The idea of making a latensification chamber for those with limited darkroom space got me to thinking.
Here's one idea...
Make a 2-part box consisting of a base unit to hold the film and a slip-on cover box with a "window" covered with plain 'ol black polyethylene film. In case you haven't noticed, the common 2 or 3 mil black plastic you buy at the local hardware store is not 100% light proof. It actually lets 1 or 2 % (???) light thru it.
With a bit of experimentation, one could figure out how many layers will be needed to reduce the light inside the chamber to the proper level for 15 to 20 minutes of latensification. (I'd guess that 2 layers will do it in an indoor situation). The "window" in the cover box should be the full length of the film to avoid uneven light.
In use, you'll need to find a place where the room light is uniform and repeatable. Bouncing a light off of the ceiling and making sure no bright light shines directly onto the plastic "window" is probably the easiest way.
The base could be a simple affair, made from 1/8" masonite or 1/4" plywood with a frame made from 1 x 4 boards. By making the frame with a 1/4" space between the boards, a light trap is created. The cover box could be made from black foam core with the black plastic taped over the "window".
The sketch shows the chamber used horizontally where large format films are merely laid on the floor, but roll films will argue as you try to clamp them down. It could also be wall mounted where roll films will behave, but large format films will take more care. Lighting for a wall mounted unit will take more thought.
A colleague and I did a modest search of available information on perborate latensification then mulled it over for a while. Here is our lazy, hurried plagiarism of other people’s hard work, with a few insignificant additions, fantasies and elaborations of mine:
Perborate exists in solution as a peroxide-perborate equilibrium. The latensification effect appears to be caused by the perborate reducing silver halide onto the existing silver atoms in the latent sub-image specks, thus making them developable. Therefore it is advantageous to adjust the pH to favour the perborate: pH 10 is the minimum pH necessary, and pH 11 gives some improvement. Lower pH favours peroxide which is less selective: therefore it produces lower speed gain and higher fog.
The faster the film, the less the speed gain.
The action does not involve the conversion of photons to electrons, so is independent of the method of spectral sensitisation – ie it is not spectrally selective.
Latensification will not work if the intensity of the light is low enough to require long exposures – it seems that one second or thereabouts is the upper limit.
Because the latensification action is on image specks with few silver atoms it is important to do it as soon as possible after the formation of the latent image. Storage temperature has an effect on the stability of clusters with few silver atoms. The potential speed gain may be reduced by 50% after a week at 20°C.
2 g/l sodium perborate was effective if the emulsion was dried after treatment and before developing.
10 g/l sodium perborate was as effective, but didn’t need to be dried.
30 seconds at 20°C was adequate. Most of the action occurred during the first 5 seconds. If the film is not subsequently dried, it should be left for a couple of minutes after the perborate bath before development. If the film is dried, additional speed gain is possible by repeating the perborate/drying cycle.
You should carry out tests to determine optimum development time. The longer the film spends in the developer, the less effect the perborate has on the final image. (my thought: does this favour a developing agent like Metol that has rapid shadow action, and a low density range in the negative? )
The perborate does increase the graininess of the images, but it is estimated to be less than the increase in graininess by using faster film.
All the ‘speed increase’ happens in the toe. There is little change in the midtones and nothing in the shoulder.
I'm continuing with my own comparisons, but a very minor leg injury has slowed me down a little, pathetic wimp that I am.
Fascinating. Thanks for doing the research.
The bit about latent image decay reducing the perborate's potential is especially interesting to me. A few years ago I toyed with the idea, but never got around to trying perborate (I've still got a bottle of solution I mixed up 5 or 6 years ago).
Now, I suspect that if I would have tried it on the 50 rolls of 220 film I usually shoot in Europe, the results would've been disappointing. It usually takes at least one month from the time I start shooting the first roll until I get home and start developing.
I can testify that latent image decay is not a problem with low light latensification on films that have been exposed as much as 6 to 8 weeks before development. I guess I could theorize that low light latensification is one way to compensate for holding films for extended periods before developing. (Obviously, I'm not suggesting that latensification should be used on a roll of film that grandma took 10 years ago... the fog would be horrendous)
Originally Posted by Helen B
Thanks very much for doing this (ahem) leg work on your injured leg.
Your research offers several explanations for why my first attempt at perborate latensification didn't seem to have a great impact:
1. I was using Delta 400, and so your remarks about faster film showing less effect are probably apt.
2. I didn't check the pH of my perborate solution prior to using; could have had something to do with it.
3. Slow shutter speeds. I can't remember, but I may have been shooting at 1 or 2 second exposures.
Did you discover, or do you know, whether the perborate solution must be used as a one-shot, or can it be re-used several times?
Helen! - I do so enjoy your posts. Thanks
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Thanks for the kind comments.
I suspect that the numbers I quoted for limiting shutter speed and loss of effect over time are highly variable from one film to another.
Image stability: I wonder how noticeable the loss of latent sub-image is - as far as I know, the fewer the silver atoms clustered together, the less stable they are. The '50% loss of speed increase in a week' example I gave was a 'may' not a 'shall'. The stability of the image centres in the less exposed grains will depend a lot on the film itself, I guess. A few years ago I left some exposed APX 400 and Tri-X for a few weeks in the tropics before developing. The APX 400 lost a lot of shadow detail while the Tri-X stood up very well.
I don't know if the perborate would be re-usable. There are all kinds of things going on between borates and hydrogen peroxide and stuff so it might be that the pH falls over time and that the reduction-oxidation properties change. This particular aspect is getting outside my current understanding.
Sodium perborate forms an alkaline solution, hydrogen peroxide an acid solution. Bear in mind, if using a borate-peroxide mix instead of perborate, that commercial H2O2 may include an acid stabiliser.
Oh, Persil got its name from perborate-silicate.
(later addition: hydrogen peroxide is more stable in acid solutions than alkaline ones; pharmaceutical grade has low amounts of stabiliser, cosmetic grade has the most)
Last edited by Helen B; 01-28-2005 at 10:48 AM. Click to view previous post history.
Originally Posted by Helen B
Very interesting info. Will have to experiment. American Cinematographer had some articles about low-light latensification in the 1950s. Apparently Columbia Studios used it especially for productions like "Sunset Boulevard". I have not been able to find a library with ASC that went back that far.
'American Cinematographer had some articles about low-light latensification in the 1950s.'
The 1948 Kodak research note that we dug up appeared to be for movie film.
a method for testing and some untested ideas
David Vestal wrote about his adaptation of Ralph Steiner's latensification techniques in Darkroom & Creative Camera Techniques, Nov/Dec 1993, page 43 e.p. (now Photo Techniques, whose web site shows the back issue still available). He uses the Kodak 5.5" round safelight and 7W lamp, half covered for Plus-X, and masked to a 1.75 inch square for Tri-X, at 15 to 20 minutes (old emulsions). He also finds that twice the time at half the light intensity gives the same speed gain in the shadows with less fog.
To test, he lights something like a Kodak Q-13 grey scale suspended well in front of a black background and shoots identically underexposed negatives so that he can compare the grey scale against the "absolutely" black background. He puts sections of this roll at distances of 7, 8.5, 10, and 12 feet from the safelight, making sure they don't cast shadows on each other, saving one strip with no latensification for comparison. He gives 15 minutes of exposure to the safelight. He develops the strips identically and suggests trying 10% to 15% extra development time as an option when latensifying. The best latensification exposure is the one with greatest density and detail increase in the shadows, but with minimal fog. Keep the light source at least 5 feet from a 36 exposure roll of 35mm or risk uneven coverage due to the inverse square law.
One thing I'm considering for myself is using a green LED off an adjustable 12V power source. A great source for these is superbrightleds.com, and I'm considering trying versions at:
(I also intend to try their amber and red versions for safelights and a dimmed green for development by inspection.)
DO NOT buy the 120VAC Edison screw base versions expecting to dim them, it won't work properly because of the electronics used to drive the LED. The 12VDC versions should be dimmable to the appropriate levels with a variable voltage DC "wall wart" transformer. Be sure the coverage of the LED is wide enough to cover the film evenly. A little sandpaper or emery cloth on the dome of the LED should work to diffuse it, or some frosted material in front of the LED. Some LEDs come with frosted or milky diffusion built in. The light bars at superbrightleds have 50 and 90 degree beams, so should be fine as is. Note that the cheaper 12" bar is lower output, while the more expensive one is brighter, but can be broken up into sections of 3 LEDs each. Either will need significant dimming.
Great thread. Now, a few months later, did anyone have any luck with latensification contraptions for use in limited darkroom space?
I thought that both the pinhole device and the centrally lit cylinder device were particularly interesting. A hybrid of these two might be a convenient and cheap device--a pizza box or thin cylinder with a diffused pinhole. One could load it in a darkened bathroom and expose it on the table under your dining room light as long as your spouse/child/roommate doesn't open the box mistaking it for dinner!