Why does color movie film have rem-jet backing and B&W film doesn't?

I hear people say that it's hard to use color movie film in still cameras because you have to remove the rem-jet backing which, I guess, is some kind of antistatic coating. When people talk about using B&W short ends in their cameras I don't hear anything about this, so B&W must not have the coating. Why? What is different about color film compared to B&W film?

Anti-halation.

As light strikes film it can pass through the layers of emulsion and reflect off the base, back into the emulsion. This causes little halos around images on the film. (Especially in bright areas or specular highlights.) The rem-jet/anti-halation layer absorbs said stray light so it doesn't affect the final image.

None of the B&W movie films I've seen have a remjet backing. I'm going to go out on a limb and guess it's because they can do the antihalation function with a grey dyed substrate on B&W, which they can't do on the color film. As for the antistatic function... well, not sure. I know some of the very most recent Kodak films have a "process-surviving antistatic coating" (that is obviously clear) but that is not the case on older films.

Duncan

i use re-cans from work of MP film 99% of the time, the backing comes off in processing and many times just requires a longer wash/pass wet sponge. Many older B&W stocks did't have a backing, now many B&W films like like Tri-X Reversal have safety backing (anti scratch & anti h) and even caranauba wax for smoother camera operation, i process it neg w/ hc110 and the backing comes off very easily. In my case w/ TriX R the only dif when shooting as still & processing negative is a slight loss in speed and higher grain.

As light strikes film it can pass through the layers of emulsion and reflect off the base, back into the emulsion. This causes little halos around images on the film. (Especially in bright areas or specular highlights.) The rem-jet/anti-halation layer absorbs said stray light so it doesn't affect the final image.

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Then why don't still camera negative films have such a backing?

BetterSense said:

Then why don't still camera negative films have such a backing?

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Perhaps it was more cost efficient to avoid pre bath and remjet washing off stage in minilabs.

BetterSense said:

Then why don't still camera negative films have such a backing?

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Movie films actually have to worry about halation from the other direction too. In a "daylight load" spool, you have light striking the base side of the outermost loop of the film, while the full metal spool sides and backing prevent it from burrowing too far in to the remaining film. (Much the same way the spool and backing paper work with 120 film.) Not an issue with still camera films.

Duncan

wow, I didn't know that. That's how my 8mm camera works, but I have always been scared to load it in bright light. I didn't know movie cameras worked like that too.

Remjet, a removable jet black layer, is the coating of carbonblack particles in a water-soluble binder on the bottom of thefilm. It has four purposes: antihalation, antistatic, lubrication,and scratch protection. Light entering the film can reflect offthe front or back surface of the film base and return into theimaging layers to expose them. When light spreads laterallybeyond its intended boundary, an image appears to have a haloaround it (halation). The antihalation layer prevents this byabsorbing light that reaches it. The remjet carbon layer isconductive and prevents the build-up and discharge of staticcharges that can fog film. This is especially important inconditions of low relative humidity. Remjet also has lubricatingproperties. Like the supercoat on top of the emulsion, remjetresists scratching on the base side and helps transport the filmthrough cameras, scanners, and printers.--Kodak

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From Kodak

BetterSense said:

Then why don't still camera negative films have such a backing?

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Rem jet is an anti static coating, still cameras don't move film at 24 FPS creating a huge static charge to build up.

Oh yes, Kodachrome had Rem jet on it.

BetterSense said:

Then why don't still camera negative films have such a backing?

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Not an entirely authoritative answer but I think many still films have dye in the emulsion that is washed out upon processing.

The first time I processed Fuji Neopan 120 film my prewash came out dark blue, almost black, and I was startled for a moment when I saw it. It took a few seconds to realize that it was dye being washed out of the film.

Doesn't T-Max film have a magenta dye that is washed out on processing?

I just assumed that these dyes are being used as anti-halation measures among other things.

RemJet has some very attractive properties. It is probably the most effective antihalation backing yet made and it has some antistatic capability. It provides decided improvement in both quality and reliability over other antihalation methods for color motion picture camera films. It is very easily removed in machine processing, which is how motion picture film is processed. A little wheel scrubs it off with a sodium carbonate solution in the prewet step. Kodachrome used RemJet backing before color negative motion picture film was common, and I suspect experience with Kodachrome was a reason for using it with motion picture film. (Kodachrome was also machine processed.) Still camera color film, except Kodachrome, has always been designed for possible processing by small labs and individuals. Reliable removal of RemJet is a problem in these conditions. It can flake off and get stuck on the emulsion unless special care is taken. Notice that some motion picture camera film, like Ektachrome, has never had a RemJet backing and works quite well even at high camera speeds. Static is not that much of a problem. I notice that some color intermediate and print films now do not have RemJet backings. Maybe technology is improving, and an alternative is possible, at least for some applications.

With the possible exception of the last version of the machinery (the Kodachrome "mini-labs"), all Kodachrome was developed in large, high-speed roller processing machines. The film was removed from the individual cassettes and then spliced together to build one long roll. That long roll was then run through the machines at high speed.

A significant percentage of Kodachrome was movie film. The remjet removal step was integral to the Kodachrome movie film work flow. Most likely it made more sense to have the same work flow for both the still and movie film.

To give a sense of scale to this, consider this fact - the machines were set up and run regularly with leader, rather than films requiring processing. This permitted calibration and checks for tolerances. The leader rolls themselves were more than a mile long.

BetterSense said:

I hear people say that it's hard to use color movie film in still cameras because you have to remove the rem-jet backing which, I guess, is some kind of antistatic coating.

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To be precise, the rem-jet coating doesn't make operation difficult in still cameras, but processing. I'm sure that's what you meant, but thought it could be misinterpreted.

The quote from Kodak is exactly correct. Kodachrome had the rem-jet backing as well due to its origins as a movie film! All Kodachrome stock was at one time used as both still and movie.

Many modern films use carbon impregnated film support that is somewhat conductive. This gives it a gray coloration after processing. It is roughly equivalent to rem-jet but lacks some of its properties such as the ability to withstand abrasion.

Rem-jet is rather more expensive and gives a high quality result. It is reserved for the best film products and is also reserved for factory (professional) processed products, eg. Kodachrome and ECN.

PE

MattKing said:

Kodachrome was developed in large, high-speed roller processing machines.

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I thought the large Kodachrome machines where cine machines, which used the sprocket holes to forward the film, which has the added advantage of no contact to the image area.

Ed

Neanderman said:

I thought the large Kodachrome machines where cine machines, which used the sprocket holes to forward the film, which has the added advantage of no contact to the image area.

Ed

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I don't think so, because they were used for so many sizes (110, Regular 8 (in several varieties), Super 8, 16mm, 127, 828, 135, etc.)

Recall that the B&W film was in use first, (back to Edison) and so it would be difficult to phase in a rem-jet b&W film as all the existing processing equipment would have to be retrofitted. Kodachrome was introduced with only Kodak processing, so they could easily require the rem jet removal step. ECN came out with the rem-jet from the start so the removal was always in the process.

Most still camera roll film has a backing that will dissolve or clear in the normal chemicals. Often the gelatine on the back remains, as an Anti-curl layer. Cut off a sample of undeveloped 120 B&W and the back is likely purple.

EFKE (Adox) uses a purple anti-halo even on their 35mm still films.

Since the 35mm film is thicker they can use the grey base for anti-halo. The data sheet for Double-X 5222 mentions that the anti-static backing must be removed to splice the film.

Some colour film is also on a grey or blue base - read the data-sheets closely.

C-41 colour negative sometimes uses a silver layer under the ensuion which is cleared in the bleach step. I belive that is also the case for some of the e-6 stuff. I would not be surprised to see that technique also used on the Motion Camera negative. MP neg is the prestige product for both Kodak and Fuji and so gets the most advanced technology.