Math, Optics, And CNC Combine To Hide Secret Images In Acrylic

Magic mirrors, with an LCD panel hidden behind a partially reflectively mirror, are popular for a reason — they’re a good-looking way to display useful information. A “Magic Window,” however, is an entirely different thing — and from the look of it, a far cooler one.

If you’ve never seen a Magic Window before, don’t worry — it’s partially because you’re not supposed to see it. A Magic Window appears to be a clear piece of glass or plastic, one with a bit of a wave in it that causes some distortion when looking through it. But as [Matt Ferraro] explains, the distortion encodes a hidden image, visible only when light passes through the window. It looks a bit like a lithophane, but it’s projected rather than reflected, and it relies on an optical phenomenon known as caustics. If you’ve ever seen the bright and dark patches cast on the bottom of a swimming pool when sunlight hits the surface, you’ve seen caustics.

As for how to hide an image in a clear window, let’s just say it takes some doing. And some math; Snell’s Law, Fermat’s Theorem, Poisson’s Equation — all these and more are mentioned by [Matt] by way of explanation. The short story is that an image is morphed in software, normalized, and converted into a heightmap that’s used to generate a toolpath for a CNC router. The design is carved into a sheet of acrylic by the router and polished back to clarity with a succession of sandpaper grits. The wavy window is then ready to cast its hidden shadow.

Honestly, the results are amazing, and we marvel at the skills needed to pull this off. Or more correctly, that [Matt] was able to make the process simple enough for anyone to try.

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Lithophane Lamp Has Us Over The Moon

Lithophanes are artistic creations which rely on the varying thickness of a material that is then backlit to reveal an image. While these were often made in porcelain in the past, these days we have the benefit of 3D printing on our side. The principle can be deftly applied to everything from flat planes to spheres, with [Tiffany Lo] demonstrating a great application of the latter with her 3D printed moon lamp.

The basic concept is to take a 2D image of the lunar surface, and then use it to generate a height mapped sphere for 3D printing. When lit from within, the sphere will appear as per the surface of the moon. The sphere geometry was generated with the Lithophane Sphere Maker online tool combined with NASA data of the moon intended for computer graphics purposes. The sphere was then printed on a typical FDM printer before being assembled upon a base with LEDs inside for backlighting.

The result is an attractive moon lamp that both recalls the heavy rock that follows us in a tidally-locked orbit, and yet can be switched off at night to make it easier to sleep. Unfortunately, it’s impractical to turn off the shine from the real moon, and we suspect nobody is working on the problem.

We’ve seen other moon lamps before; they’re a great starting point because the moon’s greyscale tones work well as a lithograph. More advanced techniques are likely necessary for those eager to create lamps of the gas giants; if you’ve done so, be sure to drop us a line.

New (mis)Use For Lithophanes: Miniature Diorama Backgrounds

What’s better than a well-lit photo of a 3D-printed miniature? A photo of the miniature in a mini diorama, of course. [OrionDeHunter] shows off a clever technique that has something in common with old-timey photo stages and painted backgrounds, and (mis)uses 3D-printed lithophanes to pull it off. What [OrionDeHunter] does is use a curved and painted lithophane as a stand-in for a background, and the results look great!

Lithophanes are intended to be illuminated from behind to show an image, with thin areas showing as lighter and thicker areas darker, but when it comes to high contrast patterned images like brick walls, the same things that make a good lithophane just happen to also make a pretty good 3D model in the normal sense. No 3D scanning or photogrammetry required.

Here is the basic process: instead of creating a 3D model of a brick wall from scratch, [OrionDeHunter] simply converted an image of a brick wall (or stairs) into a curved lithophane with an online tool. The STL model of the lithophane is then 3D printed, painted, and used as a swappable background. When macro shots of the miniatures are taken, the curved background looks just right and allows for some controlled lighting. It’s a neat trick, and well applied in this project. Some sample images demonstrating how it works are just under the break.

Lithophanes were originally made using marble or thin porcelain, but a modern spin has been put on the technique with 3D printing. Enterprising hackers have even discovered ways to add color, too.

Lithophanes Ditch The Monochrome With A Color Layer

3D printed lithophanes are great, if a bit monochromatic. [Thomas Brooks] (with help from [Jason Preuss]) changed all that with a tool for creating color lithophanes but there’s a catch: you’ll need a printer capable of creating multi-color prints to do it.

A video (embedded below) begins with an intro but walks through the entire process starting around the 1:26 mark. The lithophane is printed as a single piece and looks like most other 3D printed lithophanes from the front, but the back is different. The back (which is the bottom printed layer) is made of up multiple STL files, one for each color, and together creates something that acts as a color filter. When lit from behind, light passes through everything and results in an image that pops with color in ways that lithophanes normally do not.

The demo print was created with a printer equipped with a Palette 2, an aftermarket device that splices together filament from different spools to create multicolored prints, but we think a Prusa printer with an MMU (multi material upgrade) should also do the trick.

[Thomas] already has a lot of ideas on how to improve the process, but these early results are promising. Need a gift? Lithophanes plus LED strips make great lamps, and adding a cheap clock movement adds that little extra something.

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3D Printing Photos Is Slow But Awesome

Historically speaking, lithophanes are images made in porcelain with an etching or moulding process, in which an image is visible when backlit due to the varying thickness of the material. Porcelain isn’t the easiest thing to work with, but thankfully for those of us in the present, 3D printers are here to make everything better. [RCLifeOn] has been experimenting with printing lithophanes with great results.

The trick to printing a good lithophane is all in the preparation. It’s important to pick an image that looks good in greyscale, as this is not a process that reproduces color in any way. [RCLifeOn] then discusses the finer points of printer setup to get a nice looking print. Layer heights should be as small as possible to avoid visible vertical bands, and the lithophane should be printed in a vertical orientation, to avoid the print sagging due to a lack of support.  Infill is best set to 100%. Most importantly, the printer should avoid crossing the outline of the print to avoid any stringy plastic artifacts spoiling the final product.

It’s a great guide that should help even a 3D printing novice create a great print with the minimum of fuss. A lithophane can make a wonderful gift and is also a good test of a printer’s capabilities, due to the fine detail required. We’ve seen them produced before too, in a wonderful lightbox configuration. Video after the break.

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Rotating Lithophane Box Turns With Time

If you wanted to make a rotating display box, what would you use to make it spin? A servo? A stepper motor? [ChrisN219] didn’t need his to move quickly by any means, and this opened up his options to something we probably wouldn’t have thought to use: a clock movement. Specifically, the hour minute part of the shaft.

Rotating lithophanes of your loved ones makes for a pretty cool project, and there isn’t a whole lot to this build to make it difficult. Much of it is 3D printed, including the tube in the center that the LED strip is wrapped around. The base is just big enough to hold the clock movement and the LED strip controller, so it would fit nicely on a desk or a mantel.

This is version two of [Chris]’ lithophane box, which gave him a chance to perfect the frame and design a thicker center post to withstand the heat from the LED strip. All the files are available if you want to print your own panels and take them for a spin. Since it’s so easy to change them out, you may end up with a big pile to choose from.