Do you mean that it must be IR absorbing?Originally Posted by Mark J
Do you mean that it must be IR absorbing?
Yes, but in the near infra-red, I posted above about the details of what the KG glasses do.
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.
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
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.
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.
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.
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.
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:
DURST 138S BULBS.pdf
'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
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