Durst 138 / 138S heat filter question

[Drew Wiley]

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 All my own hand spec books are somewhat out of date, but were from the largest coating facility in the world at the time, not far from here.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]

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]

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]

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]

The heat absorbing glass is indeed 3mm, and comes mounted on a metal carrier, as you guessed...

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 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]

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]

Thickness of holder frame is 1,55 mm indeed. Glass inside it is 3 mm.

[Mark J]

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]

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]

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.