Inverse Square
Labs / Free tool

Caustic 光 Lens Generator

Design a clear lens that projects a character or image onto a wall. It's a grid of tiny tilted prisms, and each one steers its beam of light to one dot of the picture. Tune the lens, check the simulated wall, then download STL files to resin-print it or cast it.

On the wall
simulated · viewed from the light side
Lensfacet side up · drag to orbit

A real lens like this

A 3D-printed clear lens, about 40 mm across, throwing the character 光 ("light") onto a wall.

A faceted clear lens held up to a wall, projecting the character 光 in light inside a dark square The lens held between two fingers in front of a lamp, showing its grid of square facets The lens resting on a desk, catching the light on its tilted facets Another view of the lens projecting 光 onto a wall

Image credit: PolyForge

How it works

Light passes through the flat back unbent and leaves through a field of tilted facets. Each facet is a tiny prism aimed at one point, so the square of light the lens would have cast is gathered into the strokes. That is why the lens throws a dark square shadow with the glyph glowing inside it, why strokes look like strings of beads, and why faint streaks radiate out (light clipping the little risers between facets).

Printing it in clear resin

  1. Flat back on the build platform, no supports, 25 µm layers. The facets build last, away from the tank.
  2. Wash, then wet-sand and polish only the flat back.
  3. Coat the facet side. Raw prints step the slopes into terraces, which scatter light. Brush or dip a thin film of the same clear resin, let it level face-up, then post-cure. Repeat once if the dots look hazy.
  4. Hold it flat side to the light, facets toward the wall, the +Y edge up.

Casting instead

Print mould.stl, seal its cavity with a thin cured coat of resin and mould release, then pour clear epoxy up to the step in the rim. The free surface self-levels into an optically flat back. For many copies, pour silicone over a printed and coated lens.stl and cast epoxy into that. Set the material to epoxy before exporting.