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Mark J
16-Aug-2023, 14:01
I have been slowly working through the various steps to get my 138 S ( purchased in approx May ) commissioned in the darkroom.
This is the unit with condenser head.
One minor issue that keeps bothering me is this :
I would like to install a heat filter, either absorbing or reflecting, in the allocated slots in the inside of the head.
Every time I go looking for material , I see that across the board, the glass thickness is around 3mm.
Now the slots in the inside of the light box look like they would not take much more than 1mm thickness.

I went looking through the manuals this evening to see if there was some sort of metal carrier into which the filter is mounted, which then slots into the 'spring-slots' in the head.

Best I could find is this shot from the 138 manual - but it's hard to see.
Does anybody have more hands-on knowledge on this ?

I am just about to talk to the local sub-contractor for optics test gear etc who can help me with some internal parts and ideas for the light-source mounting, so it would be good to get a bit of background info on this part, if we need to add it to the list.

241593

ic-racer
16-Aug-2023, 16:45
If you ask me, the heat absorbing glass is to protect your color filters. If not using filters in the drawer, you would not need the heat absorbing glass.

How about this one:

241596

Mark J
16-Aug-2023, 16:52
Thanks for looking but I will quite likely be using colour filters in the drawer, and that filter is too small, the port in the head is 85 x 75mm, all the bigger filters are 3mm thick.

Jason Greenberg Motamedi
16-Aug-2023, 19:21
The heat absorbing glass is indeed 3mm, and comes mounted on a metal carrier, as you guessed...
https://uploads.tapatalk-cdn.com/20230817/9f76aa91d5ce92405025db713777a102.jpg
What do you need to know? I was confused initially by the two pieces of glass, but from what I have read on this forum (https://www.largeformatphotography.info/forum/showthread.php?134344-Durst-138-Heat-Absorbing-Glass) it is intentional and helps dissipate heat without a notable effect on light distribution...

Mark J
17-Aug-2023, 02:35
Oh fantastic - thanks !
Isn't that interesting...
In that case I will order some 3mm glass and discuss a holder for it with John.

Mark J
17-Aug-2023, 04:56
Very odd about the two pieces of glass - wonder if it is something to do with either heat expansion or an alignment aid of some sort ?
I will most likely be buying a 'hot mirror' ( dielectric reflect ) for this, so the glass will not get as hot. There are 100mm square pieces you can buy, I can get it cut down to size accurately.

Drew Wiley
17-Aug-2023, 11:15
Hot mirrors are sometimes combined with UV coating on the opposite surface - another option.

interneg
17-Aug-2023, 11:25
The heat filters in the Ilford MG500 dichroic head are also two piece - whereas LPL used 1.

Mark J
17-Aug-2023, 12:44
Hot mirrors are sometimes combined with UV coating on the opposite surface - another option.

Is that a UV reject or a UV AR coating, do you think ?

dsphotog
17-Aug-2023, 20:42
The HAG in the Durst 184 condenser head is also 2 piece.

Drew Wiley
18-Aug-2023, 17:52
Mark - depends what you want. All my own hand spec books are somewhat out of date, but were from the largest coating facility in the world at the time, not far from here. The nice thing about them was that they had so many leftovers, that a simple small seemingly custom filter order might be shipped free as a courtesy sample. But now you'd need to check other sources. The whole point is to trim of some UV too, but not enough to affect visible colors in the print. Most of the heat needs to be extracted via a pull fan duct anyway. You can't rely on passive methods if a modern halogen bulb is involved.

Mark J
20-Aug-2023, 08:03
OK thanks. If it's trimming/rejection then I don't think it's really what I need - looking at the emission curve of a 2950K or 3000K bulb, it only just reaches into the UV, there's some useful output in the 375nm to 400nm range that is valuable to help decent exposure times for the harder grades, so I don't think I need to worry about any excess UV coming through. I'm not printing colour, and can see that it would be a different issue in that case.

On the split heat filter design, I'm starting to think it may have been as simple as the Schott material only being made in a limited size ( width ) . There were definitely limitations on some filters, that I can vaguely remember from the past in design work. So it may not be a mysterious technical solution to something !

ic-racer
20-Aug-2023, 14:30
I have a few devices with multi piece heat filters. Movie project with like 3 and Durst color head in like six or seven pieces.

Mark J
31-Aug-2023, 11:05
Some progress on this in two areas -
1. Last week I went to a local guy in our work orbit who does test equipment ( 'OTI' ). He's got the enlarger head on-site and the pics, and will draw and cost-up a new filter holder from pressed/folded steel. Just waiting on that.
2. Checked the 'Hot Mirrors' from Knight Optical, and the stock ones are not quite right because they knock out the 380 to 405nm at the short end of the visible spectrum too - just the sort of area I wanted to get and bought a 3000K bulb to achieve. So the choice is to go for absorbing filter again, even if in two pieces.
3. The same company were able to offer me their last piece of KG3 glass at a price I couldn't refuse, on the basis I get it cut myself when the final dimensions are sorted. It's a big enough part to cut two pieces from, which will be finished with minimal chamfers on one edge (each) , in order to butt them up together, as per the Durst part.

The KG3 material absorption starts a little earlier in the red ( ~ 600nm ) but it's of no consequence when working with B&W paper.

Louie Powell
1-Sep-2023, 05:39
The heat absorbing glass is indeed 3mm, and comes mounted on a metal carrier, as you guessed...
https://uploads.tapatalk-cdn.com/20230817/9f76aa91d5ce92405025db713777a102.jpg
What do you need to know? I was confused initially by the two pieces of glass, but from what I have read on this forum (https://www.largeformatphotography.info/forum/showthread.php?134344-Durst-138-Heat-Absorbing-Glass) it is intentional and helps dissipate heat without a notable effect on light distribution...

I have a Durst F60 - the heat absorbing glass is one piece and sized to fit in the filter drawer which sits above the condenser.

I bought (special ordered) the HAG because I thought it was necessary to minimize negative heating and popping. But in retrospect, I really wonder if it is all that essential given the reflex design that Durst used in their enlarger heads. I would think that most of the heat from the lamp would flow upward. Also, the metal reflector plate that directs light from the side of the lamp downward through the condenser, negative and lens would absorb some of the heat. Finally, I picked up the habit of always turning on the enlarger lamp for a few second to preheat the negative before actually exposing prints, so I never actually saw evidence of negative popping.

As to idea of the HAG being in two pieces - I can understand how that might help heat dissipation. The only concern I would have is that the seam might cause a discontinuity in illumination that would show up in the print, but as long as it is above the condenser, that won't happen.

Mark J
1-Sep-2023, 06:36
The heat absorbing glass is absorbing near infra-red ( 0.7 to 2.0 µm ), not thermal ( 5 to 12 µm ) infra-red. This NIR will reflect off the mirror and pass through the condenser lenses quite freely, but will create some heat when it interacts with the negative or any multigrade filters. So the 'HAG' is just designed to reduce the burden of unwanted radiation through the system.
It's true that the mirror and the lenses will absorb thermal infra-red, but the bulk of the energy output from a 3000K bulb is in the visible and NIR.

I don't believe now that the two-part filter has anything to do with heat-dissipation. Splitting a filter into two, as shown, will do next-to-nothing to increase its heat dissipation in any case. The reason is just that the common Schott filter materials like KG1 were only made in limited strip widths, they are made in a different process to the normal optical glass.

Nilpson
20-Oct-2023, 11:24
Thickness of holder frame is 1,55 mm indeed. Glass inside it is 3 mm.243267

Mark J
20-Oct-2023, 12:27
Thanks for confirming those numbers.
I emailed the fabricator early this week as the project was drifting, I got a surprise when he said he had made the parts now ( hadn't contacted me ! ). Hopefully I have a filter holder that will do the job, similar to this one. I couldn't get to him this week ( he closes early on a Friday ) but hopefully early next week. Next job is to get another optical friend of mine to cut my heat filter into two suitable pieces to fit the holder.

phdgent
20-Oct-2023, 23:06
One piece of my DURST 138G's heat absorbing filter is broken in two parts (now I have a 3 part filter...).
I went to a glass merchant and he suggested to use a piece of protecting glass as used in coal- or wood stoves, 'normal' glass would not stand the heat of a 300W to 500W bulb...

Mark J
21-Oct-2023, 05:40
Not sure if that helps. Resistance to heat is one thing, but it also needs to absorb in the right part of the spectrum, to be effective.

phdgent
21-Oct-2023, 22:58
Not sure if that helps. Resistance to heat is one thing, but it also needs to absorb in the right part of the spectrum, to be effective.

Do you mean that it must be IR absorbing?

Mark J
22-Oct-2023, 05:24
Yes, but in the near infra-red, I posted above about the details of what the KG glasses do.

Mark J
1-Mar-2024, 06:32
At the end of my experiments here, i've actually got the Durst working and I'm not so far using the heat filter.
It turns out that for prints at 16x20" , I am OK using a 160W linear halogen bulb in a security light as the source, and this is enough to give approx 15 sec exposure at f/11 for 5x7" negs.
This is with Opal glass ( Schott ) as the diffuser.

I should put up a small thread on this soon, the details are worth seeing if anyone wants to refurb a 138 S condenser set-up without building an LED source.
There is a thread running at FADU forum, in the darkroom section, showing all the details.

Bob Wagner
7-Apr-2024, 06:33
If I could hijack this thread for a closely related question, my 138S was set up for point source lighting with a 1,000 watt halogen bulb. The filter drawer holder has 3 pieces of opaque glass, one is slightly more opaque than the other two. Gives very short exposure times. I changed the light bulb socket and installed the opal large diameter bulbs but exposure times are surprisingly long. Are the glass pieces necessary for use with the Opal bulb? Do they affect exposure times? Working on some more experimentation to see whether I should go back to point source or not. I do B&W only

Mark J
7-Apr-2024, 11:02
I'm assuming the glass in the filter drawer is not really opaque ! Otherwise nothing would come through.
Are they neutral, ND glass ?
They may be just taming the amount of light ( and some heat ) from that 1000w bulb.
Heat filter glass if present will be a pale blue or pale blue-green, but mainly transparent in the visible.

You can probably remove one or two of those if they are ND's.
What is your opal bulb, it could be from 300W to 500W ?
At 300W the heat might not be too bad.

notorius
8-Apr-2024, 00:01
I am using large opal bulb 300W with no glass in the multigrade filter drawer. IR heat filter is present. There is maximum power recommendation by Durst(written on the head housing), the bulb should be no more than 200W. With the 300W bulb exposure times are quite short. 10s @f16, head on the column at the maximum height and the projection board at cca 85 cm (32") from the floor. With Ilford multigrade filter in the drawer. I am asuming any glass filter in the filter drawer would lenghten exposure times. The heat is not excessive with short times, but I would like to have longer times, especially when making smaller prints.
My plan is to replace the incandescent bulb with LED bulb or to make custom board from high CRI LED strip and incorporate dimming function.

Bob Wagner
8-Apr-2024, 05:34
My mistake, translucent, not opaque, 'frosted' you could say to about the extent of a camera ground glass, and 3mm thick each. Opal bulb is 500w, removing the glass significantly shortened exposure times.
I think I will put one of the 3 back in for heat absorption (?) and to reduce the amount of visible light, it just seemed odd there were 3. Have to research how to tell if glass is heat absorbing or not.

Mark J
8-Apr-2024, 10:22
If it's frosted or translucent , then it acts like another diffuser, so some light will be lost that way.
Are you printing 5x72 or 4x5" ? How big is your opal bulb ? For 4x5" I think you only need 3" diameter.

phdgent
8-Apr-2024, 22:22
DURST bulbs are the outmost unfindable objects in photography, even in the 220V version which was the original!
That's why I hardly dear to use that enlarger, even with a Volt stabiliser, as I have only ONE spare bulb left...

Here is a description as published by by DURST USA, I think so but I am nor sure:

248808

Bob Wagner
9-Apr-2024, 05:16
'Unfindable' indeed, but I have two and they are big enough for 5x7 so I think I will use them and get to printing. I got the 138 free, but it has been a bit of chore to get it operational. Not that I am complaining too much, it's a problem many might like to have - but research and development time are over. Thanks everyone for your input!
BW

Mark J
9-Apr-2024, 10:48
My solution to the bulb problem is in this thread -

http://www.film-and-darkroom-user.org.uk/forum/showthread.php?t=14606&page=5

It's fine enlarger, you were lucky to get one free !
The condenser head has been a step change for me in terms of print quality, and I am re-assessing many archived negs that I thought were too soft or where the highlights were shouldered in the past.