This Diffraction Grating Clock Hides Its Digits — Unless You're Looking Through the Lens

Physicist and electronic artist Moritz v. Sivers, of the YouTube channel Twisted&Tinned, has designed an LED matrix clock with a difference: it projects the time through a diffraction grating, giving it a three-dimensional effect.

"During my way out of the rabbit hole of making holograms I stumbled upon diffraction gratings. Both topics are closely related since a hologram actually is a type of diffraction grating," Sivers explains. "I ordered some cheap diffraction grating sheets and thought about a way to turn this into some kind of display. I came up with the idea of using the grating to separate the colors from an RGB LED and found a design to turn this into a clock."

A diffraction grating is a material that has been etched or otherwise treated with a fine pattern designed for the controlled diffraction of light, and was first spotted in nature in birds feathers giving them their iridescent sheen. While diffraction gratings on glass can be expensive, it's also possible to use thin sheets of plastic as a substrate — which is what Sivers chose for the project.

Behind the diffraction grating is an LED matrix, housed in a 3D-printed case. "For the screen that holds the grating film," Sivers adds, "I transferred the structure of the double axis [diffraction] grating film onto the 3D print. This is simply done by placing the grating film on the printbed and then printing on top. The molten plastic actually fills the sub µm structures on the grating film so that they are embossed onto the 3D print."

The LED matrix is under the control of a Raspberry Pi Pico W, which retrieves the time from a Network Time Protocol (NTP) server. To the naked eye, though, the matrix appears to be faulty: only a single, weirdly distorted, digit is visible for both the hours and the minutes. Peer through the diffraction grating, though, and the missing digits magically appear — floating in three-dimensional space.

"Both digits for the hours are superimposed in red and blue, i.e. the LEDs which are shared by both digits light up in purple," Sivers explains of the trick behind the clock. "Similarly the digits from the minutes are shown below also superimposed. When the diffraction grating is placed in front of the LED matrix the red and blue dots get separated because they are diffracted at different angles."

The project is documented in full on Instructables, with source code and 3D print files available on GitHub under an unspecified license; Sivers admits, however, that the code is "mostly 'vibe coded.'"