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 Originally Posted by Mr Bill
PE, if this is in reference to my post, it is just a misunderstanding. I spoke of no such system, I referred separately to them.
It was a reference to placing a yellow filter over a blue filter which will not work properly due to the cutoff differences of the two filters. The other combinations will not work either.
PE
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I have a copy of Kodak Tech. Pub. E-81N that describes making separation negatives for use on Matrix Film. In the instructions, it recommends adjusting your exposures so that the times are as close to equal as possible across channels. To achieve this while using a dichroic color head it recommends dialing in filtration opposite the color of the separation filter used under the lens to act as a type of ND filter if an ND filter is unavailable and the lens is to be maintained at it's optimum aperture.
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Hi PE. To be more clear, I don't know if you were in reference to my post, or not. Since you have just said "placing a yellow filter over a blue filter..." I'm guessing that you are in reference to my post #5. If so, perhaps you misread, possibly this part?
[excerpt from post #5] So the exposing method is: start with white light, essentially red+green+blue combined together. When enough of one color, say 2 seconds of blue light, is achieved, then the yellow filter paddle swings into the light path - this immediately terminates any blue light exposure, while still allowing red and green to come through
I believe that everything I have stated is correct, so if you are disputing anything, please give me the opportunity to defend it. If you were not refering to my post, then I apologize for raising the issue.
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Bill;
I was referring to it. And, the method you suggest was usable but not very successful due to the broad cut of the filter compared to the blue light. The filter did not cut off all of the blue light. That is my point. The subtractive filters are broader in bandwidth than the additiive filters and thus "leak". You need a special, high density yellow, magenta and cyan filter set to get it to work so that when superposed, all 3 = BLACK and not gray! Test it yourself and you will see.
PE
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Originally Posted by Greg Davis
I have a copy of Kodak Tech. Pub. E-81N that describes making separation negatives for use on Matrix Film. In the instructions, it recommends adjusting your exposures so that the times are as close to equal as possible across channels. To achieve this while using a dichroic color head it recommends dialing in filtration opposite the color of the separation filter used under the lens to act as a type of ND filter if an ND filter is unavailable and the lens is to be maintained at it's optimum aperture.
Again, this is possible with separation negatives where you are striving to make 3 B&W negatives or transparencies for DT or other reproduction methods. When used with color negative positive systems, this is difficult due to the speeds of the three layers.
If you take Cyan as = 0 speed, then magenta = +0.8 Log E and yellow = + 1.2 log E in speed. You have to balance this somehow. It can be done, but juggling this becomes a feat for someone who can do matrix algebra in their head! 
See my previous post.
PE
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I don't disagree, Ron, I just posted that to illustrate that adding the opposite color does indeed act as ND rather than serve to balance color for printing.
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I was more concerned that there are people out there that might misunderstand you, me and Bill.
That about sum it up.
PE
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Originally Posted by BetterSense
Suppose I make an automatic shutter mechanism to make the 3 exposure sequence through the different filters for me. Since I am good at electronics, I make myself an enlarger timer to drive my color-exposure-sequencer. I can make the electronics do whatever I want.
I would use RGB leds. It would be so much easier mechanically. However, you probably need to analyze the wavelengths a bit. If you buy the most usual, cheapest LEDs, I can say that the red is probably a bit too short and blue is a bit too long, and green is quite optimal. However there are longer reds available, some people use them for plant growing where 625 nm red is not optimum. Check for Ebay, Dealextreme and http://www.satisled.com/ for cheap leds.
This is based on the absorption maximums of color negative film dyes. They are designed to match with paper sensitivities.
But anyhow, I would GUESS that even with the little wavelength mismatch, the RGB leds will work very well, comparable to CMY filtered white light sources. I have yet to make a RGB LED color head so I don't have experience in practice, though.
I would start with 5 to 10 Watt R, G and B (something like this: http://www.satisled.com/30w-watt-rgb...olor_p288.html ; I haven't bought this so I cannot comment further!), a diffuser very close to them, a small gap (an inch maybe?), a second diffuser (or an 90 deg angle with white surface at 45 degrees, as used in some dichroic enlargers) and then either a condenser lens system or a third diffuser. Then I'd use either PWM or analog circuitry to control the color; or, as the current papers have very low HIRF, you can just use the leds full power and stop them one at a time when the time for that color is full.
With RGB LED system, you actually have CMY knobs that works like adjustable CMY filters! It's just in which way you write the labels to knobs. The red knob at full power is Cyan 0%, and red knob at zero power is Cyan 100%, and so on.
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I would use RGB leds. It would be so much easier mechanically.
I disagree, Coming up with a RGB array that is bright enough to print 4x5 would be fun, but it would he hard to design and expensive in parts and in power electronics. By contrast, I already made my 3-filter servo-drive colorwheel, which I can use with any enlarger.
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Another option (for simple light-source-independent conversion to a color enlarger) would be to get a Janpol.
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