Film "hypersensitization" What is it??

[htmlguru4242]

After posting my question about pushing films for night shots, I did some research, and stumbled upon something mentioned as "hyping", "hypering" and :hypersensitizing" film (I'm assuming all of these terms are synonyms ... ). It seems to be used in astrophotography...

What exactly is it? The name sounds self - explainatory, but I've never heard of it before ... does anybody know anything?

 

[David A. Goldfarb]

What it is is enough of pain that I don't see myself doing it other than to satisfy an idle curiosity any time soon.

There's a good explanation in Steve Anchell's _The Darkroom Cookbook_. One method involves exposing exposed film to hydrogen peroxide vapors under pressure before development, and another involves exposing developed film to acid vapors before fixation.

 

[htmlguru4242]

It did seem like it would be a decidedly dificult thing to do / pain. It was not something I was planning on trying, but it seems like an intersting idea.

 

[Donald Qualls]

Actually, "hypering" as used in astrophotography is a pre-exposure process of soaking the film in "forming gas" (nitrogen, 2% hydrogen, as I recall) for some period of time. Some films gain by it, some don't, but Tech Pan was the king of hypering; it gained two stops in speed (to ISO 100 or so) and the reciprocity failure was reduced to the point it was the fastest film there was for exposures calculated at more than an hour.

Unfortunately, most of the microfilm replacements for Tech Pan are said to be non-hyperable, as are many of the most current slide and color negative films.

All the stuff in Anchell or Anchell & Troop is post-exposure processing -- peroxide treatement, either before or after development, perborate treatment, and latensification can all gain about one stop, but do nothing for reciprocity failure; only a pre-exposure treatment can affect reciprocity failure because it's a case of the image not being recorded due to loss of sensitivity at very low light intensities.

 

[Lee L]

Hypersensitizing is a term used for a number of procedures to increase the sensitivity of films, or perhaps more accurately put, to dramatically lower reciprocity failure. These include:

baking in nitrogen or forming gas (92%N+8%H)
soaking in nitrogen or hydrogen
a nitrogen bake followed by a hydrogen soak
bathing in water, ammonium hydroxide or silver nitrate
a water bath followed by a hydrogen soak
pre-flashing

An explosion proof vacuum chamber is employed in some of the procedures. Driving oxygen and moisture out of the film is an important part of some methods.

These procedures were more effective on older emulsions and films specifically produced for astrophotography, the latest and most common films to be effectively treated with some of these methods being Tri-X and Tech-Pan. These methods are less effective with modern T-grain films which have lower reciprocity failure to begin with, and with the advent of these films and CCD astrocameras, hypersensitizing has fallen out of favor and is less common than in the 1970's through 1990's. For more detailed info, try Wallis & Provin "A manual of advanced celestial photography" (out of print), or Covington "Astrophotography for the Amateur".

Lee

 

[htmlguru4242]

Seems interesting, but too bad that modern films dfon't work well with it; I've read stuff [just now] about Tech Pan working well with it, so It's too bad that its not being made anymore. I will be shortly getting a roll of Tech Pan.. but I'm certainly not using it for this.

There's a bunch of stuff on Photo.net about hydrogen peroxide hypering, and it sounds pretty easy, though there's not concrete info. as to the quality of the results.

 

[Tom Hoskinson]

I've done both. I've used hydrogen peroxide (post exposure) and baking the film (pre-exposure) in a vacuum chamber back-filled with metallurgical forming gas (92%N+8%H).

As others have said, the effectiveness is very dependent on the type of film.

I didn't like the results I got with the hydrogen peroxide method.

I did like the results I got with forming gas hypering using Tech Pan, Fuji color neg and Konica color neg.

I also got good results by preflashing the film, followed by normal development.

 

[avandesande]

I have a theory at least (I used to be a monomer chemist) that the real objective is to remove oxygen from the film. When we polymerized monomer we would use nitrogen to remove all the oxygen from the batch (we confirmed this with an oxygen test). Oxygen itself will terminate free radicals.

I believe that oxygen interferes with the electrons on the surface of the silver crystals. Using inexpensive nitrogen to purge the film should work fine.

 

[Tom Hoskinson]

That is my understanding as well. The gas hypersensitization process places the film in a reducing (N2 H2) metallurgical forming gas environment that "grabs" oxygen. The reduction of silver oxide has the effect of increasing the sensitivity of the silver crystal surfaces. The effect goes away as the film sits (and re-oxidizes) under ambient conditions.

 

[Photo Engineer]

IIRC, you need the hydrogen.

PE

 

[Tom Hoskinson]

Yes, you need the hydrogen - hence metallurgical forming gas - (a sufficient amount of H2 in a non-explosive mixture dominated by Nitrogen). I pull a vacuum, then backfill with N2/H2. You want a reducing atmosphere in the heated chamber.

 

[htmlguru4242]

what were you using for a vacuum chamber?

 

[Tom Hoskinson]

I am using a small chamber made specifically for hypering film. it was sold to me by an astrophotography suppy store. a heater is built into the chamber.