Jon Hilty
Introduction
Inferential Photography, aka "Lippmann Plates", is a form of color photography devised and pioneered by Gabriel Lippmann in the late 19th century. Rather than a 3-color model such as RGB (red, green and blue) or CMY (cyan, magenta, yellow), it is able to record and reproduce the full visible spectrum. While there were a few other processes that use similar approaches, such as Edmond Becquerel's color daguerreotypes, they all fell short of their goal for various reasons.
I will quote from page 3 of "The True Colour of Photography" (Bjelkhagen, Green), as the authors are far more capable of explaining how they work succinctly:
"The process exploits the formation of standing light waves, by means of a mirror, and records these in a
single-layer, panchromatic, ultra-fine grain, but black-and-white photographic emulsion. The developed
plate consists of an array of volume gratings containing a coding of the actual spectral distribution of the
light for each discrete region of the optical image. When illuminated by white light, the gratings select
and reflect the original spectra registered in the image, and these construct, by Bragg diffraction, the
colours of the original scene."
In a massively reductionist sense, Lippmann plates are the answer to the question, "What if you could make a black and white plate with such an insanely high resolution, that it could take a picture of the light waves themselves"?
When we cook up a Lippmann emulsion, we need to keep the silver halide grains as small as possible - both to keep the emulsion almost perfectly transparent, but also to keep the grains small enough that they can resolve the standing waves of light from the scene.
If a ray of light is allowed to pass through the emulsion, hit a reflector, and pass back out of the emulsion, it will interfere with itself as it travels through. Due to this interference, there are areas within the emulsion that receive a lot of energy, and there are areas that receive very little, due to the action of the wave cancelling itself out. The areas with the higher energy expose the silver halide grains in the vicinity. When the plate is developed, those exposed grains are reduced to little metallic silver "mirrors". Later, when the plate is illuminated with diffuse white light, the photons in that light that correspond to the patterns formed in the emulsion will be reflected right back at you, with the rest being scattered in different directions.
Because of this, the colors of a Lippmann plate can only be viewed at a specific angle - otherwise the appears to be an odd sort of negative.
Lippmann plates are also susceptible to changes in humidity - the gelatin will swell slightly in more humid environments, causing the colors to "redshift". The gelatin expanding causes those little fringe patterns to spread out a bit, and they will begin to reflect light corresponding to longer wavelengths than were recorded in the original exposure. Particularly excessive swelling will even cause reds to turn "black" - they have shifted to reflecting infrared light instead! The colors restore when the plate is allowed to dry out again.
Furthermore, viewing the plate at a wider angle will cause the colors to blueshift.
Typically, a prism is mounted to the plate with Canada balsam - this helps to protect the delicate surface and stabilize it from humidity-based color shifts. The prism also serves an important second function as well - it separates the surface reflection from the "metallic reflection", massively improving color saturation and contrast.
The Reflector
As stated previously, for standing waves to form within the emulsion, there needs to be some sort of reflector present. Any old mirror won't work here though - the mirror needs to be in more or less perfect optical contact with the emulsion. This really limits our options here, as the mirror needs to be removed at some point for the colors to be visible.
Mercury
Mercury was the choice back in Gabriel Lippmann's day. Using a special camera back, the mercury could be flowed in right before the exposure was performed, and flowed back out after the exposure was complete. Negative heath effects of mercury aside, the mercury could also wreak havoc on the emulsion, causing fog or desensitization of the plate. Though there are a few modern examples, most modern Lippmann photographers have chosen to forgo mercury in favor of other options. It must be said though, that images using mercury as a reflector are bright and saturated, reds in particular. The use of mercury as a reflector allows the user to omit any post-swelling baths during processing.
Mercury could conceivably be replaced with gallium or a gallium-based alloy like galinstan, though I have had issues being able to remove the gallium oxide that stubbornly sticks to everything. You can watch more about that here if you are interested.
Air-gelatin reflection
The vast majority of modern Lippmann photographers have opted to make images using absolutely nothing at all as a reflector - that is, except an air gap behind the plate. This is because the Fresnel reflection that occurs at the air-gelatin boundary reflects 4% of light back into the plate - just enough to form a fringe pattern.
The vast majority of my work up until quite recently has revolved around using the air-gelatin method. Very bright and saturated images can certainly be obtained, but I always found it quite difficult to consistently get good results with it. Though this is not always the case, I found myself often trying to fight away overwhelming cool tones that the image takes on, and I have yet to really determine what exactly is causing them.
Due to processing-induced shrinkage, air-gelatin Lippmann plates require a swelling bath after development, otherwise the image will blueshift considerably.
Mica
This is a relatively new development in Lippmann photography, and no doubt there will be a ton of stuff I get wrong as I learn the ins and outs of this new variant. "Silver" reflective mica powder can be stuck to the back of the plate, and behave like a diffuse mirror. Though I had my doubts before trying it myself, the mica undoubtedly produces fringes that are quite similar to mercury - very strong reds, and no requirement of post-swelling the plate as the air-gelatin plates need. Though my implementation now is far from perfect, it has been considerably easier to record bright, saturated, and accurate colors with it present. The recipe in this guide will be largely focused around this mica variant of the process.
Some brief notes on plate size
Though the plate sizes were somewhat limited back in Lippmann's day by how much mercury you were willing to carry around, the air-gelatin and mica methods allow for the user to create plates as big or as small as you want, so long as you have a camera that can accommodate. It might be surprising to some that I tend to shoot these in a fairly small format - 6cm x 6cm. In the past I have shot larger ones, 4x5 and occasionally 5x7, though it was quite difficult to see the entire scene all at once without bobbing your head around a bit. Nick Brandreth showed me that this process can look very good in medium format, especially when a dedicated viewer is used, and I have stuck with it since then. I do plan on scaling up in size in the near future, but probably only up to about quarter plate at the biggest. I wouldn't insist that the user necessarily follow directly in my footsteps on this point, and I would encourage you to experiment a bit and find a size that suits your tastes.