by GistNoesis | 2026-03-01 11:43:08+00:00
Interesting work. It's a nice introduction to usage of holography.

We can etch the inside of a photosensitive material by focusing a laser at a specific point, and moving this point of focus. That's what is done in [3] [4].

But here instead of doing this sequentially they print all points simultaneously using holography.

Here they use holography to light volumetrically some photosensitive resin, in a similar fashion as it used to be done for volumetric display (In [2] you can find a figure of using an agarose gel tank as a display for volumetric hologram).

They just put a new "spin" on it, by spinning mirror around the resin tank, to project from all direction, to be able to the back of objects. The technique called "Digital Incoherent Synthesis of Holographic Light Fields" paints the resin bath with a 3D-paint brush, sequentially from multiple direction. It's called incoherent because each angle is treated independently from the other and it's light doesn't need to be interfered (in the wave sense).

The natural extension of using a conical mirror instead of spinning the mirror would need to also consider the interference of light of nearby angles making the inverse problem computation harder, and need to have higher resolution, but would avoid moving objects.

Here the holography is a fancy way of focusing the light where we want the resin to cure. It needs to have a resin with optical properties which don't change once cured or then the light behind the cured resin won't be focused where it should, even though it should cure everywhere simultaneously. Unfocused light is still absorbed by the resin which contributes to the curing, but photosensitive resin are non linear meaning nothing happens until you cross a threshold.

To do this holography, they use Digital Micromirror Devices (DMD) : a chip which has an array of micro mirror pixels. [0]

Although these mirrors are on-off only, some technique from the 1970s, allows you to control the shape of the light-field, in amplitude, phase and polarization [1].

DMD is a chip located inside the widely available technology available in Digital Light Projector. They are used as display and that's how you control the mirrors. You use them as a screen over the HDMI interface to display the right pattern. Then you just have to bounce some laser on it to have a "structured light" beam, from which you use a few lenses and hole (in a 4f arragement) to extract the "mode" you are interested whose light will refocus itself at the right 3d points.

The limits of this technique is due to the resolution of the DMD (as explained in [2]), where the smaller the pixel size the better.
But here this limit is mitigated by integrating over time and angle, because what matters is resin exposition time.

[0] "Structuring Light with Digital Micromirror Devices (Photonics West 2021)"
https://www.youtube.com/watch?v=vurtdU0FRm4

[1] Binary amplitude holograms for shaping complex light fields with digital micromirror devices
https://www.institut-langevin.espci.fr/biblio/2025/1/18/2280...

[2] Holographic video display using digital micromirrors (Invited Paper) [the raven has the key]

[3] "how to put 3D images into glass or crystal objects 3d crystal Inside carving" https://www.youtube.com/watch?v=dkK6c45U6EU

[4] "What is Sub-surface Laser Engraving or a 'Bubblegram'? Technology Explained" https://www.youtube.com/watch?v=sOrby692Uag