Archived from groups: alt.comp.periphs.dcameras (
More info?)
"Old Nick" <nsnfwhite@dodo.net.au> wrote in message
news:g0ob70d42lr34t5ln5865hpdkll5c8bqh2@4ax.com...
> On Thu, 8 Apr 2004 11:34:06 -0400, "Marvin Margoshes"
> <physnospamchem@cloud9.net> vaguely proposed a theory
> ......and in reply I say!:
> remove ns from my header address to reply via email
>
>
> >> I suppose there must be a molecular or cellular discrete level
> >> involved! <G>
>
> >Each silver halide particle in a photographic emulsion is digital. It
must
> >absorb two photons to become developable. It is a go-no go situation.
> >Absorbing one photon won't do the job, and absorbing more than two won't
be
> >different than absorbing two. When the blackening in the developed
emulsion
> >is measured in small areas, one is effectively counting silver particles,
> >and the counting statistics control the measurement noise.
> >
> >
>
> OK. As I said, I feel that using "analogue" to describe film is
> questionable, or not usual.
>
> But I went looking to see how things had fooled my failing brain.
>
> The stuff below seems to contradict you in both essence and fact,
> unless you feel tha molecular/atom level (which I had mentioned)
> counts. If electron levels and atoms are what you are talking about,
> then everything is digital. Photons are digital.
>
> I get the distinct impression that the _grains_ can have many levels
> of interaction with light, and behave accordingly.
>
> This is how I remembered it, but I use quotes to try to back me up.
>
>
http://science.howstuffworks.com/film4.htm
>
> "When a photon of light is absorbed by the spectral sensitizer sitting
> on the surface of a silver-halide grain, the energy of an electron is
> raised into the conduction band from the valence band, where it can be
> transferred to the conduction band of the silver-halide-grain
> electronic structure. A conduction-band electron can then go on to
> combine with a positive hole in the silver-halide lattice and form a
> single atom of silver. This single atom of silver is unstable.
> However, if enough photoelectrons are present at the same time in the
> crystal lattice, they may combine with enough positive holes to form a
> stable latent-image site. It is generally felt that a stable
> latent-image site is at least two to four silver atoms per grain. A
> silver-halide grain contains billions of silver-halide molecules, and
> it only takes two to four atoms of uncombined silver to form the
> latent-image site.
It doesn't disagree at all: "It is generally felt that a stable
latent-image site is at least two to four silver atoms per grain."
Regardless of whether two, three or four photons absorbed, you get one
silver particle after development.
Generally speaking, I'd expect that a photon flux large enough to result in
three or more latent image sites in more than a very few silver halide
grains will be over-exposure.
> In color film, this process happens separately for exposure to the
> red, green and blue portions of the reflected light. There is a
> separate layer in the film for each color: Red light forms a latent
> image in the red-sensitive layer of the film; green light forms a
> latent image in the green-sensitive layer; blue light forms a latent
> image in the blue-sensitive layer. The image is called "latent"
> because you can't detect its presence until the film is processed. The
> true photoefficiency of a film is measured by its performance as a
> photon detector. Any photon that reaches the film but does not form a
> latent image is lost information. Modern color films generally take
> from 20 to 60 photons per grain to produce a developable latent
> image."
>
>
> ****************************************************
> I went on a guided tour not long ago.The guide got
> us lost. He was a non-compass mentor.........sorry
> ........no I'm not.
Facebook
Twitter
Instagram