Small optical wedge prisms for cheap - AVS Forum
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post #1 of 29 Old 08-22-2007, 10:51 AM - Thread Starter
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I found on ebay this product:
http://*******.com/2u8q5f

It seems to be of good quality and for $8 it is cheap for sure.
The only problem is that the wedges are kind of small (36 mm) and I am not sure of whether they would be suitable for an anamorphic lens.
With a sheet of paper I checked the size of the image coming out from my projector (optoma h79) and those prisms would probably be high enough to fit the whole image only if I'd put them quite close to the lens (let's say 5-7cm or 2"-3"). Would that be too close?

EDIT: it seems the tiny url link is being automatically censured. Is it just some spam protection thing or am I breaking some rule?
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post #2 of 29 Old 08-22-2007, 11:13 AM
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I think ebay links are blocked. I believe I know which ones your referring to, the problem would be the second prism I believe as the beam expands I don't think you'll be able to get the whole image in. The enclosure I created for my French prisms places the 1st prism about an inch away from the projector lens and if I do a vertical compression lens I think it will be right against the housing so I don't think you'll have a problem with that part.

check this link for similar options.
http://www.surplusshed.com/pages/category/prisms_1.html
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post #3 of 29 Old 08-22-2007, 12:06 PM - Thread Starter
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oman321, the prisms I linked on ebay were sold by surplusshed indeed. They are these ones: http://www.surplusshed.com/pages/item/l3442.html
and I share with you the same concern that the second one could possibly be too small .

On their website I saw they have many more but most of them are either way too small or right angled which, I think, will not be suited. (Honestly I couldn't figure out how to calculate the correct theoretical angle of the prisms; I wish I could find some good mathematical reference).
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post #4 of 29 Old 08-22-2007, 12:25 PM
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Send Mark CAVX a pm maybe he can give you some guidance with that, I rely on others for that type of stuff.
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post #5 of 29 Old 08-22-2007, 02:25 PM
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What the heck--it's only 8 bucks, I went ahead and ordered one. I am pretty sure the prism is much too small to be used as a second element, but may be useful as the first element. I will post what I find out.

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post #6 of 29 Old 08-22-2007, 03:47 PM
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Oh I can't wait to hear the results. I love my coated prisms but I wish I would have gone with the smaller prisms.
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post #7 of 29 Old 08-22-2007, 11:14 PM
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Don't they need to be different sizes? Or would it be okay to use two prisms of equal dimension?

-Sean
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post #8 of 29 Old 08-23-2007, 07:00 AM
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They could be different dimensions, but I believe they need the same angle. As with the French prisms they both have a 20 degree angle but are one is larger than the other.

Steve Scherrer, when you say you ordered one, do you mean one pair cause i'm sure you realize you need two prisms.
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post #9 of 29 Old 08-23-2007, 08:18 AM
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Of course I know that you need two.

I only ordered one, as I do have other larger prisms to play with, and I am absolutely convinced that the wedge would simply be too small for the second element. I am going to experiment with the wedge along with the other prisms that I got, which I know do not have the same angle, just to see what happens. If I get it and decide that I need a second one, I'll go ahead and order that one as well.

EDIT: after thinking a little bit about this, I am not convinced that the angles need to be the same, and I think in some situations, differently angled prisms may actually help with CA. If you think about it, the first prism splits the beam based on the angle of incidence, the angle of the prism and the refractive index of the prism. The angle of incidence for both prisms actually changes based on how much stretch you want, so this is pretty much dictated by the result you want to achieve in terms of the picture. Playing with some of the prisms already, it has been seen that one of the prisms actually provides the stretch for one side of the screen, and the other provides the stretch for the other side of the screen.

The refractive index of the materials is constant, if we are talking about the same optical glass.

But I think the reason you see CA at all is because you are using a second prism that doesn't have the ideal angle. If you use two prisms of identical angles, the second angle can't "recombine" the light sufficiently because the second prisms is displaced and angled with respect to the first prism. The only way that the second prism could "recombine" the light is if you placed it face-to-face with the first prism to form a glass box. Once you introduce space between the first prism and the second prism, the light beam will have separated to a too-large degree to be completely recombinable when it enters the second prism. Therefore, this actually means that it seems you want a larger angled prism for the second prism than the first prism to more effectively recombine the light source.

As it is, the first prism using the $8 wedge would actually be the first prism, and my lower angle prisms would actually be the second prism, so I have it reversed. Oh well, I will mess around anyway (perhaps combine my larger prisms to get a larger angle) to see what happens.

Of course, this is all very uneducated thinking. Someone with some physics knowledge may be able to debunk my line of reasoning pretty easily. But it makes sense to me, right now.

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post #10 of 29 Old 08-23-2007, 08:40 AM
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Cool, look foward to your results.

What kind of prism's do you have already?
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post #11 of 29 Old 08-23-2007, 09:06 AM
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Massillon plaque prisms (uncoated) that I am currently incorporating into a housing that will provide for pass through mode and stretch mode. It has been a slow process, but I finally found a great housing that I think will work great. I have one of the prisms mounted inside already, and am working on getting the second (larger) one in there as well. I will post results soon either here or on the DIY Audio site (or both) when I am done. I actually quite pleased with what I have done so far.

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post #12 of 29 Old 08-23-2007, 11:42 AM
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Quote:
Originally Posted by Steve Scherrer View Post

Massillon plaque prisms (uncoated) that I am currently incorporating into a housing that will provide for pass through mode and stretch mode. It has been a slow process, but I finally found a great housing that I think will work great. I have one of the prisms mounted inside already, and am working on getting the second (larger) one in there as well. I will post results soon either here or on the DIY Audio site (or both) when I am done. I actually quite pleased with what I have done so far.

what housing?
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post #13 of 29 Old 08-23-2007, 12:21 PM
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It's a rigid trapezoidal basket (with faux leather finishing) that I found at a store in my area called "Hobby Lobby". It is actually too long to be effective, but I cut it in half and doubled it up to shorten it (and provide a more rigid structure to hold the prisms). I will try to post some pictures of what I have so far when I get home tonight. I have the first prism mounted and rotatable for pass through mode and stretch mode. Now I just need to do the same thing for the front prism (which will be a larger prism). But it is interesting that the size of the basket (which is trapezoidal) actually fits the prisms very well.

I am a little concerned about the pivot posts that I glued, using very strong adhesive, to the prisms. I am fashioning a housing for the front prism that will provide additional support for the pivot post in case the adhesive fails. I am just afraid of the prism falling out of the housing and hurting someone sitting or standing beneath the prism. The back prism (the one closest to the projector) won't fall out if the adhesive fails because it is between the back wall of the housing and the front prism, so I am less concerned about that.

EDIT: I can't find the actual basket that I am using, but this was roughly the shape of it (although not weaved). I cut it in half and doubled it up, gluing the two pieces together and finally screwing them together.

http://www.target.com/gp/detail.html...sin=B000M48BDW

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post #14 of 29 Old 08-23-2007, 07:15 PM
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Here are photos of the housing I was talking about. I have to taped together with duct tape (and I use spray adhesive as well), but I will have to come up with something a little more solid--like screwing the two halves together:

I only have one of the prisms in the housing so far. This is pass through mode:


This is stretch mode:


This is the top, so you can see the adjustment dial:

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post #15 of 29 Old 08-23-2007, 10:43 PM
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That is different Steve, it is easier to work than wood?

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post #16 of 29 Old 08-24-2007, 05:31 AM
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Quote:
Originally Posted by CAVX View Post

That is different Steve, it is easier to work than wood?

Mark

I put some screws in the top and bottom to hold together a little better, but I think overall it's easier to work with--but more importantly, it is fairly strong and MUCH lighter than wood or particle board.

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post #17 of 29 Old 08-27-2007, 07:31 PM
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More pictures of my housing.

And one shot of the wedge I got from Surplusshed (below). It appears much too small to be useful, but the quality of the glass is great.






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post #18 of 29 Old 08-27-2007, 08:18 PM - Thread Starter
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It is supposed to be optical glass, so good quality. Unfortunately this is one of those cases where size does matter.
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post #19 of 29 Old 08-27-2007, 08:33 PM
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Steve,

It appears that you have attached the pivot to the prisms in just one place. Would it be better if you were to have made profiles (that same shape as the prisms) out of some sort of plastic and bonded that piece to the entire side of the prism?

Though I am guessing that there is not allot of resistance when you turn the dials...

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post #20 of 29 Old 08-27-2007, 10:39 PM
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Anyone think those little guys might work with an AE900?
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post #21 of 29 Old 08-27-2007, 11:49 PM
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Quote:
Originally Posted by 230-SEAN View Post

Anyone think those little guys might work with an AE900?

They might at the absolutely smallest image end of the zoom range...

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post #22 of 29 Old 08-28-2007, 07:39 AM
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I don't know about the image size of the AE900, but I would imagine that even if the image fits the first prism element (closest to the projector) there is no way the image would fit in the second element. But hey, I have be way off, since I am only guessing.

With respect to the housing, I will take the discussion over to the DIYAudio site, since it is way off topic here.

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post #23 of 29 Old 08-28-2007, 09:23 AM
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Give a link Steve, I'd like to follow your results.
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post #24 of 29 Old 08-28-2007, 10:25 AM
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Sure:

http://www.diyaudio.com/forums/showt...68#post1288368


This is a great (and very long thread) relating to DIY anamorphic lenses. I understand that some of the detritus has been removed, but it is still fairly long. I believe I linked you to the latest post in the thread.

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post #25 of 29 Old 09-27-2007, 01:57 PM
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Would the $8.00 prizms work for a vertical compression setup on an IN72? Throw range is 14' to a 92" 16:9 screen.

How to identify a future projector owner, he's the kid sitting two feet from the 42" plasma.
http://www.avsforum.com/avs-vb/showthread.php?t=954837
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post #26 of 29 Old 10-06-2007, 07:10 PM
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post #27 of 29 Old 10-06-2007, 10:38 PM
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Quote:
Originally Posted by Jim J View Post

new offering there:

http://www.surplusshed.com/pages/item/l9113d.html

Those are the same prisms as the so called "French" prisms...

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post #28 of 29 Old 10-07-2007, 12:46 PM
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$125 and completely and AR-coated. Is this what those of you that went through the French site paid?

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post #29 of 29 Old 10-07-2007, 12:53 PM
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Ya, it was something like $120.00 plus $5.00 for shipping. Due to conversion to Euro though I ended up paying more like $135.00.
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