Re: Some points of ponder

From: bsinger <bsinger@sasktel.net>
Subject: Re: Some points of ponder
Date: Thu, 15 Jul 2004 09:32:54 -0600

> You say you can "dye films yourself". I've never heard of this.
> Can you point me in the direction of information on how to dye
> ordinary film to make it more IR sensitive?

Although I discourage you to go this route if the motivation is the
apparent cost, this may be the only option in some cases. The
procedure is to make a very dilute solution of infrared dye of
suitable kind, usually in methanol or other solvent, and immerse the
film in thie bath for a certain amount of time (typically 1-3 minutes)
and dry, all in total darkness. This works the best with undyed or
"color blind" material, but it is reported to work with commercial
panchromatic films.

Silver iodobromide is practically only sensitive to blue-green and
this is called "color blind." If the film or plate needs to be
sensitive to gree, yellow, orange, red or infrared, there needs to be
something adsorbed onto the silver halide crystal that accepts photon
of lower energy (such as red light) and inject electron into the
silver halide crystal in an efficient way. This is called sensitizing
dye. Dyes like eosin and erythrosin are known as among the earliest
sensitizing dyes to make dry plates sensitive to green light. Great
many dyes were subsequently found for the same purpose with better
performance, different sensitivity pattern, etc. Indeed, silver halide
photography was the leading engine for dye research as well as gelatin
research.

Even a same dye can exhibit different spectral sensitivity depending
on how and how much of the dye is adsorbed on the AgX crystals. One
extreme is highly aggregated adsorption of dyes utilized in color
materials (and multicontrast enlarging papers). This type of dye use
gives very sharp cutoff at the longer wavelength end of the sensitive
range. But the other extreme of very sparse dye adsorption is
preferred for infrared plates and films. This makes dying materials
for IR by immersion very practical. Of course the dye can be added to
emulsion while emulsion is being chemically digested or at some other
stages.

Many early dyes were inferior in that their adsorption was poorer,
especially in panchromatic and infrared dyes. This is why early
panchromatic plates were very slow and high contrast. Very fine
grained emulsion was the only practical way to increase surface area
to enhance dye adsorption. Infrared dye molecules are larger and some
of the disadvantages remain. If you immerse commercially available
films in infrared dye bath, I'd try a slow film first.

Classic infrared dyes are dicyanine, kryptocyanine and neocyanine.
When I wrote him last time, Filip Uhlik had success in immersing Tri-X
or something in neocyanine solution to sensitize the film to infrared.
The dye I have experience with is 3,3'-diethylthiatricarbocyanine
iodide. This type of dye can be systematically "tuned" to different
wavelengths by changing the length of the bridge (referring to the
shape of the molecule) and sensitizing property is also perferrable.

I don't know about dicyanine and kryptocyanine, but these dyes can be
purchased from some specialty research supply houses because these
dyes are used in laser research today. They typically cost $40 to $100
per gram.

I skipped a lot of details but I should note that only some dyes are
suitable for AgX sensitizing purpose. Even if the spectral absorption
is close to what you need for sensitization, the key is that the dye
must adsorb well onto the AgX crystals, and must be able to transfer
electrons (and in some special cases energy) to the AgX
crystals. (Dyes are not coloring the film base or gelatin binder --
these can only decrease the sensitivity.)

I would like to repeat -- if the money is the motivation, forget about
it. But if you want to do IR in 6.5x8.5 or 5x7 negative, there may be
no other way than DIY approach. (this is my case...)

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)