Multicontrast paper: contrast x wavelength

[lungovw]

Does anybody know a source/document that would show how different wavelengths affect contrast while using multi contrast paper? I searched data sheets for AGFA/Ilford/Kodak and could only explanations about their correspondent filter sets. The reason I am asking is that I made a diffuser to my Durst Laborator 1000 that fits like a drawer replacing the condensers (so I have now a two in one). I used regular rgb LED strips. The result was so good that I want to make an improved one, with monochrome leds and actual control on the amount/mix of different wavelengths in green/ blue region.

[ic-racer]

You will have to test you LED with the paper. No big deal, just contact print a step wedge using the LED you want to test. See how many gray bands you can see. Since a 21 step wedge has 0.5 stops per step, counting the number of steps visible in the print and multiplying by 0.15 then by 100 will give you the ISO(R) value. ISO(R) is based on the log of the exposure range.

[lungovw]

Ic-racer, thank you, that will be useful once I have a new diffuser. By now there is a previous decision to be made in regards to which leds should I buy. Inside the meaningful part of the spectrum, the available options are:

440 - 450 nm = royal blue
460 - 470 = blue
490 - 495 = cyan
520 - 523 = green

The RGB strip uses blue and green, the ones listed above, the ones I used. The blue yielded very short exposure times (shorter than condenser with 150W bulb) and hight contrast. The green one demanded very long exposure and no contrast. My question is whether would be the case of assembling a diffuser using cyan and royal blue as well, or instead of blue, 460-470. Later I can mix them in different proportions using something like Arduino. For that purpose, a graph showing how contrast behaves according to wavelength would be very helpful.

I am attaching two pictures. One shows the strips glued to a board before soldering and the other with the diffuser installed already inside the enlarger in place of condenser lenses.163906163907

[jp]

http://www.ilfordphoto.com/Webfiles/20131161234331282.pdf

show some sensitivity info but does not differentiate between blue and green. But it would let you pick LEDs in the proper light range.

[Larry Gebhardt]

When I built my LED head I searched around and eventually found some charts that helped me decide on Green and Royal Blue LEDs as the closest matches to the paper, though I seem to recall that I wanted to try getting even shorter wavelength blue/violet LEDs if I could find some. At this point I'm not sure which source I was looking at, but if I find it I'll post it.

[lungovw]

Larry, following the link jp mentioned I found this graph attached. I tells me you are right: royal blue might be interesting in combination with green. At least Ilford papers react well on those regions. I found another Ilford doc that sheds more light on the subject. http://www.ilfordphoto.com/Webfiles/2006130201152306.pdf. By now I am testing and printing in two steps with regular blue and green - using very low power leds. But the idea is about making a head that will combine both in pre selected proportions in one or two steps as desired.163938

[Eric Woodbury]

Hard graphs to find. I looked once and found some hints. Main thing is that your blue LED cuts off before the green sensitive layer 'turns on'. Royal blue is the right choice. Any more blue and you won't be able to see it, especially if you have fair skin and blue eyes.

If you are interested, I have designed and simulated (never built) a circuit that provides current drive to LEDs. It is easily adapted to different power supplies and current levels. It is all analog and designed to be simple (it wastes some power). It has a rotary switch such that you can choose various current levels from maximum to minimum in 8 settings one stop apart. Also, it has a second adjust to ratio the green/blue, i.e. blue would go from zero to max (maximum for that particular rotary switch setting) in half a turn with green full-on. For the second half, the green is full-on and the blue output goes from max (for that level) to off. I'm happy to give you the circuit and a parts list. There is no pcb, so you'd have to wire point-to-point but as mentioned, circuit is simple. If you would like a different style of blue/green ratio, I can probably make such alterations.

[jp]

http://www.freestylephoto.biz/pdf/product_pdfs/ilford/Ilford_MGIVRC.pdf

p1 show how density is made by different wavelengths; 400-450nm works about the same, 520-525 ish nm works very different

[lungovw]

Thank you Eric for your kind offer. But the case is that for me, great deal of the the fun in this project is designing a digital controller with Arduino. I have already a timer for film development made with that platform and I can add more programming lines getting the same board to control color and time of exposure with a led head in single or multiple exposures. A third deployment will be a enlarger light meter so I will have a "all in one". It is possible to send commands with an old DVD remote control and have the parameters shown in an old mobile phone LCD. It is incredible what can be done with these processors. Today it is in a protoboard and looks like the attached picture. Soon I will move it to a little box with a real printed circuit board and some style. My time-O-Lite will be jealous.

The good thing is that now, based on what I just learnt in this thread, I will definitely go with Royal Blue and Green only. Ordering from China today. Thanks for helping me out. ( I am not a professional photographer so I have time to play around with those things)

163979

[Eric Woodbury]

Very good. I have modified the circuit for a complimentary output; where the total LED current is constant and it shifts from GREEN to BLUE. Made the circuit simpler, actually. Old circuit is for a different project. No code in either. Good luck. Happy snapping. EW

[Larry Gebhardt]

Here's what I wrote up on my LED head http://www.trippingthroughthedark.com/category/equipment/led-head/

I started a rebuild, but progress has stalled. The idea of the rebuild is to separate the LED power circuits from the controller(s). The controller will simply send commands with I2C to an I2C compatible PWM chip, which will control the LED drivers. It's not necessary, but it the idea is to simplify things enough to make easy use of different controllers such as Raspberry Pi touchscreen or Arduino with keypad and basic display as I'm doing it now. It will also allow differing head configurations with a range of LED options. Using an inexpensive driver such as the Meanwell LDD series will give a lot of flexibility in LED wiring and power levels.