Hi!
I'm new here, comming back to film.
I used to print variable contrast in black and white (ilford and kodak) on my old b&w condenser enlarger.
I still have a box of polymax but I've lost the filter pack.
I first came here searching for variable contrast printing. I see that some have had success using leds as light source to control contrast so I'm certainy going to experiment.
I wonder if anyone experimented printing RA-4 with colour red, green and blue leds. I think it should be possible but I'm newbie to colour printing.
All comments welcome!

Hello Pitagors

I make my LED head for B&W printing based on the sensitivity curve of many B&W papers. I would do the same for a DIY color RGB head.
The sensitivity of some color papers were published in this thread
You might have already read it. It contains many useful information regarding color printing

LED have many advantages but also some drawbacks. Their drawback for color printing seems to be a too wide spectrum. This problem can only be solved by using dichroic filters or by using lasers but lasers have also their own drawbacks (including their expensive price). The emitting spectrum of LEDs drifts with the junction temperature increasing (need a good heatsink to solve this problem).
The LEDs have also to be properly controlled to keep their emitting power stable. This can be done by using a current source driver (this is what I use in my DIY head) or by using a feedback photodiode.

Few solutions have been proposed in the thread, but quite curiously nobody proposed rotating filters which is a commonly used solution in spectroscopy.

This is the way I would do it:
* make a board with green leds, blue leds and red leds wavelength matching the graph above.
* add three dichroic filter on a motorized disk (motor=stepper motor)
* add a diffusing glass
* add a photodiode to control the emitting power of the source (closed loop)

Print color sequentially.
If sequential printing is not possible then thing will become more difficult.

The electronic would be quite simple to make.

I guess that adding filters and a stepper makes the idea too complex.
I didn´t know the leds drifted significantly with juncture temperature. Sounds very reasonable of course. I wonder if during a normal exposure time this would be a problem. In any case maybe a "preheat" phase (somehow shutting-off the enlarger out - with a stepper motor and some blades hehe) might mitigate the problem.
For what I read so far, it seems that the main problem is the green contents of blue leds. This is indeed a problem for contrast but for color, rather I was thinking: unless the printing needs no green exposure at all, it could be compensated with less exposure from the green led. This could be easily be solved with software once the system is calibrated... of course calibration would need some extravagant instrument like an spectrometer.
I also read (I think PE was saying somewhere) that there is sensitivity in the paper outside the leds range, and thus the paper will not work as expected.
Then I wonder: which kind of leds do ra-4 digital printer use??

With the temperature increasing in the junction, the wavelength of the LED drift a bit and the light output decreases significantly.
Considering the narrow peaks of the paper sensitivity, this might be an issue. A large heat sink will solve this problem!

The stepper motor seems quite an overkill solution, but you could replace the filters easily if necessary ( filter wheel) which would be a great feature for experimenting. Note than a stepper motor + driver cost less than $50. you can also recycle a stepper motor and its belt from an old ink jet printer Check here for a low cost driver. This would be very easy to make for an omega enlarger as you can easily replace the head.

If only the blue LED is a problem, there is also the option to add filters on your blue LEDs only (check the B370 http://www.hoyaoptics.com/color_filter/blue.htm) ?
This would be easy to do?

The ra-4 digital printer might use lasers.

Dominique



What is your enlarger