telecine condenser lens

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telecine condenser lens

Postby Ralph » Mon Jan 03, 2005 5:59 pm

I am trying to complete a home telecine machine. I have modified the projector to run at 20fps with a diffused low watt light source I have also constructed a condenser lens box(double convex) with a 45 degree angled mirror. I am trying to mimic a cinemate, no offence Roger, but at this time I cannot afford one so I figured I would try to build one out of spare parts. So far so good except one thing. In certain light conditions(silouettes) a double image reveals itself slightly offset. By rotating the lens box, the offset image moves from left to right or vice versa. Can anyone tell me why.
I appreciate any help.
Ralph
 

Re: telecine condenser lens

Postby GeneratriX » Mon Jan 03, 2005 7:44 pm

Ralph wrote:I am trying to complete a home telecine machine. I have modified the projector to run at 20fps with a diffused low watt light source I have also constructed a condenser lens box(double convex) with a 45 degree angled mirror. I am trying to mimic a cinemate, no offence Roger, but at this time I cannot afford one so I figured I would try to build one out of spare parts. So far so good except one thing. In certain light conditions(silouettes) a double image reveals itself slightly offset. By rotating the lens box, the offset image moves from left to right or vice versa. Can anyone tell me why.
I appreciate any help.


Sure, Ralph! ;)

looks like a low quality mirror between the projector lens and the condenser lens. Is a well known fact by optic designers, the existence of those "Phantom Effects" in low quality mirror surfaces.

The scientific explanation is very simply: the crystal supporting the reflecting surface has their own refraction coefficient, so, it can reflex a low intensity image from the light rays, before they can "touch" the real mirroring surface. Because the slightly different path longitude both images looks displaced between they. Something as a half-translucent mirror before a real mirror: you'll get two images displaced: one weaker, one stronger.

Replace your mirror by one of professional quality, and Voila'! ;)
But I could suggest ask to more people before spent new money!

Just my two cents!
Cheers! ;)
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Re: telecine condenser lens

Postby MovieStuff » Mon Jan 03, 2005 7:58 pm

Ralph wrote:In certain light conditions(silouettes) a double image reveals itself slightly offset. By rotating the lens box, the offset image moves from left to right or vice versa. Can anyone tell me why.


Well, there are a variety of potential reasons. Assuming that your condenser lens isn't creating internal bounce, the reason most likely is that you used a standard mirror and not a front surface mirror. If you use a standard mirror, there is a layer of glass that covers the silvering and light bounces off both the glass and the silvering, which creates two images. If you use a front surface mirror (also known as a 1st surface mirror), then the silvering is on top of the glass so you get only one image. Contact a local mirror store. They often have "two way" mirrors that are really just semi chromed mirrors that have no backing. You can flip them over and spray paint the glass side black and they will do a pretty decent job. A true front surface mirror can be found in Edmunds Scientific but be prepared to pay $$$.

Good luck!

Roger
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Postby Dusty » Tue Jan 04, 2005 2:15 am

Also check the optics section of American Science & Surplus, sciplus.com. I don't know what size mirror you'll need; they always have some first surface mirrors, but only occasionally very large ones.
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Postby Actor » Tue Jan 04, 2005 6:01 am

Another possibility is a 45 degree prism. The reflection is 100% internal so you won't get a second image. Edmund carries them under the name "tank prism" because they were used in tank periscopes during WWII.
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Postby Ralphie » Tue Jan 04, 2005 4:24 pm

the reason most likely is that you used a standard mirror and not a front surface mirror.



This seems to explain a lot. I will try and find a front surface mirror.
Thanks to everyone for your help.

Ralph.
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Postby Actor » Tue Jan 04, 2005 4:54 pm

By the way, a simple test to tell if a mirror is a first surface mirror is to simply touch the mirror with a pen. If the tip of the pen appears to touch its reflected image then it's a first surface mirror. If the tip of the pen appears a couple of millimeters away from its image, it's not.
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Postby Marco P » Fri Jan 07, 2005 5:20 pm

Another way to get a cheap front-coated mirror is to salvage a Polaroid cam. Those with integral film packs contain quite large mirrors. You can usually get them for almost nothing ...
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