What is the purpose of the colored base?

[brofkand]

What is the purpose of film having a colored base? I notice most of the films I work with have a blue-greenish base. Why?

Wouldn't it be better if the base were just clear?

 

[keithwms]

I assume that since this is posted in the b&w forum that you are not referring to the 'mask' in most c41 films.

On b&w films, there is a dye to prevent halation. That is the 'antihalation' dye. There may also be some anti-curl substance.

 

[BetterSense]

Some films either don't have it or it washes out completely. Fuji films seem to have a crystal clear base after washing; my TriX is purple, my Tmax is clear but is pink if I don't fix and wash it to near excess, and Foma is bright blue even after fixing and washing.

I've heard the antihalation theory, but Fomapan is bright blue even in medium format, which I understood as not having an antihalation dye in the base itself due to the backing paper.

 

[AgX]

An Antihalation Dye is added to a separate layer. But there can also be a dye added to the base itself.

The former is intended at preventing reflection of image forming light at the surfaces of the base, especially the base/air transition.

The latter reduces light transmission within the base, thus unwanted reflected light as well as non-image-forming light entering the base.

 

[cmacd123]

Different makers have different methods of preventing light from getting a second chance at exposing the emulsion. Roll film often has a dye on the back as the base if thin, and would not stop much light unless VERY dark. Many films have dye, or other materials just under the emulion, (sometimes refered to as AHU) colur materials can have a silver layer as it will come out in the bleach, as do some B&W reversal materials. (if they do - they cannot be processed as a negative)

The older style EFKE films (25/50/100) are the most common ones to have a dye backing in 35mm, which shows as a purple colour on the undeveloped film.

The data sheet for the film will often describe if the base has a tint.

Materials for positives often omit the tint.

 

[brofkand]

I understand. Answers my question perfectly!

So, if this dye's purpose is to cut down on halation, why doesn't it wash out when you do the prewash like an antihalation layer?

Also, with it being dye and all, does it affect the stability of the film base? I have heard this is a reason why color films in general are not as archivally stable as black and white films...but maybe I'm just reading too much.

 

[Photo Engineer]

Don't forget one additional reason.

Film is tinted with a colored conductive material to prevent static electricity and also to prevent light piping. This introduced a slight tint into the support itself which cannot be washed out.

PE

 

[AgX]

So, if this dye's purpose is to cut down on halation, why doesn't it wash out when you do the prewash like an antihalation layer?

The antihalation layer is the layer that contains the antihalation dye.

There are two ways to place this layer.

-) at the back of the film. (here it can even serve as anti-curl layer etc.)

-) between the base and the emulsion (this should be most effective)

In the former case diffusion stability of the dye is less important (though there is contact with the emulsion side of another winding) than in the latter.

So a means must be sought to keep the dye at its place until processing and sure while coating.

One can either fix the dye and dissolve the whole layer (in case of back layer) or dissolve or bleach the dye in one of the processing baths.

 

[AgX]

Also, with it being dye and all, does it affect the stability of the film base? I have heard this is a reason why color films in general are not as archivally stable as black and white films.

I'm not aware of a dye de-stabilizing the base on long-term, however there may have been an issue with a certain subbing layer on a TAC base.