Latensification

Hypersensitizing doesn't give you much of a film speed increase, but it does give you a vast reduction in reciprocity failure. This is a big issue if you are photographing comets with hour-long exposures and clockwork pointing mechanisms, but it doesn't buy you much at 24 fps.

There are all sorts of different recipes....a pressurized hydrogen/helium mixture is popular, as are mercury vapor and ammonia. Each have advantages and disadvantages but none of them really are useful for cine work.

--scott

I've based this on a piece by David Vestal and Ralph Steiner from a forthcoming book. Their line is that the only way to keep the threshold exposure low enough is to do it very very slowly, and, of course, only after exposure -- never before. 7 to 15 minutes, 10 feet from a very very dark (they suggested a green safelight filter over a 7.5w bulb with a black mask.

Then there is the other technique, Concurrent Photon Amplification, which was discussed in Pop Photo in the 70s. Involved tiny tiny lamps at the film plane that exposed the film right as you shot. I actually built this into a CP for testing, which showed promise, but since we were able to shoot 7250 instead of 7247 for the film in question, it became a moot point.

I believe that Deluxe General's AL400 system was just a big room with rollers and a dim lamp, that the film cruised through on its way to the processing machine. Anyone here ever use it?

>Also, anyone know anything about hypersensitising? It's a technique of exposing the >rawstock before exposure to a gas (Helium?). A colleague tried it to shoot Halley's >comet some years back. But the comet was such a fizzer, all the helium in the world >couldn't help.

It is written up in many Astronomy magazines, but the problem is that the film must be loosely wound, so cine film is a problem. It's done with gas (forget which, but not helium) and in the old days it was done with -- eek! -- mercury vapors. Apparently it works well, but must be done in advance. Also, might not keep well after hyping.

Anyone else have other info?

Jeff "hyped up himself" Kreines

The gas is Hydrogen. There was an article in Scientific American Magazine in the late 70's (???) which described the process. I tried it at the time with some B&W emulsion - and can say that it works....but was more bother than it was worth.

Paul Gaffney

I was recently told that it takes Three photons to Activate a grain of silver. Of course this has to be a generalization as Film ( color film anyway) is made up of ten different layers. with three different layers for for each of the colors. A slow , medium, and fast. Latensification sounds like just an extremely low level flash. Is it only for Black and white? Does there make a difference Pre or Post exposure?

Steven ( Nit picking over three photons) Gladstone

If it gets much less than 3, we'll have the quantum physicists after us. Never mind about the speed of each layer. The faster emulsion layers have larger grains in them (strictly crystals of AgBr, not grains yet). Being larger, they present a larger surface area to the stream of photons, and collect more direct hits sooner. 6 (or 3) photons is enough for any crystal, however big. That's why fast films are grainier.

I've learnt (since my last posting, and thanks to Walls & Attridge, Basic Photo Science, Focal Press) that latensification in normal photography is done _after_ the image exposure. The long duration of 15-30 mins at very low intensity takes advantage of reciprocity failure to minimise the fogging effect. 6 photons in less than a second will expose a grain: 6 (or even more) photons in half an hour won't, as the effect of the frist one has decayed before the last one arrives. So unexposed film isn't fogged at all. However, slightly exposed film already has a few photon captures recorded, so it only takes a couple more to start the image effect.

But in practice, how effective is it? Anyone know?

Dominic Case Atlab Australia

Explain this to me. As I understood it, what flashing does essentially is that it raises the toe of you curve into the latitude, as a straight horizontal line at, say, 3 1/2 under within the appropriate exposure time (pre OR post). I have post-flashed before, but never pre-flashed. I can't understand why, in theory, pre wouldn't do the same.

Flashing in basically a double-exposure so what does it matter the order in which both exposures are taken? In theory, pre and post should yield the exact same result. No?

Serge Marcotte.

Apparently it does work. Gives perhaps 2 stops speed gain with no additional grain, which is why I am interested.

You should do a little test... and tell us all!

--Jeff "that's low LIGHT, not low life" Kreines

I have Pre flashed, I never Post flash. this is based on tests done with stills. I felt that Post flashing brought out more grain, even though they were flashed the same amount. The nice thing about the stills, was that by happy accident, I had offset the frames and so only half of each frame was flashed. Flashed some 7277 in a test once with an extremely low level of CTB ( Accidentally, I meant for a higher flash). The shoot was tungsten balanced. Gave the film a really nice snap though. Much nicer than unflashed, and certainly better than the heavier flashes.

Steven Gladstone

It's not the same thing as flashing at all. Flashing increases the base fog and shortens the dynamic range.

See Dominic Case's excellent explanation to understand how Latensification works -- it's the slow exposure that permits the photons to accumulate and kick over those grains that have gotten some exposure.

Jeff Kreines

Gentlemen,

have been following this thread. Thanks for dredging this one up Jeff K. and thanks for following it down Dominic- anyone know of a lab that offers this procedure currently?

2 stops with no fog. But only in the lower blacks, toe, (ZONES 1-2?)

I wonder what this looks like... anyone suggest a film known to have undergone this process for a video rent look see?

Caleb "no plans to build a lab anytime soon" Crosby

Sounds intriguing, OK for stills, but not really practical for motion picture. Also, I worked out the exposure, it's a normal fogging light plus a 4.60ND filter. (That's a stack of 5 x ND9s then a bit more). That's about as dark as my darkroom anyway. The calculation's there, free for anyone who wants to try.

Dominic Case Atlab Australia

My e-mail has been fouled up for the past 2-1/2 days (local problem) and as a result it can't read out a good number of the posts that are in the In box, including several on the latensification thread. So someone else may have covered the following:

These are some gleanings on the subject taken from C. B. Neblett's book, PHOTOGRAPHY PRINCIPLES AND PRACTICES, a 1942 edition (these techniques seemed to have been more widely attempted back in those days, due to limited emulsion speeds.)

Neblette: Hypersensitizing was done with ammonia, ammoniacal silver chloride, mercury, or exposure to a weak source for a period of 30 min. to 1 hr. after exposure in the camera. In the case of the ammoniacal silver chloride, plates were immersed for 2 min. at 65 F, & dried as quickly as possible w/o heat. Speed increase was 2x to 7x, depending upon the emulsion; slower emulsions showed greater effect than did faster emulsions. Treated films would keep only a day or two before fog began to show.

Exposure to mercury after exposure for 20 to 30 hours at room temp. increased speed 2x - 2-1/2x varying with emulsion type; some emulsions didn't respond at all.

Bathing an exposed emulsion in a dilute solution of hydrogen peroxide for a few minutes at room temp. increased the speed, but it varied with the emulsion and the pH, so it was impractical.

After exposure in the camera, an exposure of 30 to 60 min. to light of such an intensity as to produce a fog density of approx. 0.2 increased the speed 2x to 4x. The effect was greater on slow emulsions than on faster ones, and contrast was reduced, so greater development was needed. The speed increase was NOT obtained with shorter times of exposure at greater intensity levels, nor if done before camera exposure.

Again, these are of 1942 vintage. They would seem to be of limited value today, in view of the fast emulsions and special processes available.

--Wade Ramsey

>At 15 minutes per frame exposure?

Well, on a tall continuous rack in its own room, with lights on both sides, the throughput wouldn't be all that bad. That's how I'm building mine, if I ever do. (The room is built... but it's become a bit of a storage area...)

Brings to mind a lab that decided to do flashing the cheapo way. The built a chamber onto their processing machine. It did work, but the speed variances led to exposure variances...

Jeff "someday someday" Kreines

Jeff,

thanks for the vote of do-ability on latensifying. The way I figured it if you ran frame one thru the box at a governed speed and arranged an equitable light path for the train to follow- it could travel steadily- if not quickly. (like you say some hieght would be imprtant- but that would create a prob with the even light path. yes?)

Ideally, I guess it would be long and flat, like a stretched railcar and go thru several successive boxes up or down along a wall. say 20 18" X 18" boxes stacked atop eachother - each about what? 50 feet long? depending on what you had more of to spare, headroom or carpet area.

The flat design would let the light be on top (and bottom) and remain even- speed then would just be a function of travel duration. low powered fourescent tubes would seem ideal. either that or lots of sockets.

I'm just speculating but why would this take any longer than the process bath?

just have to make plenty of long boxes that are wired and light proof- or light proof the room.

Either that or a centrally located lab, preferably in ohio, could install junction boxes (reallly long 18 x 18's) that follow the phone lines out to several states- and we could feed our underexposed footage right out of the changing bag into a spigot type thingo that would latensify the film en route (kind of a slow boat to Ohio type deal) and the lights could be on dimmable system depending on the mileage incurred. One easy swipe of the bank card and...

wait, isn't there any way to latensify IN POST??

Vote Crosby in '98 "a lab on every block."

Helmbelly wrote :

>I'm just speculating but why would this take any longer than the process bath?

You're right - it wouldn't. I'm just intrigued by the low light levels required - enough NOT to fog film in 15 or 30 minutes. I reckon (real back of an envelope stuff here) that a _single_ 100W tungsten lamp would do the job in ten minutes at a distance of ten metres with a 3.00ND filter. (That's black with a capital B). Smaller chamber, more filters needed. Any light leak would spell disaster!

Easier to buy a faster film stock.

Dominic Case Atlab Australia

The still people use a 7.5W light bulb, in a safelight housing, at 10 feet. But there are smaller bulbs than that...

And there ISN'T a faster stock! We're talking available darkness!

Jeff "make mine toe" Kreines

>just have to make plenty of long boxes that are wired and light proof- or light proof >the room.

Mine is a room 15 feet long, 4 feet wide. Rollers in the center, very dim lamps at each end. Depending on whether it's 16mm or 35mm, up to 48 strands, 8 feet tall, so, that's nearly 800 feet of exposed surface footage.

At a 15 minutes exposure, the speed would be 53 feet per minute. My racks probably will be shorter, but it's not that bad. This is a personal lab, not looking to run a lot of volume.

Boxes isn't a great idea, because you really need some serious distance between the lamp and bulb. In my case, I was folding the path -- lamp right near the film, facing away from the film, bouncing off a wall and back, for a nicely diffuse light source from an effective distance of 14 feet or so.

I'll try and test it, at least with short strips, in the next 3 months.

Jeff "darkness on the edge of town" Kreines

Years ago, Neblette claimed fast stocks didn't show much improvement, compared to slower stocks. Interesting to see if that still holds true with fast Vision stocks! If 79 can be exposed at EI 2000 or so with no increase in grain, someone better go into business latensifying it!

Wade Ramsey