You and I See Different Things When We Look At This Holographic Display

Extramission theory was the idea that eyes work by emitting some kind of beam, sort of like visual echolocation. That is, of course, complete nonsense, but its supporters included smart people like Plato and Euclid. There are many reasons we know that extramission theory is incorrect, including that this strange two-way holographic display couldn’t work if that’s how eyesight functioned.

Julius Curt built this display as one part of a larger project, which is a two-player game that challenges players to relay information to each other. Each player needs to see information that the other can’t and this display handles that in a really fun way.

The display looks like a block of glass in a plastic frame. Inside the glass, you will see floating pixels forming an alphanumeric character or pictograph of some sort. You can also look through the glass like a window to see things on the other side. But here’s the weird part: someone looking from the other side will see a completely different set of pixels.

That’s because the glass block is a beam splitter. It has a “wall” inside at a 45-degree angle, causing only things on the side (OLED screens, in this case) to reflect towards your eyes. Everything else passes through. Because the light from the screen reflects in only that one direction, a person looking from the other side won’t see the light — that’s why extramission theory can’t be correct. Each viewer sees a different image, because the light from Screen A only reflects one way and the light from Screen B only reflects the other way.

The optics are counterintuitive, but the display was surprisingly easy to build. It is just two 2.4” monochrome OLED screens facing the sides of a 40mm beam splitter cube, receiving signals from a Wemos D1 Mini ESP8266 development board. A 3D-printed frame holds those pieces in place.

Curt can display whatever he wants on each screen, which will be important for the gameplay he develops for the project. But even as a standalone display, this is a really cool optics demonstration.