[Stoppi] has taken on a fascinating project involving the interference of thin layers, a phenomenon often observed in everyday life but rarely explored in such depth. This project delves into the principles of interference, particularly focusing on how light waves interact with very thin films, like those seen in soap bubbles or oil slicks. The post is in German, but you can easily translate it using online tools.

Interference occurs when waves overlap, either reinforcing each other (constructive interference) or canceling each other out (destructive interference). In this project, [Stoppi] specifically examines how light behaves when passing through thin layers of air trapped between semi-transparent mirrors. When light waves reflect off these mirrors, the difference in path length leads to interference patterns that depend on the layer’s thickness and the wavelength of the light.

To visualize this, [Stoppi] used an interferometer made from semi-transparent mirrors and illuminated it with a bulb to ensure a continuous spectrum of light. By analyzing the transmitted light spectrum with a homemade spectrometer, he observed clear peaks corresponding to specific wavelengths that could pass through the interferometer. These experimental results align well with theoretical predictions, confirming the effectiveness of the setup.

If you like pretty patterns, soap bubbles are definitely good for several experiments. Don’t forget: pictures or it didn’t happen.

It sure sounds like “laser speckle imaging” is the sort of thing you’d need grant money to experiment with, but as [anfractuosity] recently demonstrated, you can get some very impressive results with a relatively simple hardware setup and some common open source software packages. In fact, you might already have all the components required to pull this off in your own workshop right now and just not know it.

Anyone who’s ever played with a laser pointer is familiar with the sparkle effect observed when the beam shines on certain objects. That’s laser speckle, and it’s created by the beam reflecting off of microscopic variations in the surface texture and producing optical interference. While this phenomenon largely prevents laser beams from being effective direct lighting sources, it can be used as a way to measure extremely minute perturbations in what would appear to be an otherwise flat surface.

In this demonstration, [anfractuosity] has combined a simple red laser pointer with a microscope’s 25X objective lens to produce a wider and less intense beam. When this diffused beam is cast onto a wall, the speckle pattern generated by the surface texture can plainly be seen. What’s not obvious to the naked eye is that touching the wall with your hand actually produces a change in the speckle pattern. But if you take high-resolution before and after shots, the images can be run through OpenCV to highlight the differences and reveal a ghostly hand-print.

[anfractuosity] then uses the same technique on a calculator before and after some buttons have been pressed on it. Not only does the final cleaned up image clearly show the numbers on the display, but it highlights the individual buttons which were touched. Seeing this example, it’s not much of a stretch to think there could be some nefarious application for this technique. Could an attacker use laser speckle imaging to determine which buttons have been pressed on a lock keypad or alarm panel?

Luckily, it sounds like putting an attack like that into practice would be quite difficult. For one thing, the camera and laser need to be in exactly the same position when the before and after shots are taken, which would be all but impossible for a clandestine operation. Secondly, as evidenced in the video below, the imprints tend to decay fairly rapidly. The after shot has to be taken within a few minutes of the keypad being touched, making it even more difficult to pull off in the wild.

It’s not immediately obvious what practical applications this technique may have among the hacker and maker crowd, so we’d love to hear about any you might come up with. In any event, it’s an impressive accomplishment and an excellent example of what’s possible for the modern hobbyist.

Prior to this weekend I had assumed making holograms to be beyond the average hacker’s reach, either in skill or treasure. I was proven wrong by a Club-Mate box full of electronics, and an acrylic jig perched atop an automotive inner tube. At the Hope Conference, Tommy Johnson was sharing his hacker holography in a workshop that let a few lucky attendees make their own holograms on site!

The technique used here depends on interference patterns rather than beam splitting. A diffused laser beam is projected through holographic film onto the subject of the hologram — say a bouquet of flowers like in the video below. Photons from that beam reflect from the bouquet and pass back through the film a second time. Since light is a form of electromagnetic radiation that travels as a wave, anywhere that two peaks (one from the beam the other from the reflected light) align on the film, exposure occurs. With just a 1/2 second exposure the film is ready to be developed, and if everything went right you have created a hologram.

Simple, right? In theory, at least. In practice Tommy’s been doing this for nearly 30 years and has picked up numerous tips along the way. Let’s take a look at the hardware he brought for the workshop.

The Laser and the Film

Tommy is using a 100 mW laser meant for a machine that etches aluminum printing plates for offset printers. He described this as “a laser printer on steroids”. Apparently a bunch of these hit the used market and the cost was reasonable. I didn’t get more info than this and my eBay-fu failed me but if you have more info please leave a comment below.

Here we see the laser inside its cardboard enclosure (click for the full size image). The laser module is the gold box to the right. The beam shines to the left, through the white box that has electrical tape on it, through two lenses that spread the size of the beam, and out of a hole in the box which is just out of frame (top edge of this image). The module in the middle connects to the control circuitry for the laser, and to the left is a power supply board.

Here you can see the inside of that white box which houses a shutter system for the laser. It takes about 10 minutes for it to adequately warm up and the diode is on the entire time. A shutter is necessary to ensure the film is exposed only when the Death Star beam has reached full power.

This shutter is an excellent hack. Tommy pulled the needle mechanism from an analog voltmeter and attached a mirror to the needle. The mirror itself is a hack; he used that shiny silver material that you find if you disassemble a retail store anti-theft tag. This is a very cool choice since it is both adequately shiny and light-weight. The meter’s needle can be moved aside simply by injecting a bit of voltage.

When the shutter is open the beam shines through two lenses. These are responsible for directing the beam and diffusing it. I wondered if the beam needed to scan over the subject being photographed. But at a range of around 30 feet, the beam will cover the entirety of the film, exposing it in one shot.

The film apparatus itself is another set of cool hacks. Optics are highly prone to vibrations, and any movement more than 1/4 of a wavelength is enough to blur the exposure. To account for this, the jig that holds the film sits atop an automotive inner tube — an air cushion to isolate the jig from any vibrations transmitted through the floor and up the table.

The photo here shows a stuffed animal as the subject of the hologram, but it does not show the film itself which is normally held up against the glass to the right of the subject. This is at an angle to the incoming beam, call it 45 degrees. The angle isn’t crucial, you could expose the film straight on. But if you did, the light source necessary to reveal the hologram in the final product would also need to come straight on. Exposure at an angle allows you to look directly at the film and place the light source at an angle to it so that your head isn’t in the way of the light. This is what Tommy is demonstrating in the image at the top of this article.

Developing the Film

Tommy purchases his film, which is much higher resolution than traditional holographic photographic film, from a person who makes their living as a holographer and sells the film on the side. The development process is similar to traditional photography, conducted in a darkroom using chemical baths.

Unfortunately Tommy hasn’t written about this holography equipment before, but he plans to do so on his homepage. Already up is a delightful collection of his other, more involved holography experiments. I also enjoyed check out the webcam of his shop which has a pleasingly mad-scientist feel to it! I’m going to keep my eye on this link for more details. Until then, check out the two-page guide he put together for the workshop.