Film-Tech Forum ARCHIVE: Negative Lens

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Posted by Ian Bailey (Member # 1752) on 05-07-2004, 03:47 AM:

In a Strong Lume-X lamphouse there is a thick,round piece of glass mounted just behind the output cone that is called a Negative Lens.What does it actually do?
Mine is badly cracked(into 3 pieces),can I just remove it and realign the Xenon or would I have to replace it.
We do not get a very bright picture in this auditorium.

Posted by Mark Gulbrandsen (Member # 72) on 05-07-2004, 09:20 AM:

I have sucessfully glued them back together with the red RTV whihc is similar to the RTV used on the space shuttle tiles. No you won't see a shadow in the light from the join areas but yes it will cut donwon efficiency a bit more. Unfortunately for proper coverage you do need that lens in there. As for your light level on screen you dodn't mention your screen size, type of surface, projector nor if you had checked lamphouse alignment to verify if it was correct. The Lum-Ex can do pretty good light levels on a medium sized gain type screen. Its biggest downfall is its small size and the need for alot of cooling air flow with a 2KW lamp and the need for very critical lamphouse to projector and bulb to reflector alignment due to its small mirror.

Mark

Posted by Ian Bailey (Member # 1752) on 05-10-2004, 04:48 AM:

Hi Mark
The Lume-x is connected to a Simplex projector with a Kelmar gate and turret.To the best of my knowledge this equipment has never been aligned,we also have trouble getting uniform focus.The screen is 18ftx10ft with a 26ft throw(screen is perforated matt finish).
I did an alignment and lens adjustment in one of my large auditoriums with much improved success but it seemed a bit hit and miss as I do not have an alignment tool and there is still some room for improvement in the focus.Is the Align-o-tron the answer for these sort of problems?
Just one more thing about the negative lens does the concave side face the lamphouse or the projector when it is installed?

Posted by Brad Miller (Member # 2) on 05-10-2004, 02:14 PM:

Absolutely get yourself an Align-O-Tron. You will be amazed at the difference and it is very inexpensive.

Posted by Mark Gulbrandsen (Member # 72) on 05-10-2004, 06:47 PM:

Actually Brad,

This may rustle some feathers but I have found that if you know the proper steps to lamphouse alignment that the Alignotron is not needed at all. There is one exception in which I use it and that is to get the lamphouse table on a DP-70, or other machine to within being very close. The rest I do by eye and then double check and touch up with the PSA Light Meter. Although some lamphouses with small diameter mirrors are touchy I find that the time spent with the Alignotron is often wasted time. Yes, I do own one and yes, I always used to use it but not any more. Doing it the "old fashioned way" is quicker and just as accurate if you know how.
If you really want to do a "proper" alignment than you need the Kinoton Alignment laser and kit(built by some german fellow btw). which runs about 800 bucks and still do a follow up and touch up with the PSA which is the most accurate method anyway.

Mark

Posted by Jack Ondracek (Member # 1466) on 05-11-2004, 10:04 PM:

As many lamps as you deal with, your methods are probably correct, Mark. You've found a system that produces predictable results based on your daily habits.

I find the Align-O-Tron a good tool to double-check my work. For me, at least, it is a second method to verify what I've learned to do by feel or sight, whether I use it first or not.

Posted by Brad Miller (Member # 2) on 05-11-2004, 10:22 PM:

Well mark it's your opinion and if your eye provides the same quality, then more power to ya. Still I've found the Align-O-Tron is just as effective with lamp alignment as image focus. I would like to see you align a turret/lens by eye and obtain the same results with focus uniformity. [evil]

Posted by Mark Gulbrandsen (Member # 72) on 05-12-2004, 07:51 AM:

Brad,
I use other types of gauges to align a turret before aligning a lamphouse. Digital depth micrometer with a fixture plate for parallelism, and a few others dial indicators. Strong has special fixtures available for setting up their turretts, even they don't use a laser at the factory! I prefer to do it more like a film cameras lens is adjusted as its extremely accurate. I think you'd find that even Phil would agree on this. I don't go by my eyes, I go by the PSA on final tweeking, the PSA is the best there is for final tweeking of a lamphouse. I do use the Alignotron occasionally though for projector turrets in which the gauges won't work, or for other reasons as I mentioned above. The alignotron is good, just ALOT SLOWER, most of the time it just lives in my service van.

Mark

Posted by Lindsay Morris (Member # 1430) on 05-13-2004, 03:32 AM:

Ian,
I have the same lamphouse as yours (Lumex) the little yellow dog as it is often called.
I light up a 32' wide outdoor screen with this lamp with a 2k lamp fitted running at just under 70 amps.
Lighting is brilliant so your smaller sized screen should be an easy one to do.
So as you have guessed something is not quite right.
The negative lens goes into the lamphouse with concave side towards the projector (flat side faces the reflector.. according to the manual I downloaded from Film Tech).
Checked mine today and that is indeed how it is fitted (not screening just doing a security check on the site over winter).
When I was setting things up I had it out and tried the lamp without it... big drop off in light just on the back of my garage door about 10' wide.
So it would seem that it is essential and if yours is busted you are probably loosing a hell of a lot of light.
Mine comes down to a quite tight spot on the back of the fire shutter when the machine is stationary and although I could not verify it exactly would say that the spot is smaller than what I used to get on the carbon arc lamps I have retired. They gave good light but the transistion to xenon was good step forward as I have more light than I need and the lamp is just cruising.
Even full moonlit nights are not a problem and I reckon I had much better screen detail this season with LOTR III than I did with LOTR II the season before and then I was running the carbons as hard as I could and even upped the amperage of the rectifier to 55 amps to try and screw some more light out of things.
Some cinemas over here complained about "dark" scenes in Master & Commander but it was brilliant on our screen [thumbsup]

I am running a twin shutter Century which does in theory give some more light due to the faster shutter open time.. maybe 10% but who really knows anyway.
I can practically read a newspaper in the back row with the light coming off the screen during bright scenes and this is with a 100' throw. If you cannot find the RTV stuff Mark mentioned I stuck a condensor lens from the slide arc lamp back
together with clear silicone after it went into 2 pieces and I could not easily find a replacement.
That worked for quite a few years in there before I found another suitable focal length one to replace it.
That slide arc gets VERY hot when in use..about 70amps AC on the carbons & slight spindling as it is over the limit for that size carbon.
The silicone never gave way... just a thought.
It was not the usual household silicone but an engineering quality stuff and I think I still have a tube stashed away somewhere at the cinema as I use that stuff to stick the steel plates for the seat legs to sit on in place (so they do not sink into the grass).. a few fall off each season but the stuff sticks like hell as a rule.
You would not need much and the lens I stuck back almost looked OK until you turned it in an angled light and then you could see the crack but could never pick it on the screen.
Email me if you want to know the silicone code number.
Lindsay

Posted by Ian Bailey (Member # 1752) on 05-13-2004, 08:44 PM:

Lyndsay
The negative lens is in three pieces but it is able to sit in its mount in the lamphouse.I havn't run the lamphouse without the negative lens in position.The cracks in the negative lens are not fine horizontal cracks, the cracks take up a lot of the surface area when looking straight at the negative lens.I can only assume that this would be deflecting the light in all directions.The first thing I want to do is an alignment of the whole machine and then I can decide whether the light output and focus is acceptable,if not I will replace the negative lens.As I have stated in the "Achieving Uniform Focus" thread running at the moment I still can't understand how a lack of light can affect focus.

Posted by Steve Kraus (Member # 476) on 05-13-2004, 11:09 PM:

What are his options to upgrade with a new, up to date reflector? Then he could eliminate the condensor lens and probably get a lot more light.

Posted by Mark Gulbrandsen (Member # 72) on 05-14-2004, 06:40 AM:

The only option thats available is a dichroic reflector and you still need the nrgative lens with that. No other reflectors will fit, the housing is way to small. Fortunately all the parts are still available as its the same lamphouse thats used on the Strong Super Trooper spotlight.

Mark