If the aerial trick works on one point of light, it'll work on many, right?  If one point is a pixel, let's show a screen!

That was a pretty easy leap.  Problem was, it didn't work so great.  LCD screens are roughly one hundred billion times dimmer than an LED (not sure I did my math right there, check me).  The cheap retroreflective fabric actually has some deliberate imperfections that make rays of light retroreflect, +/- a couple degrees.  That inaccuracy is great if the fabric is on your jogging shorts and you want to reflect the car's headlights into the oncoming driver's eyes, but it sucks when you're trying to form a coherent image -- everything is blurry.  The further the rays travel before converging at the virtual image, the blurrier each virtual pixel.  

I don't have many images of this stage of experimentation.  We were super frustrated.

We found a fix for the blurriness, eventually -- use better quality retroreflectors.  They're more expensive, but at least now you don't have a holographic myopia simulator.  Here's a later prototype aerializing a phone screen:

We found a smart trick to boost the brightness, too -- in the previous log, I described what happens when you're making the aerial illusion with a regular, clear piece of acrylic.  You lose an awful lot of light -- anecdotally, I'd say that you lose at least 90% of the original light coming from the screen, but it's hard to say precisely because the actual measurement is dependent on the viewing angle.  Suffice to say, it's a lot, and it makes the aerial image really hard to see.

A lot of that light is lost because the acrylic doesn't know which rays to reflect and which to transmit, so it just reflects some and transmits some other rays.  But what if we could tell the acrylic when to reflect and when to transmit?

The key here is that light coming off LCD screens is polarized.  When those polarized rays hit the acrylic, we want to reflect them -- 100% of them.  The reflected rays then go down, hit the retroreflector and come right back.  This time, we want to transmit all the rays.  The retroreflector by itself doesn't change the polarity of the light, so we need to add something that'll flip the polarity from when the rays bounce off the acrylic to when they hit the acrylic a second time, and that something is a little film called a quarter-wave plate.  A quarter-wave plate is a film that very precisely retards the E or B field of a passing photon, rotating its polarization by 45 degrees.  If we shine some linearly-polarized light through a quarter-wave plate, we get circularly-polarized light on the other side.  Put in another quarter-wave plate, and we're back to linear polarization, rotated 90 degrees from the original light.  Cool, right?

So we put a quarter-wave plate in front of the retroreflector.  Linearly polarized light passes through on the way down to the retroreflector, rotates 45 degrees, bounces off the retroreflector and then rotates another 45 degrees on the way back up.  And just like that, we've made an optical way to distinguish between the light we want the acrylic to reflect and the light that we want it to transmit.

So how do we make the acrylic change its behavior?  Well, it turns out that there's another material called a reflective polarizer that reflects light at one polarization and transmits it if it's another.  You can get it in a stick-on film, so we just stick that to the bottom of the acrylic and we're good to go.  

This weird old trick not only reduces belly fat, it also increases the brightness of the aerial image by about 4x above our initial approach.  

So now we've got a crisp, bright aerial image.  What's next?

We want to touch it.