Color enlarger using laser - Additive method

[koraks]

what's the point of teasing someone along with how something analogous might, or more likely, might not work?

Nobody's teasing anyone. Several people have explicitly stated that lasers are a dead-end street for this particular application.
The talk about digital exposure systems for color printing has zero relevance for the construction of an enlarger light source.

 

[DREW WILEY]

It all does, even laser scalpels as an illustration. Why go to all the tech and trouble to make an intense linear point light source, and then go through all the trouble just to undo that, and revert back to a diffuse source? Why do you take issue with examples which more easily spell out the lack of logic,
which might be not so apparent to someone who has probably never designed an enlarger before?

Lasers do have imaging applications; but they work on a different premise than optical enlarging. And there is also a significant practical distinction between simultaneous and sequential additive printing; so I cited a few examples of that too.

 

[Milpool]

You seem to be very into “additive” printing. However unless you are using R, G and B emitters it’s all subtractive anyway whether you see R, G and B on the baseboard or use a CMY filter pack. The paper only cares about the wavelengths reaching it.

It all does, even laser scalpels as an illustration. Why go to all the tech and trouble to make an intense linear point light source, and then go through all the trouble just to undo that, and revert back to a diffuse source? Why do you take issue with examples which more easily spell out the lack of logic,
which might be not so apparent to someone who has probably never designed an enlarger before?

Lasers do have imaging applications; but they work on a different premise than optical enlarging. And there is also a significant practical distinction between simultaneous and sequential additive printing; so I cited a few examples of that too.

 

[DREW WILEY]

There's potentially quite a difference. Additive can be designed more precisely targeted. With subtractive CMY, there can be some white light spillover, potentially contaminating the other paper layers a noticeable amount. That is particularly the case with older subtractive colorheads, or those where some of their dichroic coatings have spalled off due to age or repetitive overheating, allowing more unfiltered, undifferentiated white light through.

And if you open up a subtractive head, you'll see how the amount of filtration of each respective color is dependent upon how much light it intercepts before going into the mixing chamber. That is not the case with additive, where you have separate lamps, each entirely covered with their own respective filter (or in my case, their own 2-filter sandwiched sets, trimming off unwanted nm band width both directions, fore and aft).

Therefore, additive versus subtractive filtration transpires much differently; and how you control the respective output of each color channel differs too. Simultaneous additive systems are harder to design.
Recent LED array innovations potentially simplify the problem.

My additive heads were built during the Cibachome era, with that medium in mind. If you looked at side by side images, additive versus subtractive, the difference is quite obvious in the much cleaner hues and saturation of the additive version. Of course, that is also an esthetic call dependent on which specific image is in play; so I kept both styles of colorheads in use. Ciba also shifted its color balance from greenish fresh to magenta-weighted over, say, six months after opening a fresh box. It was an idiosyncratic but beautiful medium in several respects. Additive printing made it more predictable to control.

Current Fuji RA4 chromogenic papers respond much differently. They have very steeply distinguished spectral sensitivity peaks (although a degree crossover muddiness remains characteristic of most color neg films themselves). Given something like a master color interneg of an appropriately masked original chrome positive, it's feasible to get outstanding color repro, in which case the result from a modern subtractive CMY colorhead is nearly as cleanly saturated as the additive RGB version. (Later Durst and Omega CMY colorheads filtered light better than their earlier equivalents.) There will still be some difference in contrast, with additive having more.

And now, with a highly saturated color neg film itself, namely, Ektar 100, there are times additive printing can be over the top - just too saturated and contrasty - depending on the subject. So now I use additive printing routinely for smaller negatives - 35mm to 6X9, and generally 4x5 too, since the degree of enlargement inherently reduces the contrast somewhat. But on the other hand, now I do most of my 8x10 film RA4 color printing using the conventional Durst CMY 10X10 colorhead instead. That is not as precisely repeatable as my custom 8x10 additive unit; but it doesn't throw the occasional electronics temper tantrums either. That big additive machine has six lamp circuits along with feedback, making it a lot more sensitive to EMI than my 5x7 3-lamp additive system. I won't go into the technical details here. But on the positive side, it runs much cooler than commercial colorheads of comparable wattage. I designed it that way.

The other significant advantage of true additive involves making precise color separation negatives and matching masks, as well as for exceptionally high quality color duplicate chromes, as well as precise internegatives - exceeding the heyday of commercial quality. But that is getting too expensive to do much longer, due to the skyrocketing price of sheet film itself, especially the 8x10 Portra 160 needed for the internegative step. I still sometimes make 4x5 internegs from my stash of old chromes; but for some time now, all my color shooting itself has involved Ektar color neg film of various formats, including 8x10, which can be printed straight onto RA4 print media.

 

[Milpool]

I’m just saying everything you are describing is subtractive. It’s all filtering of white light down to an appropriate mix of the wavelengths the emulsion components are sensitive to, using RGB or CMY filtration.

 

[DREW WILEY]

NO - It is not subtractive. Subtractive means removing something from white to comprise something OPPOSITE across the color wheel. But Additive means just that - you add relatively pure primary red, green, and blue light as needed. We're talking basic color and optical theory here, with its own specific terminology. How those primary illuminants are constituted depends the specific light engineering approach. But once it is converted into actual RGB light, in their separate respective roles, these behave as separate primaries, not as CMY secondaries.

Color paper emulsions are sensitized to red, green, and blue light, NOT to yellow, magenta, and cyan light. What are visually PRODUCED or realized in the paper are yellow, magenta, and cyan dyes. Therefore, purer actual RGB illuminants will produce, at least in theory, cleaner better targeted color in a print than the ordinary subtractive approach.

With ordinary subtractive colorheads, by increasing the yellow portion of white light, you are truncating the blue; by increasing the magenta, you are truncating and removing green; increasing blue decreases the yellow. Whole different ballgame than additive printing, which does not truncate any opposites in white light to gain their effect, with a certain amount of unfiltered white light left over. How well the two different approaches correspond in the end result all depends. I gave a few examples on my preceding post.

 

[mshchem]

All I know is I'm using what I have, colorhead wise. With Fuji who knows what could happen with distribution of paper. I'm not saying RA4 paper goes away, just that it may be sold in increasingly larger volumes.

Some of the big folks use huge wide rolls to print everything from billfold to poster size on the same machine simultaneously.

 

[Milpool]

Perhaps we are talking past each other in our uses of the words additive / subtractive. Since they are models for color viewed by reflection vs emission / projection they aren’t all that applicable to the exposure of photo paper. By “subtractive” I meant only that the light source emits white light, from which wavelengths are attenuated.

My point is what matters are the sensitivities of the emulsion components and the incident wavelengths. The paper is sensitive predominantly to R, G, B. The enlarger light source emits a full spectrum (ie “white” light). Filters are used between the white light and the paper to control the proportions of R, G and B passing through. This can be done with R, G and B filters or C, M and Y filters, so all I’m saying is RGB filtering isn’t more “additive” than CMY filtering. If you want to pass red wavelengths you can use a R filter or M+Y (ie additive secondaries adjacent to R).

Maybe you are referring to RGB filtration as additive and CMY filtration as subtractive because they are the primaries on the additive and subtractive color wheels respectively. I don’t know, but we’re off topic anyway.

NO - It is not subtractive. Subtractive means removing something from white to comprise something OPPOSITE across the color wheel. But Additive means just that - you add relatively pure primary red, green, and blue light as needed. We're talking basic color and optical theory here, with its own specific terminology. How those primary illuminants are constituted depends the specific light engineering approach. But once it is converted into actual RGB light, in their separate respective roles, these behave as separate primaries, not as CMY secondaries.

Color paper emulsions are sensitized to red, green, and blue light, NOT to yellow, magenta, and cyan light. What are visually PRODUCED or realized in the paper are yellow, magenta, and cyan dyes. Therefore, purer actual RGB illuminants will produce, at least in theory, cleaner better targeted color in a print than the ordinary subtractive approach.

With ordinary subtractive colorheads, by increasing the yellow portion of white light, you are truncating the blue; by increasing the magenta, you are truncating and removing green; increasing blue decreases the yellow. Whole different ballgame than additive printing, which does not truncate any opposites in white light to gain their effect, with a certain amount of unfiltered white light left over. How well the two different approaches correspond in the end result all depends. I gave a few examples on my preceding post.

 

[DREW WILEY]

You basically misunderstand both the standard vocabulary and specific distinct methodology, Milpool.
And it's not really off topic. In terms of spectral effect, my RGB colorheads behave much like RBG lasers. The respective target bandwidths are quite narrow. Additive filtration is quite dense in comparison to subtractive filtration, and needs quite a bit more light. It's a lot more selective, providing purer light, without residual white light. That results in purer hues in the print.

But like I hinted earlier, it's not a make or break difference anymore, since Fuji papers have been tweaked with a more defined response. From an engineering standpoint, however, it's important to stick with the standardized vocabulary.

Dichroic filtration operates a little differently; but for illustrative purposes, just grab the most yellow contrast filter you have and hold it up to a scene, and quite a bit of other colors will be visible through it. It's hard to find a totally magenta filter either, or a cyan one which only lets cyan through. But if you take true color separation filters like a 61 green, 29 red, and 47B blue, almost nothing else gets through them.

Well, in effect, my RGB dichroic colorhead filtration works like that. Dialing in full M and Y filtration on a conventional subtractive enlarger does not and cannot achieve the same full effect. (However, the internal feedback loop on my additive machine allows me to use a control panel programmed with intuitive CMY cc settings. But if I punch in full 200 cc C and M each, that is not what I get, but pure blue light itself. A conventional CMY colorhead cannot do that, but only partially simulates it, without the same degree of purity).

 

[mshchem]

Drew, you speak with due authority on this matter. Additive printing is more precise. I have the Beseler Universal heads, three lamps, red, green and blue. These heads have control panels that "speak to the operator" in CMY after 50 years it's easier to understand.

My completely uninformed question is how powerful would these lasers need to be and what sort of diffuser could handle this???

It just seems backwards to try and make a laser diffuse and uniform, to cover something like a medium or large format negative.

 

[Milpool]

Whether RGB exposure or CMY exposure produces better results would have to do with the relationships between the light source spectral output, filter curves and spectral sensitivity of the paper. The physical principle, however, is the same in both cases. If you choose to call RGB filters additive, I’m afraid CMY filters are just as additive. I think you’re just confusing this with additive/subtractive color wheels and/or color mixing.

Calling a system additive sort of makes more sense for RGB LEDs (or RGB lasers) where these are individually controlled since in theory you don’t need to filter.

 

[koraks]

Whether RGB exposure or CMY exposure produces better results would have to do with the relationships between the light source spectral output, filter curves and spectral sensitivity of the paper. The physical principle, however, is the same in both cases.

This is correct.

If you choose to call RGB filters additive, I’m afraid CMY filters are just as additive.

I understand what you mean, but Drew is right w.r.t. the standard vocabulary. At some point, "additive" and "subtractive" made their way into this particular vocabulary and while they are technically speaking confusing in the sense that they confound characteristics of the light source with the process of exposing a light-sensitive material, it's the terminology we're kind of stuck with. The background to this somewhat odd 'additive' construct can relate to two concepts:
1: Exposure systems with three discrete light sources whose color is mixed (added up) to make the exposing light. A practical example is the Philips PCS2000 enlarger, which in fact used 3 bulbs.
2: Exposure methods in which the exposure is built from three consecutive exposures, one after the other, each with a distinct wavelength distribution.
It's not entirely clear (and practically, it doesn't really matter either) which of these gave rise to the additive/subtractive printing nomenclature we use today. In common parlance, we have either way ended up with the notion that 'additive exposure' refers to the use of narrow-band red, green and blue light, while 'subtractive exposure' refers to the use of a single white light source that's filtered with M, Y and optionally C filters. Again, is it technically entirely correct or conceptually clear? No. In that sense you have a point.

Calling a system additive sort of makes more sense for RGB LEDs (or RGB lasers) where these are individually controlled since in theory you don’t need to filter.

Yes, I understand what you mean and that's technically more correct. It's also not just in theory either. You really don't need to filter with (the right) LEDs or lasers. It's true additive in the most strict sense of the word - although not necessarily in a temporal sense (see #2 above).

My completely uninformed question is how powerful would these lasers need to be and what sort of diffuser could handle this???

The required power is ultimately a question of what kind of printing times you want on what kind of surface area of the paper, keeping the sensitivity of the paper in mind (this is more or less constant for available papers in the West). Diffusion need not be very complex as I've indicated earlier, but it'll come at the cost of light output since a considerable amount of light is lost in a diffusor.
In practice, if you're talking about a solid state (semiconductor) laser, you'd be looking at power levels of well over 10W per color channel to get exposures similar to a system like a 35mm Durst M305. I do NOT recommend anyone to experiment with lasers at such power levels; it's a major hazard to one's vision as it takes just a fraction of a second of unfortunate stray exposure via a mirrored surface etc. to cause permanent eye damage. It's (almost literally) playing with fire!

Additive printing is more precise.

Not by definition and in practice it really depends on the wavelength distribution of the color channels, as @Milpool points out.

 

[cowanw]

"When I use a word, it means just what I choose it to mean—neither more nor less"
Humpty Dumpty

 

[Milpool]

Thanks - I wasn’t aware of the standard / convention in darkroom-speak.

This is correct.


I understand what you mean, but Drew is right w.r.t. the standard vocabulary. At some point, "additive" and "subtractive" made their way into this particular vocabulary and while they are technically speaking confusing in the sense that they confound characteristics of the light source with the process of exposing a light-sensitive material, it's the terminology we're kind of stuck with. The background to this somewhat odd 'additive' construct can relate to two concepts:
1: Exposure systems with three discrete light sources whose color is mixed (added up) to make the exposing light. A practical example is the Philips PCS2000 enlarger, which in fact used 3 bulbs.
2: Exposure methods in which the exposure is built from three consecutive exposures, one after the other, each with a distinct wavelength distribution.
It's not entirely clear (and practically, it doesn't really matter either) which of these gave rise to the additive/subtractive printing nomenclature we use today. In common parlance, we have either way ended up with the notion that 'additive exposure' refers to the use of narrow-band red, green and blue light, while 'subtractive exposure' refers to the use of a single white light source that's filtered with M, Y and optionally C filters. Again, is it technically entirely correct or conceptually clear? No. In that sense you have a point.


Yes, I understand what you mean and that's technically more correct. It's also not just in theory either. You really don't need to filter with (the right) LEDs or lasers. It's true additive in the most strict sense of the word - although not necessarily in a temporal sense (see #2 above).


The required power is ultimately a question of what kind of printing times you want on what kind of surface area of the paper, keeping the sensitivity of the paper in mind (this is more or less constant for available papers in the West). Diffusion need not be very complex as I've indicated earlier, but it'll come at the cost of light output since a considerable amount of light is lost in a diffusor.
In practice, if you're talking about a solid state (semiconductor) laser, you'd be looking at power levels of well over 10W per color channel to get exposures similar to a system like a 35mm Durst M305. I do NOT recommend anyone to experiment with lasers at such power levels; it's a major hazard to one's vision as it takes just a fraction of a second of unfortunate stray exposure via a mirrored surface etc. to cause permanent eye damage. It's (almost literally) playing with fire!


Not by definition and in practice it really depends on the wavelength distribution of the color channels, as @Milpool points out.

 

[DREW WILEY]

It isn't just "darkroom speak". It's baked into both engineering and the human physiology of vision - we ourselves see in color additive mode with RGB receptors. Bees and moths have to develop their own version of all this if they want to reproduce how they see color in pictures on their own walls and computer screens; that's up to them. But the entire human industry of what you're looking at on your computer screens right now uses the same underlying terminology and the technology related to it. So does every artist who know mixing paint theory. It's basic to understanding how color works.

Yet from a practical standpoint, the manufacturers of pre-coated color film and color paper have made life a lot easier by designing these products with peak sensitivities to red, green, and blue light. After that, it's up to us to employ enlarging light sources which reasonably approximate this themselves. I already gave an example of how much true narrow band additive printing improved Cibachrome results, but also stated that it's not such a big deal anymore with current Fuji RA4 papers.

But if you're going to do any "assembly" color printing process like dye transfer or tricolor carbon, or even make in-camera color separations, you will need to religiously comprehend the real distinction between additive versus subtractive light. The entire offset printing industry would have gone broke at its inception 150 years ago if it didn't understand that. We're talking Color Repro 101 here, not anything esoteric.