Film-Tech Forum ARCHIVE: Condenser Problem magnified by Tungsten Light Source?

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Posted by Mitchell Cope (Member # 32) on 04-23-2001, 05:29 PM:

The 35mm portable projector that I use (A.V.E. Transportable a.k.a. a Japanese Shinkyo) uses a 1000W tungsten filament bulb (DPT). When I'm projecting a clear area such as sky, you can see slight, but noticeable, vertical rainbow stripes from the filament. My condenser lenses are cracked, but the pattern of the crack is not on the screen. What's the best way to eliminate these stripes? Can they be eliminated by fixing the condenser lens elements? Is there a refraction problem in the condenser lens elements that can be addressed?

Posted by Josh Jones (Member # 447) on 04-23-2001, 05:58 PM:

That is one of the problems with tungsten light sources. With a xenon lamp, the image being projected on the gate is an image of the gas ball or plasma. With a tungsten lamp, the image being projected on the gate is an image of the filament. Since the image of the plasma is continuous, you do not see "ribbons" of light. With a filament, the incandescent tungsten is not a continuous flat shape, it is instead a broken line. You could slide the condensers back and forth until even distribution of light is attained. good luck in your quest.

Josh

Posted by Jerry Chase (Member # 660) on 04-23-2001, 07:37 PM:

You are on the right track.

Rainbow stripes. What else produces rainbow stripes? If you guessed a prism, you win. A cracked condenser degrades the lens of the condenser into a couple or more prisms.

I'd replace the condenser, or at least look for Canadian balsam to seal it back together.

Josh is correct as far as he goes, but no system would pass design if it had fringing or spectrum problems like this.

Posted by John Pytlak (Member # 331) on 04-23-2001, 09:48 PM:

I'm not too sure Canadian Balsam would hold up to the heat of a condenser lens, but it may be worth trying if a replacement is unavailable.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
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Posted by Josh Jones (Member # 447) on 04-23-2001, 11:37 PM:

Oops!
I must have missed the part about spectral breakup(I likes big words

) In that case, try and locate a replacement. www.edmundoptics.com try their optical division.

Josh

Posted by Randy Stankey (Member # 64) on 04-24-2001, 12:26 AM:

Could this be a manifestation of Newton's Rings? ===> Click Here

If you say you have a crack in one of the condensers, the glass could be partially out of line (bulging or sagging, for lack of a better way to describe it.)

I'm going to ask the same question I ask everybody who calls me and tells me that there is a problem with the picture: Does this happen with just ONE lens or BOTH (all) your lenses?

Thing is that this could happen in just about any part of the optical system where light passes through glass. It could be the condenser or it could be one of the lenses. (long shot)

Posted by Mitchell Cope (Member # 32) on 04-24-2001, 06:59 AM:

Randy, it's with all of my projection lenses.

I wasn't sure if one or both condenser lenses were broken. It turns out it is only one, the rear one, the one nearest the lamp. Obviously, the heat cracked it. I'm going to try to order a replacement today. I know the diameter and the approximate focal length.

Let me ask a dumb question here. For projectors that use a carbon arc or a xenon gas plasma, do they use, or need, condenser lenses?

Posted by John Pytlak (Member # 331) on 04-24-2001, 07:14 AM:

A few lamps have used condensers. For example, the "Peerless Hy-Candescent" carbon lamphouse. For xenon, the Zeiss Xenosol III lamp used "honeycomb" condensers with its vertical lamp and two mirror system.

But most modern lamps use some form of an eliptical mirror system.

Posted by Gordon McLeod (Member # 33) on 04-24-2001, 09:53 AM:

Also important is that one uses the correct lamp
Most of the lamphouses that use a condensor and a verticle lmap require and incandescent with a planar type filmament aray. THis is to provid overlapping uniformity.
If you replace this style of lamp with the quartz halogen replacement with the single coiled filament you will get all sorts of grief

Posted by John Eickhof (Member # 323) on 04-24-2001, 10:38 PM:

Some of the portables employ a fresnel diffuser lens also to cover the filament lines, also, slight out of focus of bulb can help this problem. If you are running a 1000
watt or higher bulb, make sure the condenser lenses are not held too tight in their mountings, they must have mountings that will allow for expansion of the glass after warm up! I would also suggest mounting a small muffin fan or blower to the condenser area.

------------------
John Eickhof President, Chief Slave
Northwest Theatre Equipment Co., Inc.
P.O.Box 258
Wendell, ID. 83355-0258
208-536-5489
email: jeickhof@nteequip.com

Posted by Mitchell Cope (Member # 32) on 04-24-2001, 11:01 PM:

One thing that has helped considerably is refocusing the mirror behind the bulb. It may not have had as much to do with the broken condenser lens as I first believed. Unfortunately, the bulb can be rotated and moved vertically, but it can not be refocused.