Between the lines – The importance of interlayers in RA4 color paper

‘Today I learned’…a lot, in fact! More than enough to fill a couple of blogs, but let’s start with this one. Did you know that color paper has layers that you can’t see, that are actually designed to be invisible, but that play a huge role in the visual aspects of the paper and the images on it? Well, now you do! They’re the interlayers, and they are quite essential, as it turns out. I’ll try to explain.

I’m going to rely on FUJIFILM’s papers, since they’re the prominent player globally right now, and it’s also the technology I’m the most familiar with. The principles will likely be the same for Kodak/SinoPromise’s papers though.

Let’s start with the basic structure of color photo paper. Essentially it’s a paper base with a polyethylene coating on either side. One of these PE coatings is subbed with a thin gelatin layer to which the actual emulsion adheres. The emulsion in turn is made up of seven layers (in the case if FUJIFILM’s papers), the key ones being the actual color-forming layers: a blue-sensitive layer that forms the yellow image, a green-sensitive layer that makes the magenta image, and a red-sensitive layer for the cyan image. So what are the other layers? Well, there’s two layers on top of the stack that perform functions like UV filtering and mechanical toughness (the top coat). They protect the layers underneath. But this still leaves two more layers. What’s up with them?

FUJIFILM paper structure, taken from a Crystal Archive Type CA datasheet

Turns out that these intermediate layers, or ‘interlayers’, are essential for the color performance of the paper. They act in two ways: they filter a little bit of light so that each layer ‘sees’ mostly the light it intends to see. But most importantly, these layers block the diffusion of developer-derived molecules into neighboring layers, where they could perform the ‘wrong’ colors. That’s it, in a nutshell.

But the above may benefit from a little more explanation. First of all, how is the color image made, anyway? There are three dyes in the final color image – cyan, magenta and yellow. Essentially, one half of the each dye molecule comes with the paper, as the so-called ‘dye coupler‘. These couplers are in themselves virtually invisible, and they need a second component to become an actual dye. The other half is produced by the color developer, which, as it develops silver halide, creates a byproduct that chemically links itself to the dye coupler and thus forms the visible dye.

So in a color material (film or paper), there are silver halide molecules that are light sensitive, and as such, they capture the image-forming light. When developed, these silver halide molecules make the color developer produce the second part of the color dyes, which migrate to the nearest color coupler, and link up with them to form the visible dye.

To make the separate color dye layers, the color couplers are coated onto the paper in separate layers. The bottom color layer is the blue-sensitive one, that will form the yellow dye in the final image. It contains silver halide molecules that are sensitive to blue light, and color coupler molecules that can form a yellow dye when combined with the color developer byproduct. The middle layer has silver halide molecules that are engineered to respond mostly to green light and magenta dye coupler molecules. The top color layer has silver halide that is sensitized to red light, and cyan-forming dye couplers.

Now, it turns out that the color sensitization of the light-sensitive silver halide molecules isn’t perfect. Particularly, the green-sensitive halide molecules are also somewhat sensitive to blue light, and vice versa (but to a much lesser extent). Some filtering using color dyes is apparently done in the interlayers between the color layers. This prevents light of one color activating the emulsion in another layer.

Typical color sensitivity curves for FUJIFILM Crystal Archive paper, taken from one of their datasheets. Note the overlap in sensitivity between green and blue curves.

But I’m told that this filtering effect is very limited and plays only a small role in comparison with color film, where the color filter layers are a very influential part of the package. Most of the importance of the interlayers is actually of a chemical nature.

So there are dye couplers for each color, as explained above. But…there’s only one type of developer-produced ‘second half’ molecule! Now, what happens if the developer-byproduct that emerges from the bottom yellow-forming layer finds its way to the magenta-forming layer right on top of it? Well, our developer-derived molecule will happily marry itself with the first magenta dye coupler it encounters, and form a magenta dye molecule. Even though the developer-derived part emerged from a silver halide molecule in the yellow-forming layer. In other words: the molecule that should have formed a yellow dye molecule, ends up forming a magenta molecule. Not good!

Inter-layer effects: the diamonds represent the molecules that form from the color developer as it develops exposed silver halide. These migrate a short distance until they hit upon a color coupler, and then together form a visible color dye molecule. The green arrows represent developer byproducts finding a coupler in the intended layer. The red arrows show developer byproduct molecules from one layer migrating to another layer and forming an (unintended) color dye there.

Here’s where the interlayers really come into play. In the developer bath, the gelatin emulsion on the paper is relatively permeable. After all, the developer molecules must be able to travel to the exposed silver halide molecules, the developer byproducts must be able to then travel on to the nearest dye coupler molecules, and when all that is done, any remaining silver needs to be bleached and fixed out. Hence, the gelatin emulsion must allow for migration of molecules through it. But this permeability at the same time carries the risk of a developer-derived molecule finding the ‘wrong color’ coupler.

The interlayers actually prevent the developer-derived molecules from making it to the other color layers. The interlayers do this by containing compounds the dveloper-derived molecules bind to while forming a colorless ‘dye’. I.e., chemically, it’s a dye, but since it’s invisible, it’s also harmless to the final color image. In a way, the developer-derived byproducts are trapped in these intermediate emulsion layers, thereby preventing the problem of incorrect dye formation.

What’s the net effect of all this? If you look at a CIELAB color wheel, then the problem of incorrect dye formation presents itself in the form of hues that shift to a different place than where they should be. The primary colors (i.e. the pure yellow, magenta and cyan) become less pure. While it’s possible to correct for colors shifting around a little bit, e.g. through filter adjustments in optical printing or ICC profile adjustments in digital exposure, there’s one effect that cannot be fixed – ever. It’s the loss of chroma, or saturation. Due to the inherent saturation costs when mixing colors, a loss of chroma in the primary constituents will induce a similar loss in chroma of all mixed colors. In other words: the gamut shrinks, and fewer visible hues can be represented.

Simulated gamut of lack of interlayer effectiveness on a CIELAB wheel. In black, a hypothetical CMY gamut without cross-contamination. The red gamut has contamination of yellow with some magenta (shifting the yellow towards orange), magenta is contaminated with cyan (shifting it towards blue) and cyan is contaminated with magenta (also shifted to blue). Contaminations like these will always result in a smaller total gamut.

In conclusion, the interlayers play a huge roll in color paper by preventing chroma loss due to incorrect dye formation. This way, they ensure that the paper is capable of producing as wide as possible a gamut of hues. To add to this, it also turns out that not all papers are created equally. For instance, the FUJIFILM Chrystal Archive Supreme I often print on has thicker interlayers than the ‘regular’ FUJIFILM Crystal Archive (‘Type CA’). The actual color emulsions and dyes are exactly the same in both papers, but due to the thicker interlayers of the Supreme, this paper is capable of producing a wider gamut and purer hues than the regular Type CA paper. Nifty, huh? So size does matter, in a way!

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