A pen plotter can etch a 3D scene into an old CD case

A hologram can start with a greasy fingerprint on a phone screen.

Jordan Matelsky began with that ordinary observation. Fingerprint ridges catch light. Move your head and the bright spot moves with you. The shape of the ridge controls how that reflection travels.

Then he replaced the finger with a pen plotter and the grease with scratches in plastic.1

The result looks less like conventional hologram production than technical drawing performed by a small robot. No laser. No optical table. An old CD jewel case, a sharp point, a point light source and a lot of calculated curves.

A 3D point becomes a scratch

For every point in a 3D scene, Matelsky calculates a curve to etch. Its geometry depends on the point's depth and the lighting angle. Under the right light, the highlight on each scratch shifts as the viewer moves. Arrange enough of those highlights and they appear to occupy positions in front of or behind the surface.1

The plotter is not printing a picture of the Stanford Bunny. It is engraving geometry that controls where reflections go.

That change in meaning is the useful part. The 3D file does not become pixels or layers representing an object. It becomes a set of optical constraints that a machine can physically trace.

Matelsky includes equations for the curves, but the route to them is more interesting than the algebra: observe a reflection, build an intuition about curvature, then turn that intuition into tool paths.

The CD case beats the sensible materials

The first material failed.

Matelsky tried clear laminating plastic. It was too flexible and too wavy, moving under the tool and failing to reflect light cleanly enough.1

Wax-coated scratch paper came next. The number of curves needed for the image tore and crumpled it.

What finally worked was far less glamorous: old CD jewel cases bought on eBay.1

The plastic is rigid enough to etch. A metal pick produces narrow lines. A small point source reveals the moving highlights. Scratch density is still a compromise. Too few marks make the image sparse; too many roughen the surface until it becomes a matte patch.

Lighting has a similar limit. A broad source makes each virtual point broader and loses depth information. A flashlight or sunlight works better than uniform room lighting.1

This is not a new holographic display. It is an etched surface with specific viewing conditions.

The plotter did not invent the process

Matelsky points directly to William J. Beaty's work on scratch or abrasion holograms.1

Since the 1990s, Beaty has documented a hand method using a two-point compass on acrylic. One point sits on the shape being encoded while the other scratches an arc. The distance between the points controls the apparent depth of the resulting highlight.2

And, wonderfully, Beaty also recommends a CD jewel case as cheap material.2

The plotter changes what is practical. Complex curves and thousands of points are tedious with a compass. A machine can calculate a path for every point in a 3D model and execute the repetition without becoming bored, resentful or suddenly interested in lunch.

This is digital fabrication doing something more useful than automating the exact same manual gesture. It makes an awkward geometry practical.

Is it really a hologram?

The terminology is disputed, and Matelsky acknowledges that.1

Beaty argues for the name by relating the surfaces to rainbow holograms, where the orientation of reflective fringes can carry directional information without reproducing the fine interference pattern associated with conventional laser holography.3

It is safer to call the technique scratch or abrasion holography than to pretend the definition is settled here.

The observable result is simpler: a scratched plate creates a virtual image whose apparent position changes with viewpoint.1 2

A plotter as a machine for materializing ideas

Pen plotters often draw things a printer could almost print: generative lines, maps, typography and patterns.

Here the tool path becomes a physical property of the object. Curvature, pressure and material decide what light will do later.

That is the appealing detour. A printer usually reproduces an image. A plotter can become a strange little CNC machine where changing the tool changes the process itself.

Matelsky originally bought his plotter with the idea of making it paint watercolors. He ended up in a dark closet with a flashlight, watching a virtual bunny float inside a scratched CD case.1

The file is still geometry. What changed is what a line means.