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N OVE L l\lE TH ODS FOR THE CRE TION OF

S ILVE R F RE E Il\I GES IN HOLOGR PHY USI N CONVENTION LS I LV E R H L I DE E l\ lU LS IO NS

Nicholas J. PhillipsZane ColemanCe \VangChristopher A. Barnett

Dept of PhysicsLoughborough University of TechnologyLoughboroughLeicestershire UK

Abst rac t

In this paper, we revisit the ar ea of silver halide processtechnology associated with the formation of voidedgelatin - conventionally known as silver halidesensitized gelatin (SHSG). Comparative studies haveshown that this method is capable of creating masterholograms in transmission mode that are virtually fre eof the grain scattering noise tha t besets convent iona lsilver halide gelatin SHG holograms. Despite intuitiveconclusions that such noise reduction must be associatedwith a proportional reduction of diffraction efficiency, itis found t ha t h ol og ra ms of h ig h e ff ic ie nc y ar.dsimultaneous low noise are achieved. Such methods areideal ly sui ted to mas te ring for embossing or recordingonto the substantially noiseless Du Pont photopolyrners.

1 . Introduct ion

Subst an ti al p ub li sh ed w ork exists to show that theremoval of s il ve r h al id e c ry st al s from the gelatinstructure of conventional halide hologram can leave aresidue of voids that endow the holograms with amoderately high diffrac tion eff ic iency not unl ike tha tapparent in regular dichromated gelat in structures. Thereader is r efe rr ed to the b ib li og ra ph y at the end of treepaper to provide some of th e r ele va nt h is to ri ca linformation. It is evident that the use of tanning bleachesbased on the dichromates is essential to the process sincethe voided gelat in layer will collapse into a state ofspatial homogeneity in the absence of tanning.

Much of o ur work at Loughborough over a pe ri od ofma ny years has been developed with the aim ofunderstanding the physico-chemical mechanisms t tunderlie the process of development and bl eac hi ng insilver hal ide holography. With this exper ience in mindand a basic desire to least understand the process optimization of the SHSG technique, the contents of thispaper were created.In the l iterature, it has become clear [1] that transmissionholograms with their relat ively low spatial frequencies

are somewhat eas ie r to s truc ture in the SHSG mode thanref lect ion holograms. Careful s tudies , by for example

Fimia and colleagues [2], suggest that a spatial frequencylimit of some 1500 lines/mm roughly definesbandwidths of s impl is ti c SHSG hol og rams t hat areendowed with acceptable diffraction efficiency.

In our approach to the problem, we haw scanned a largenumber of chemical r out es so 2S to gain at least anunderstanding of the chemical barriers to success. At theoutse t, we shall make it clear that we too have founddifficu lty with the ref lect ion hologram case but at leastwe have a view of what the major chemical problems areand we shall off er a wor king solution to this mode ofproblem in forthcoming publications.

It is t imely to take a s eri ous view of the SHSG methodsbecause mastering for the recording of copy holograms inDu Pont s photopolymer is now limited by theindifferent, if not totally inadequate quality of the silverhalide materials. We must remember the wide differencesof sensitivity of t he s il ve r halides and the Du Pontmater ia ls . \Ve must expect to record ho log ram s in thefaster of the Du Pont materials at a sensitivity ofapproximately s uI/crn , whereas we can easilydemonstrate good diffraction efficiency of silver halideholograms at a level of some 25 ul/cm . The differenceis profound and m ea ns t ha t on ly the most stable ofob ject s can be r eco rde d in the Du Po nt material for thepurposes of display holography. Very stable objects areof course suitable fo r d ir ec t m as te ri ng in th ephotopolymer b ut t he tec hno lo gy of the recordingapparatus for such ultra-stable recordings is high and thecasual worker wii] find it beyond his grasp.

When we began our work, it was with the fear that e itherSHSG would be impractical and highly complex, or thatit would resul t in holograms of low eff ic iency due to theremoval of the highly refract ive halide. That neither oft he se p ro bl em s is of impa rl anc e c re at es a helpfulscenario and it is the purpose of this p ap er to pr ovi desome usabl e r ec ipes for t he che mi st ry t ha t i ns pi re

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confidence in the method so that the general worker .villbe e n o u r ~ e dto use it.

2 . The SHSG Process - Some Histor icalApproaches

Historically, two routes appear to have a place ofprominence; these can be outlined as follows:

Ascorbic acidSodium sulphite anhydrous)CatecholUreaSodium carbonate anhydrous): \fake up to 1

Bleaches

55aoo

The method of Graver et al [3] These we have optimized and the formulae are ginn withrelevant comments on rationale.

Develop the h ol og ra m to a density D 2.Solvent bleach the hologram to remove the developedsilver - using a tanning solvent bleach.Fix out th e remaining unexposed (undeveloped)silver.

Non tanning relialogenating bleaches

::\ TBI

In t hi s m et ho d, the inherent effects of high gammaleading to holographic non-linearity are endemic. Resultsreported by Graver and co-workers indicated diffractionefficiencies for gratings of the order of 35 7c

Themethod

of hang an d Winnick [ 4]

Distilled waterCopper sulphatePotassium BromideGlacial acetic acidPotassium persulphateMake up to 1

::\ TB2

8 ml.1 g.17 g.

10 ml.10 g

t this point it is timely to ask how such formulationsare arrived at.

Develop the layer.Rehalcgenate in a tanning rehalogenating bleach.Fix out the remaining halide.

This method has re c eive d much atte ntion sinceefficiencies as high as 75 have been r ep on ed . T hereader is referred to the recent and detai led studies ofFimia and co-workers for an update.

Distilled waterFerric nitratePotassium bromidePotassium persulphateMake up to I

sao ml.50 g.17 g~ o g

The ideas embodied in these two methods cover 111:= :;y ofthe tr ic ks th at C2n be played with the method of SHSGbut the chemical formulations appea in ei the r c ase to beeither flawed or a trifle primitive. What we shall do h er eis to show hO\\I to construct optimized chemical bathsfor the process from which further developments C2n beconstructed.

Firstly, we note the use of the agent potassiumpersulphate. This is used to regenerate the oxidizing

agent e.g. cupric sulphate or fer ricnitrate)

after itsreduction as a resul t of attacking the silver of the image.Th e merit of this regeneration process has been discussedat length [6].

Optimized Chemical Baths for the Silve rHalides and th e SHSG Method

De ve l o p e r s

Ou r experience here is that the o pt im al developingchemistry is still somewhat empirical but some clearwinners have emerged.

Th e hal ide KEr) level is set so as to avoid the format ionof coarse crystallites of silver bromide without too muchloss of efficiency.

Th e overall rate of bleaching is a lso important and hasbeen found to be optimized somewhere around a clearancetime of 5 minutes . Overa ll d ilut ion exert s the necessarycontrol of rate.

Fo r the moment, we can give w or ke rs a comfortableprediction of success in the t ransmiss ion mode ofrecording) by the use of the developer CWC2 below):

Fim ia and co-w orkers claim that for the correctmanifestation of the SHSG method then tanning effectsin development should be avoided. s a result, detailedstudies of developers based on ascorbic acid a developerknown no t to tan ) have been conducted by his group.Some novel advances in this area are to be published Phillips, Fimia, et al [5]) in the near future.

Tanning bleaches

Despite a detai led search by the authors [6], we concludedthat only the dichrornates have the effect of hardening thegelatin in the co rre ct m od e for use with SHSG. Weevolved the tanning bleaches below 3S follows:

8 rnlS g.

40 g.

Distilled waterPotassium dichromateGlacial acetic acidPotassium persulphaie

i) Solvent tanning bleach (STB)

8 mlistilled water

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Potass ium hydrogen sulphateMake up 10 I

5 g suitable c le ar in g so lu tio n 111;:) be pr e parcd 3Sfollows:

Experimental Stra tegies

For the average worker, effective results can be obtainedusing the following procedure in transmission mode:

Th e subtlety of the need for 3. cJearing bath lies in thefact that the precipitation takes place ext er nal to therecording layer and also wi thin the voids of the gelatin.Thus in fact, lack of clearing also leads to a loss of indexmodulation of the bleached layer.

Here, the rann i ng is influenced by the pH of thesolution. As the pH is raised, the r ate of s ilver solventaction diminishes, /1. five minute clearance lime has beenfound to be a minimum for serious gelatin hardening totake place.

Th e basic level of dichromate determines the overall rateof clearance together with the following constraints:

It is found that the glaci al acetic acid exer ts cont rolover pH and enhances the hardening reaction.Potassium persulphate regenerates the reduced Cr

ions (from their Cr:+ state .Potassium hydrogen sulphate exerts a crude buteffective control over the overall level of acidity.

Distilled waterSodium metasilic ateSodium sulphite (anhydrous)Make up to I

900 J]]2 g.

:;0 g.

It is clearly observed that tanning action is enhanced bys lo wi ng t he rate of clearance. Such observations arerelated to the clear indication of chromium attachment tothe gelat in made observable by yel lowing of the layer.Since potassium persulphate is capable of reoxidizing theCr 3 ion back to the Cr 6 s ta te (a consequence itslarge Redox potential), this agent has 3 role to play inresolution enhancement of the overal l process. Thus theuncontroJled diffusion of the Cr:+ ion, which might leadto uniform tanning, C3n be inhibited.

(ii) Rehalogenating tanning bleach RTB

n this form the bleach must be subject a degree ofint rn l conflict It mus t tan or harden the gelatin l ay erand yet must also promote the physical tran sfer of halidethat l eads to the modulation of the image. With thes efeatures in mind, we evolved the following bleach.

Develop in CWC2 to a density D 2.5.Rebalogenate in RTB with agitation.Fix for 2 mins.Dry (see later).

An examina tion of the bleached but unf ixed hologramlayer clearly points o ut th e undesirable scatteringconsequences of l arge and imperfect ly formed s ilverhal ide grains . In fact, the Agfa mater ia ls 8E75HD and8E56HD, both exhibit poor control of improbable butlarge grains in the tail of their statistical distribution.

Th e removal of the halide grains at the fix stage stilll eaves a brigh t and c lear void image that can be fur theramplified at th e drying stage. Th e net result is aremarkable reduction of image noise but with l ittle or noreduction of the s ignal if proper drying procedures areadopted.

Equal pans are mixed immedia te ly prior to use and themixture diluted I ..j. with distilled water.

Part A.Distilled waterPotassium bromideDi-sodium ED T AMake up to 500 ml.

Pa11 B.Distilled waterAmmonium dichromateGlacial acetic acidPotassium hydrogen sulphatePotassium per sulphate\1ake up to 50 0 ml.

- 00 T ll

17 g.10 g.

- 00 ml...j. g.

0 ml.2.5 g.10 g.

Th e solvent bleach route is outlined below:

Develop in CWC2 to a densi ty D 2.0.Solvent bleach in STB with agitation.Cle3r.Fix.Dry (see later .

Using th e solvent bleach also produces results ofi mp or ta nc e, b ut b ec au se of the precipitat ion problemmentioned earlier, it is best just to rehalcgenate with thebleach RTB when no such precipitation occurs.

5 A New E xp er im en ta l S tra te gy tha tCombines th e Best of All Approaches

With either th e solvent or rehalogenat ing tanningbleaches, powerful agitation is needed during the bleachprocess. Fo r the reasons outlined above, failure to agitatewill create stagnation du e to the inhibit ion of physicaltransfer in the b le ac hed layer and into the bath. Whensolvent bleaches are applied, an insoluble precipi tate isf or me d. S uc h p re ci pi ta ti on must be washed away(cleared) in an appropriate solut ion. We have found that a

In this case the image modulat ion is f irst established bydiffusion transfer in a non-tanning bleach (e.g. }.lTB I orNTB2 . The layer is then reactivated arid totallyredeveloped prior to final tanning, rehalogenation, andfixing:

Develop in CWC2 to a density D 2.0-2.5.Wash.BJe3ch }.lTB I or NTB2).

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Wash.Reactivate and expose 1 strong white or UV light.Redevelop to completion.Wash.Rehalogeriate using RTB.Wash.Fix.Wash.Dry see later).

Th e novelty of this approach lies in the hardening levelsachievable for the layer. Since al l the ini tial silverpopulat ion is rehalogenated, the hardening level is nowat its max imum for such a layer.

T hi s m et ho d has been found to be very effective increating holograms of very low noise and simul taneoushigh efficiencies. For this method we introduce two newstages of processing. These are formulated as follows:

Rea ct iva to r bath.

packaging of the finished result is impcr;,ti\c.

The drying process can be as follows:

Immerse the plate in a warm bath 4S-55 C) ofpropano l-water mix tu re at a specif ic g rav ity of O,S6.Then immerse, with agit ation, in at least two baths ofwarm dry propano l. Remove s lowly to avoid dryingmarks.

We have also evolved the following method for dryingwhich has yielded outstanding results.

Fir st immerse the wet bye I in methano l ;H roomtemperature) to remove excess water and droplets etc.Second, immerse in ethyl methyl ketone at roomtemperature). Both these steps C2n be for say 2 minuteswith agitation. Finally, immerse in a bath of either ethylme thyl ketone or me thyl methacrylate at an elevatedtemperature 6S C) for a few minutes and then lift outcarefully to avoid drying marks or streaks,

Immerse bleached hologram in bath for 2 minutes undersafelighting. \Vash 2;Jd transfer to second developer.

R ed ev el op er b at h.

An effective redeveloper has been const ructed from amodified D 19 solution as follows:

Distilled waterSodium sulphite anhydrous)Calgon sodium hexametaphosphate)~ a k eup to I

Distilled waterSodium sulphite anhydrous)MetolQuinolSodium carbonate anhydrous)UreaFe w drops of wetting agent.Make up to 1

900 ml.50 g.

0.5 g.

SOO ml.10 g.2 g.

14 g.50 g.50 g.

The dried layer is, of course, moisture sensi tive but infact considerable use C2n be made of an uncovered mastermade by this method so long as the relative humidity ofthe working environment is low enough. It is preferableto cover the hologram quickly using a UV cu rab lecement and glass cover.

7 . Conclus ions

In this paper, we have outlined our exper ience andrationale behind the manufacture of SHSG holograms.Th e results o ffer noise levels not a tta inable by grainysilver halide holograms. The reflection hologram case isno t so simple and will be the topic of furtherpublications.

With the increasing demands of mastering for embossingand exposures into photoresist or photopolymers, theseexotic methods take on a pragmatic and important role.

References

Th e hologram is inserted into the developer afterr eact ivat ion in safel ight ing and then placed under anintense white or UV lamp. Th e effect of the l ight ing isto create almost instant high silver density. Immersionin the redeveloper can be extended 10 approximately 30seconds - 1 minute.

Though this me thod may seem tedious in the number ofsteps necessary, it produces outstanding results.

6 . D rying and Packaging of the F in ishedSHSG Hologram

If rapid drying of the l ayer is invoked, say by the use ofh ot p ro pa no l, then th e voids created during theprocessing of the holographic layer can be propped upso as to remain as the modulation mechanism of thehologram [7]. Because these voids are subject to collapseif moisture is a llowed to permeate the layer, specia lized

I. A. Fimia et 2 Proc. SPIE 215 1980).2. A. Fimia, A. Belendez , and 1. Pascual, J. :\fod. Opt.

10, 2043-2051 197/).3. W. R. Grayer et aJ, pp1. Opt. 19, 9 1980).4. B. J. C hang and K. W in nick , Proc. SPIE 215

1980).5. N. J. Phil1ips, A. Fimia, et al , be published

1993).6. H.1. Bjelkhagen, ~ J. Phi ll ips, and Ce \Vang, Proc.

SPlE San Jose 1991).7. T. A. Shankoff, AppJ. Opt. 7, 2101 1968).

Bibl iography

B. J. Chang , Froc. SFIE 177, 71-81 1979).D. K. Angell, P roc. SPJE 883, 106 1988).J. Kosar, i gh t Sen si ti ve Sys tems Wiley, 1965).N. J. PhilJjps ct 3 Proc. SPIE San Jose 1993).