Re: Sensitivity spectrum of gum (was: Re: Gum problem(s)

----- Original Message -----
From: "Katharine Thayer" <kthayer@pacifier.com>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Thursday, December 01, 2005 1:18 AM
Subject: Sensitivity spectrum of gum (was: Re: Gum problem(s)

> Browsing through an old chapter on dichromated colloids in the printing
> industry this afternoon, I came across something that caught my eye,
> that relates back to this discussion of a couple of weeks ago, and
> after reading that, then I re-read the section in Kosar on sensitivity
> as well. It looks to me from my reading that Martin has an
> interesting point here, although the data don't speak entirely with
> one voice.
>
> According to Kosar, the dichromates are "efficient absorbers of
> ultraviolet, violet and blue radiation, with maximum absorption by
> ammonium dichromate at 3670 A and potassium dichromate at 3575 A." It
> seems, as Martin says, that most people just measure the sensitivity
> spectrum of the dichromate solution rather than the sensitivity of the
> dichromated colloid layers. Two different sets of researchers who
> measured the dichromated colloid film found, for albumen and for some
> unnamed "resins," that the sensitivity of the mixed colloid is the same
> as for the dichromate. " But for other colloids, it seems that's not
> the case, and here's the part that made me sit up and take notice: A
> researcher named Jorgenson found that the spectral sensitivity of
> dichromated gum arabic was shifted toward the green or longer wave
> lengths compared with albumen. "Spectrophotometric curves of albumen
> and gum arabic showed that this shift was probably due to the
> difference in the spectral absorption of the colloids. "
>
> I can't seem to find in Kosar any direct information about dichromated
> gelatin, although he cites one study of "dichromated colloid film,"
> without specifying the colloid, as finding the sensitivity to be
> maximum between 3500 and 4500 A, another study, again with colloid not
> specified, that finds a spectrum similar to what Sandy describes,
> except it goes to zero at 580 rather than 520.
>
As I already mentioned, exposing dichromated gelatin at 532nm (green) is a
fairly common thing in holography. And the light sources involved there are
by no means necessarily in the watts of power range. Incidentally, US
4254193 explicitly describes the (holographic) exposure of dichromated gum
arabic at 488nm (blue).
With regards to the recording material I don't think it matters a lot
whether the recording is photographic, holographic or lithographic.
By the way among the recording materials used by Gabriel Lippmann (for
"Lippmann photography") were dichromated colloids (albumin and cellulose).
So it's certainly fair to assume at least some sensitivity to visible
wavelengths.

There are not many places to get commercial dichromated gelatin plates ready
for recording. One would be Slavich in Russia. If you go to
http://slavich.com/technical.htm, click on "PFG-04"..
Figure 1 shows a diagram of the spectral sensitivity of those plates. Not
sure why, but curiously, it shows a peak sensitivity around 410nm. That's
undoubtedly a still visible wavelength...

Martin
Received on Thu Dec 1 05:39:45 2005