How Does Direct Positive Film Work?

[htmlguru4242]

I've been reading around online (and here in the topic about Kodak rapid copy) about direct positive films.

How do these work?? I can't come up with any logical idea, and i don't see anythting online.

Does anybody know?

 

[KenS]

I've been reading around online (and here in the topic about Kodak rapid copy) about direct positive films.

How do these work?? I can't come up with any logical idea, and i don't see anythting online.

If my ageing memory serves me well enough, the film is irradiated to the point where any added exposure by use, is "solarisation" (in the correct sense of the word) to provide you with a positive image. I have seen two images made with regular pan film where the sun, in a clear sky, prints darker than the sky.

Ken

 

[Donald Qualls]

Interesting -- I understood they used a non-silver chemistry that was activated by the alkalinity of Dektol, but in fact would develop just as readily in a plain sodium carbonate solution (that'd be easy to check, if you have some of the film).

ISTM it'd be rather tricky to pre-expose a film *precisely* to the downturn point of the H&D curve; for one thing, the resulting film would have very poor keeping qualities, because of latent image fading effects, which would cause effective loss of speed as more and more additional exposure is required to push back through the reversal point.

 

[George Papantoniou]

That's very interesting Donald... I always thought (like Ken) that the thing had something to do with the solarisation that happens with overexposure, although I thought that the contrast had to be really hard to control...

Can you explain more about the way those films really worked, Donald ? What did they use for making the photo sensitive emulsion ?

 

[Helen B]

I was also under the impression that direct positive materials were pre-fogged silver halide emulsions. Agfa Copyproof reversal film certainly was, and I thought that the Kodak emulsions were similar.

Here's an excerpt from L P Clerc: "A fine-grain silver chloride emulsion is uniformly fogged so as to obtain on processing the maximum density required afterwards in the positive image. It is then bathed in a solution containing 0.01% of safranine and 1% of potassium bromide. The material is dried without washing.

The emulsion layer so treated is then exposed in contact with the negative or positive one wants to duplicate. According to the density of the original, the exposure should be some 30 to 50 times as heavy as that needed to produce the fog. If the intensity is kept low and time of exposure long, the tone range of the process is considerably longr than for short exposures and high intensities; in a certain instance an exposure of 36 minutes allowed four times the range to be reproduces as an exposure of one minute with a correspondingly adjusted intensity.

...

One should avoid, however, developers which act very slowly, and those containing a silver halide solvent. These latter are liable to develop the internal latent image of the first exposure, since it is only the superficial latent-image specks which are destroyed by the second exposure."

That is by no means the only method of producing a direct positive using pre-fogged silver halide, but it is conveniently brief and clear.

Best,
Helen

 

[Photo Engineer]

Kinda all right and all wrong. There are several types of direct reversal emulsions, but they basically involve a core-shell structured precipitation.

The core emulsion, of whatever type is fogged during sensitization and then a second normal emulsion is precipitated on top. The developer or the coating usually contains a 'nucleating' agent that prevents development on the surface where there is exposure, but reveals the fogged silver where there is not any exposure.

This is a rather simplified verson of some rather complex physical chemistry. I hope it works for you.

This gives a direct positive image.

Kodak made a color print material sold in Europe under the name Directachrome which was used to make prints directly in malls and kiosks with a simple 2 step process. Kodak's instant film used this technology, which the attorneys said was different than Polaroids and the subject of many Kodak patents.

The judge didn't see a difference between direct reversal and normal emulsions, but saw no malice in Kodak's work to get around Polaroids work therefore he fined Kodak but did not treble damages. At this point Kodak abandoned work on direct reversal.

Very high speeds could be obtained, but the emulsions had to be rather large coarse grained.

At the time of introduction of Ektacolor 30/37 paper, Directachrome was scheduled to be introduced as a companion "R" print material using the same process as the "C" paper, and actually the same emulsions but the "C" paper did not have the reversal chemistry built in. Due to problems with "re-reversal" and dmin, the Directachrome introduction as a professional product was delayed and finally cancelled when it was found that the nucleating agent caused unexpected effects on the "C" paper after a process became seasoned, if one tried to intercut the two products. Also, the developer had a keeping problem related to the nucleating agent.

Another product that never made it. For every one you see on the market, there are sometimes 10 or more that don't make it.

PE

 

[nworth]

PE:

Was the same technology used with Kodak's other direct reversal materials? I'm thinking of materials that have appeared over quite a span of time: Kodagraph Autopositive, SO-132, Aerographic Direct Duplicating Film 2422. Are the products that were made by Agfa and DuPont of similar technology?

 

[htmlguru4242]

oops ... double post

 

[htmlguru4242]

Ah, now I know how it works ( I wasn't actually going to try doing this [unless it's super-easy], I was just looking for the theory behind it)

While looking around on Photo.net on the subject of solarization for reversal, I found this topic: Look at the bottom post with the picture
Dead Link Removed

The guy CLAIMS to have obtained positives by leaving the negatives to soak in water of a low pH for a few days and then developing normally. This sounds REDICULOUS. Is this guy just BSing, or can this actually be done?

 

[Helen B]

One of the other ways of achieving direct positives does involve a similar process to the one described in the photo.net thread. From Mees, referring to a British patent of 1946: Knott and Stevens found that it was possible to produce a reversed image with some emulsions by soaking the exposed material in water, then giving a second diffuse exposure, then developing. Mees gives little information, and I hope that I have interpreted his words correctly.

Best,
Helen

 

[Photo Engineer]

Nworth, there were many types of direct reversal. I have no idea what was used in individual products or by other companies. I seem to remember that some of the Agfa stuff was similar to Kodak stuff or vice versa, whoever invented it first. Internally, we used code names to define the type of direct reversal chemistry used in a given product. Then, beyond that there were different types of emulsion that used a given type of chemistry and the chemistry could be used in the coating, the process and etc. So it was a broad area.

As for the reversal in water, I suspect that we are seeing the reversal portion of a dichroic image or that we are seeing an extreme example of the Sabattier effect.

PE

 

[Photo Engineer]

One of the other ways of achieving direct positives does involve a similar process to the one described in the photo.net thread. From Mees, referring to a British patent of 1946: Knott and Stevens found that it was possible to produce a reversed image with some emulsions by soaking the exposed material in water, then giving a second diffuse exposure, then developing. Mees gives little information, and I hope that I have interpreted his words correctly.

Best,
Helen

Helen, IIRC, Ed Knott was one of the early EK workers on direct reversal systems. I seem to remember talking to him about it at one time or another. It was when I was working on Ektacolor 30/37 and the other group was working with the Directachrome. He stopped by on a visit to talk to all of us.

I hasten to add that the methodology you describe above is classic Sabattier. I use it with some of my color work to get interesting results and have posted some of the results on PN. I was not trying for reversal, but you can see the classsic positive image behind the negative image.

PE

 

[Donald Qualls]

Can you explain more about the way those films really worked, Donald ? What did they use for making the photo sensitive emulsion ?

I have no idea how they actually worked, I'd just read it wasn't a common silver-based emulsion. OTOH, if it's preexposed past the downturn, it would account for the extremely low speed and relatively poor contrast...

 

[nworth]

There are obviously a number of tweaks to the process and possibly a number of totally different processes. The process PE outlined makes a lot of sense to me on the gut feeling level, without a good understanding. A couple of things of note - SO-132 had a pretty high base fog level, consistent with the sort of thing Helen B outlined, but I suspect the emulsion was constructed more on the PE lines; Kodagraph Autopositive required very low fog levels, and it had to be exposed to green (yellow filtered) light or it would turn black; both these materials were very slow;

 

[mfobrien]

If anyone would like some of that film to play with... I think I have a bunch that I inherited from a friend.

 

[jason314159]

Kodak made a color print material sold in Europe under the name Directachrome which was used to make prints directly in malls and kiosks with a simple 2 step process.

PE

Was this related to Vernon Bissonette's patents in the mid-70s?

 

[Phillip P. Dimor]

I've used a particular direct duplicating film before, I forget the CAT number and name but it was daylight-loading safe. I exposed it with a platemaker and developed in dektol. It was interesting (and a quick way to make a duplicate) as the duplicates (and film itself) really had no grain. It reminded me almost of diazo film (and perhaps it was? don't think you can develop diazo film in dektol though..) Ah well.

 

[Donald Qualls]

Phillip, that sounds like some of the dye-based duplicating films Kodak sells in its microfilm family. Pretty darned slow, as I recall, but as you say, essentially grainless (the dye is continuously distributed in the emulsion, so the "grain" would be near molecular size). No idea how the chemistry works, though...

 

[Photo Engineer]

Was this related to Vernon Bissonette's patents in the mid-70s?

Jason, Vern's patents were related to catalytic amplification of color images using a special oxidizing agent. There was no relationship between Vern's work and Directachrome which predated his work by at least a decade.

Some of the work on direct reversal was done at the Kodak Pathe plant in France.

All of this work was silver halide based.

PE

 

[jason314159]

PE,

I found 4,089,685 searching for something else. Thanks for explaining the difference.

 

[Photo Engineer]

Jason, just FYI, you will find my name as co-inventor on some of those patents, and I delivered the talk on the subject of our patents at the ICPS in Rochester a while back. There are quite a few regarding the cobalt amplification of color images. But, they are totally unrelated to direct reversal.

PE

 

[General Treegan]

Many years pass but here is my first test following Mees notes.

One of the other ways of achieving direct positives does involve a similar process to the one described in the photo.net thread. From Mees, referring to a British patent of 1946: Knott and Stevens found that it was possible to produce a reversed image with some emulsions by soaking the exposed material in water, then giving a second diffuse exposure, then developing. Mees gives little information, and I hope that I have interpreted his words correctly.

Best,
Helen

 

[NedL]

Many years pass but here is my first test following Mees notes.

nice! Soaked then developed?

 

[General Treegan]

Yes, for 2 hours in a warm water bath, flash the film and then process in a diluted developer.

nice! Soaked then developed?

 

[drmoss_ca]

Just once, out of curiosity (or perhaps stupidity), I took a colour negative film and did a homemade basic reversal processing. I think I used Rodinal as the B&W developer, flashed the film on the reel for a guessed 20 seconds in front of a bright light, then ran it through a C-41 presskit. The film came out almost black, but with a very bright light you can just see there are positive colour images. The Flextight managed to extract something more than my eyes could:

I can't say I recommend doing it! But it was a fun experiment.