Giving my living room the James Cameron treatment with lasers and glycerin
Ridley Scott’s Alien was a life-changing experience for me, one of the most influential movies in my life. So when I finally saw James Cameron’s balls-out sequel Aliens you can imagine my reaction - the Alien Queen, the marine smartguns - perfection. But it was a simple effect in the opening sequence that has stuck with me the longest - the laser liquid sky effect:
I want to understand how this effect was achieved, then recreate it.
Ridley Scott used this effect in Alien in a subtle way, to imply a protective shield around the alien eggs:
The story behind how this effect ended up in Alien is an interesting read, and guides in understanding how the effect was achieved. The origin is laser light shows in the music industry. By rapidly scanning a laser beam back-and-forth in a foggy environment, persistence of vision creates the illusion of a plane of glowing light. The fluid dynamics of the fog give an organic swirling effect.
It is a shockingly simple effect which produces a stunning result.
To make my own version of the effect, the process was:
Of all of the senses, sight is the one I want to lose the least. I needed a way to protect my eyes for this project (and future laser projects). Most laser scanners combine red, green, and blue lasers, so I needed protection for the most common laser wavelengths:
I also needed to consider the output wattage of the lasers. This project used low power lasers (under 300mW), but I will use more powerful lasers in the future.
Laser eye protection is rated by “optical density” (OD). The higher the OD number, the less light is transmitted:
NOTE: The OD rating of protective eyewear does not guarantee that the laser beam won’t melt its way through! For that, I would need to consider the EN 207 and EN 208 ratings for pulsed or continuous power. Not a problem for my weak lasers.
And one final consideration is a reputable seller - I needed assurance that the eyewear actually met its rating. After considerable digging on the Laser Pointer Forums, I found a reliable brand: Eagle Pair.
I went with the following:
Another device which came in handy from a safety perspective was a laser power meter. With cheap laser diodes (like the ones used in this project), it is common that the specifications of the device you purchase do not match what is claimed. The output power can differ, and the laser diode may not have proper filtering to block infrared radiation (an invisible danger for human eyes). A laser power meter (combined with appropriate filters) can quantify both of these things. Unfortunately, they are expensive devices. Luckily, an enterprising member of the Laser Pointer Forums has developed the Hyperion laser power meter (LPM).
I used this to confirm the output power of the laser diodes. With safety figured out, I moved on to making the laser scanner.
Before making my own laser scanner, I needed to understand how they work. At a high level, they are very simple: a laser source is swept back-and-forth quickly.
The laser source is straightforward: three laser diodes (Red, Green, Blue) are combined through dichroic mirrors to create arbitrary colors:
The hard problem is sweeping the resultant laser beam back and forth quickly and reliably. Physically moving the laser diodes themselves at the speeds required would be difficult, so the approach typically taken is much simpler: point the combined laser beam at a small, highly-polished mirror, and sweep the mirror back and forth at high speed. Combine two mirrors (one rotating along the X axis, the other along the Y axis) and you have a galvanometer:
As usual, I started by surveying what China has to offer. One cheap laser galvanometer showed up across all the suppliers, the AT20:
The important specifications are:
A 60 degree sweep was enough for this project, but the required scanning speed was unclear. Kpps (Kilo Points per second) is a measure of how many thousand points per second the galvo mirrors can accurately move between while scanning at an 8° test angle. To determine if this was good enough for this project I compared to typical specifications for laser galvos:
This confirmed that 20 Kpps was good enough, so I moved forward with the AT20.
Once again I surveyed what China had to offer on the cheap end of things. Laser pointers of dubious quality (and power ratings) were available, but they weren’t powerful enough for this project. Even if I had looked for higher power red, green, and blue laser diodes I would still have needed to build all the supporting optics for combining the three laser beams to make arbitrary colors. Once again the users at the LaserPointerForum came to the rescue: Laserland sells an all-in-one RGB laser module with sufficient power (300mW total) at a reasonable price:
The specifications:
The green laser has a significantly lower output power, but this is expected - the human eye has a peak sensitivity at ~555nm which causes green light (at 520nm) to appear significantly brighter than blue (~450nm) or red (~660nm) light as the same optical power.
With the galvo and laser source figured out, I moved onto the control electronics.
Each galvo axis (X and Y) expects an analog input voltage ranging from 0V to 5V. Since I was using an Arduino Uno R3 (which does not offer true analog outputs) as the microcontroller board, I needed to use an external DAC (digital-to-analog converter) to provide the required signal. The MCP4822 met the requirements - with 12-bit resolution it can accept values from 0 (corresponding to bottommost position of the mirror swing angle) to 4095 (corresponding to the topmost position), with 2047 corresponding to central position.
A sine wave sweeping from 0 to 4095 at a constant frequency achieved the liquid sky effect. But I also found I could animate the beam by manipulating the min and max values of the sinusoid, as well as its frequency.
The RGB lasers were controlled by a TTL signal, with 0V corresponding to laser off, and a voltage between 3-5V corresponding to laser on. The laser power levels are not controlled with an analog voltage, the individual lasers are either at full power or off. To achieve arbitrary colors, I needed to combine the red, green, and blue laser with different power levels - I did this with PWM (pulse-width modulation). With the 8-bit resolution of the Arudino Uno R3 PWM pins, I had 256 steps of brightness to work with.
I designed and printed an enclosure to ensure the laser beam and the galvo mirrors remained aligned. This also limited the laser beam to my intended direction:
The completed assembly:
I next worked on dialing in the galvo control code. A sweep of 450Hz was the highest I could achieve:
For comparison, this was a test with a low sweep rate:
The choice of fog machine was easy: a generic 900W model from China completely dominates the low-price segment:
For fog liquid, it was simple to make my own with distilled water and pure glycerin:
If desired, I could change the ratios to achieve a desired thickness:
For this project I went with a 75/25 mix. After some mixing and shaking, I had fog juice.
I needed something interesting for the laser to scan. I’m recreating an effect from Alien/Aliens, so a sci-fi theme seemed appropriate and gave me an excuse to make some large-scale 3D prints I’ve had on my bucket list.
The DOOM helmet:
And the Scout Trooper helmet:
Thanks to the extremely large build volume of the AnyCubic Chiron (400mm x 400mm x 450mm) these models were printed at full-size without having to segment any components:
After days of printing, sanding, priming, painting, and weathering, I had my props:
We set everything up, I waited until evening, filled the room with smoke, and performed a test of the scripted animation:
Everything was ready to shoot the scene.
Unsurprisingly, there is a community of laser hobbyists who make their own laser scanners (Photonlexicon). The go-to laser galvo in that community seems to be the Dragon Tiger DT-30 Pro, with a higher scanning speed of 30kpps. I will keep this model in mind for future projects.