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??? I would have thought they produce teal, not black. I'm probably too naïve.
??? I would have thought they produce teal, not black. I'm probably too naïve.True additive blue and true additive green do not mix to produce yellow, but block each other and produce black.
??? I would have thought they produce teal, not black. I'm probably too naïve.
[...] so I assumed I would want to maximise the speed as much as possible. To do this, I opted for a holographic diffuser, which has somewhere around ~80% transmission, far more than the ~30% opal glass diffusers often have. with the holographic diffuser, and a negative developed for grade 3 papers, and a 16x20 print size I had to stop the lens all the way down to f/45, and even then, found the optimum exposure at f/45 was about 2 seconds.
So, far too fast/bright to be useful. [...]
I think it would really help if some of you would simply study up on basic color theory first. Or just buy a cheap set of RGB color separation filters from an outfit like Edmund, stack or screw them together, then look at a bright light through those. If they're pure enough color (like a 29 red, 61 green, and 47B blue set), you won't see a thing, just black.
Blue is the challenge due to its large overlap with green. Measuring with a ruler on the monitor, at 450 nm, the B/G ratio is 2.8. But at the blue-peak at 486 nm, the ratio is 4.0. So I think your idea of employing a blue LED at about 486 nm is promising, because for a given exposure of blue, it will expose green the least. BTW, I got that 486 nm with a ruler as well. That max ratio of 4.0 should yield the purest hue.
Cree makes a cyan LED that is very close to 486 nm. Here's the link to it in digikey. It's 1-amp, so it's "freaking bright."
We could probably make a good RA-4 enlarger using only LEDs from this one family.
Unfortunately, that Cree blue-green example looks like a poor candidate for a balanced color enlarging source. It might be nice for VC paper, however.
Can you dim your LEDs using PWM? Using a PIC18 microcontroller, with 12-bit PWM, I can dim the LEDs by up to 6 stops (1/64th intensity), while maintaining good accuracy of exposure. If you want to stay with analog electronics, a 555 chip can probably get you 3-4 stops of dimming (that was my first controller).
Anyway, please post a description and pictures of your LED-head and controller in a new thread. It's always interesting to see what folks have done.
Mark Overton
True additive blue and true additive green do not mix to produce yellow, but block each other and produce black.
For a few days, I've been thinking about the first graph in your article, which shows the sensitivity curves for R-G-B.
Blue is the challenge due to its large overlap with green. Measuring with a ruler on the monitor, at 450 nm, the B/G ratio is 2.8. But at the blue-peak at 486 nm, the ratio is 4.0. So I think your idea of employing a blue LED at about 486 nm is promising, because for a given exposure of blue, it will expose green the least. BTW, I got that 486 nm with a ruler as well. That max ratio of 4.0 should yield the purest hue.
The graph does not show the threshold (inertia?) of each color. If green has a higher threshold than blue, then an exposure at a ratio of 4.0 (486 nm) could produce a pure yellow.
Cree makes a cyan LED that is very close to 486 nm. Here's the link to it in digikey. It's 1-amp, so it's "freaking bright." This LED is a member of Cree's XE-G family, and here's the link. This family includes a 660 nm "photo red", and several greens. We could probably make a good RA-4 enlarger using only LEDs from this one family.
So far I've just been doing black and white printing, without the red channel of course (though it is useful for planning exposures without having to hide light sensitive papers). After reading this, I definitely want to try RA4 sometime, maybe my college has a processer hidden away somewhere I can use.
If anything, the opposite is my problem.
You should! And let me/us know how it pans out! In particular if you run into any issues, because that's how we can take this even further.
Yeah, I recognize that. Well, it's mostly not a problem, really, but it's certainly true that power is just not the limitation.
Concerning PWM, addressed by yourself and also @albada: PWM is evidently the way to go with LED, and my design of course also uses it for filtering. I found that 8 bit is far insufficient for color work. In the first successful design of my head (I had moved beyond RGB COB LEDs by then), I used 12 bit PWM as that was the resolution the PCA9685 I used offered. The current version runs on an ESP32 uC which will do happily 16 bit PWM at the frequency I selected, but for convenience's sake I'm running it at 12 bit.
There's a bit more to the PWM story than just bit resolution; you'll have to start looking at the pass current of the LED arrays on a 'scope to see what you're doing, and a lot will depend on the driver topology used. I doubt there's a lot to be gained to go beyond around 14 bit dimming resolution; I don't find it necessary in any case. But 8 bit was way insufficient and only OK for B&W work. @Ethan Brossard, you may have to address this aspect if you're going to do color on your machine.
For RA4, I imaginine I could do similar where I flash the paper for a specified amount of time with full brightness of each color. Would that work for gaining more control over color, or is there something I'm missing?
What RGB COB LEDs (wow thats a lot of acronyms) did you use?
If they make suitable RGB COBs
Nope, that would work! It's conceptually the same as the consecutive and separate exposures that were done sometimes. I mostly discarded that option because I liked the possibility for some burning & dodging. I rarely do this with color, and overdoing it tends to result in color issues, but a slight nudge here and there is nice. But I end up doing that...well, rarely, if every. So might as well do away with that requirement...
Generic Chinese junkyard ones. Which is to say, I got more expensive ones from a German seller on eBay which turned out to be identical apart from the heatsink backing, which was way better on the German units. But they didn't appear to be different in terms of actual chips. That's BTW always a gamble; you can get LEDs from whatever source you like, but you virtually never know what actual LED die you find inside the device, because that information is not released.
I am not aware of any RGB COB LEDs with red and blue wavelengths suitable for RA4 color. It's the route I tried first because it seemed attractive, but it turned out to be a dead end. That's the essence of my blog post: all devices with integrated R, G and B LEDs within a single device turn out to be the wrong color set for RA4 printing. I have not yet found a single exception to this rule.
But 8 bit was way insufficient and only OK for B&W work. @Ethan Brossard, you may have to address this aspect if you're going to do color on your machine.
Yes! In fact, I found 8-bit to be frustrating for B&W because it only provides about 4 stops of accurate dimming. 4 stops down is 1/16th intensity, so its PWM is about 256/16 = 16, so the step-factor of PWM of 1/16 is larger than I like. A 5th stop would have a step-size of 1/8, which is too coarse.
Good news: Arduino has one 16-bit PWM with two outputs, so you can get 12-bit PWM out of it (green and blue, red would still be 8-bit). But the Arduino library does not provide access to that, so you must write registers yourself. I can give you source-code if you're interested.
@Ethan Brossard: The optical system in a condenser-lamp creates an image of the white bulb on the paper. That's why it's important to have a bulb of the correct physical dimensions in there. But an image of a LED won't work; you'll just have a bright spot on the middle of the paper.
Because condensers are so much more efficient than diffusion heads, it might be possible to build a bright enough head with only 1 LED of each color
Arduino has one 16-bit PWM with two outputs
I’m a bit confused by what you're saying about condenser design.
By my understanding of condensers, the only two points where the lamp would produce a sharp image of the lamp itself are at the lamp itself, and at the lens focal point where the condenser is focused. By the time that light reaches the paper, isn’t it so out of focus that it’s just even illumination?
I could not locate the "stumbling in the dark" article
That was my fault; my memory basically sucks
Variable Contrast LED Head for Durst 138S – Condensers | Tripping Through The Dark
www.trippingthroughthedark.com
I've never done any work with PICs, but I see what you mean with the bare metal experience. Can be fun, indeed! My stm8s foray was like that.
PS: I still have a soft spot for the 328, even though it's been surpassed left and right by just about anything. It's just such a nice little piece of silicon.
Thank you for this great thread! I recently (at the beginning of 2022) built an 8x10 LED enlarger myself, and this would have been a great resource while I was designing it, though it seems I did about the right thing, though I didn't do nearly as much research as you did. I used 450, 525, and 660 nm LEDs as well, and have not had any issues with them.
So far I've just been doing black and white printing, without the red channel of course (though it is useful for planning exposures without having to hide light sensitive papers). After reading this, I definitely want to try RA4 sometime, maybe my college has a processer hidden away somewhere I can use.
Anyway, in response to the questions about LEDs being fast enough to match dichroic heads, in my experience that's not an issue at all. If anything, the opposite is my problem. When I designed my system, I hadn't done any tests to see how much LED power would be needed, so I assumed I would want to maximise the speed as much as possible. To do this, I opted for a holographic diffuser, which has somewhere around ~80% transmission, far more than the ~30% opal glass diffusers often have. with the holographic diffuser, and a negative developed for grade 3 papers, and a 16x20 print size I had to stop the lens all the way down to f/45, and even then, found the optimum exposure at f/45 was about 2 seconds.
So, far too fast/bright to be useful. Since then I've replaced the diffuser with a more traditional one I scavenged from a broken cold light I had lying around, but based on exposure times, I'd hazard to guess that it's still brighter than my DeVere 8x10 dichroic head.
Maybe I'll set them both up and measure the brightness with my spotmeter, I'll get back to you guys if I do that.
I also needed EEPROM, and the STM32 and ESP32 have none, so it's emulated using software and flash
red is specified as Wratten 25 with a wavelength of 610 nm, which seems implausibly low to me.
You wrote:I think it would really help if some of you would simply study up on basic color theory first. Or just buy a cheap set of RGB color separation filters from an outfit like Edmund, stack or screw them together, then look at a bright light through those. If they're pure enough color (like a 29 red, 61 green, and 47B blue set), you won't see a thing, just black.
True additive blue and true additive green do not mix to produce yellow, but block each other and produce black. I
brings up the Philips additive system: PCS150, PCS2000.
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