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BUILDING A COLLIMATED DISPLAY FOR PC GAMES AND By Rick Lee (click to send mail)
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BUILDING A COLLIMATED DISPLAY FOR PC GAMES AND By Rick Lee (click to send mail)
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| This all got started when Tom Kopke [70640.3205@compuserve.com] mentioned that
it might be possible to have a flight simulator display with a more
realistic, 3-dimensional sense of depth by mounting a fresnel lens in
front of the monitor. This sounded implausible to me
at first (it would just magnify and degrade the image, right?) but Tom is
a physicist who works on simulator display systems for the U.S. Navy so I
had to take his proposition seriously. He explained that the lens allows
you to view the monitor with your eyes focused out nearer to infinity
instead of two feet in front of your face. The fact that the image also
appears larger is gravy. The main benefit is that it looks far away. In
the science of simulation, this is called a "collimated display".
Collimated means that the light rays are coming out parallel instead of
radiating out in the normal way. Some professional simulators use curved
mirrors to provide the collimation and some use fresnel lenses that cost
tens of thousands of dollars, but Tom assured me that an inexpensive lens
SHOULD (he had never actually tried it) work well enough to make it
worthwhile.
If you don't know what a "fresnel" lens is (pronounced frayNELL or fruhNELL)... it's a flat lens which takes the curve of a lens and reduces it to a flat shape by sectioning the lens into concentric rings. This isn't a very good definition, but most dictionary definitions are equally lousy. See the illustration below. I don't want to expend a lot of effort here defining it since I think that most of you know what one is. See the illustrations below. |
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| The lens above is taken from an "overhead projector".
This type of lens usually does very well for this purpose provided that
the focal length is long enough.
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| Tom's idea started to make a lot of sense to
me... and being the sort of guy that just loves "a project" (I built my
own flight simulator rudder pedals long before they were commercially
available) I sent for an Edmund Scientific catalog to get a lens and get
started.
I didn't know that Edmund Scientific had two catalogs... the big science catalog and the wimpy little hobby catalog. They sent me the hobby catalog and so, not knowing any better, I ordered a cheapo $8.95 lens called a "Full Page Magnifier". The lens was floppy plastic and it sure didn't look very good holding it up to the monitor but I could vaguely see the effect that Tom was talking about so I forged ahead hoping that it would be better when it was mounted flat. It was. I built a box out of foam-core board to mount the lens on to hold it flat in front of the monitor I used black foam-core to minimize flare inside the box. You could also use black paint or paper on the inside instead. If you can't find foam-core board, you can use any type of heavy cardboard that you can make work. This isn't brain surgery. I crudely taped the box on the front of the monitor with duct tape for a while but eventually I put some self-stick Velcro on the box and on the monitor. This holds it very securely in place and also looks a lot better. The Velcro allows me to put it on and off when needed. |
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 My current monitor set-up. After experimenting with various box depths I finally settled on the box holding the lens 7 inches from the monitor. The focal length of this lens was 11 inches and the closer to the focal length you go, the more "depth" you see but the lower the quality of the image. To find the optimum distance, look through the center of the lens with your nose pressed against it and move the lens away from the monitor until the image falls apart and then slowly move it back in until you can easily focus on the image. I compromised on 7 inches which allowed for a very high quality image but also gave me a good sense of depth. Images on the screen REALLY DO look farther away looking through the lens. You can put your face up just inches from the lens and your eyes will relax into a focus many feet out. When playing Quake (or even golf or driving games like NASCAR or TestDrive) you can turn off the lights, put your computer sound through headphones and put your face up in the lens and become completely immersed in the scene. The scene will cover out to your peripheral vision. I've seen grown men recoil from playing Quake in this way saying that it's just too intense to handle. Some people who are prone to motion sickness may experience what simulation experts call "simulator sickness" due to the heightened realism. |
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| I also use Microsoft Flight Simulator a lot so I have another lens design which I sometimes use for this program. This box only covers the top portion of the monitor where the 3-D "outside" image is displayed leaving the lower control panel exposed normally. This gives me a real sense of outside/inside. You have to refocus your eyes on the inside and outside. See the illustration below. In order to make the lens smaller for this purpose, it had to be cut down. It's important to cut the lens so that the center of the concentric circles remain in the center of the lens. In other words, cut equally from the top and bottom. I use the full screen box when flying the helicopter in Microsoft Flight Simulator since I use a full-screen scenery display with that type of craft. The full screen lens works fine with all flight sims and programs, you might not want to bother with the half-lens, but it's a very interesting experiment. |
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| In addition to the magnification and the look
of depth there is also a good parallax imaging process going on. As you
move your head from side to side you can see a distinct difference in the
inside and outside views. Put your hand up in front of your face, close
one eye and move your head from side to side in order to see the effect
I'm talking about. This works with the exposed lower control panel and
also with the lens itself which looks very much like a windscreen you are
looking through.
At first, when using this with Flight Simulator, I was a little bothered by the little bits of dust and scratches on the plastic lens... but if you've ever been in a real cockpit you'll know that it's no stretch of the imagination to tell yourself that is a windscreen in front of you. In addition to the dust and scratches, the concentric circles cause some flare. You get a lot of flare from any light source in the room, so it helps to use this in a darkened room or build on a "hood" on the top and sides. Any light colored parts of the display such as the sky also cause flare with the circles but you just have to get used to that since there's nothing you can do about it. Oddly enough, this can also helps the feeling of realism with flight or car simulations. We all know what windshield wiper streaks look like with the sun shining on them. The effect of the concentric circles is quite similar to this so the display becomes a good bit more complex and realistic with a lot of interplay of light and flare going on in there instead of the rather sterile view and crisp colors we are used to seeing on the monitor. I have set up the view so that in my normal viewing distance I don't see quite all of the view. Thus, I have to sit up and lean over to see the lower portion and move my head from side to side you peer around the sides of the "window". This effect in conjunction with the depth of eye focus and parallax is really quite striking. At first I wondered if it was just me. Perhaps I was just talking myself into this so I invited one of my flight-sim-fanatic friends to come over and try it out. I set it up in a darkened room, turned the audio amplifier/subwoofer up to nice realistic roar and turned him loose with it. He immediately liked the effect even as he taxied onto the runway and he totally concurred with me that the whole business was well worth the effort. After he had flown around for a half hour or so he removed the box to compare and we both said "Yuuuuck... look how FLAT it looks!" I should add that this effect is different from other 3D effects that require the use of polarized or colored glasses. Those systems are interesting but tiring to the eyes and they suffer from jarring discontinuities when you look at the edge of the image. The collimated display is not 3D in this sense. However, because the image appears so much farther away, your brain interprets the image as being in 3 dimensions. After getting the larger Edmund catalog, I upgraded my fresnel device with a better lens. This one was 11.25 inches square. I found out it was actually a surplus (used) lens salvaged from an overhead projector. This lens was better but it cost 31 dollars (in 1992). Replacement lenses for overhead projectors are available for 30 to 50 dollars. You have to make sure that the focal length isn't too short when ordering a replacement lens. Some experimentation may be necessary. The cheap lenses such as "full page magnifiers" will work fine but the ones for overhead projectors have better sharpness at the edges and less spherical and chromatic aberration. I have also found a fairly universal source for cheap lenses. Most bookstores and office supply stores carry them as "Page Size Magnifiers". The ones marked "2X Magnification" work best, and at less than 10 dollars, it's easy just to try it out. When I stepped up to a 17 inch monitor, I thought that these overhead projector lenses would be too small. However, to my surprise it works just fine. I had to create a box with a trapezoidal shape to attach the lens to the monitor. See photo. At some time in the future, when I have more room I want to try a multiple lens/monitor setup. First 3 monitors and then maybe 5. The software exists to run this many screens (see http://wideview.00server.com/main.html ) In theory, it should be possible to have the lenses butting against each other for an almost seamless view which looks far away.
For a very sophisticated application of that theory, see the Boeing article below.
Read an article on
Boeing's experimentation
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| Places that sell fresnel
lenses:
Edu-Care of Texas sells replacement parts for overhead projectors…
which have great fresnel lenses:
Staples Office Supply sells a Staples brand 2X page-size magnifier for about 7 dollars which works pretty well. Sometimes they are listed as "Apollo" brand. Notice that it says 2X magnification and the lens is thick and rigid. The floppy plastic ones are not as good. I've heard that Office Depot sells one with the Apollo brand that is similar. I've tried the Bausch&Lomb page magnifier but it is less magnification and doesn't work as well. Edmund Scientific Maxi Aid Catalog American Science & Surplus Thunderseat makes one commercially designed for flight simulation... a bit pricey at 200 dollars but it's a good one. http://www.thunderseat.net/fresnel.htm (this site was down for a long time and now it's under a company in India which seems to be selling the seat, but they don't appear to sell the lens) Copyright (c)1992, 1997, 1999, 2000, 2002 Rick Lee
SPECIAL NOTE regarding using a fresnel lens to make a big-screen TV. I've been getting a lot of email recently from people interested in using a fresnel lens to project a TV or monitor image on the wall. This is a silly idea. Somebody on eBay has been selling plans to do this. This is based on the principle of the way that some actual large-screen TVs work, but those TVs use special monitors which are a zillion times more bright than your average TV or monitor. If you tried to project a regular TV or computer monitor on the wall... even if it were in a TOTALLY darkened room, the image would be very, very dim (and fuzzy) and therefore useless except as a curiosity. (yes, I've tried it just to see)
Back to Rick's Flight Simulator Page Read an article on Boeing's experimentation with fresnel lenses for collimated displays
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