How does DCG form an image?

[holorefugee]

I haven't seen that paper for twenty odd years, but at the time it did make an impression on me because I was trying to figure out the physical mechanism behind the effect.

How does DCG make an image?

[Johnfp]

I did quite a bit if research on the "mechanics" of the DCG imaging process. Here is what I found and posted a while ago, hoping some would ellaborate on it. So far, no one has. But it does give some basics.

http://www.holowiki.org/index.php?title ... CG_Process

[dannybee]

in simple form the Refraction of air and gel is different, basically you have harden and soften parts of the gel, when you dehydrate the gel it leaves air bubbles left where the water was, so that ther is a Refraction diffrence between the two ther by changing the phase. so this type of hologram is a phase type hologram

[Dinesh]

Yes, Danny's right. The initial exposure causes a latent image where the "bright" parts of the interference pattern cause cross-linking and the "dark" parts don't. The cross-linked parts are slightly harder than the uncrosslinked parts, roughly 1 part in a 1000. Development magnifies this small difference so that the (harder) crosslinked parts become even harder than the soft uncross linked parts. So that the exposed, but undeveloped, emulsion is about 2% efficient and the developed emulsion amplifies this small difference up to about 40 - 80% efficient (depending on how well you develop it). The actual efficiency is proportional to exp(delta_n), where delta_n is the difference in refractive index. Due to the exponential relationship, the greater the difference, the (much!) greater the efficiency.

There are two conflicting theories on how this amplification works. In one theory, the alcohol draws out the water inside the emulsion, leaving behind bubbles of air. These bubbles have a lower density, and so a lower refractive index than the surrounding material so increasing the small difference from the latent image. In another theory, the rapid extraction of water cracks the emulsion and so amplifies the difference.

So, depending on which side of the argument you fall on, you're either a crack head or a bubble-ist!

[Tony]

There are two conflicting theories on how this amplification works. In one theory, the alcohol draws out the water inside the emulsion, leaving behind bubbles of air. These bubbles have a lower density, and so a lower refractive index than the surrounding material so increasing the small difference from the latent image. In another theory, the rapid extraction of water cracks the emulsion and so amplifies the difference.

So, depending on which side of the argument you fall on, you're either a crack head or a bubble-ist!

So Dinesh, is there anything that occurs regarding cracking pre-exposure? If for example, I place store my film in dry (low RH) environment then place it in a humid one, does it crack and create air pockets? I heard this might occur by a DCG guru once.

I believe I am a crack-head sort. Bubble-ist are way too conservative anyway, besides I think Bill O'reilly might be one....

[dannybee]

crack head or a bubble-ist! maybe its both, some of the bubble's crack under the stress put on them by holographer
thus creating noise in the gel. so the trick is to crack or not to crack

[Dinesh]

So Dinesh, is there anything that occurs regarding cracking pre-exposure? If for example, I place store my film in dry (low RH) environment then place it in a humid one, does it crack and create air pockets? I heard this might occur by a DCG guru once.

If you mean pre-exposure, all you'll do is create noise. Without exposure, there is no latent image and so the entire mass of the emulsion is affected. The latent image will be "superimposed" on the affected gelatin and create noise due to scattering both in and outside the fringes.

If you mean post-exposure, but pre-development, I suspect it'll depend a lot on the difference between the two environments. If the difference is sufficent, I suspect the image will be more broadband.

I think I know the guru. He told me this once and I told him what I just told you.

so the trick is to crack or not to crack

... that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous humidity,
Or to take arms against a sea of alcohols,
And by dilution end them? To soak: to harden;
No more; and by hardening to say we end
The swelling and the thousand natural cracks
That dcg is heir to, 'tis a consummation
Devoutly to be wish'd. To soak, to harden;
To harden: perchance to develop:
(Hamlet - sort of - by Willy Shakespeare with contribution by Danny et moi)

[Tony]

Bravo