Holographic Reflex Sight from Half-Life: Alyx

Description

This project is an attempt to recreate the pistol reflex sight from the game Half Life: Alyx.

This is an in-game screenshot of the reflex sight:

To recreate it, I used the C-RT20 self-developing hologram film sold by LitiHolo. The film requires no chemicals to develop and is easy to use.

I started by taking reference screenshots of the pistol in game. From those I made a dimensional drawing of the round disk elements in Solidworks and laser-cut them out of 2.3mm clear acrylic.

Next, I modeled a holder for the disk elements and 3D printed it.

The holder has a rectangular cutout on the bottom to allow laser light to couple into the disks and a slot at the end for the holographic slide.

The edges of the disks were also sanded to improve the contrast in the final hologram.

I set up the holder and disk elements on an optical breadboard using a cheap 532nm green laser module from amazon as the illumination source. In testing with a Michelson interferometer setup I found that this module had a surprisingly long coherence length, at least several feet. I wasn’t able to move the mirror far enough away for it to lose coherence within the space of my garage. The process I used is similar to what is described here but using a fast photodetector from a previous project and oscilloscope instead of inspecting the interference pattern visually.

The C-RT20 holographic film is more sensitive to red wavelengths, so in an attempt to reduce exposure time I also tested a 100mW red diode laser module intended for laser engravers, but found that the beam coherence length was less than a millimeter using the same setup. Because of this it would not work well for holography.

The oscillscope trace showing the periodic loss of coherence as one of the mirrors is moved at near-constant speed is shown below.

To make the reference beam cover the critical area of the plate, I used a 20X infinity corrected microscope objective from ebay. In order to better match the intensity of the reference beam to the scattered light from the disks, the beam was passed through an OD=13 ND filter and bounced off a 50/50 beamsplitter to reduce its intensity by roughly 40x.
Effort was made to equalize the path length between both of the first beamsplitter outputs and the holographic plate.

The full optical path is shown below.

The beam can also be seen during exposure with the lights off:

Due to the low efficiency of this setup, a very long exposure of about 2 hours was used. This length of time may not have been completely necessary, but the film is much more forgiving to over-exposure than under-exposure.

Once the exposure was complete, I took reference photos of the placement of the hologram relative to the microscope objective and made a model for a holder that incorporates a fold mirror, some convex lenses, and a fat beam green laser module to reconstruct the hologram.

The blue lines show a ray diagram I used as a guide for placing the mirror and lenses.

This is the final result:

Unfortunately one mistake I made in the design was having the fit between the plate and the holder a little too loose:

Further improvements

This project is still a work in progress. These are some things I’d like to add/improve in the future:

• “Red dot” sight integrated into the hologram - this is present in the game but would require more optical elements to add. I was running low on optical mounts and lenses by the end of the setup process so I chose to leave it out of the first attempt.
• Trim the film and adhere it to a disk to better mimic the one in the game
• Redo the hologram with a yellow laser to match the color in the game (but these are very expensive) - or combine and use red and green lasers. I’m not sure if this would work or not.
• Recut disks with better airflow under the workpiece in the laser cutter. I think some smoke adhered to the bottom of the disks and made them look slightly foggy.
• Replace disks with fiber rings to eliminate reflections and fogginess
• Picatinny rail-style mount and integrated battery
• Change angle of reference beam to make it harder to catch yourself in the eye with the reflected reconstruction beam
• Improve fit of hologram to the illumination mount