At last, I found a moment to finish this project of LED head for a 5x7 Durst 138/139 enlarger, I have started 6 months ago.
The design is very similar to my first home made LED head I presented here on APUG except this one is far more powerful (the first prototypes were for a 4x5 Omega enlarger).
I was very close to finish the omega LED head project , when I found a Durst 138/139 on our local ebay.
Those 5x7 enlargers are so rare here in NZ, I could not resit to break my piggy bank and replace the omega with this beast

Anyway, here are the characteristics of this new head:
* 3x independent colour channels (R,G, B)
* Each channel is current controlled. The amount of light can be adjusted in real-time thanks to a PC software and a remote control
* The transfer function of each channel has been linearized
* In its current configuration, the maximum power consumption is ~85W (equivalent to a ~500W light bulb at least). I could extend it to the limit of my driver (120W). This would require me to invest in a new and big power supply but as my darkroom is quite full. I prefer to avoid this option for the moment. The head can be run at 150W with a more powerful driver (~ 900W light bulb) but this, I'm sure, won't be necessary.
* Cold design (the LEDs are mounted on a big heat sink)


The red channel has a maximum power consumption of 11Watt. The red light is only used as a safe light for B&W so there was no point to increase its power. It is composed of 4x high efficiency LZ1-00R200 LEDs (Uf=2.8V, If=1A, lambda=660nm, manufacturer: Led engine)

The green channel is composed of 30x 941-XPEGRNL100D01 high efficiency LEDs (Uf=3.55V If=0.5A, 133 lumen, lambda= 520nm, manufacturer: Cree). At its full power, the green channel consumes 35W. It might be extended to 52W if necessary but actually the current is limited to 350mA per led to fit my power supply). The amount of light produce goes up to 3200 lm!

The UV/blue channel is made of 32x unbranded UV 395nm LEDs bought on ebay. They are rated 3W but I am using them at ~1W for the moment. I suspect from the shape of the die they are EP-U4545V produced by EPILEDS but who knows. Anyway here are the EPILED UV LEDs characteristics (Uf=3.8V, If=.7A, 40lm). The actual maximum power consumption of the blue/UV channel is ~37W but I did a try at 57W and it works like a charm!


For the full story about LED wavelength choice, drivers, please read my previous post in this group.

Some pictures of the head:

On the right, the light source of the Durst139, on the left my home made light source
On the bottom left, the remote control which is connected to the driver.
The LED head is on and we can easily see the network of green and UV/blue led.

My DIY B&W proportional LED head for my Durst 139/138 enlarger by stormpetrel_geek_mode, on Flickr

At full power!
It is VERY bright indeed. The heat think and the LEDs remains at room temperature. This is very impressive as I expected more heat generated by the LED.
It might be another story once in the enlarger head box, but this enlarger came with an air duct and a fan to cool down the krypton head light of the durst 139, so it will be very easy to evacuate the heat if necessary.



My DIY B&W proportional LED head for my Durst 139/138 enlarger by stormpetrel_geek_mode, on Flickr


The red channel:

My DIY B&W proportional LED head for my Durst 139/138 enlarger by stormpetrel_geek_mode, on Flickr

The next step will be to try the head with 5x7 enlargement. However I will have to make a lens adapter for a nikkor 210mm first as it is the only 5x7 enlarging lens I own.
Unfortunately it is quite a big lens designed for the Devaer enlargers, and it does not fit on the Durst.
Maybe this weekend.

The title might be a bit confusing.
This is not a RGB colour head for colour printing but a RGB head for B&W printing only.
The LED wavelengths have been selected to optimize printing on MG paper.
They might not be appropriate for colour printing.

Update on this project
-------------------------

I have replaced the 395nm 3W LED with 440nm 3W LED (ebay Epiled).
The idea behind using UV LEDs was to make an universal head I could use also for alternative process. However after few trials, I found that was not a good idea at all for the following reasons:
* Most of the enlarger lenses cut at 400-410nm (schneider & rodenstock) so there is no point to produce UV light if it is absorbed by the lens. Nikkor lenses are better for this kind of use as they cut at 380nm-390nm but anyway better to avoid the edges.
* By choosing green & blue wavelengths to far away from each other mights result in slightly out of focus pictures. I noticed the focusing problem with the red light. When I focus with the red channel, the image produced with the green is slightly out focus. So by focusing with the green LED, the UV pictures mights be also out of focus. Now I use the cyan channel to focus and as the green & blue channel are closer to each other, the problem is not noticeable with my lenses (which are not APO). I might be a bit picky here. Focusing with the green channel would do it too!
*fluorescence of the paper. I had some concerns that the fluorescence of the paper produced by the UV light could generates photons in the green range during the exposure which could mess up all the print.


Next modification is a mezzanine board equipped with cyan 475nm LEDs (30x 3W XPEBBLL10000201, 6x banks of 5x LEDs, 16.5V 2.1A).
The cyan LEDs will be used to solve a mid-tone rendering problem which occurs with MG paper at soft grades. More info about this problem and the cyan sensitive emulsion in this document .

I have not calibrated the head with the cyan color yet, but I will one day. At the moment I use the cyan channel as my primary focusing light.

The mezzanine board is a double sided board. I left as much copper I could to dissipate the heat. As the board is not fixed on the heatsink, I have added some powerful mini-fan to dissipate the heat. I works fine and I can hardly feel any heat building up on the board.

Here is a picture of the LED light source in the head of my Durst 139 enlarger

Multigrade LED head for durst 138 /durst 139 enlarger by stormpetrel_geek_mode, on Flickr


Light emission spectrum of my Durst 138/139 LED head by stormpetrel_geek_mode, on Flickr


The LED head works perfectly well. It is stable, does produce any heat and it is accurate.
It is far more powerful than required and I use it at less than 1/4 of its max power @ f22.... but I did not make any large print yet.

Regarding the number of LEDs per channel, I tried to match the power of each channel to ~90W except for the red channel which is ~20W.
However an extensive calibration showed that haf the power would have been enough for the green channel.


Homemade Multigrade LED head for dust 138/139 by stormpetrel_geek_mode, on Flickr

I will detail the calibration in a next post