CurtPalme.com Home Theater Forum

Ile · Posted: Tue Jun 08, 2010 1:02 pm Post subject:

virusc · Joined: 11 Apr 2007 Posts: 358 Location: Massachusetts

I have a set of 1209s housings and mounting brackets that I would sell pretty cheap. I bought from hammerhead 5 years ago or so for a 1208s/2 conversion that I never did. PM me if interested.

Gerbrand · Joined: 13 May 2008 Posts: 199

@virusc: Thanks for the offer, but since you are at the other side of the Atlantic, shipping costs alone would probably be more than I would be willing to pay. (Right now, that is, I might be getting more desperate at some point...)

Someone at the Dutch htforum posted some pictures of the LC hardware and it really looks like there is a lot of space between the front side of the tube and the chamber. Basically the glue is applied to the side of the tubes.

So, I am still considering the option of replacing the C-element by a plate of Al and use that as a reference for glueing the tube.

So here comes my next question: does anyone have a proper mechanical drawing (i.e. the type you can take to a workshop) of a C-element? I am mostly interested in the flange of course.

Cheers,

Gerbrand

jkruger · Joined: 24 Oct 2007 Posts: 2435 Location: Carlsbad, CA

The barco 9" c-element flange is 148mm in diameter and 6mm thick.

Gerbrand · Joined: 13 May 2008 Posts: 199

Any idea about the diameter of the holes for the bolts and their location? Of course I could measure it myself, but it is never as good as having the actual drawing.

Gerbrand

Gerbrand · Joined: 13 May 2008 Posts: 199

Hello,

It's been a while since I posted anything in this thread. That's because I have been busy. Following suggestions by others I have designed a tool that can be used to glue the tube in the LC hardware with an accurately defined Scheimpflug angle.

Basically it is a plate of aluminium that can be screwed to the LC hardware instead of the C-element. In this plate four screws with a plastic tip are fitted that can be used to measure the location of the present tube. I can then take out the old tube and use these same screws as a reference/support when glueing in the new tube. Of course I can also make a small adjustment to the SP.

The picture, I think, explains everything.

Today I tested the system and was happy to see that everything fitted together as it was supposed to.

However, when I took my first measurements I found that the SP angle was quite different from what Barco's LENS programme predicted. The horizontal angle seemed OK, but the vertical angle was quite a bit off. I measured 0.75 degree, when LENS said it should be 0.45 degree. Since I estimate that my measurement accuracy is 0.15 degree at worst, this seems quite a significant difference.

So I wonder, how accurate BARCO actually glued these tube themselves.

Gerbrand

donaldk · Joined: 17 Jun 2008 Posts: 308

Gerbrand, how do you prevent the bolts from twisting, screwing up the reference, I don't see any lugnuts?

Gerbrand · Joined: 13 May 2008 Posts: 199

They do twist a little bit: I think the tip can move up to 1 mm sideways. This should add up to 0.1 mm accuracy, which is still a lot less than the discrepancy I found.

However, I will follow your suggestion and add a few nuts and see if it improves. This will also help me to lock the bolts at the correct length.

Gerbrand

donaldk · Joined: 17 Jun 2008 Posts: 308

Indeed, you need to lock the bolts, by adding a bolt on top of the plate, so when you remove the jig from the housing the bolts don't twist, reducing or increasing the desired length.

tse · Joined: 03 May 2006 Posts: 1014 Location: Sweatbucket, Fl.

Check page 18 for an explanation and formula for calculating your shim thickness. Warning! Trig is involved. This paper explains alot about CRT projectors.

http://www.xymox1.com/Projector/crtpaper.pdf

Scott

Gerbrand · Joined: 13 May 2008 Posts: 199

@donaldk

Yesterday I remeasured the whole thing, but now with some nuts at the back of the plate. The bolts no longer moved or twisted, but the measurement result is just a little bit closer to the number expected from LENS. (0.65 degree instead of the expecteded 0.46 degree).

@Scott

Thanks for the reference. I don't expect the trig to scare me off.

Gerbrand

Gerbrand · Joined: 13 May 2008 Posts: 199

Hi,

I think I nailed the issue: LENS is wrong.

The details:

Page 17 on the document Scott referenced, states that the SP angle is given by (here comes the trig):
tan SP_angle = n / m * tan(lens_to_screen_angle)

In this equation m is the magnification and n is the refractive index of the fluid between the tube and the lens.

If I plug in the appropriate numbers: m = 20.8, n = 1.478 for a 20/80 glycerine/glycol mixture and lens_to_screen_angle =10.5 degree I get a SP angle which is very close to what I measured initially: 0.75 degrees.

My suspicion is that the LENS program ignores the liquid coupling.

I can prove this by doing the same calculation for an air coupled system (BG808) and a liquid coupled system (BG1209s). I had to choose different screen sizes to get the same magnification because the tube size is different.

The result is shown in the attached figures. The SP angle is exactly the same in both cases!

So the SP is adjusted by Barco to the correct angle, only LENS is not to be trusted in these matters...

BR

Gerbrand

Gerbrand · Joined: 13 May 2008 Posts: 199

Hi,

The calculation of SP angles has kept me busy over the past week. What was bothering me was the following: if I substitute n=1 (refractive index for air) in the equation from the paper that Scott referenced to, I still do not get the SP angles that are reported by LENS. The difference is small (0.51 degree for the paper, vs. 0.46 degree for LENS, but still...).

After consulting wikipedia http://en.wikipedia.org/wiki/Scheimpflug_principle, I now think that also the paper is not entirely correct.

I think the correct equation should be (here it comes):