Colour sep: who does it?

[keithwms]

Ah, I wasn't thinking of the negative process necessarily, but in any case, whether positive or neg printing process is the goal, it seems like 3 is minimum for true white and true black!

I wonder if ciba process might be better with sep positives than normal slide films. Just because the b&w process gives such fine contrast control in development. Just a stray thought.

You know, when I was going through Jack Mitchell's stuff after he passed, I found some glorious positives on glass. I wonder what they were, how they were made. They are fantastic. 2 1/4" squares as I recall. My guess is that they date from before 1960 or so, I could probably find some date clues on his boxes.

 

[Photo Engineer]

In general, 3 color separation printing will give better color than white light printing from a color negative. CMYK printing is not needed with photographic processes, just with printing processes using halftones. The halftone process reduces dye density such that a black "overprint" with an amber filter is needed to "fill in" the missing density.

PE

 

[Tom Kershaw]

PE,

Could you clarify what you mean by a 'white light print'?

Tom

 

[Photo Engineer]

White light = a bulb in an enlarger. Any CC or CP filter may be used to correct color temperature.

Three Color = White light, no CC or CP filter and a set of sharp cutting R/G/B filters.

All of the above should use a WR2B UV filter (sometimes built into the enlargers today) and a heat absorbing (IR) filter (which can also be built into the enlarger).

PE

 

[Struan Gray]

Which makes me wonder whether r,g and b sep negatives really are the optimal three.

I wrote this essay by arguing from aesthetic principles:

http://struangray.com/twiglog/2008/12/05/trichromie/

But further digging has revealed that the orange-violet-green triplet was used by early tricolor methods. The US patent for Autochrome (here) suggests using this triplet, and I've seen it crop up as a standard filter set elsewhere too. I'm not sure if the triplet was dictated by the availability of suitably pure filters with low absorbtion in the passband, or if it was motivated by existing ideas of which colours were complementary to which on the prevailing colour wheel. Regarding CYM as the natural complement to RGB is a relatively recent phenomenon.

If you look at the CIE diagram and regard it as a coordinate space it is obvious two primaries can only mix to produce the colours on the line joining them in the CIE diagram - if you have three you can access the triangle between them. In principle, any three will do, but 'bad' choices will limit your gamut, and some colours have higher filter factors than others, which has practical implications.

These days a tunable laser or supercontinuum source will allow you to access the entire saturated edge of the CIE plot, and in theory at least produce any colour you like in a projected image. Land's retinex experiments got very close by adding the complementary colour to the primaries, effectively allowing negative values (just as cyan is negative magenta+yellow in RA4 printing). With prints though, you are limited to triangular gamuts.

 

[keithwms]

Fascinating. Thanks Struan!

One of the nutty things I am thinking about is creating a filter material with r,g,b sequence that could be overlaid on roll film, and you'd just feed it through a camera as usual, fire three shots...

 

[c6h6o3]

I am more interested in shooting panchromatic b&w film and building a colour image from that, in totally analogue fashion. Who does it?

Thanks in advance.

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