Sensitising Dyes and Dye Couplers (Newbie question)

Hi guys

I'm new to analog and APUG and am trying to get a very basic working knowledge of film chemistry through Internet browsing. Please forgive me if the answer to this question is obvious, or readily available - I've looked and haven't found it.

My understanding is that sensitising dyes determine the wavelengths of light that an emulsion will respond to, while dye couplers are used to form dye clouds by reaction with oxidised developer. My question is whether there is a necessary relation between the colour that a sensitising dye responds to, and the colour of the dye formed by the dye coupler in that layer?

Of course in most C41 films, the colour created by the dye coupler is the inverse of the colour which that emulsion layer is sensitive to. But is this necessary, or a choice of the emulsion chemists? The reason for the question is a vague curiosity as to whether a positive transparency film could be created that is developed by a standard C41 process, by using dye couplers in each layer that create the same colour that the layer is sensitive to, rather than its inverse? I'm aware of the comlexity of producing any emulsion, so it's not in any sense a suggestion that it be done. Just a question to enhance my understanding.

I'm an engineer (electronic, not chemical) so technical responses are most welcome. Thanks for indulging my curiosity!

Andrew

OK, I think figured it out.

Presuming that the dye couplers don't have to be the inverse colour of the sensitising dye, you would still have the problem that the more light the layer was exposed to, the more silver would be formed, therefore the more dye, therefore the darker the image. So it might be colour correct but the luminosity would still be a negative. C41 relies on the double negative of the printing process to restore the positive luminosity, while in E6 the unexposed areas form colour dye so more exposure means less dye and greater light transmission for a brighter image.

Duh!

There is a direct relationship between couplers and sensitizing dyes. This is regardless of whether the film is negative or positive.

A blue sensitive layer uses a yellow colored sensitizing dye and forms yellow dye from the coupler. The yellow dye forming coupler is generally an acetoacetate and the sensitizing dye in this layer and the other layers is generally a cyanine, carbocyanine or merocyanine dye. A green sensitive layer uses a magenta colored sensitizing dye and forms a magenta dye. The image dye couplers are generally pyrazolones. The red sensitive layer is sensitized by a cyan sensitizing dye and the imaging coupler is either a napthol or phenol. All layers are blue sensitive, but the blue sensitivity is filtered out from the light by placing a yellow dye or silver layer under the blue sensitive layer.

PE

Thanks Ron!

Andrew

You are welcome.

I did think of one exception to the rule. IR color films are false color and change the relationships above, due to the IR layer and so you have a Green/Red/IR layer setup and the coupler orders are changed as well.

PE

I've always wondered, how is the yellow filter layer removed?

My question is whether there is a necessary relation between the colour that a sensitising dye responds to, and the colour of the dye formed by the dye coupler in that layer?

Click to expand...

The relationship between sensitizing dyes and colour coupling dyes in a typical film are related only so far as they work together towards color synthesis; subtractive in this case. In other words, you could have a totally color blind emulsion (i.e. no sensitizing dye) but with colour couplers that created celadon, or any other color you wanted. They're not inextricably linked, chemistry wise, but it's a choice governed by color synthesis theory that the red record needs to become cyan, green -> magenta, blue -> yellow.

Of course in most C41 films, the colour created by the dye coupler is the inverse of the colour which that emulsion layer is sensitive to. But is this necessary, or a choice of the emulsion chemists? The reason for the question is a vague curiosity as to whether a positive transparency film could be created that is developed by a standard C41 process, by using dye couplers in each layer that create the same colour that the layer is sensitive to, rather than its inverse?

Click to expand...

It is necessary if your goal is realistic color. That the color record is turned to its inverse is not unique to negative films; positive films work the same exact way, so does dye-transfer, color carbon, offset-lithography, etc. This is the foundation of subtractive color synthesis and there's no other way to make a color image on a paper or transparency than to use cyan, magenta and yellow. The whole negative bit confuses things, but the synthesis is occuring in exactly the same way, with RGB layers and CMY dyes.

Think of it this way, using a positive film as our example; the red-sensitive layer records all things red, the highlights, and makes shadows out of everything else, therefore it makes sense that the shadow densities have to become red's inverse, cyan. In other words we want the shadows to subtract red from our white viewing light. The red parts will actually be clear in the red layer and the yellow+magenta from the blue & green layers are responsible for ultimately producing red.

If the red record became red upon developing and the blue becomes blue, etc., the result would be complete black where any two colors overlap. This is because any 2 of the additive primaries (RGB) will fully absorb the light. The subtractive primaries, or secondaries (CMY) will only create black when all 3 are overlaid, and intermediate combinations produce all the colors.

The only way that "creating the same colour that a layer is sensitive to" could work is if the layers were separated afterwards and projected on a screen with 3 different lights. This is additive synthesis. But since they are overlapping and we're shining 1 light thru them, we have to utilize the secondaries.

Hope this makes sense; let me know if something isn't clear.

Athiril said:

I've always wondered, how is the yellow filter layer removed?

Click to expand...

If it is a mordanted dye, the chemical "bond" is removed and the dye washes out. If it is CLS (Cary Lea Silver), it is bleached out in the Bleach and then fixed in the Fix.

PE

Holmburgers, thanks for the detailed and clear reply. I appreciate the time you gave to respond to my question. I had to look up "Celadon", though

Andrew

Yeah, that was the most random color I could think of...

My pleasure to reply, I hope it was helpful.