Film Thickness

Tony · Post by **Tony** » Thu Jun 07, 2012 8:49 am

Emulsion thickness - Thicker more narrow banded less light gets through, thinner more broadband

Can someone explain this to me again? I found this to be the opposite. No formal experiment but just in the formulations I have used thinner gives better narrow band.

BTW where is Dinesh? I have coffee and nothing good to read. Guess I'd better go back to my Kim kardashian blog on particle physcis

dannybee · Post by **dannybee** » Thu Jun 07, 2012 9:01 am

Tony wrote:Emulsion thickness - Thicker more narrow banded less light gets through, thinner more broadband

Can someone explain this to me again? I found this to be the opposite. No formal experiment but just in the formulations I have used thinner gives better narrow band.

BTW where is Dinesh? I have coffee and nothing good to read. Guess I'd better go back to my Kim kardashian blog on particle physcis

Dinesh is hidding in his cave

Johnfp · Post by **Johnfp** » Fri Jun 08, 2012 6:08 am

I think you may be misusing the word Thick. The difference between a 4 micron and 8 micron coating will for sure yield differences for DCG but I am not sure you can actually call 8 micron thick emulsion a thick hologram (These are just examples to get the point across not actual quotes from anyone.)

The key to broad band vs narrow band is getting the fringes gradually thicker as you go deeper. This comes down to how the emulsion is processed with alcohol. A quick processing dries the top layers and forces water down in the emulsion where it swells the fringes to then be finally dried. This change in fringe spacing yields a broadband hologram. With that, if you really want broad band you final swelling of the very top fringes need to be blue (probably best to start with the bluest wavelength) and the deeper fringes need to gradually go to red.

Now use the same process in a thinner emulsion and there aren't any deeper fringes to get swelled. So it is more likely to have all the fringes darn near the same thickness.

Now a thick hologram, lets say 25 microns or so takes a very gradual and slow processing to reach all the fringes. If you try to make it broadband then the deeper parts of the emulsion do not get dried and you will have trouble getting a hologram at all. So, in my opinion without actually trying it, a thick emulsion forces us to dry it slow which yields a narrow band hologram. The nature of thick emulsion also requires longer alcohol dry times.

dannybee · Post by **dannybee** » Sun Jun 10, 2012 8:47 am

I think the confusion here is that thick film is use for narrow band because in a thick film you have more mirrors stacks, but it also dependent on how it’s processed. With a thinner film you have less mirror stacks and if you use 99% iso one bath hot you will get broad band holograms, coins almost look gold, copper, and silver.
For master it is best to get as many narrow band (mirror stacks) as possible to get a bright narrow band master…maybe Dave could comment on this he has work out a method to come up with very good masters
Also is the type of dichromate used if you shooting in 457 you might try potassium dichromate

Tony · Post by **Tony** » Mon Jun 11, 2012 8:34 am

quote="Johnfp"]If you try to make it broadband then the deeper parts of the emulsion do not get dried[/quote]

Thanks Danny and John

John in reading your comment here I had another question. I measured my heat gun once and it is well north of a 100C for sure. If the emulsion assumingly gets that hot, where does the water go? I find it hard to believe it stays trapped and not dry.

dannybee wrote:For master it is best to get as many narrow band (mirror stacks) as possible to get a bright narrow band master…maybe Dave could comment on this he has work out a method to come up with very good masters

Thanks Danny, tryed again with a thicker film and fixed it for a long time, got a nice yellow/green and when I reprocessed it, it was pretty dim. Really odd.
I can only think that my making it thicker I've effectively have more AmDi and that could be adding to the variable.

Johnfp · Post by **Johnfp** » Wed Jun 13, 2012 4:27 am

The key to broad band vs narrow band is getting the fringes gradually thicker as you go deeper. This comes down to how the emulsion is processed with alcohol. A quick processing dries the top layers and forces water down in the emulsion where it swells the fringes to then be finally dried.

Now a thick hologram, lets say 25 microns or so takes a very gradual and slow processing to reach all the fringes. If you try to make it broadband then the deeper parts of the emulsion do not get dried and you will have trouble getting a hologram at all.

I highlighted points to ponder.
I am not sure where you think that the inner fringes do not get dry. First of all, most of the "water" drying occures in the alcohol baths not the air drying part. While I am sure a very little bit of remaining water (1 or 2% after the 99% alcohol) does get removed but it's mainly the alcohol you are forcing out of the emulsion at that time.

In the case of a thick hologram. Trying to dry it quick with alcohol, does force some water down in the emulsion where it will remain and ruin the hologram. Unless you leave it in the alcohol for a very long time. SO to recap. Say I have a thick emulsion (30um) and put it directly in the 99% alcohol. Then I leave it in there a minute or so like you would so with a thin hologram. I doubt seriously the water deep in the emulsion has had a chance to get removed via the alcohol. So, the hologram will not appear. We may be able to leave it in the alcohol for quite some time to dry it out but that may not be the best process for that hologram. Better off taking a thick hologram and drying it very slowly, that is, multiple alcohol percentages and long times in each bath.

Johnfp · Post by **Johnfp** » Wed Jun 13, 2012 4:42 am

I think I should start using a different word for drying when it comes to alcohol. I looked up such words as dehydrating, solubilize, etc and could not really find one. Do we have a consensus on using a different work for drying with alcohol and drying with air such that there is not confusion in the DCG process?

I am not a chemist but in looking up Isopropyl in the wiki I found some interesting things.

http://en.wikipedia.org/wiki/Isopropyl_alcohol

Unlike ethanol or methanol, isopropyl alcohol can be separated from aqueous solutions by adding a salt such as sodium chloride, sodium sulfate, or any of several other inorganic salts, since the alcohol is much less soluble in saline solutions than in salt-free water[2] The process is colloquially called salting out, and causes concentrated isopropyl alcohol to separate into a distinct layer.

Like most alcohols, isopropyl alcohol reacts with active metals such as potassium to form alkoxides which can be called isopropoxides.

It does not remove water from gasoline; rather, the alcohol solubilizes water in gasoline.

Isopropyl alcohol vapor is denser than air and is highly flammable with a very wide combustible range. It should be kept away from heat and open flame. When mixed with air or other oxidizers it can explode through deflagration. (This is why I do not recommend microwaving it. Just one little spark and you may have a very bad issue.)

holomaker · Post by **holomaker** » Wed Jun 13, 2012 8:13 am

Hi Guys! The films processing speed (fixing step), has a lot to do with the final holograms replay color. If the emulsion is thick it will require a longer time period for the fixer to do its magic deep into the bottom layers of the emulsion causing the top layers to receive longer fixing than the deep layers, hence causing a gradient effect through out the films thickness. Now when using a thin emulsion its quite easy to produce a narrow band hologram as all the layers receive the approximate same time being fixed, causing a non gradient effect ...................

As far as the microwaving Alc, I’ve done it for years with no problems, what scares me are baths of 99% alch sitting on a heater of some kind producing constant flammable vapors. With each hologram i shoot i place my glass vessel in the oven w/a glass plate cover, its easy to see if it starts to boil, now if you walk away and allow it to continue boil, surely bad things may happen.By keeping a record of volume of ALC used and time in the M-Oven the Alc can be heated easily/safely every time.

dannybee · Post by **dannybee** » Wed Jun 13, 2012 8:20 am

ditoos on the microwaving I do it too with no problems...yes fixing is the key! and if you dont fix the thick ones right you will come up with white reflectors

Tony · Post by **Tony** » Wed Jun 13, 2012 8:28 am

holomaker wrote: If the emulsion is thick it will require a longer time period for the fixer to do its magic deep into the bottom layers of the emulsion causing the top layers to receive longer fixing than the deep layers, hence causing a gradient effect through out the films thickness.

Great to see a DCG discussion.

A quick question Dave. Would this apply to light fixing or heat fixing? I would imagine that those methods would not result in as much layers. Light fixing you can flip the plate if you want.