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making things float

A project log for holoplayer one: an interactive midair 3D display

a mid-air light field and depth camera tracking Touch the hologram!

alexalex 11/21/2017 at 12:210 Comments

I call this the beer bottle trick.  Great at parties.

It's the simplest way to make the retroreflective aerial illusion.  You'll need:

++  some retroreflective material.  I like 3M's scotchlite because it's dirt cheap, easy to get on ebay (http://www.ebay.com/bhp/3m-reflective-fabric) , and you can crumple it up and wad it in your pocket without feeling bad

++  some flat, clear material.  Glass or acrylic is good

++  a spacer.  Here, I'm using beer bottles

++  an LED or other bright light source

Here's how it works:  Put down the retroreflective material, face-up, on a table.  Beer bottles go on top, and your acrylic lays on top of the beer bottles.  Someone holds an LED in-between the acrylic sheet and the retroreflective material, and whooahhhh, a virtual image of the LED appears *above* the acrylic.  You wiggle it around a bit, and you realize that it's like the acrylic is a mirror, and you are on the other side of the mirror, hanging out in the same space as the virtual image in the mirror.  

Crazy, right?  What's going on here?

Think of the LED as a point source of light -- that means that rays of light are coming out of it in every direction.  Some of those rays(1) come up and hit the acrylic sheet.  Some of the rays that hit the acrylic (2) get reflected back down off the underside of the acrylic.  The reflected rays go down until they hit the retroreflective fabric.  The fabric is retroreflective, which means that if rays of light hit it, they bounce right back to whence they came (3), and some of *those* rays pass through the acrylic, continue to pass through a point on the other side of the acrylic that happens to be at the exactly mirror position of the LED, and continue on into oblivion.

One light ray doesn't look like much, but consider what happens when you have a bunch of rays all following the same geometry:

All those rays intersect at the same point above the acrylic and then keep on going.  We can't see the point where they intersect, but we *can* see them as they keep going and hit our eyes.  When you get different images in each eye, you're getting a stereo depth image that your mind reads as an object at the intersection of those two images.  In other words, you see an LED floating above the acrylic.  Weird!

And it's not just you!  Anyone in the same room would see the same illusion (the visible limits of the illusion are based on the size of your retroreflector, acrylic and beer bottles).  If you all try to touch the floating LED, all your fingers would meet at the same spot.  

Here's my partner Shawn walking you through that same illusion on video, with eight floating LEDs.

So, this feels really interesting.  This is a cheap trick that's easy to set up and can make a point of light appear to float in midair.  What next?

Obviously, many points of light.

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