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[jrburns47]

Re papers on hologram conservation from Northwestern,
https://www.researchgate.net/publicatio ... ngine_no_9

[ConservationTech]

Hi,

I think this would be a very interesting area of research. As far as I know, there has never been an independent and unbiased evaluation of the permanence or longevity of holograms.

http://resources.culturalheritage.org/p ... 5_Wise.pdf

Thank you, Din, yes it's a great article!

Very useful and kind of the only resource within the art conservation field. As mentioned in the article, the black mount on our SHG hologram is concerning for display and light exposure. Museum conservators like to place mathematical lux maximums on objects, parameters that display teams must not exceed, it would be useful to quantify the threshold of deleterious light exposure through my destructive testing. To aid conservators in quantifying their display parameters.

[ConservationTech]

Thanks, Din, this paper is new to me, and I appreciate it.

The paper is disappointing, because it's more of a "survey" of art, rather than a technical study. It did have an interesting list of potential problems with silver halide materials. I think this might be useful for the original poster.

I've had many a conversation about conservation of holography with Edwina Orr, Jody Burns, Stephen Hart and others. The point I felt that display holographers didn't see was that the method of image production is different from that of a photograph. In photographs, the image is produced by a grey scale variation in the oxidation of the silver based on the grey scale variation of the focused image, while in holography, there is no grey scale, but (ideally) a sinusoidal profile. Therefore the plane (fringe) structure of the hologram was quite important. In technical holography, the plane (fringe) structure is vital to achieve the optical function and whenever I designed a hologram, I was always studied and designed around the desired planar structure. In display holography, the spatial frequency of the planes (fringes) is a factor that does not exist in a photograph and is important, because any decay in the spatial structure will cause aberrations, while any decay of the grey scale in a photograph will not cause aberrations but intensity variations - a photographic image may fade, for example, but not distort or colour shift, over time.

There were interesting points in the paper:

" Holograms do have many physical similarities to photographs and so comparable
considerations can apply"
I'm not sure I agree here, as I've described above

"Accelerated ageing indicated that there was a change in silver particle morphology
and that hologram deterioration was largely catalysed by residual processing chemicals,
accentuated by the small developed grain size in the image silver, resulting in a loss of resolution."
Agree. Stephen Hart used accelerated ageing methods and found a similar result.

"However, it has been suggested, that long term exposure to light can also cause this shift from red to blue, and that this
change can be irreversible, as a result of permanent shrinkage of the gelatin in the emulsion layer.
(Orr, 2012)"
This is presumably Edwina Orr and David Trayner (who, by the way, were my mentors in 1982 along with Peter Miller). I have thought about this, but i couldn't see a straightforward mechanism. One possibility is that light causes hardening and so causes shrinkage, and one way I thought of testing this theory was to record a hologram on just gelatin - no photo-receptors. Of course it needs to be a great deal of light. We (Joy and I) did coat glass with just gelatin and hoped to have Ron Olson record a hologram with a pulse. Unfortunately I never contacted Ron and nothing came of it. So, reading this statement by Edwina, she must have tried exposing a hologram to light and seen a blueshift

Heat is particularly a problem for holograms with a black backing, so lighting of negligible heat as well as UV emission
should be chosen.
This is a statement by Hans Bjelkhagen and I agree up to a point. But, we (Joy and I again) did carry out accelerated ageing under several scenarios and we found that ageing occurred more rapidly with UV than with any other accelerated ageing method. So, UV is more important than heat. The answer is, of course, obvious, since AgBr is particular sensitive to UV (I believe the sensitivity peaks at around 300nm)

I talked to Joy this morning about the spatial frequency issue, and she brought out another point. The original conservator says the original artist would be recognised. But, several of these older holograms are in collections and not owned by the actual artists. Also, older holograms were never "signed", in that very few know who the actual artists were. There was an exhibition of holograms at the Museum of Contemporary Art (MOCA) in Los Angeles, and it was disappointing that none of the holograms on display had the artist's name displayed.

Very rich response. Thank you. I agree that there is a strong discrepancy in understanding the authorship of holograms. Especially those authored by partnership of modern artists and holographic laboratories. As with our hologram, it was only after some digging that we linked its creation to Polaroid Labs and Benton. And for the efforts of conservation, this technical link, to the lab, to the material data -- is of utmost importance.

Given your extensive experience in creating and testing --
1. Would you agree that UV light exposure is the biggest risk to black-backed SHG holograms?
P.S. We found that UV filtering plexiglass did not impair our image reconstruction.
2. Do you agree with the Wise article's statement about heat exposure darkening/dimming the image?
3. What are your preferred reconstruction sources for white light reconstructed holograms?
4. Any ideas for display sustainability?

[ConservationTech]

Re papers on hologram conservation from Northwestern,
https://www.researchgate.net/publicatio ... ngine_no_9

Thank you. Very relevant as it's Benton.

[Din]

Given your extensive experience in creating and testing --
1. Would you agree that UV light exposure is the biggest risk to black-backed SHG holograms?
P.S. We found that UV filtering plexiglass did not impair our image reconstruction.
2. Do you agree with the Wise article's statement about heat exposure darkening/dimming the image?
3. What are your preferred reconstruction sources for white light reconstructed holograms?
4. Any ideas for display sustainability?

To record a hologram, two laser illuminations have to impinge on the plate - one from the uniform reference beam and the other from reflections off the object. This variation causes a physical/chemical changes inside the emulsion; in the case of silver halide, it causes a variation of density (darkness) caused by the silver halide crystals (white) ionising to silver (black). The important point here is that all of these results are caused by physical and chemical changes - called actinic reactions - based on the energy of the laser beam and the physics and chemistry of the silver halide. Therefore all of these effects are radiometric - alterations based on physics, chemistry and measurement. Radiometric is quantifiable by instrument

Now, you process the plate with various chemicals. At this point, either all of the unexposed silver halide crystals have been washed away, assuming the plate is thoroughly washed, or there are still unexposed silver halide crystals (as Joe Farina has mentioned); there are two mechanisms in which the halide crystal still exists in the emulsion. In conversations with Jeff Blyth (Cambridge University) he mentioned that the remaining halide crystals are smaller than the original crystals which may have something to do with the crystals not darkening subsequently. Once developed and dried, the image of the object is seen when reconstructed. However, the seeing of the object relies on vision parameters - how bright, what colour, what contrast etc. This mechanism of seeing something is photometric, based on human perception, which is subjective and involves the brain/eye interaction. It's important to understand that a display hologram is radiometric in the recording, but photometric in viewing.

1. Would you agree that UV light exposure is the biggest risk to black-backed SHG holograms?
P.S. We found that UV filtering plexiglass did not impair our image reconstruction.
I don't think it's the biggest risk, and it probably makes little difference whether the hologram is backed, especially for a reflection hologram. Consider that the hologram is blacked on the back, emulsion side and to view the hologram you allow light to pass through the front glass plate and interact with the emulsion. Any uv exposure is based on the uv passing through the glass and onto the emulsion. At any rate, there are two mechanisms where uv light can damage a hologram: by reducing any unexposed silver halide crystals to silver and by breaking down the gelatin structure. The silver halide sensitivity in Agfa film to uv rises from about 350nm, peaks at about 400nm and drops at about 450. Thus, uv will start darkening the plate due to any remaining silver halide crystals - and there will be remaining silver halide crystals depending on the processing scheme - most processing schemes involve bleaching which retains some silver halide. Also, unless the plate is thoroughly washed, there will be halide crystals and it's well-nigh impossible to wash it that thoroughly. However, silver halide to silver reduction is a slow process, and effects can take between weeks to months to years, depending on the level of uv exposure. I have a silver reflection hologram from 1983 that's still unaffected, but I rarely take it out into sunlight or expose it to a uv source. In terms of the breakdown of gelatin, the gelatin itself hardens, gets brittle and so shrinks, causing the hologram to blue shift. Again, this is a slow process. In other words, simply placing a plexiglass screen and viewing the hologram is not going to show any ill effects. But, prolonged exposure to halogen lights or fluorescent lights (both strong in the uv) will show long term effects. If the hologram is designed to be displayed under uv-rich lighting, be sure to check the transmission of the plexiglass - you want plexiglass to block roughly in the 350 - 450 nm range.

2. Do you agree with the Wise article's statement about heat exposure darkening/dimming the image?
Heat exposure will not directly affect the physics and chemistry of silver halide to silver reduction, unless a lot of heat is present. However, heat will dry the gelatin by driving off water. This drying effect will cause the gelatin to shrink. It's important not to place a heated lamp too near the hologram, but you shouldn't anyway since placing the light source too near the hologram will cause blurring. The sharpness of the hologram is a function of the source size as "seen" by the hologram. If the bulb is too close, the hologram "sees" a larger source.

3. What are your preferred reconstruction sources for white light reconstructed holograms?
Again, it depends. A display hologram is generally recorded by a collimated beam, ie a flat beam. In order to prevent any distortion of the hologram, the reconstruction source should be as close to the original recording source; if the reference is collimated, so should the reconstruction source. But, it's not easy to illuminate the hologram with a collimated source, requiring mirrors and lenses. So, generally, a point source - one with ideally no width - should be placed a longish way from the hologram (but see my comment above on distortion). So, if a bulb with a filament is used, it'll cause blurring because every part of the filament will cause it's own reconstruction. The best option today is an led because it's both small and does not emit heat. But, you have to be careful that the white led is a single emitter. Some 'white' leds are packaged with RGB leds and will create multiple images.

4. Any ideas for display sustainability?
Basically process it very well - wash the bejesus out of it! Be careful how you dry the plate, I lost a whole box of Slavich film by drying it in alcohol, but drying it with a surfactant, such as photoflo, causes *dark reaction and blue-shifts the hologram. Also, limit the exposure of the hologram to light, since prolonged light exposure will cause dark reaction.

*'dark reaction - the tendency of the unexposed silver halide to gradually reduce to silver over a long period. This happens even for visible (not uv) light, but over longer time scales