Tips for DIY diffuse head for Durst 138s?

Both are on their way to me. I don't mind the height and I still have to catch up on the weight to make the movement smooth.

Yes, Lacap will be good for use with my Nega138.

This material might interest you. It was developed specifically to achieve the best compromise between diffusion and transmission.

bernard_L said:

This material might interest you. It was developed specifically to achieve the best compromise between diffusion and transmission.

https://www.exolongroup.com/index.php/fuseaction/download/lrn_file/mf0365_e_190704.pdf

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That looks interesting. It's not exactly commonly available, but I'll take a look at it.

koraks said:

For the star/bead LEDs, the best choice is a dimmable power supply; you calculate the voltage and current you need (depending on how you put the LEDs in parallel & series), then pick a power supply that will yield the desired amount of current and can work up the voltage needed under that load (this will likely not be a limit, really). For 3W LEDs the current is typically 700mA-1A per LED, or string of series-connected LEDs. Check the specifications of the LEDs you buy. I would recommend running them at <80% of their rated current. Then when selecting the power supply, make sure it's one that offers dimming, either linear or through PWM. There are undoubtedly power supplies with a knob/pot meter on them, or else you can connect one easily if it has a dimming input.

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Do you think I could benefit from your knowledge a little more?

I have some material already on its way to me, I'm currently choosing white LEDs.....but I still have in mind the possibility of controlling the contrast with colored LEDs.
But I've gotten so used to working with Darkroom Automation F-stop timer and enlarging meter that I don't want to change it.

The timer switches 1x230V for the safety light and 1x 230V for the enlarger, so both are powered from it.
All measurements with the meter are done with white light, only for the exposure itself do I use filters or adjust the color head on the second enlarger.

The calibration of my process consists of making step wedge contacts for all grades 00-5 under white light, the intensity of which I measure with a meter.
So I have a clear relationship between the intensity of white light and resulting tones/density on paper under specific grades.

Do you think something like this is technically feasible? Use white light for measurements/focusing, be able to set the ratio of B+G LEDs and then control it using the 230V output on the Timer?

Sure, there's a couple of ways to switch that. One very simple/low-tech way would be to just wire an additional light switch to your white light 'focus' array. Then have the timer control the B+G power supplies. Set the ratio B+G by using dimmable power supplies, each with a knob/pot meter to set the output. It may not be the fanciest setup, but it can be implemented without a lot of technical know-how from commonly available parts.

Thanks.....but I'll probably humbly go back to the solution with only white LEDs.
In the meantime, I've been looking into what's needed for precise regulation of B+G LEDs and I probably wouldn't be satisfied with potentiometer control. Now when I make a HD curve of paper and I achieved such predictability that I am able to work with a minimum number of test strips (in case of a good negative, no tests at all).
I would probably have a hard time putting together an x bit PWM solution and even harder time implementing it into my workflow.

White LEDs a d VC filters should work just fine; nothing wrong with starting there.

The inside of the Latibox 138, originally used with a color head, is just shiny metal. Does it make sense to paint it somehow? Maybe with matte white paint?

No, don't paint it. These were well designed. You get your white diffusion from the diffusion sheet itself. I don't know Euro brands (Rohm & Haas plastics are probably sold there too); but here we have something called Sign White Acrylite. It's more translucent than ordinary white acrylic or Plexiglas; and I use it in the equivalent of about 1.5 mm thickness, easy to size by scoring and snapping. You should have slots in the bottom of your mirror box to slide in one of these. But for extra diffusion, you could fit another one near the top, without significant light loss.

It will probably take some experimentation to get an ideal result, once you've factored in your light source itself. And perfectly even diffusion requires fine-tuning it in relation to the falloff factor of your preferred enlarging lens itself, at the specific aperture range you're most likely to use.

I happen to own a 180 Rodagon which is perhaps the only shorter than "normal" focal length lens for 5x7 capable of full evenness of field with the 5x7 Latibox. Ordinarily, you'd want a 210 or even 240 lens instead (I do own an excellent 240 as well).

Going fancy, one can sand or grind diffusion sheets thinner at the corners and edges than at the center to even better equalize falloff; but that's a chore. I've got mine within 1/10th stop. Most people simply burn in the edges and corners a little instead, during exposure.