Collimation Experiments

The visual systems used in a professional Level-D system consists of a spherical mirror and an ellipsoid shaped projector screen.

Design and optimisation for ellipsoid back projection screen of collimated display system in flight simulator. B. Ma, Li Lin, Q. Ma

On a traditional display system, such as a projected curve screen or television, the image is only aligned from the perspective of the Captain or First Officer POV. This presents a parallax error – out of window views of airport runways or identification of objects may be observed offset from its genuine position. Each pilot has a different viewpoint because of the angle of the projected light reflecting from the screen. Parallax correction can be integrated into the simulator with realtime POV changes, however this is not a passive system and will require manual user input.

Collimation eliminates the parallax error because light rays are reflected parallel from or near infinity. This effect also provides depth to the image. When looking at a collimated screen the ciliary muscles of your eyes relax; your brain interprets the image far away in the distance (an illusion) when in reality it is a reflection on a mirror placed a few feet in front of you.

Unfortunately we don’t see many enthusiast flight simulator builds with collimation displays; most likely because of the difficulty with designing and implementing a visual system requiring a high degree of precision. Collimation displays outside of professional Level D Flight Simulators are also not observed because of its active patent, however this ultimately shouldn’t prevent you from designing and building one yourself.

It has been a fun few weeks investigating and experimenting with collimation (on a smaller scale) to determine if such a challenging optic system could be integrated into the Soarbywire Airbus flight simulator.

There are two ways collimation can be applied – with a fresnel lens, or a thin film mirror (mylar).

Collimation with a Fresnel Lens

Fresnel lenses, like thin film mirrors, are not a very commonly interfaced into the visual systems of flight simulation training devices. I have previously written how this is a missed opportunity.

Large fresnel lenses are now available in a variety of sizes and can be purchased affordably from e-commerce platforms such as Aliexpress.

400 mm x 300 mm (600 mm focal length) Fresnel Lens placed in front of a 4K resolution computer monitor

When mounted in front of a high resolution monitor, the effect is immersive. You are drawn into the 3D image; the 400 mm x 300 mm fresnel lens is an optimal size for a 4K monitor. The quality of the image through the fresnel lens remains clear with minimal aberrations.

Unfortunately when placed in front of a 1080p projector the effect was less impressive. With the image resolution spread out over a larger area, individual pixels become more apparent when viewed and enlarged with the fresnel lens. A projected image from a 4K projector should theoretically provide a better image quality.

Individual pixels are observed when a fresnel lens is applied onto a projected 1080p image

Nevertheless fresnel lenses provide a collimated effect which can be implemented in a flight visual system with minimal planning or engineering. Simply mount the lens appropriately in front of the projector screen or TVs and you should get a relatively good result.

Collimation with Thin Film

Mylar thin film can be purchased at your local garden store or online retailer. Unfortunately you will usually be limited by the width of the roll – the largest usually available is 54″ (137 cm) ; this limits the size of the FOV and the application of the visual system to smaller type jets and general aviation aircraft. One could possibly experiment with the vertical FOV and draw the screen closer to the pilot POV to determine if this screen size can still work with larger flight deck setups.

To experiment with a thin film visual system a small card box was shaped spherically, mylar was applied by tape and an inlet was created to fit a portable home vacuum. When applying suction with the vacuum, the thin film forms into a spherical shape providing it with collimated mirror properties. Reflect an offset display unit from the mirror (an iPad was used in this example) and at a certain viewing angle the collimated effect is realised.

Experimental mini-collimated system; mirror aberrations due to an imperfect spherical shape

The collimation is extraordinary and the effect can only be fully appreciated in person. One inherent character of collimation is the constant distant focus. With a conventional non-collimated image moving physically closer will cause your eyes to refocus. As you transition to your near point of vision the image will blur as your eye lenses are unable to accomodate further. However with collimation, parallel rays travel to your eyes allowing you to always focus on the image no matter how far or close you are to the display.

High fidelity flight simulators incorporate a collimated visual system for greater immersion and realism. Projected images on flat or curve screens appear flat-like without depth compared to a collimated display system. A home built collimated device should also be able to be achieved on lower end enthusiast (home) devices with careful project design and planning.

One thought on “Collimation Experiments

  1. Pingback: Prototype Collimation Display Test – iGamezone

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