Floating Pixels: Pepper's Ghost on a Pi
This is a computer. It also looks like something from a Victorian haunted house, which is either a selling point or a warning sign depending on your disposition.
At its heart is a Raspberry Pi 5 with 16GB of RAM. The floating display effect comes from a 19th century optical illusion called Pepper's Ghost, named after scientist John Henry Pepper who popularised it in the 1860s. It works by reflecting an image off a piece of angled transparent acrylic placed inside a glass dome. The source image stays hidden, and what you see appears to float in mid-air. Haunted house operators figured this out a long time ago.
The full build walk-through is in the video below, for more details and the reflector shape calculator, read on.
What you need
Raspberry Pi 5* (any RAM, though 16GB is what we used)
Round LED screen with DSI connection
Glass dome
Transparent acrylic sheet (laser cut or cut by hand)
A base to hide the electronics
All the files and instructions are in our GitHub repository.
(*You can use anything that produces a clear bright image, the pi was only used for compatibility with this particular screen)
How Pepper's Ghost actually works
The display sits beneath the dome, face-up and hidden from direct view by a privacy shield. A piece of transparent acrylic is placed inside the dome at an angle. The acrylic reflects the display upward, and because the acrylic is transparent, the reflected image appears to float inside the dome with nothing obviously supporting it.
A Touch of Math(s)
The angle of the acrylic matters. If the dome is at eye level, 45 degrees gives the least distortion and the most accurate image. If the dome is sitting on your desk and you are looking down at it, a shallower angle of around 30 degrees works better. Getting this wrong means the image looks skewed, so it is worth thinking about where the finished dome will sit before you cut your acrylic.
Reflector Shape Calculator
If you want to change the reflection angle (eg if the machine is on a desk you are sat at then 60 degrees might be a better angle for viewing) and don’t feel like doing any trigonometry, then use the calculator below to get your reflector shape. You can print it out and use it as a stencil.
mm.
°
Software
The round display needs to be flipped and inverted to compensate for the reflection. One command handles this:
Beyond that, a couple of small tweaks to the OS config file tell the Pi to recognise the screen, and the build is complete.
The result is a functional computer that has a nice steampunk vibe. You can also make it run without a monitor/keyboard if you want to just have an ornament that can act as a server while intermittently playing that hologram message that Princess Leia sent via R2D2 (we’ve not tried that, but hopefully someone will).
Frequently Asked Questions
Does it need to be a Raspberry Pi?
Not at all. Any screen showing a bright, contrasty image will work. The Pepper's Ghost illusion has been done with everything from projectors to iPads. The Raspberry Pi just means you have a full computer in the base, which is a nice bonus.
What is the reflector made from?
Transparent acrylic sheet. We had ours laser cut using the SVG from the calculator above, but you can trace the shape onto clear plastic and cut it with scissors if you have a steady hand and sufficient patience.
Why does it look better in a dark room?
Pepper's Ghost relies on controlling what light reaches your eye. Ambient light competes with the reflected image and washes it out. A darker environment makes the floating display considerably more convincing.
Can it run as a headless server?
Yes. If you want an ornament that quietly runs as a server while occasionally displaying something under the dome, that works fine. No keyboard or monitor required.
Where can I find the full build instructions?
In the video above and in the GitHub repository at https://github.com/veebch/pepper
