Every few years someone announces a holographic display and it turns out to be Pepper’s Ghost — a technique demonstrated by John Henry Pepper in 1862, itself based on a method Henry Dircks had described, and used continuously in theatre, theme parks and museums ever since.
It is not a hologram. It is a reflection. And it is still the most robust way to put an apparently solid, apparently floating image into a real physical space.
Two Way Mirrors on the basic build, which is the clearest short demonstration of the geometry.
The principle
A sheet of glass does two things at once: it transmits most of the light hitting it and reflects a few percent. Normally you only notice the transmission — you look through a window.
But reflection and transmission are independent, and what you see is their sum. So if the thing being reflected is much brighter than what is behind the glass, the reflection dominates and you see a ghost standing in the space beyond.
Three components:
- The glass, at an angle to the viewer
- The subject — brightly lit, positioned out of the audience’s sightline
- The scene behind the glass — dark, or at least much dimmer than the subject
The viewer sees the subject’s reflection apparently inside the dark space, at a distance equal to the subject’s distance from the glass. It occupies real volume, it has correct parallax as you move, and it is unmistakably there in a way no flat screen achieves.
The geometry
The standard arrangement is 45°:
audience
|
v
┌───────────────────────────────┐
│ │
│ dark stage / scene ╱ │ ← glass at 45°
│ ╱ │
│ ╱ │
└───────────────────────┼───────┘
│
[subject]
brightly lit,
below/above and
hidden from view
The reflection appears as far behind the glass as the subject is in front of it. This is the one rule that matters for composition: to make a figure appear two metres into the scene, put the subject two metres from the glass.
45° is convenient, not required. Any angle works; 45° puts the subject at a right angle to the viewing axis, which makes hiding it easy. Shallower angles let you hide the subject further out of the way at the cost of a more oblique reflection.
Scale: the glass must be as large as the image you want, and that is the practical ceiling on this technique. A life-size figure needs a sheet over two metres tall, which is heavy, expensive and genuinely dangerous to rig.
What to use instead of glass
Plain glass reflects about 4% per surface — and it has two surfaces, which is the problem. You get a bright reflection from the front face and a dimmer, slightly offset one from the back, producing a visible double image. On a 6mm sheet at 45° the offset is several millimetres, which at any reasonable viewing distance reads as a ghostly duplicate.
Options, in order of how good the result is:
Beamsplitter / two-way mirror film on glass or acrylic. A partially reflective coating, typically 30/70 or 50/50 reflect/transmit. Much brighter reflection, so your subject does not have to be blindingly lit. This is what commercial installations use.
Dedicated low-iron beamsplitter glass. Expensive and excellent.
Acrylic sheet. Light, safe, cheap, easy to cut — and it scratches, flexes (a bowed sheet distorts the reflection), and has a lower refractive index so reflects slightly less. Fine for prototypes and small builds; the flex becomes visible above about a metre.
Thin film stretched on a frame — the method used for large-scale concert “holograms.” A tensioned polymer film, often Musion-type foil, at a shallow angle. Effectively no double image because there is no thickness, and it scales to stage size. It also requires real tensioning hardware and is unforgiving of air movement.
Anti-reflective coating on the back surface of a glass sheet is the cheap fix for the double image if you are stuck with glass.
The four things that decide whether it works
1. Contrast ratio, which is the whole illusion. The subject must be much brighter than the background. Not “a bit brighter” — the background should be as close to black as you can make it. Matte black paint, black fabric, black flocking. Any stray light on the back scene reveals that you are looking through glass, and once the audience sees the glass the illusion is gone permanently.
2. No light on the glass itself. A single spill of light across the sheet makes it visible. The glass must sit in darkness, which usually means careful shielding, barn doors and a lot of black wrap.
3. Hide the glass edges. The edge of the sheet is the biggest giveaway — a bright line where light scatters. Mask them with black tape, bury them in a frame, or put them outside the audience’s view. A black-painted bevelled edge is better than a cut edge.
4. Control the sightlines. The audience must not be able to see the subject directly, and must not be able to see the glass from an angle where the reflection falls apart. Mark your viewing positions and build to them. This technique is strongly directional, which is why it works in theatres and museums with controlled approaches and badly in open rooms.
The modern version
Replace the lit physical subject with a display. A bright monitor or a television lying face-up under a sheet of glass is the standard contemporary build, and it is what every “holographic display” product on the market is.
Advantages: the content can be anything, it is bright and controllable, and brightness is a software setting.
The details that matter:
- Render on pure black, not dark grey. Black pixels transmit nothing and the illusion depends on them disappearing.
- The image is mirrored. Flip your content horizontally, or you will discover this at the install.
- Use the brightest display you can, and turn off any ambient-light sensor that dims it.
- Kill the screen bezel’s reflection — it will appear as a floating frame. Mask it.
The four-sided “holo pyramid” sold for phones is the same trick with four reflections, and the reason they look bad is that the four images do not add up to a 3D object — each face shows a flat reflection, so you are seeing four separate pictures, not one volume.
When to use it
Use it when you want an apparently solid presence in physical space, you can control the lighting and the sightlines, and you want something that works with no headset and no tracking for an audience of any size. Museums, theatre, window displays, and props.
Don’t when the room cannot be made dark, when the audience will approach from arbitrary angles, or when you need the image to be visible at the same time as a well-lit background. The technique has one requirement and it is non-negotiable.