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# Projection Glass

5 replies to this topic

### #1 of 6 Shiminskika

Shiminskika

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Posted February 20 2011 - 06:24 AM

I am working on a research project for school, and i have an image being projected at a 270 degree angle on to a piece of glass that is angled at about a 30 degree angle itself. I need to find the angle of refraction as the projection enters the glass and then exits.

Picture of Problem:

http://static.hometh...um.com/imgrepo/

Also is there a special type of glass i should buy for the project so that there is no reflections from around the room and to make sure i get the best quality image. Also i am afraid that if i get 99% transmissive glass that the projection will go right through and not refract through the glass.

Any help would be greatly appreciated. Thank you.

### #2 of 6 Leo Kerr

Leo Kerr

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Posted February 21 2011 - 07:03 AM

you realize, of course, that you're going to get reflections, anyway?

Okay, here's what I think you might be missing.

99% transmission glass means very "white", very clear glass, where 1% of the light passing through it gets converted into heat.

Different glasses, and there are countless varieties of glass, have different indicies of refraction; this is the number that tells you how much the light will be "bent."  And perhaps how much might be "bounced" in a first-surface reflection.

The amount of reflection depends a great deal on the angle at which the light strikes the glass.  A very shallow angle, and the glass will be hugely reflective.  30° is a pretty shallow angle, so there will be a relative lot of first-surface reflection, bouncing off and away, toward where you have the "30°" on your drawing.

The IoR, however, will tell you the angle of the "bend" toward your arrow marked with the question marks.  However, most glasses will not be able to bend anywhere near as much as you're showing!

If you need to see the image in the glass, you'll need something for the light to interact with, which means a less transmissive piece of glass.

What is it you're trying to do?

And, sort of as a repeat, the only way to not have reflections is to physically "kill" them after they happen.  Think lots and lots of black velvet light-traps.

### #3 of 6 Leo Kerr

Leo Kerr

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Posted February 21 2011 - 07:07 AM

wait, are you trying to make the image appear in the glass?

Then your best bet is to not worry about the glass as much as getting one of the Hitachi or 3M films to apply to glass or acrylic to make the otherwise relatively clear surface also act as a projection screen.

### #4 of 6 Shiminskika

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Posted February 21 2011 - 07:32 AM

im making a 3d volumetric display. its a cube with a hollow triangular prism inside, and a projector will project onto the sides of the prism and the innate properties of the prism will put the sides together to create a "hologram" so to speak inside the prism.

im trying to recreate this project.

i know i need a coating on the glass that enhances color and contrast as well as reduces ghosting.

### #5 of 6 Shiminskika

Shiminskika

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Posted February 21 2011 - 09:59 AM

im also not sure of the rejection range i should get.

### #6 of 6 Leo Kerr

Leo Kerr

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Posted February 22 2011 - 09:35 AM

My understanding of volumetric displays is that they don't do any bending of the light.  Or, rather, they don't try to bend the light.

Instead, you take a piece of, say, borosilicate float glass, very very thin (it's relatively easy to get 0.4mm, I think, I know 0.6mm isn't a problem.)  And hold this at an angle -- and 45° is the usual angle I've heard -- to the projector, projecting the image through the angled glass.

Then, spin the glass, as if there was a turn-table mounted to the lens's front.  Spin it relatively fast, and at a very precise relationship to your video scan-rate.  Indexed to the scan-rate, so that you know what content you need to render to make sure the volumetric image does what you think it should do.

The image "appears" merely where it interferes with the piece of float-glass as it spins in the beam.  You might need to apply a very very faint coating of, well, pixie-dust to the glass to help make the image appear.  (essentially, just a little bit of very very fine crud, whatever it is, to make the glass not perfectly clean, and thereby help reveal the image -- by "revealing" the glass as it spins through the illuminated pixels.)

Some other bits that might be useful; it's probably advantageous to have the spinning glass in a low-pressure atmosphere, or better yet, a vacuum.  Lowers the air resistance, and makes it easier for you to keep up the several thousand RPM spin of the image-glass.

Making it small helps, but makes it potentially harder to focus your actual image in the volume.

Leo

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