Different anti-halation philosophies (rem-jet, etc)

[jim_causey]

I've been reading with some interest lately about the technical

properties of Kodachrome -- in particular, the rem-jet anti-halation

layer.

 

I did not previously realize that different films took different

approaches to anti-halation. Apparently, other films (such as motion

picture films) sometimes also use rem-jet.

 

My questions:

 

1) How many different approaches to anti-halation are there?

 

2) Why do different films use these different approaches?

 

I could see, for instance, if rem-jet were invented before other

technologies, why it would have been used in early Kodachrome, but if

that were the case surely Kodak would have used a more modern

technology when they reformulated the emulsion later on?

 

Thanks,

 

Jim

[reuben_c]

Remjet has an advantage when used for daylight loading movie film spools. If you've ever used the old double-8 film you've seen it. You have a roll of film, and you wonder why it isn't fogged when you load it. The reason is that the remjet backing is opaque, and keeps the light from going in and fogging the film.

 

It also works for antihalation purposes, and I think it has a lubricant benefit in movie cameras too. But my personal belief is that its primary (original) purpose in life was to allow for daylight-loading movie film reels.

[reuben_c]

For your other questions, some other types of antihalation are dyed support (most common for negative film), which also helps prevent light piping, dyed emulsion, and dyed gelatine backing (on opposite side of support from emulsion). Some films probably use a combination.

 

For positive (reversal and print) films, dyed support is detrimental. Even though it does not effect the image proper, it does reduce the total amount of light passing through the image, meaning you will need brighter lamp and-or faster projection lens. For negatives this isn't an issue, because you can simply adjust your exposure time to compensate. A dyed base does not affect the image at all, even though some people mistakenly believe that it does.

[silent1]

Reuben has it, for the most part, though why the remjet was retained in the still photo version of Kodachrome is probably a highly technical question.

 

In general, though, you see remjet only on movie film, dyes of one sort or another in still film and some movie films based on still emulsions. However, some few movie films (both color and B&W), instead of rem-jet or dyes, use a colloidal silver antihalation layer; this is metallic silver, like that of the final image, but is embedded in a layer beneath the emulsion proper; it bleaches away with the image silver during a reversal or chromogenic process, but in a B&W film makes it difficult to obtain a negative from the film. And some reversal films, apparently including at least some versions of Kodachrome, use a colloidal silver yellow filter layer that, under some circumstances of development, can become opaque black like a silver antihalation. Again, in a reversal film this is normally a non-issue, because the bleach step will remove this silver along with the image silver, but if you try to develop such a film to a B&W negative, you'll get solid black even after removing a rem-jet layer, if present.

[rowland_mowrey]

Guys;

 

Dyed support is not really a viable option.

 

Rem jet usage is as you describe and also has antistatic properties because it is conductive just like the black bags used to package some electronic parts.

 

Most color films use grey silver in an undercoat. It stops back reflection from the support and is removed by the bleach / fix process steps. Rem jet allows light to pass through the support and is then absorbed, so there can be a tiny bit of light piping in the support.

 

B&W films use a grey dye instead of grey silver. It is usually in an undercoat.

 

In addition, most films use colored dyes in the emulsions themselves to lower multiple internal reflections and enhance sharpness. They are termed 'acutance dyes'. They wash out rapidly in the process.

 

They have another purpose as well. They tweak the speed of the emulsion(s) to make sure that the film is exactly on aim. This type of dye is also used in color paper.

 

Ron Mowrey

[reuben_c]

<i>why the remjet was retained in the still photo version of Kodachrome is probably a highly technical question. </I>

<p>

Perhaps Mr. Mowrey can confirm it (I copied and pasted his name this time to prevent my inveterate spelling problems ;) but my guess is that it is simple logistics; one less master spool to coat.

<p>

<i>Dyed support is not really a viable option. </I>

<p>

If this is the case, then how come it is so widely used?

[rowland_mowrey]

Reuben;

 

I knew that would come back to haunt me. I didn't make the comment about dyed support clear.

 

Dyed support intended for antihalation would have to be so dense that the film would be opaque. You see how dense that remjet is and the grey silver layer in color film? That is how dense a dyed support would have to be.

 

There is, in some supports for some products (IIRC), a tiny bit of carbon to prevent light piping, and to act as an antistat. That is about it. It really does not work as an antihalation layer. Carbon is not used in the support for color films for example. The grey silver is sufficient in the undercoat to prevent light from 'leaking' into the support.

 

Remjet is used for Kodachrome so that the film can be exposed through the base during processing. If grey silver were used, that would be impossible as it would not be removed until the bleach / fix steps.

 

Ron Mowrey

[reuben_c]

Then why is it also used in other movie film stocks that don't require Kodachrome's unique reversal processing?

 

Seems like the one common denominator is daylight loading at some point in thier history (meaning that there are still some cameras or magazines that need it).

[rowland_mowrey]

Reuben;

 

Motion picture film moves through a camera at very high speed. The remjet acts as an antistat during this motion. In some cases, film speeds are extremely fast. I have seen high speed cameras use up a 1000 ft roll of color film in seconds. I have seen a jam of estar based film actually break teeth off sprockets at that speed.

 

Ron Mowrey