Spectral peaks

[interneg]

swmcl & Pere: it's pretty clear that you don't understand why or how the specific bandpass filtration of RGB separation filters was intended to work in the way it does - & playing around at emulating tungsten lighting is irrelevant to the things that matter here. You need an appropriate set of RGB sources & to appropriately control them with enough precision for the purpose in hand. That's all there is to it. The difficult bit is making it work accurately & reliably enough to give the precise control necessary.

[Drew Wiley]

Nope. It might simulate a continuous spectrum, just like certain LED architectual lighting claims to do. But what fools an untrained eye won't likely fool color paper engineered for three specific spectral frequencies. Get a dozen different nm illuminants involved, you'll still have problems. You either hit the baseball or not. Getting close doesn't count. And even when you do manage a hit, if it isn't right on, the ball doesn't go where you hope it will. I should know. My grammar school when I was a little kid always flew the flag half mast when I came up to bat! Then I joined the Indian Reservation little league team, and was the only white kid on it. They put me out in left field - and I mean way out, over the next barbed wire fence.

[Pere Casals]

swmcl & Pere: it's pretty clear that you don't understand why or how the specific bandpass filtration ...

Interneg, just consider channel crosstalk in rgb led vs continuous spectrum...

[ic-racer]

Which came first, chicken or egg? With multigrade paper egg, I think it was the chicken that came first, then they designed the multigrade egg to be exposed to the dichroic chickens already in use in color labs. Most had tungsten light sources with dichroic filters of Magenta and Yellow.

I'm all for new stuff on the market. I'd like to see LED PAPER!! Sure, just a marketing hype like DIGITAL LENS, but the spectrum of the paper could be altered to match the most popular LEDs on the market, yet still be useable with conventional dichoric color heads.

In the color printing world, already Kodak has a laser paper (Royal) and paper for both laser and enlarger (Ektacolor Edge).

[interneg]

In the color printing world, already Kodak has a laser paper (Royal) and paper for both laser and enlarger (Ektacolor Edge).

The difference between them in large part relates to their reciprocity error - one designed for short exposure by laser or led (has no one noticed that the chromira & several other paper exposure devices have been successfully using an RGB led exposure setup for 20 years?) and the other for slower exposure under an enlarger or exposure device (laser, led etc). This differentiation between papers has apparently been the case for a long time - paper for minilab use was designed for high intensity short exposures & the paper for hand printing etc was designed for longer exposures - Ilford has a similar difference in BW papers - Multigrade Express being the minilab paper.

[interneg]

Interneg, just consider channel crosstalk in rgb led vs continuous spectrum...

The success of both the Chromira & the Frontier over the last 20+ years which both use RGB LED exposure systems suggest that you are probably running after an irrelevance. Both use systems to ensure the purity of the RGB output - the Chromira using trimming filtration & the Frontier a kind of digital closed loop analysis (at least that's what the patents seem to suggest).

[Pere Casals]

The success of both the Chromira & the Frontier over the last 20+ years which both use RGB LED exposure systems suggest that you are probably running after an irrelevance. Both use systems to ensure the purity of the RGB output - the Chromira using trimming filtration & the Frontier a kind of digital closed loop analysis (at least that's what the patents seem to suggest).

Digital minilabs/printers have profiles and this allows perfect control to print with 3 monochromatic colors. A digital printer may prefer saturated paper as saturation can be controlled digitally. The digital calibration allows a totally flexible job with monochromatic light sources.


Analog printing is different... while with monochromatic (or narrow band) light sources we can still balance the average tone, depending on the spectral nature of the light the tonal nuances will be different, as channel crosstalk will be different.

This is, IMHO, if adjusting illumination to have same red and green in the print it may happen that the orange in the prints can be different, depending on the crosstalk. In the digital case this can be calibrated, but with enlarger printing we have some footprint from the light spectrum.

For this reason "my guess" is that a LED color head intended to print R4-4 may benefit from Cyan and Orange additional light sources to adjust the R-G and G-B crosstalks. Monochromatic (or narrow band) sources expose less the neighbour channels.

This is what I see from the color sensitometry point of view, anyway we should see what happens in practice, of course.

What it's true (IMHO) is that we can retrofit a color head illumination with a 3400k warm white CRI 98 LED, this is a good substitution generating little heat an throwing a close spectrum, but I'm not sure if a R-G-B led bulb (with spectral valleys) would require the cyan-orange additions...

I've been exploring that because I've some treasured CDU-II 8x10, planning to enlarge 45 and MF slides to 8x10, I've never printed RA-4, but I guess it's the same issue.

[Ted Baker]

Let's see what spectral peaks we'd like to be able to switch on in our most desirable enlarger light source. If you could switch on any in particular which ones would they be ? Of course, if all switches were 'on' we'd like a white light with a flat response curve.

Perhaps an indication of which ones are the most important would be another thing to think about ...

For color negative, as opposed to colour separations from positive originals:

#70, #98, #99 but keep in mind the paper already has sensitivity that already matches these with broad spectrum light that has lots of long wavelength light (i.e. light bulb). If you use narrow spectrum unless you get it right you risk never being able to get a constant matching gamma for all three dyes in the print, the colour coupler will get in the way. Of course if you get it right that won't be a problem.

[swmcl]

Drew,

If certain phosphors were added to certain LEDs wouldn't that spread the spectrum making it more continuous ? It is going to be almost impossible to replicate a continuous-spectrum light source. I think we all know that, but, it is an issue of getting close enough. I would think that the combination of LEDs and phosphors would sort it out to a large extent.

I do realize that too much phosphor would make timing issues more difficult because they glow for a period afterwards.

Its funny how we hold onto grievances from childhood. Twas a nastier world back then for some. Nowadays everyone including the drinks-carrier and sandwich-mum get a 'participation award'.

[Drew Wiley]

A bunch of dead-end ideas here. And I can stipulate dead-end because there are already a bunch of dry bones down some of those alleys. Traditional colorhead printing takes full spectrum light, albeit toward the warm end, and subtractively picks out discrete sections of it. Additive design is quite a bit trickier, but has its precedents too. But with LED's or lasers, you have to make the individual sources correspond to those exact points. Not so easy, because you either hit or miss them - there's no true continuous spectrum to choose within. And no, I certainly wouldn't want to see a paper specifically balanced for extant LED's, because then you'd have to invent a correspondingly skewed line of films with odd dye sensitivity, and would have to do this over and over again as the technology steadily improves. And sorry to rain on your parade again too, Pere, but that CDUII you've got probably went bad twenty years ago. You can certainly use it; but expect some highlight crossover and potential loss of overall saturation. It is basically just tungsten-balanced OLD-style Astia, not the superior and fresher Astia II. Even if it's still good, you need to mask your slides to get good reproduction. I probably know how to do this better than anyone else alive, but have specialized equipment. No point now.
Regular Astia 100F is an even better duping film; you just balance your colorhead to 5000K. Both have their sweet spot between ten and about forty seconds of exposure. I used em mostly via contact; but for enlarging 4x5 film to 8x10, I use a 240 Apo Nikkor process lens (dialyte-type). Kodak EDupe was decent too, but ran into expiration issues even sooner, long ago. Nowadays I sometimes make internegs instead, for RA4 printing. Masking skills are essential for the best results - a complicated subject in its own right. Flashing was used by the commercial labs, who generally didn't do a good job with either dupes or internegs. But if the flashing is relatively circumspect, it will work in some Ektachrome instances, probably not ideally with high-contrast examples of Kodachrome or Velvia. But no harm in trying. You might end up with something interesting, even if a bit technically off. If you choose to outright shoot CDUII film instead, obviously start with an 80A blue conversion filter.