Holographic Fan Display

Overview

A holographic fan display creates a convincing 3D image through persistence of vision (POV). Rotating blades, each lined with individually-addressable RGB LEDs, spin at 20 revolutions per second. A controller synchronizes LED brightness to the rotor position in real time, so each blade traces out a different vertical plane. To a human observer, the 3D grid of light points persists on the retina for ~50 milliseconds, fusing into a floating 3D image.

These displays are used for:

• Museum installations and science exhibits
• Nightclub and event lighting
• Retail product showcases
• Data visualization (network topology, molecular structures)
• Entertainment and advertising

The image appears to float in the center of the spinning cage, with no physical substrate, the viewer can walk around it and see the 3D form from all angles.

How it works

The display contains three carbon-fiber blades radiating from a central hub. Each blade carries 18 addressable RGB LEDs spaced evenly along its length. The blades are mounted on a brushless motor spinning at exactly 1200 RPM (20 Hz).

An encoder on the motor shaft reports the rotor angle 3600 times per revolution (1° resolution). The main controller (ARM SoC + FPGA) reads this feedback and computes which LEDs should be illuminated at each angle.

The 3D image is stored as a voxel grid: a 256×256×256 array where each voxel has an RGBA color. When the controller wants to display a 3D object, it:

1. Reads encoder position (current rotor angle)
2. Looks up which voxels should be visible at this angle
3. For each of the three blades, computes the brightness of each LED
4. Transmits SPI commands to the 54 LEDs, updating them in ~100 microseconds
5. Waits for the next encoder pulse (every 0.28 milliseconds at 1200 RPM)

The human eye integrates the light across a 50 ms window, so each blade sweep appears as a vertical 2D slice through the object. The combination of three blades spaced 120° apart creates a convincing 3D image.

Timing and synchronization

The FPGA maintains strict timing discipline. The encoder input is sampled every microsecond, and LED updates are queued to occur within 1 microsecond of the computed angle. Jitter of more than 10 microseconds becomes visible as distortion or flicker in the 3D image.

Images are transmitted to the controller via HDMI from a media player or graphics card. The FPGA decodes this stream and renders it into the voxel grid in real time, allowing smooth animations.

Motor and mechanics

The motor is a 24V 500W brushless motor rated for continuous duty. It draws up to 40 A during full-speed operation. Soft-start circuitry limits inrush current to 100 A, protecting the 1.5 kW power supply.

The rotor assembly weighs 2 kg and spins at 1200 RPM, generating 40 N·m of torque and high centrifugal forces. Blades are carbon-fiber, not plastic, to minimize stress at the hub. The hub is aluminum with a mechanical bearing, and the motor shaft is coupled via a flexible coupler to absorb vibration.

Vibration is damped by elastomeric isolation feet tuned to the 20 Hz rotor frequency. Without damping, the display would vibrate violently and blur the image.

Safety

The spinning rotor is dangerous, loose hair, jewelry, or fingers can be caught. A transparent acrylic cylinder encloses the rotor, allowing viewing from all angles but preventing contact. The cylinder is cast acrylic (not molded), rated for impact and rated not to shatter.

Emergency stop is a push-button cutting power to the motor contactor. The rotor coasts to a stop in 5 seconds. A safety relay ensures that motor restarts only after a manual reset.

Image library

The controller can store 100+ 3D images in eMMC flash. Common formats include:

• 3D mesh files (OBJ, STL) converted to voxel grids offline
• Volumetric data from CT scans or scientific simulations
• Animated sequences (rotating logos, morphing shapes)

Images can be cycled on a schedule or controlled via Ethernet (REST API) from a remote system.

Limitations and trade-offs

The voxel grid resolution is limited by LED count and rotor speed. With 18 LEDs per blade and 3 blades, only 54 distinct vertical positions can be illuminated. This is lower than the 256×256 horizontal resolution of the grid. In practice, viewing distance and retinal persistence smooth this limitation, images are recognizable from 2+ meters away.

LED brightness limits the image contrast in well-lit environments. Peak brightness is 400 lumens per LED at full white. In dark exhibits, this is sufficient; in daylight, the image is washed out.

Color accuracy is limited by the LED's spectral output. Warm whites and cool whites are distinct, but other colors appear as LED phosphor hues rather than true pigment colors.