The Diffraction Process of Colour Photography - Prof. R.W. Wood, 1900

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• 01-14-2011, 01:09 PM
holmburgers
The Diffraction Process of Colour Photography - Prof. R.W. Wood, 1900
The iridescence found in soap bubbles, crow's feathers, CD's, oil slicks, butterfly wings and countless other places is created by diffraction.

Diffraction is nature's digital; in much the same way that a series of 1's and 0's make up digital information, diffraction relies on a sort of ON and OFF system to create any colour of the spectrum.

In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings.

If said "small obstacles" & "small openings" are arranged in a constant & repeating manner, only certain waves will "make it out alive" upon passing through a diffraction grating, thus we will see a specific, predominating colour.

This of course is the modus operandi of the Lippmann color photograph. But there is another method in which to produce colour photographs by diffraction, invented by Professor R.W. Wood of the University of Wisconsin. The most complete account of this procedure can be found in A Handbook of Photography in Colours by Thomas Bolas, Alexander A. K. Tallent, Edgar Senior; published in 1900 by Marion & Co.

It begins on page 290; luckily it is on Google books -> http://books.google.com/books?id=1J0...page&q&f=false

In short; you have 3 ruled line screens, or gratings printed on transparent film. The gratings have lines that are 2000, 2400 and 2750 to the inch. These are the "masters" to create the diffraction gratings.

The novel part of the process is in these screens, which will be printed onto dichromated gelatin (DCG). With these fine lines impressed into the gelatin, a diffraction grating is created; meaning that light will go thru it, scatter, and depending on the viewing angle you will see different colours. Each of the 3 screens is capable of creating all colours, but not from the same angle of view. By distributing the lines differently in each screen, red, green & blue are created simultaneously from each respective screen if viewed from a specific angle.

As usual with 3 color work, you have 3 separation negatives of the subject, taken thru the standard separation filters. The negatives must be made into positives however, and each one is then placed in contact with the appropriate line screen, red w/ 2000 lpi, green w/ 2400 lpi and blue w/ 2750 lpi. These are then contact printed onto unpigmented dichromated gelatin.

Just as in the gum or carbon process, the DCG is then developed by etching with hot water to dissolve the unhardened gel. You are now left with 3 clear images. Due to the extremely fine lines courtesy of the line screens, these DCG plates are now diffraction gratings with diffraction "zones" relating to the separation positives which they were printed in tandem with.

Imagine a b&w separation positive of a red apple on a black background. The apple will be very transparent, surrounded by heavy density. Therefore, the clear portion will let thru the line screen and a diffraction grating corresponding to red will be made in relation to the apple.

This is the basis of the process. The only trick being that it requires a simple viewing apparatus; though I think that it basically just limits your viewing angle to a small degree, which could be achieved with a steady hand, or at most a piece of carboard with a hole in it.

The result is pure, permanent colour photographs!

I've attached a Word DOC with all figures and text from the book. I did the OCR from the Google books version.

Enjoy!
• 01-14-2011, 01:37 PM
holmburgers
edit: technically speaking, oil & soap would be examples of thin-film interference, not diffraction. Forgive me...
• 02-25-2011, 10:55 AM
holmburgers
So, I'd just like to reiterate how (relatively) easy this process would be for someone who already does carbon or other dichromated colloid processes. The result is a color picture that relies on diffraction, and thus cannot fade.

Quote, "The colours are extremely brilliant, and there is a peculiar fascination in the pictures, since, if the viewing apparatus be slowly turned so that its direction with reference to the light varies, the colours change in a most delightful manner, giving us, for example, green roses with red leaves, or blue roses with purple leaves, a feature which should appeal to the impressionists."
• 03-09-2011, 02:22 PM
DarkroomExperimente
isn't this how the polachrome film worked? ...or was that something different
• 03-09-2011, 02:30 PM
holmburgers
No, this system was definitely never used commercially. Very much a "lab curiousity". For one thing, the viewing angle has to be very specific for correct color, hence the need for a specialized viewer. All the viewer does is confine the angle of view however.

I believe Polachrome worked off the screen-plate principle.
• 03-09-2011, 04:51 PM
Joe VanCleave
I disagree with the OP's inference that the diffraction effect is "digital" because it produces discrete, light and dark, positively and negatively reinforced interference patterns.

Assuming it were true, such a whole number unity effect would not be properly termed"digital" but rather an artifact of Boolean logic. It would be "Boolean," not "digital".

But of course, the diffraction of photons is a wave-like phenomenon, governed not by Boolean principles at all, and is most decidedly "analog" in nature, in the sense that multiple waves interfering with one another out of phase produce additive and subtractive vectors whose output space might resemble some discrete function only superficially, absent a deeper understanding of the underlying principles.

~Joe
• 03-09-2011, 05:07 PM
Lionel1972
Thanks Chris for digging out again another forgotten amazing color photography process!
• 03-09-2011, 06:34 PM
holmburgers
Quote:

Originally Posted by Joe VanCleave
I disagree with the OP's inference that the diffraction effect is "digital" because it produces discrete, light and dark, positively and negatively reinforced interference patterns...

Joe, I feel that you're failing to see my statement as an analogy. But I do agree with you that it's not a very good analogy. I think that this statement...

the diffraction of photons is a wave-like phenomenon, governed not by Boolean principles at all, and is most decidedly "analog" in nature, in the sense that multiple waves interfering with one another out of phase produce additive and subtractive vectors whose output space might resemble some discrete function only superficially

....eloquently describes the principle. That is of course, after I looked up some of the terms.

Thanks Lionel, my pleasure!
• 06-09-2011, 03:10 PM
holmburgers
Hey all.

So I'm curious about how one would make the "line gratings" that are 2000, 2400 & 2750 lines to the inch. That's beyond the scope of a lens+film or a conventional printer if I'm not mistaken.

Professor Wood's gratings were made by Cornell's Dividing Engine, which I guess is basically a machine that is used to cut divisions in instruments like rulers, sextants, the like.
• 06-09-2011, 07:26 PM
gmikol
Just as an example, 2540 lines per inch (really, line pairs), is 100 lp/mm.

This is certainly within the realm of high-resolution document/copy/litho film and lenses, so currently-available materials would be able to support the line gratings. In fact, in the text, the author states "photographic copies of these (gratings) were used for the experimental work."

As for creating gratings of the required frequency, this is more difficult. Commercially, you'd want to look for custom "Ronchi Rulings", but these are not inexpensive (\$1000 per grating for 4x4" chrome-on-glass at Edmund).

A crazy idea I've had floating around in my head for a while would be to get 600- or 900- line-pairs per inch films on a 3600 DPI imagesetter (3-dots black/3-clear or 2-black/2-clear, respectively, or maybe 2-black/3-clear to account for dot gain [720 lp/in]), which you could then make reasonable-ratio reductions (in the range of 2x to 5x, depending on what your imagesetter films are) using a 4x5 camera or enlarger onto litho film. If you're using a white light source, best to use an APO enlarging lens. Can't guarantee the quality you'd get, but I can't see any other way to get there.

--Greg
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