In the days of Rembrandt, paints were colored with mineral pigments. These were made from certain chemical compounds. For example, titanium oxides give a very neutral white, various cadmium compounds give yellows, and cobalt compounds give blue. Also, some colorants were derived from biologi- cal sources. Some blues and purples were derived from extracts of the bodies of snails. Some reds are from berries. The list is long and many of the actual recipes were lost long ago. (For example, the blue dye used in the Jewish prayer shawl is supposed to be made according to a formula based on a cer- tain sea creature. However, for the past 1000 years or so, no one has known just which sea creature that is supposed to be.)
Dyes
A dye is an agglomeration of a large number of special molecules in a trans- parent material. Typically those used in making images have a benzene ring with a tail attached at one (or more) of the six apex points. The make-up of the tail generally dictates which wavelength(s) of light the molecule will absorb. These are shown schematically in the upper part of Figure 8.3 . In the case of photographic fi lms and papers, the medium is clear gelatin. In dishwashing liquids, it is the water in the liquid detergent. In some fi lters, it is a plastic. And in stained glass, it is the glass itself.
The color of a suspension of dye molecules is determined by the wave- lengths that are absorbed. If a dye absorbs blue light, it will appear yellow. That means it is absorbing blue and letting red and green pass unaffected. If the dye absorbs green light, it will appear magenta. It is blocking green light and letting blue and red light pass. A blue fi lter blocks yellow (that is, red and green) and lets blue light pass. The color of the fi lter is the color of the light that it lets pass and the opposite of the color it absorbs.
The suspension is very close to being a solution. In any event, the dye molecules have no discernable, external shape. The light-absorbing material appears simply to be dispersed in the medium. Since it has no external shape, it has no front surface and, therefore, no front surface refl ection occurs when light approaches the dye molecule. It simply absorbs some of the photons at certain wavelengths and does not affect any other photons. The suspension appears colored but otherwise clear in that the light that does go through, goes straight through. We can clearly see right through the suspension at
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the nonabsorbed wavelengths. The only front surface refl ection effects occur when the incoming light reaches the boundary of the total suspension. No such effects occur when the light reaches the dye molecule.
Dyes can be mixed together or applied in layers or dye can be super- imposed. One can put two different dye molecules into the same suspen- sion and there is no interaction. They each act as if they were there alone. Put a cyan and a yellow dye in the same gelatin layer and it will appear green. That is to say, red and blue light is blocked and green is allowed to pass.
If the dye is dispersed in a plastic and coated on a clear base, we can make a dye-based fi lter. If a cyan dye is put into one fi lter and a yellow dye is put into another, the two fi lters can be put into the same beam of light and the effects will combine just as if they were in the same layer. The cyan fi lter will pass green and blue light and the yellow fi lter will pass red and green light. Together they will pass only green and block red and blue. The result is essentially a green fi lter. To render grass in a photographic paper print, it is accomplished with dyes in the cyan and yellow layers, leaving the green to come through. The same is true with a dye sublimation printer.
The important factor is that dyes do not scatter light. They appear clear and absorb only some of the incoming light at the selected wavelengths. Accordingly they can be mixed together and superimposed.
FIGURE 8.3 Dyes vs. Pigments. Dyes are shown in the top image dispersed in a clear fi lm. The dye does not cause scattering of incoming light. The lower image shows pigment particles on a surface. There will be scattering off the various surfaces of the particles, causing the color to be diluted with white light and preventing the placement of a different layer of pigment under the top layer.
One downside to dyes is that they fade over time. The effects of light, heat, and humidity accelerate the fading. Special chemistry is employed to make them more stable, but as it says on the warning labels, “ in time, all dyes will fade. ”
Inks
The walls of the ancient Egyptian burial chambers still show vibrant col- ors, but the colors in the photos of you as a young child are not what they used to be — especially if the picture was hanging on a wall in sunlight. The ancients used pigments, or inks, whereas your photos are made with dyes. Inks are less susceptible to fading.
To make an ink, the dye is attached to a more stable, solid molecule. Take the dye used to make Easter eggs. It starts out as a dye, but the eggs — with their carbonate shells — are treated with vinegar (acid) and then the dye is applied. The dye attaches to the partially dissolved carbonate material (a mordent process) to produce a new combination that is more stable than the dye alone. Inks are made by attaching dye molecules to more stable materi- als in order to make the colorants more stable. Ink particles are shown in the lower part of Figure 8.1 . This has certain side effects, however. The colorants are no longer as clear as the original dyes.
Consider the reverse process. In a hobby shop, you can fi nd paints to be used on pottery. The label has to show what color the material will come out to be after it is fi red in a kiln. Looking at the slurry in the bottle will give only a vague idea of what the ultimate color will be. The material appears chalky, grayish, and dense. But after the material is melted in the kiln and a layer of the colorant is dispersed in the glaze, it will be much more vibrant. If we were to take the fi lter discussed earlier (dye in plastic) and grind up the plastic, it would lose its vibrant color and start to approach a faintly col- ored grayish powder of the potter ’s paint. If the powder were to be melted down and allowed to cool into a smooth layer again, the vibrant color would return. The point is that inks are bound in rough, randomly shaped par- ticles like sugar crystals, and they scatter light of all wavelengths before that light gets to the imbedded dyes. Inks will absorb some of the photons of the selected color of those landing on them, but they will also scatter light of all wavelengths. Some of the scattered light will go back to the observer. Very little light will actually penetrate the layer of ink (as was the case in a dye layer).
Because of the scattering, it is not feasible to put one ink on top of another as was done with the dyes. If a yellow ink were deposited on top of a cyan ink, the resulting deposit will essentially look yellow and only faintly green. This is because the yellow layer will absorb blue and scatter all colors. The scattering prevents much light from penetrating down to the cyan layer. It also prevents any light that had been modulated by the cyan layer from coming back through the yellow layer (it scatters on the way back as well). To make a green spot using inks, one would put down
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a very small spot of yellow ink and right next to it, a very small spot of cyan ink. One spot will provide blue absorption and the other will, in very close proximity, provide red absorption. If the result is too fi ne for the eye to resolve, the effects will be visually perceived as green. The scat- tering nature of inks also makes them marginal for transparencies. The projected images appear blurry. This pointillism effect is well known to magazine printers, certain impressionist painters, and computer screen manufacturers.
Remember that inks comprise small particles of material with imbedded dyes. Front surface refl ection of light landing on the particles from any light that enters the patch is what causes the scatter. It is much harder to get vibrant colors from inks than from dyes.