Safelight

[Dinesh]

[quote:ded5955832]I guess most holographic silver halide emulsions are very forgiving with
respect to additional incoherent background white light illumination
(provided that it's below a certain threshold of intensity/time)[/quote:ded5955832]
It never occured to me, but thinking about it, I assume a small increase in the background "DC" level would not much matter.
The intensity is given as
I = I_1 + I_2 + 2I_1I_2cos(<phase difference>).
the last term produce the fringes, but the first two terms woud produce an increased background intensity - what I call a "DC" level. If the incoherent exposure is at a low enough level that it takes place at the 'toe' of the H&D curve, it'll be non-linear. This would mean that the increase in overall density will be small relative to the fringe density provided that the incoherent exposure is small relative to the coherent exposure. That is, the ratio
(I_1 + I_2)/(I_1I_2) would not change much.
This would require that the fringes themselves were as 'contrasty' as possible which, in turn, has polarisation implications.

[Sergio]

I'm not sure about the similar result when you stated that incoherent illumination on the fringe structure and silver halide crystal folow the formula: The intensity is given as
I = I_1 + I_2 + 2I_1I_2cos(<phase difference>).

the phase difference does not counts but the silver halide conduction band have a wide channel energy for the incoherent light, mainly blue, so the effect is "holographic" on fringes or "semicoductor" diretly over grain?

[Dinesh]

Actually, thinking about it further, I should have said that for two coherent beams the intensity,
I = I_1 + I_2 + 2I_1I_2cos(<phase difference>).
However, if there is simultaneous uniform incoherent illumination I_3(lambda) then the total is:
I + I_3(lambda).
Since I_3 is incoherent, the sensitivity of the medium will determine the effect at any given wavelength, which would depend on the wavelength distribution in I_3, hence I_3(lambda). If the proportion of the "actinic" wavelenght in I_3 is not very strong in the incoherent illumination, the amount of light that hits the film from the incoherent illumination is relatively weak. However, the gamma of the film determines tha increase in density due to I_3 and the fringe contrast, determined by I, gives the DE. So if the incoherent I_3(lambda) at lambda = (for example)633 is low enough to be at the foot of the H&D curve, the developer gamma may not increase density due to exposure by I_3 alone by very much and so the decrease in fringe contrast from just exposure by I as a ratio of exposure by I+I_3 may be very low. if we let density (after development) be rho:
d(rho)/dI_3 << d(rho)/d(I)

[Martin]

"It never occured to me, but thinking about it, I assume a small increase in the background "DC" level would not much matter."

Yes. It might be interesting to find out wether techniques used for certain
photopolymer systems (e.g. broadband pre-exposure etc.) equally work
for DCG.
I think we briefly discussed this in the context of latensification
and hypersensitization. Given the little that is really known of the Cr
crosslinking process, I wonder whether or how much techniques like using additional incoherent light exposure impact on issues like DE and
sensitivity (with respect to both speed and recording wavelength).

[Dinesh]

I was also wondering what the rate of "actininc reaction" was, ie the physical change caused by the actinic reaction. There were some posts a while back dealing with table stability which caused me to think that there is an assumption that the exposure is linear. I wondered whther the physical change - crosslinking, Silver speck creation etc - was a linear function of the exposure. If so, motion throughout the exposure would be equally important. However, if it wasn't, then assuming an exponentially increasing physical effect due to the exposure, motion in the early part of the exposure may be more crucial than motion in a later part of the exposure. Also, does this rate of physical change depend on the kind of reaction? Is it, for example, linear for Silver and exponential for DCG?

[Sergio]

I was also wondering what the rate of "actininc reaction" was, ie the physical change caused by the actinic reaction. There were some posts a while back dealing with table stability which caused me to think that there is an assumption that the exposure is linear. I wondered whther the physical change - crosslinking, Silver speck creation etc - was a linear function of the exposure. If so, motion throughout the exposure would be equally important. However, if it wasn't, then assuming an exponentially increasing physical effect due to the exposure, motion in the early part of the exposure may be more crucial than motion in a later part of the exposure. Also, does this rate of physical change depend on the kind of reaction? Is it, for example, linear for Silver and exponential for DCG?

No, for some polymer holography the ratio is not linear, per example the silicon oligomer material have most of polymerization made after the END of exposure, here the polymerization is greatly enhanced by the heat apllied, here there is another holographic polymerization model, the actual limits the diffusion equation under exposure, Loukas Paraschis counts the Post-exposure Grating Development Contributions to describe with success the polymerization.

The Polygrama photopolymer uses another mechanism that is almost a self enhanced development, itself related to the physical structure, so the final result is not linear with exposure, although the DE was "amplificated".

[Dinesh]

[quote:162b63709d]No, for some polymer holography the ratio is not linear, per example the silicon oligomer material have most of polymerization made after the END of exposure [/quote:162b63709d]
Interesting. Then how does the monomer know where to polymerise?

[quote:162b63709d]The Polygrama photopolymer uses another mechanism that is almost a self enhanced development, itself related to the physical structure, so the final result is not linear with exposure, although the DE was "amplificated".[/quote:162b63709d]
So there is some kind of "latent image" weak pre-polymerisation? Does the latent image polymerisation act as a catalyst of nearby monomers ?

[Sergio]

Interesting. Then how does the monomer know where to polymerise?

Of course; initially, as with cationic ring openning polymerization mechanism the initial chain is formed at light areas, then since the polymerization evolution is high temperature dependent, you could "develop" the hologram with heat, up to 90% of polymerization value, a very interesting effect.

So there is some kind of "latent image" weak pre-polymerisation? Does the latent image polymerisation act as a catalyst of nearby monomers

Matter of fact all the photoinitiation mechanism, polymerization and chaintransfer are realized at same time in this emulsion, no latent effect, however I believe that silicon acid mechanism have space for development of a efficient one.

[Martin]

Matter of fact all the photoinitiation mechanism, polymerization and chaintransfer are realized at same time in this emulsion, no latent effect, however I believe that silicon acid mechanism have space for development of a efficient one.

The question then becomes, what would a latent image in photopolymers "look" like (in contrast say to silver halides and DCG)?

[Sergio]

I do not see in the "traditional" polymerization method, the chain reaction is a triggered mechanism that will not stop in vinyl acrylate systems until termination, on other side in cationic ring open mechanism (epoxies), due the heat sensitivity, the total polymerization can be delayed at some extent that we can talk in latent image formation here. Transmission holograms can become high sensitive as the border of some silver halide materials.