Last month I successfully developed a Denisyuk single beam reflection hologram using a coffee-vitamin-c developer. If you are not familiar with coffee developer (also called "caffenol") -it's a well known fact that coffee has a developing action very similar to metol. When combined with vitamin C (ascorbic acid), another "alternative" developer, it forms a superadditive combination whereby the resulting mix yields better results than the sum of its individual parts. For more see: http://caffenol.blogspot.com/
Anyhow, the success here was limited -more of a proof of concept than anything else... the image was very faint -almost invisible in white light. In laser light it reconstructs more brightly -and this reveals a slightly grainy/noisy image (though with good depth).
Photographers have reported that coffee developer will "fog" film slightly -generally producing low contrast, low density images. As far as I understand it, this is the opposite of what we want in a holographic developer -higher contrast and higher density would yield clearer brighter fringes right? Some photographers have suggested the use of a restrainer (such as potassium bromide) to yield higher contrast with coffee developer -I have not tried this with holography yet.
I also wonder about the appropriate bleach to pair with a coffee developer. I used what I believe is a "re-halogenating" or "diffusion-transfer" bleach (from the integraff JD-4 processing kit). I selected this bleach because I thought the re-halogenating action would increase contrast and density....
I wonder if there may be a better bleach to use with the coffee to improve brightness/reduce noise?
I wonder if anyone has any suggestions for improving the method -especially related to the best bleach and improved recipes for the developer?
Setup:
I used a laser diode (labeled 15mw, in practice I estimate it is more like 8-10mw, 635nM, ordered from allelectronics) at a distance of about 35cm, using the diverging beam straight from the diode incident to the plate at 56 degrees from above. 10.6 volts supplied to laser by a lab power supply. Subject was a white porcelain curved lid, in contact with the plate and falling away to a depth of 2cm. Plates were PFG-03m 63mm square glass plates ordered from integraf.
Exposure and Processing
Laser was allowed to warm-up for 1 min. Tests were made at 2,4,8 and 16 seconds. The best exposure was at 16 seconds. Prewashed in distilled water 1min. Developed for 6 min 30 seconds to a density just over that of an ND 1.2 or 6.25 percent transmittance (judged by comparison by eye through green safelight). Washed in distilled water 2 min. Bleached 1 min. Washed in distilled water 2 min. Photoflo 20 sec.
Chemistry:
Coffee Developer (I apologize for the rough measurements. I plan to repeat experiments with a gram scale):
2 cups Albertson's brand "bold and robust" french roast ground coffee
add four cups boiling distilled water -steep one hour.
Filtered 3 times -through doubled up coffee filters.
Add 3 heaping teaspoons ascorbic acid
Add 1 cup Baking Soda (or use Sodium Carbonate)
Add water to make 6 cups working solution with a PH of about 8.
Bleach Bleach Solution from JD-4 kit (1000ml)(see http://www.integraf.com/Downloads/JD-4.pdf):
Copper Sulfate, Pentahydrate 35 grams
Potassium Bromide 100 grams
Sodium Bisulfate, Monohydrate 5 grams
ADD Distilled water to make 1000 ml
Sincerely,
Tristan Duke
Coffee Developed Hologram -proof of concept
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Dinesh
Coffee Developed Hologram -proof of concept
There is a difference between holographic developers and photographic developers. Holography is an imaging of very fine structures on the order of 3000 lp/mm - approx 10 milli-inches. So the holography developer has to be able to resolve this very tiny structure size. Whereas photography concerns itself with much larger structures on the order of about 200 lp/mm. A developer that works reasonably well for photography may not work so well on holography because it doesn't have the necessary resolution, ie it can't resolve very small structures.
When you drop a film into a developer, the film begins to darken slowly. After a while it darkens at a pretty constant rate - twice the time results in twice the darkness. After a still longer while the darkeness flattens out, which means that as you go longer and longer, there is no correspondent increase in darkness. Finally, it gets to a point where there is no increase in darkness at all. The rate at which it goes through these stages depends on the exposure. The longer the exposure, the faster does the film in the developer go through these stages. Under-exposed film takes a long time to get to the last stage of no increase in darkness while over-exposed film may get to this stage in seconds. The ability of a developer to go through these stages is encapsulated on what's called the H and D curve ("Hurter Driffield curve"). This is a curve of the density (in the sense of how dark the developed film becomes) against the log of the exposure. That is, it's a D-logE curve. This curve looks like a backwards "S" with an elongated straight line portion. The "toe" of the H and D curve is flat because for the initial time there is no increase in density (darkness) - in fact hardly any density at all. The curve then rises uniformly on the "straight line portion" because now the increase in density is proportional to the time in the developer. Finally the top flattening of the curve happens when there is no increase in density at all. The slope of the straight line portion is referred to as the gamma of the developer.
For holography, the straight line portion, the gamma, needs to be very steep while for photography it can be quite shallow. In your case of the coffee developer, you need to find the h and D curve (if anyone has one!) and see if it has a high gamma. If not, the film may develop, but the results will be poor. Thus the statement:
The bleach does not increase contrast or density. Actually, it reduces them! The function of the bleach is to convert the hologram from an amplitude hologram to a phase hologram. When you first pull the hologram out of the developer it consists of actual dark lines with grey spaces between them. The light has to go through these dark regions and there's a lot of absorption of the light resulting in a weak image. When you bleach, you convert the dark/grey structure to a clear but less hard/more hard structure. You convert the density profile of the lines to a corresponding hardness profile. Now that the "data" in the hologram has been convert from a series of dark lines to a series of transparent lines of varying hardness, the hologram is much brighter.
Assuming your making a reflection hologram, the rehal bleach is probably a little cleaner. The trick in reducing noise is the density to which you develop it. The darker it gets in the developer, the longer it needs to bleach and that introduces noise. This has to do with effects such as grain growth. However, you need to take the film to a density of 2, so you can't develop too light.
I believe Colin put up several bleach formulas from the old forum so they're probably somewhere in the archives. Personally, I've always like the Van Ranesse bleach but that's just a personal preference. Some time back I wrote an essay on silver halide called "Grains", it's probably lurking somewhere also and may (stress: may) be worth a read.
When you drop a film into a developer, the film begins to darken slowly. After a while it darkens at a pretty constant rate - twice the time results in twice the darkness. After a still longer while the darkeness flattens out, which means that as you go longer and longer, there is no correspondent increase in darkness. Finally, it gets to a point where there is no increase in darkness at all. The rate at which it goes through these stages depends on the exposure. The longer the exposure, the faster does the film in the developer go through these stages. Under-exposed film takes a long time to get to the last stage of no increase in darkness while over-exposed film may get to this stage in seconds. The ability of a developer to go through these stages is encapsulated on what's called the H and D curve ("Hurter Driffield curve"). This is a curve of the density (in the sense of how dark the developed film becomes) against the log of the exposure. That is, it's a D-logE curve. This curve looks like a backwards "S" with an elongated straight line portion. The "toe" of the H and D curve is flat because for the initial time there is no increase in density (darkness) - in fact hardly any density at all. The curve then rises uniformly on the "straight line portion" because now the increase in density is proportional to the time in the developer. Finally the top flattening of the curve happens when there is no increase in density at all. The slope of the straight line portion is referred to as the gamma of the developer.
For holography, the straight line portion, the gamma, needs to be very steep while for photography it can be quite shallow. In your case of the coffee developer, you need to find the h and D curve (if anyone has one!) and see if it has a high gamma. If not, the film may develop, but the results will be poor. Thus the statement:
means that holographic film needs a high resolution (to resolve the tiny structural size) and a high gamma (for high contrast). The meaning of the term "contrast" and "density" may be confusing in that the contrast and density of an image is not the same as the contrast (gamma) and density (resolution) of the developer. If you want to know the efficacy of the coffee developer, you need to get the H and D curve.tristanduke wrote: As far as I understand it, this is the opposite of what we want in a holographic developer -higher contrast and higher density would yield clearer brighter fringes right?
The bleach does not increase contrast or density. Actually, it reduces them! The function of the bleach is to convert the hologram from an amplitude hologram to a phase hologram. When you first pull the hologram out of the developer it consists of actual dark lines with grey spaces between them. The light has to go through these dark regions and there's a lot of absorption of the light resulting in a weak image. When you bleach, you convert the dark/grey structure to a clear but less hard/more hard structure. You convert the density profile of the lines to a corresponding hardness profile. Now that the "data" in the hologram has been convert from a series of dark lines to a series of transparent lines of varying hardness, the hologram is much brighter.
Assuming your making a reflection hologram, the rehal bleach is probably a little cleaner. The trick in reducing noise is the density to which you develop it. The darker it gets in the developer, the longer it needs to bleach and that introduces noise. This has to do with effects such as grain growth. However, you need to take the film to a density of 2, so you can't develop too light.
I believe Colin put up several bleach formulas from the old forum so they're probably somewhere in the archives. Personally, I've always like the Van Ranesse bleach but that's just a personal preference. Some time back I wrote an essay on silver halide called "Grains", it's probably lurking somewhere also and may (stress: may) be worth a read.
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Martin
Coffee Developed Hologram -proof of concept
The pH of your developer is in my opinion far too low for solvent development. In order to get high image contrast I'd look for values > 10.5. By the way, you can leave out the coffee (at least in the developer) and make a developer based on ascorbic acid (plus sodium carbonate etc.) only.tristanduke wrote:
Coffee Developer (I apologize for the rough measurements. I plan to repeat experiments with a gram scale):
2 cups Albertson's brand "bold and robust" french roast ground coffee
add four cups boiling distilled water -steep one hour.
Filtered 3 times -through doubled up coffee filters.
Add 3 heaping teaspoons ascorbic acid
Add 1 cup Baking Soda (or use Sodium Carbonate)
Add water to make 6 cups working solution with a PH of about 8.
Due to its low silver content PFG-03 may not be the material of choice for this kind of tests. Solvent-physical development might be a better way of processing PFG-03 when playing with ascorbate/coffee.
I remember having prepared an ascorbate based colloidal developer (unfortunately I can't seem to find that formula anymore but it was made of ascorbic acid, amidol and sodium sulfite). I believe pH was in the 7.0 - 8.0 range. It worked reasonably well - though not on par with the GP-2-like developers.
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Dinesh
Coffee Developed Hologram -proof of concept
Isn't this AAC?Martin wrote: By the way, you can leave out the coffee (at least in the developer) and make a developer based on ascorbic acid (plus sodium carbonate etc.) only.
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Dinesh
Coffee Developed Hologram -proof of concept
So, Martin, what is the pH of coffee?Martin wrote:The pH of your developer is in my opinion far too low for solvent development. In order to get high image contrast I'd look for values > 10.5.
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Martin
Coffee Developed Hologram -proof of concept
Yes - for example:Dinesh wrote:Isn't this AAC?Martin wrote: By the way, you can leave out the coffee (at least in the developer) and make a developer based on ascorbic acid (plus sodium carbonate etc.) only.
ascorbic acid................18g
sodium carbonate ..........80g
water..........................1L
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Martin
Coffee Developed Hologram -proof of concept
At the strength I use for drinking, I measured a pH value of ~5.5 for an instant coffee...Dinesh wrote:So, Martin, what is the pH of coffee?
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konsew
Coffee Developed Hologram -proof of concept
About half year ago I developed hologram in Nescafe classic with excellent result. This is my topic from old forum (with picture example)
http://holoforum.org/oldforum/viewtopic ... e95#p56916
I used Ferric EDTA bleach and Ascorbic acid bath.
http://holoforum.org/oldforum/viewtopic ... e95#p56916
I used Ferric EDTA bleach and Ascorbic acid bath.