Excel tutorials, Photoshop tutorials, even how to edit videos like I do with Premiere.

This is not the stuff you're gonna find on YouTube, this is really high quality tutorials.

They've got over 100,000 of them.

So if you want to support Smarter Every Day,

please consider going to lynda.com/smarter

You'll get a 10 day free trial which you can cancel at any time.

I wanted to make sure that if you're supporting Smarter Every Day

you're gonna have a really good experience

so that's lynda.com/smarter

You can cancel at any time.

I'm Destin,

I hope you enjoyed this, feel free to subscribe,

if not, no big deal.

Hope you're getting Smarter Every Day

Have a good one.

OK is there any truth to the rumor that you actually proposed using one of these?

- There is some truth. [laughs]

As I was finishing up the project I thought to myself,

Hey it'd be pretty cool if I could like write messages on here