I’ve now made a crude LED head modification for the Durst M605 which works quite well, even in the higher grades. I’ve used 3 RGBW LEDS from Cree http://www.cree.com/LED-Components-a...Lamp-XML-Color I think those are the successors of the ones used in Heiland cold light head for the Focomat V35. As planned, I control them out of the application, using the PWM outputs of the same arduino that does the sensor readings.

I am a big teststrip aficionado, as I get so much information from them. So the workflow I do now basically consist of my normal steps described earlier, now using the convenience functionality of a teststrip mode for finding the base exposure and contrast:

  1. I take a sensor highlight reading to get in the ballpark for base exposure (still to-do: get a sensor based indication for density range / contrast grade). To get into reasonable times, either the aperture can be adjusted, or now the overall intensity of the light source can be electronically dimmed / increased.
  2. Then I make 5 teststrips, 1/3 F-stop apart around that measured highlight time to find optimal base exposure. For this, I use a contrast settings about 2,5 using the auto teststrip mode for exposure time.
  3. I decide on best base exposure time.
  4. I make teststrips for contrast settings using the teststrip mode for contrast variants. *
    How that mode works: I set a range of contrast (e.g. 2-4) and it sets up the number of teststrips required in ˝ grade steps. (e.g. grade 2-4 would set up teststrips for grade 2, 2.5, 3, 3.5 and 4 = 5 teststrips) Contrast teststrips are always done in full exposures, as incremental teststrips are not possible here. The contrast teststrip mode automatically sets the time (the base exposure time determined in step 2 above** for all the contrast teststrips. It then automatically adjusts the contrast setting by modifying the PWM output (read: light intensity) to the blue and green leds, changing the ratio of light intensity between blue and green.
  5. I carry over the evaluated base exposure and contrast setting to the “productive” print section. I then decide on initial burn and dodge sequences and make the first full print attempt.
  6. I make corrections based on this print.

This is my workflow for single grade printing.
A question: Is the ratio between intensity of green vs. blue light the same ratio as the ratio for times in separate exposures would be? E.g. if the ratio for a 10 seconds exposure at a certain grade is 30% intensity blue light to 70% intensity green light. Would it be the same as the ratio between separate exposures? E.g. 3s at 100% intensity of blue light, followed by 7s 100% intensity of green light. Then the single grade to split grade conversion would be straight forward and the same system could be used e.g. for separate burn and dodge sequences for hard and soft filters.

*) I don’t use calibrated Filter equivalent numbers or paper grades yet for this step, but mainly record the raw ratio of light intensity between green and blue for now. I have some rough presets numbers though, where a certain ratio is assigned to an arbitrary “grade” setting (e.g. 50% green and 50% blue = preset 2,5). For this text I use the term “grade”, although this does not mean any standardized / speed matched paper grade. The assignment of ratios to the grade settings was done on some rough tests to give nearly even separation of grades across the spectrum for that specific paper (MG IV RC). For fine-tuning the contrast, any settings of the presets can be overridden with specific ratio settings if necessary – e.g. dial in a bit more blue and dial down green accordingly to get more contrast.
Clearly quite some calibration needs to be done for the papers I use, as the response to the ratio is dependent on the paper. I think this calibration is what systems like the Heiland splitgrade already have built in / pre-programmed in. But then again, I don’t know yet if I’ll want to go through this effort myself at all. Maybe I just stick with the intuitive approach and compensate based on experience. I have already developed some gut feeling about how much I have to compensate with the Ilford filters I currently use. I guess over time I’ll develop the same for the new material. Not very scientific – but I think that’s a good thing.

**) Maybe I’ll introduce an exposure compensation factor for the various contrast changes. Before I can do this, I have to do some more testing / calibration with the LED filtration method to find the correct factors following the method described in Way beyond monochrome. Again, probably not worth the effort for me.