Here's a little something to wet your intellectual whistle for the end of the week.

US Patent 4,242,428 by Chester Davis, Additive System of Color Photography Based on Iridescent Pigments.

Don't ask me how I came across this, but suffice it to say I found myself reading it, and was quickly intrigued by this interesting process.

Lemme see if I can describe it in a nutshell...

Ok, so we're all familiar with "nacreous" pigments, a.k.a. iridescent pigments, pearlescent pigments, etc. These are thin wafers of mica (typically) and coated with a very thin and precise layer of Titanium Dioxide (again, usually). Other compounds can be used, but the basic idea is the same. Light goes through the thin coating and is refracted into an array of colors that change with our angle of view [see here]. You can get pigments which tend more towards a certain color, determined by the thickness of the coating.

Now, there's something else going on here . A certain amount of light makes it through the thin coating unscathed, then goes through the mica (which is relatively transparent) and is reflected off the inside surface of the thin TiO2 coating, where it's sent back through the mica, to our eye. This secondary internal reflection is necessarily complementary in color to the initial reflection and serves to desaturate the color from these types of pigments. Thus we get the familiar whitish-blue, whitish-red, whitish-green, appearance that is typical of these kind of pigments. They are not intense and deeply saturated... ...yet.

What the inventor discovered is that by applying a neutral dye to these pigments, the initial reflection from the TiO2 coating is preserved, but the secondary internal reflection is diminished by the absorbing nature of a black dye, thereby eliminating the addition of complementary light and creating an intense and vivid color from the initial reflection alone.

The real kicker here is that a black or neutral dye can be substituted for, you guessed it, silver halides! The finely divided silver serves as a filter to strengthen the inherent interference-reinforced reflection color of the pigment. Now we have a means to make this intense and vivid color appear in proportion to a light exposure; the necessary requirement for forming a photograph.

So this process definitely takes a little bit of imagination, but take for granted that applying a black dye or (metallic silver) to these pigments will give an intense vivid color, which happens to be incredibly light fast since it's completely inorganic and based on interference. Also, the amount of silver needed to create an intense color is claimed to be exceptionally low compared to normal b&w imaging, yet the chemicals used are no different.

Imagine if you will an emulsion of a silver-salt, and within it a dispersion of these iridescent pigments. The coating might have a pearlescent quality, but it would generally be quite white, if not invisible (the author mentions that its refractive index in gelatin makes it dissappear). However, upon exposure and development, vivid colors are formed image-wise as the developed silver takes its effect on the "micaceous" pigments.

I encourage you to read the patent, and use your noodle to imagine the applications of this, particularly to color photography. The author does mention its parallels to Lippmann photography, though it's not entirely the same. But potentially, you could create a positive color image direct out of a camera, with nothing more than b&w photochemicals. Heck, make it a monobath, shoot it in your 8x10", and we're talking about the ability to pull a completely permanent color photograph out of a tank in less than a couple minutes.