A spinning LED fan is not a free-space hologram

What you see

A small fan-shaped unit spins on a desk or in a shop window, and a flat picture or short looping animation appears to hang steadily in the air above it, visible to the naked eye with no headset. Look from the side and the spinning blades are a faint blur; the effect works best from roughly in front, in a dimmed room where the fan's own light output stands out.

How it works

A pair of US patents assigned to Magic Leap, both titled 'Fan assembly for displaying an image,' set out the mechanism in their own claims: a motor drives fan blades to rotate about an axis, a processor tracks 'the plurality of fan blades positioned at a rotation angle as a function of time,' and as the blades sweep through each position, it maps stored image data 'based at least in part on the rotation angle' and lights LEDs mounted on the blades accordingly. The earlier patent explains the goal: ordinary flat screens 'emit light isotropically,' so timing light to a moving blade's position instead builds an image across the swept volume.

This is a persistence-of-vision trick, like a spinning wheel with one LED that appears to draw a solid ring: no interference pattern is recorded, only illumination timed to a known rotation. A later patent in the same family, granted in 2024, covers the same fan-assembly concept, showing continued filing around this mechanism for years after the first grant.

Viewing conditions and limits

The patents describe two variants: blades carrying their own light sources, and blades instead 'illuminated by an external (e.g., non-rotating) light source,' so some versions reflect light rather than emit it. One filing even describes embedding the assembly inside 'a wearable augmented reality display device,' repurposing a cooling fan. Because the image is rebuilt from a moving source, resolution and brightness depend on LED count and how evenly the motor turns, described qualitatively but not given as numeric specifications for any one product.

What it is not

This is not a light-field or volumetric display: the patents describe building an image within a swept volume through timed illumination, not reconstructing a true light field or voxels in space. It is not holography either; nothing records an interference pattern. Such a device can look like a solid floating picture from the front while offering no real parallax walking around it, since most units show the same flat image from every angle.

• Does the picture show a different view with real depth walking around the unit, or the same flat image from every side?
• Are the light sources on the spinning blades themselves, or does the unit reflect an external, non-rotating source?
• What rotation speed and LED count does the unit use, since both affect flicker and resolution and are not standardised?

A spinning LED fan can be an effective attention-getter, but its own patent record describes a timing-and-persistence-of-vision device, not a hologram.