Atmosphere Effects: Holographic Projection
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  1. #1

    Default Holographic Projection

    Here's a technique I've used for several decades in a number of applications that I've never seen anyone talk about for Halloween but is a natural - holographic projection. It's possible, and fairly easy, to project full 3D solid-appearing full parallax images into space without any screens, fog, or anything. The trick is a large lens such as a Fresnel lens with a diameter as close to the focal length as you can get. I've used the lenses from the writing surface (NOT the projection lens) of old overhead projectors with great success. New plastic lenses about that size will run from $60 on up but junk burned out overheads can be had for free if you're lucky. Some old large-screen TV's used Fresnel lenses as screens and sometimes you can find a lens several feet across.

    You want to find the focal length of the lens first. Put a flashlight or other point source of light far away (at least 30 feet) - I've used street lights outside - and focus the image of it onto a flat surface. Measure the distance from the lens to the image. That is approximately the focal length.

    Now here is the trick. If you place an object at twice the focal length on one side of the lens, there will be a copy of everything the lens sees at twice the focal length hanging in free space on the other side, a full 3D solid object. The observer has to be facing the lens from that side because the projection exists only in the area framed by the width of the lens - the bigger the lens (or shorter the focal length) the better it works. Also, objects that give off light work the best, especially a single color as large Fresnel lenses tend to have chromatic aberration and multiple colors will result in some color fringing. Keep this in mind to stage you setup for best effect. Don't let people move too far off center as the object disappears as the projection moves off the edge of the lens.

    This is a true holographic projection! If you project an incandescent light bulb the projection will feel hot! I have a clock that I made that projects the time 16" into space in front of it and if you place a tissue at the location of the projected digits they shine as if they were actually there. You can make a working (albeit clunky) virtual flashlight that way! The reason for this is that the lens actually recreates the wavefront of the source at the projected location within the limits of the angle of the lens and the viewer. That virtual flashlight actually shines a beam forward!

    BIG BIG WARNING!!! IF YOU PROJECT THE SUN IT WILL INSTANTLY BURN WHATEVER IT TOUCHES AND INSTANTLY START FIRES!! The temperature of the projected sun can reach several thousand degrees!!! You can do welding with this setup! I've been able to burn text into the roadway writing with a 12 inch lens. Make sure your setup can't start a fire during the day. Be mindful that even if you cover the lens some materials are opaque to visible light but transparent to infrared and plastic lenses will still focus the heat even if covered!

    The possibilities for Halloween displays are endless and the effect is amazing! It's very strange to see something hanging in the air, especially if it is moving, that you can put your hand through, and not have any kind of screen or smoke. With a large lens you can project an entire live person! I'll leave up to your imagination to decide what to project. I would love to hear or see photos of what you do.

    ijv

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  3. #2
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    This sounds incredible! I did a bit of searching on youtube; the results were some pretty impressive videos! And I agree, for haunting the possibilities are endless.

  4. #3

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    Great video! Looks like there are more videos to check out, too! Thanks.

    The important things: big lens as close to f/1.0 as possible to get the widest viewing angle range and a good subject to project.

    Plastic Fresnel lenses are not the highest quality so expect the image to be slightly blurred. That can be addressed to a great extent by having monochromatic point-source lighting to eliminate fringing and other lens issues and keeping the subject simple and small. The other important thing is to stage the display to keep the viewer within the best working range from side to side. It can be really dramatic to have the projection in front of any barriers to that the viewer can actually try to touch it. Very effective!

    Remember to use a real solid object to project. A flat picture will be projected as a very thin flat image in space. I've used a small CRT this way and it's pretty weird to see an electric image just hanging there without any mechanism creating it, but solids look solid and very real if your lighting is good.

    One thing I didn't mention before is that the two sides of most fresnel lenses are not the same. Projected on way the object is more accurate, but projected the other way it looks like it is stuck on the surface of a big bubble. Also, make sure you have a positive lens, not a negative one (like those stuck to the back of a van window to give a wide angle view). The negative ones won't work.

    By the way, speaking of negative lenses, a few years back I made a helmet for a space suit with a negative lens as the faceplate and put a small UV light at the base shining up. If you used fluorescent makeup you could outline the features of someones face and when they wore the helmet people would see a tiny but very real cartoonish head inside that looked very, very alien! No one could figure out how this shrunken head could be so totally real! Of course, it was a real person but all that could be seen was the moving makeup reduced to a third its size.

    ijv

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  6. #4

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    Wow this has gotten my interest peaked! I have not heard of this. I just happen to have a big screen TV fresnel lens in my shed that I got out of a TV to use in just goofing around with. Melting pennies and catching fire to stuff, and it does do that very well. I was going to use it and make a solar cooker at some point.

    I am going to have to do some tests with my lens in the next few days. I have to see what this looks like in person. If I can figure out how to make this work like in those videos....oh boy wait until next Halloween! (no way I have time for it this year, too many other projects right now)

    Thanks for posting the info

  7. #5

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    Another thing - the projection image is upside down from the source object.

    I wrote a couple of short papers detailing some of the formulas and geometry involved back in the '80's and might still have some stereoscopic pictures showing the effect I took from back then. If I can dig them up I'll post them to my website and post a link here.

    At the time I was working on a camera for making 3D images using Integral Photography. The camera had to create a solid image in space of a scene which could then be captured on film covered with an array of pixel-sized lenslets cutting through the middle of the projection. The camera was huge - about as big as a desk - and had 16 inch diameter lenses. You could half climb inside to get a good view of the projection so you could position the film plane where you wanted it. Initial images were eerie, because they were pseudoscopic a person's face would appear dead still but would smoothly rotate in all directions to follow you around the room. I still have one hanging in my office. New visitors find it unnerving. I see them keep glancing at it but you can't escape it - the face is always turning and following you.

  8. #6

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    Very interesting.
    So who's going to be first to try this?
    Be sure to post your results.......with a short description of set up

  9. #7
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    Quote Originally Posted by icemanfred View Post
    Very interesting.
    So who's going to be first to try this?
    Be sure to post your results.......with a short description of set up
    Not me but I'm intrigued. :-)

  10. #8
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    I'd love to give this a try. I've looked up replacement lenses for overhead projectors and found one for $15. The lens is 11" square and has a focal length of 14". So, does that mean it can only project 28"?

    As I think I understand the set-up- point light shines at lens - object placed between point light and lens - projection of object occurs on otherside of lens - viewer must be on that side looking at projection/lens/point light.

    Maybe I am confused? Please let me know.

  11. #9

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    I've had great success using those kinds of lenses. That's a great price - hopefully the quality is good enough for imaging. Make sure it's a lens and not a mirror! I've tried using some sheet magnifiers, too, but they tend to have a longer focal length compared to their size and the image quality is not so great, but they do work within their limits and they are cheap and easy to get to play around with, just not as stunning.

    First, hold the lens flat with the surface facing you at arms length in front of you and move it around a bit. With one side of the lens facing you there will appear to be a virtual bulge coming out of the middle. this is actually the optical shape of the lens if it wasn't flat. That is probably not the side you will want to face the viewer because of the distortion it introduces.

    Next, try looking at something bright like a window around 10 feet away or more. You'll see a small image of the window, upside down, located at about the focal length or so on your side of the lens! Because you have a lot of light around it might take a minute to notice that it is in space between you and the lens.
    With the lens one way the virtual image will be floating on the surface of the blob you saw before, with the lens the other way it will be about the same location but the blob doesn't distort it or show up so much. That's the orientation you will probably want.

    Here's the math: something located many focal lengths away will be projected upside down at the focal length distance on the other side of the lens. Something at twice the focal length will be projected at the same size as the original at twice the focal length on the other side. As you get closer than twice the focal length with the object, the projection goes farther away from the lens and the projection gets larger (very quickly!) on the other side until when the object is at the focal length, the projection is essentially at infinity and the effect falls apart.

    Play with a flashlight in a dark room - set the flashlight where it shines at the lens. Stand on the other side. Depending on the location as just described, you'll see the flashlight bulb and beam reproduced at the location in front of the lens. If you move side to side it will work until the projected image moves off the edge of the lens, where it just disappears. This works really well with an LED light source like a red digit alarm clock. Because you want to keep the projection within the bounds of the lens, I've found that something the size of the digits on an alarm clock is about the best size for a lens from an overhead, it projects nicely at 1:1 and there is enough room for the viewer to move back and forth to nail the illusion. Play with it and you'll find what works best for you and what you are trying to do.

    Depending on what it is you want to project, you can use the geometry to advantage, just remember that at twice the focal length on both sides you get a 1:1 reproduction and as you move away from that various distortions show up that ultimately mess up the illusion, with getting closer to the lens being more distorted than getting farther away. Play with it to get a feel for it.

    Make sure you cover up the area from the lens out to behind the object you are projecting - any loose light is also projected and it tends to confuse the image you are trying to achieve. I've had best results with objects that give off their own light, although lighting an object can be made to work.

    Let me know if you have more questions. Good luck. Waiting to here how it goes!

    ijv

  12. #10
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    ijv,
    Thank you for your detailed reply. I believe now I understand the concept. The only aspect I have reservations about is getting people to notice the effect from the sidewalk as they walk by.

    I could build a black box for the object being projected. I then could light the object inside the box while aligning the lens with the object. Maybe that will filter out any unwanted ambient light. I imagine this to be part of our graveyard and would be one of the effects for visitors walking by at night. So, I could build a small ghostly doll, place it in the box, light it and blow a fan on it, project it and see what happens.


    The lens in question is the writing surface of an overhead lens. That is the lens with the concentric circles. I'll report back in a few weeks if I can pull this together.

    R

 

 
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