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  1. #21
    Photo Engineer's Avatar
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    Another factor in this design is the need for identical reciprocity in the 3 components used. If they don't have the same response to HIRF or LIRF then the characteristic curve pulls apart and bumps appear or contrast changes strangely. In addition, the latent image keeping of all 3 emulsions must be the same or you get similar bad things taking place.

    PE

  2. #22

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    Quote Originally Posted by Photo Engineer View Post
    Interestingly, a grain has to be absorbant to light at the wavelength needed, in order to have imaging take place, so the grain is not 100% transparent. In fact it cannot be. So, if you were to observe holographic emulsions at the wavelength being used, the emulsion must have some absorption, even if it is UV or IR.

    PE
    Yes, obviously some degree of (visible light) absorption must take place. I am not sure about the AgX crystals, things can get incredibly complicated with grains substantially smaller than the recording wavelength (Raleigh, Mie scattering)!
    I guess the main part of (red-green) light absorption is due to the sensitizing dye. But this is barely noticeable. A coating with 10nm grains, 7um thickness, looks completely transparent to the naked eye. And even at the 633nm recording wavelength there's no absorption measurable (at least not with a common lab power meter). By the way the low light absorption is key to the recording of simple reflection (Denisyuk) holograms: the recording light enters the emulsion and hits the object behind the recording plate. There, it will be reflected, part of it will reenter the emulsion and interfere with the incoming light – making the holographic recording. The more object light reaching the recording plate in that scheme, the better the condition for high diffraction efficiency of the hologram.

  3. #23
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    Some exitation of something must take place, and that is due to absorption of light. Sensitzing dyes, if present, can absorb a lot of light and that is why they can increase the speed in their primary region of absorption. The layer is thin, the emulsion is small and the energy is very high. This allows the coating to be essentially invisible to the human eye.

    PE

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    Layer thickness aside, the main reason for the “invisibility” undoubtedly is the extremely small grain size. Let's not forget, if you develop this kind of emulsion with a physical or colloidal developer you'll get beautiful yellow filters that absorb light from UV - blue. Looking through such a layer at a point light source, you will notice no haze.
    There's practically no absorption at any visible wavelength. For example even at the 633nm recording wavelength for a red sensitive emulsion overall absorption is extremely low – though there must be high absorption on each light hit (spectrally sensitized) AgX crystal.
    By the way, the energy levels we're dealing with, are pretty weak most of the time. A great many people are doing their exposures with lasers of a few Milliwatts of power only.

  5. #25
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    You are taking a Lippmann emulsion (the tiny AgX crystals) and converting them into Carey Lea silver. This is what usually happens when reducing agent is added to a Lippmann emulsion and you can do it to form red, green or blue silver or gray silver. The anti halation layer in film is made by such a process to form gray colloidal silver in gelatin from fine silver halide.

    PE

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    Yes exactly – with the difference though that Carey Lea formed his colloidal silver layers by direct silver nitrate reduction.

    By the way, I noticed that many patents dealing with the making of “regular” silver halide emulsions, mention the addition of colloidal silver or a small part of Lippmann emulsion.

  7. #27
    AgX
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    PE, Hologram,

    When this Lippmann thing started, I thought everything was said and clear (sic), but amazingly every post reveals new aspects!

  8. #28
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    Quote Originally Posted by Hologram View Post
    Yes exactly – with the difference though that Carey Lea formed his colloidal silver layers by direct silver nitrate reduction.

    By the way, I noticed that many patents dealing with the making of “regular” silver halide emulsions, mention the addition of colloidal silver or a small part of Lippmann emulsion.

    I'm not quite sure I follow this. I can say that a "regular" emulsion often adds silver iodide crystals as one step. This must be done carefully though.

    PE

  9. #29

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    Here are some excerpts from a couple of patents:

    US 6,248,507:
    To facilitate Ostwald ripening it is contemplated to employ fine grain silver iodide emulsions having a mean grain size of less than 0.1 micrometer. The small sizes of the silver iodide grains are chosen to maximize available grain surface area per unit volume and to improve the distribution of the silver iodide at the time emulsions are blended. In a preferred form the silver iodide grain emulsion is a Lippmann emulsion. Lippmann emulsions with mean grain sizes down to about 30 angstroms have been reported, although the typical mean grain size of Lippmann emulsions is about 0.05 micrometer. While the iodochloride prior art referenced above typically states that any iodide ion source may be used in the preparation thereof, it is a critical feature of the invention that fine silver iodide grains be used as the iodide source for~the preparation of the region of the grains containing a maximum iodide concentration.

    US 4,082,553:
    Nicholas et al points out that iodide-free Lippmann emulsions have been used as overcoats to inhibit release of iodide to the developer solution. Nicholas et al notes, however, that these Lippmann emulsions in turn produce disadvantages by silver plating out on transport rollers during processing. To obviate this, Nicholas et al teaches the coating of the Lippmann emulsion layer with a silver precipitating agent, such as metal sulfides, selenides, polysulfides and polyselenides, thiourea; heavy metals and heavy metal salts; fogged silver halide and Carey Lea silver.

    US 6,472,137:
    In another preferred embodiment addition of iodide to emulsion grains rich in silver bromide is performed by adding fine preformed grains of silver iodide, whether or not including bromide and/or chloride in minor amounts, said grains having a grain diameter of not more than 100 nm, and more preferably, not more than 50 nm. Such fine grains are so-called "Lippmann" emulsions. Addition of iodide making use from such fine grains rich in silver iodide has been described for the preparation of {111} tabular grains in JP-A's 04-251241 and 08-029904 and in EP-A's 0 662 632 and 0 658 805, wherein an outermost phase rich in silver iodide has been added to {111} tabular grains rich in silver bromide (optionally comprising up to less than 10 mole % of silver chloride). Addition of said fine AgI-Lippmann emulsions to the surface of the silver halide crystals in order to get a global iodide content of less than 1 mole % in the grain may advantageously proceed as disclosed in EP-A 0 475 191, wherein an excellent speed/fog ratio and a high covering power are attained.


    US 5,879,873:
    A conventional growth step for the precipitation of high bromide grains with {100} crystal faces can then be effected, but with vAg preferably maintained in the range of from 150 to 220 mV to minimize thickening of the tabular grains. Typically growth is achieved by the addition of jets, at a constant or accelerated flow rate, of silver nitrate and alkaline metal halides or, again, a fine-grain emulsion of the Lippmann type.


    US 6,518,009:
    A method of imaging employing sensitized high chloride silver halide emulsions which exhibit improved speed and high intensity reciprocity performance for use with short duration, high intensity exposure optical and digital exposure systems. The method comprising providing a photographic element comprising at least one high chloride silver halide emulsion layer, exposing said element utilizing a high intensity actinic radiation exposure for an exposure time of less than 1/100 second, and developing said element to produce a photographic image, wherein the high chloride silver halide emulsion layer is comprised of silver halide emulsion grains containing at least 90 mole percent chloride, based on silver, obtained by providing a high chloride host emulsion, bringing a Lippmann emulsion comprising primarily fine silver bromide grains doped with iridium into contact with said high chloride host emulsion, and subsequently chemically sensitizing the high chloride emulsion.

  10. #30
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    Yes, this is what I referred to in my post above yours.

    After the nuclei are formed in the initial silver addition to halide, the run is stopped and held. Then AgI or KI may be added as described in these patents. Then the emulsion is diluted to the right level of gelatin and halide concentration for the rest of the run to continue.

    PE

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