Control of DCG and non silver holographic materials

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ABSTRACT

We review the properties and relative usefulness of 3 non silver Volume holographic recording materials that are available today. Dichromated Gelatin (DCG) will receive the most attention followed by Dupont Omnidex products and a light treatment of Polyvinyl Carbazole (PVK). Enhancement and control of color, bandwidth and diffraction efficiency of volume reflection holograms recorded in DCG and photopolymers is discussed. Methods of increasing the bandwidth while shifting the center frequency toward the red is given for photopolymers. Red pseudo color will be covered thoroughly so that the practitioner will have all the elementary tools to make full color and broadband DCG holograms from scratch. The entire DCG technology is disclosed as it relates to production of high quality display holograms that span the spectrum and may be narrowband and very deep or shallow and broadband.

MATERIAL

The list of volume holographic recording materials includes the silver halide films, photopolymers, photocrosslinkers, photochromics, ferroelectric crystals and a few less well known oddities. While useful devices may be made in all these materials only a few will provide a holographer with a sufficiently high index modulation, resolution, signal to noise ratio, spectral response, and archival properties. The silver halide films have grain size limited resolution, scatter short wavelengths excessively, have only a moderate index modulation and tend to print out in UV light. They are popular for many applications because they can be made panchromatic, are much more sensitive than any other materials and are commercially available.

We ocassionally use silver halide films to make master holograms and we reduce the blue scatter by converting them to a gelatin hologram (SHG). Used in this manner they are nearly equivalent to dichromated gelatin (DCG). They enable the production of DCG masters shot with red light at silver speeds that can be copied with green light in DCG and reconstructed finally at the original red color. SHG processes are covered in previous publications.

The three materials detailed in this paper were chosen because they each have more or less all the properties needed for the construction of high quality or high efficiency broadband or narrowband color controlled holograms. They are not without faults however and the purpose of this work is to provide information to a holographer to assist him to make a better material choice for a particular task or to set up for production in DCG. Some information is given relating to the peculiar drop in average index in DCG, PVK and DMP128 due to the nature of the index modulation in these materials. Very thick coatings, up to about 50 microns are also dealt with in a special section.

Comments made about preparation, processing, fine tuning, protection and applications are based on many years of working with DCG and many months of working with the other two materials, Dupont's Omnidex materials and Polyvinyl Carbazole (PVK). DMP 128 is another well documented material. It is a proprietary Polaroid formulation of lithium acrylate in combination with a branched polyethylenimine. Although it is not generally available commercially Polaroid has allowed laboratory evaluations and it is now quite common to get holograms mass produced in the material at Polaroid. Several other photopolymer systems are possibilities for holographic work but these 3 represent the readily available and practical recording materials of the day.

REFLECTION HOLOGRAM FABRICATION STEPS

Mixing and coating

DCG is a mixture of Ammonium dichromate, gelatin and water. It is stirred together, heated, filtered and spun on or drawn down with a bar. Drying with or without heat from gel or liquid states makes little difference. Bright yellow lights are permissable. A 10 micron film can be made by mixing one gram dichromate with 3 grams of gelatin and 20 grams of water and spinning a flooded 8x10 glass plate at about 80 RPM. The solution and the coated plate is useful for about 3 days, much longer with refrigeration.

PVK is dissolved in chlorobenzene along with Carbon tetraiodide, it is difficult to get into solution even with heat and agitation. It is also very unstable and may gel in as little as 15 minutes after mixing. The solution may be further thinned for convenient spinning but is easily applied with a bar. A number 28 bar will yield a 5 - 7 micron coating when 2 grams of PVK are dissolved in 25 grams of chlorobenzene. The patent disclosure mentions nothing about mixing the sensitizer with the PVK while still dry but if this is not done the solution will likely not be photosensitive. The solution will usually remain stable enough for coating for about 1 hour after mixing. The coated substrate is good for at least 8 hours.

Dupont's materials can be purchased already to use on plastic substrates or may be obtained in liquid form for coating by any of the techniques covered here.

Storage after coating

DCG stores well at low humidity in a refrigerator or freezer but containers must prevent contamination, condensation, freezer burn and frost which can all destroy surface quality. Film of 10 to 20 microns or more store best and are good for at least a year. At room temperature and 50% RH, thin films are good for a few hours, thick films typically last a week or more. The addition of a small quantity of TMG to the mixture will greatly increase storage time at room temperature by increasing the PH.

PVK does not store well as a rule, sometimes it has lasted a week in the refrigerator. Typically it moves suddenly to insensitivity within 24 hours and it does not seem to age gradually. The instabilities in coating and storing could be alleviated by adding more antioxidants to retard the spontaneous formation of free radicals but at the cost of reduced photo sensitivity.

Dupont's materials seem to store well under refrigeration for a year or more depending on type, their newest film is also very sensitive, requiring only a few mj/cm*cm to expose in red or green.

Exposure characteristics

DCG is most sensitive in a hot moist environment. At 50% RH and 68 degrees F it may require 60 mj/cm*cm of 488 nm light, while at 75% RH and 80° F 4mJ/cm2 will do the same job. At 441 nm less than 1 mJ is enough and at 514 or 532, 50 to 100 may be necessary. The percentage of dichromate affects speed more or less linearly, a 25% mixture is typical but mixtures of from 10 to 30 percent are necessary to control color. Gross overexposure will cause a decrease in efficiency all the way to zero. Overexposure causes an initial increase then a decrease in bandwidth and a blue shift plus a compression of contrast or dynamic range.

PVK is totally insensitive to moisture, water can be used as an index matching fluid with no effect on the hologram. It requires about 20 mj/cm*cm at 488 nm to make a good single beam reflection hologram and because it is a cross linker it can be overexposed. Over exposure also results in a blue shift, a wider, then narrower bandwidth and loss of contrast.

The Dupont material is a real time material and as a result can interfere with itself during an exposure. As the hologram is forming it is also reconstructing and making small dimensional changes. The results can be that at some time during an exposure the reconstruction could be out of phase with the construction momentarily. The energy required varies from about 8 mJ/cm2 to a 100 or so mJ. It has good resistance to water and readable holograms have been made by us in 100ms.

Processing procedures

DCG is hardened briefly in Kodak Fixer with hardener then rinsed and plunged into several hot or cool alcohol baths. Cool baths produce better uniformity and lower noise, hot baths can yield tremendous index modulations with large chirps in Bragg spacing but often with increased noise. Depending on the mixtures, temperatures and times spent in each bath, a wide range of effects can be had. Processing and reprocessing affects bandwidth and center frequency over 100 nm or more and index modulation can be varied from near nothing to .25. If the first pass in the baths produces a shift to red, a second pass can shift it to blue and a 100 nm bandwidth can be reprocessed to yield a 30 nm bandwidth.

Typically "master" holograms are processed to construct near the recording wavelength but processing and dichromate content allow the control of color to range from 650 to 450 nm for a straight on exposure at 488 nm. This much versatility in color control is certainly useful and will be covered in more detail. Conformal mirrors on flat substrates can be color shifted easily with angle but more complex shapes must be tuned with processing and film formula juggling. As an example of a reprocessing procedure, a red shifted broadband mirror may be narrowed and blue shifted by agitation for 30 seconds in a room temp 50:50 mixture of water and alcohol then plunged into 99% hot alcohol with agitation for 30 seconds and pulled slowly from the hot bath.

The direction of the shift can be controlled by the ratio of alcohol and water in the first bath and the amount of shift can be controlled by the time in the same bath. A near ideal tuning bath has a specific gravity of .86 when it is warmed to about 55 degrees C. This process can be repeated many times if necessary, especially if the last hot bath is not hot enough to cause excessive scattering center buildup. Multiple buffer baths between the first color control bath and the last dehydration bath help to keep the last bath clean.

PVK is swollen in Xylene or Toluene for a few seconds then dried in warm Hexanes or Hexanes mixed with alcohols. Like DCG the latent image must be enhanced by swelling in the first solvent and then replacing that solvent with a miscible but nonswelling solvent. Again, color and modulation control is by temperature and time. Color control is similar to DCG methods and either broadband red shifts or narrowband blue shifts are possible by altering time and temperature. Reprocessing is possible but scattering centers build up rather quickly in this material. The first solvent probably dissolves away material as it causes swelling. Signal to noise is usually good in narrow band processing but not so good for broadband reconstructions.

A recent proprietary improvement in processing involves the use of a clever monobath made up of two miscible solvents. One of the solvents will swell the PVK and is more volatile than the other solvent which will not soften or swell the PVK. The most volatile solvent evaporates first and leaves the hologram structure in rigid uniform shape while the second solvent is driven out with warm dry air.

Dupont films are developed with UV light and heat. They may then be brightened and color shifted by the addition of monomers and or solvents. It is common practice to laminate a cover glass over gelatin holograms to protect them from moisture, abrasion and chemicals. Many common epoxies have been identified as safe for this purpose as well as a broad class of adhesives described as UV polymerizable substances,( monomers, epoxies, resins, adhesives, etc). I accidentally caused an enhancement of several photopolymer holograms while attempting to laminate them. In one case the bandwidth widened from 40 nm to 150 nm and the optical density remained almost as high as the original structure.

The photopolymers behave a little like sponges that can be dampened and swollen or alternatively soaked and saturated while the shock dried DCG and PVK structures are more like a stack of Ruffles potato chips that get damp, go limp and then collapse.

Enhancement of Dupont photopolymer is the easiest and most reliable. The holograms produced from blue exposures originally playback blue but the enhanced holograms playback at a longer wavelength and are noticeably brighter. Solvents alone brighten and shift the reconstructions to the red but they are temporary treatments and not generally as effective as UV curable monomer type adhesives. A Dupont product is now available to make predictable shifts in playback color. They provide a monomer on a cover sheet that will diffuse into the exposed film where it causes swelling and can then be fixed by polymerization under a UV source.

Some Dupont film reflection holograms will respond to the following recipe with a red shift and increase in total diffracted energy. Apply Lightweld 401 evenly and cover then wait for a color change and cure with strong UV source. If this substance is left uncured it may destroy the original structure. It is also anaerobic and therefore requires a cover to cure.

Protection

DCG is notoriously bad at remaining stable in normal environments. Moisture will cause the Bragg structure to collapse and the gelatin grabs moisture easily from the air right through most plastics and glue. This material usually requires lamination between glass with enough gelatin removed around the edges to form an "O" ring seal. Thick plastics, such as 30 mil mylar, will also work and certain fluorinated plastics such as "Aclar" in thin 5 mill layers are satisfactory provided that the edges have been cleared of gelatin before laminating.

PVK needs protection from abrasion but it stands alone as the only holographic material we ever worked with that is completely waterproof. It requires only a 4 mil mylar laminate for adequate abrasion protection from the environment.

Dupont's materials may be used as is or uncovered and rolled down onto a glass substrate. They need very little protection after being fully polymerized with UV light but a stiff flat backing helps with image distortion. Water can affect them temporarily but the structure is essentially humidity proof.

APPLICATION NOTES

DCG is easily the most versatile material, just about any kind of HOE or hologram can be made in it. Unfortunately it has poor environmental stability and must be well protected or it may not be intact when you need it. As long as glass or thick plastic is acceptable in the finished product DCG is the number one choice. With some difficulty it can be made panchromatic for full color work and under warm moist conditions it is a little more sensitive than the other two in the blue-green region. The SHG versions are much more sensitive and represent the only fast "non silver" medium useful for pulse holography applications.

PVK is not so pleasant to work with as DCG but it goes onto plastic substrates easily, has a high delta n and needs only minimal protection. It should be very good for such things as eyewear, solar collection and other outdoor applications or anyplace where superb environmental stability is required.

Dupont's materials come with an ever wider range of properties. They are durable and panchromatic but lack a little in dynamic range. The maximum available index modulation is lower than the other two materials and display holograms are typically less bright.

Initially we tried to determine the index modulation of simple reflectors made in each material by fitting them to the simple one dimensional Kogelnick expression for diffraction efficiency (D.E.) of reflection volume holograms.

Kogelnick Expression
${\displaystyle DE=\tanh ^{2}\,{{\pi \Delta nT} \over {\lambda \sin \Theta }}}$

Where

• T = thickness of material in microns
• λ = .5 microns (typical)
• sin θ = 1 (for conformal mirror)

This relationship seldom describes real reflection structures because it does not describe the effects of a gradient on the index modulation (delta n) or a chirp in the grating spacing (d). A gradient in delta n such as is caused by the absorption of light by the sensitizing dye results in a smooth broadening of the angular and spectral bandwidths and a smoothing of sideband peaks (when DE is held constant.) A chirp in the Bragg plane spacing also broadens the bandwidth though not so smoothly and the combination produces a highly asymmetric spectral bandwidth. The data and a description of the computer model is given in a previous publication.

All three materials exhibit a useful range of index modulation and color control and each has found multiple commercial uses. The differences lie mainly in sensitometric characteristics, environmental stability and in the degree of difficulty to obtain or use. The balance of this paper will detail the use of DCG. We will try to give instructions that can be followed by anyone that is already familiar with more conventional holographic fabrication techniques and materials. Some details are left out for the sake of brevity but can be found elsewhere in the references or other literature.

SPIN COATING APPARATUS

A variable speed turntable capable of 50 to 100 RPM will coat films of gelatin or PVK from 4 to 50 microns on 8 x 10 inch glass or plastic substrates. Plates as small as 3 inch diameter or as large as 16 x 16 inches have also been successfully coated with this range and technique. The turntable should be equipped with a surface or arms that will mate to a removable tray that is one or two inches larger than the substrates being coated. We have used ordinary variable speed phonograph players with pie tins turned upside down and glued to the turntables and we have used Dayton variable speed gear motors with heavy duty arms attached. Both devices worked very well.

Trays have been made up of stainless steel, plexiglass, polyethylene dishpans or modified from aluminum cake and pizza pans. The best trays have straight sides measuring 2 1/2 to 4 inches high and are fitted with 3 rubber posts inside and outside. The posts inside hold the substrate an inch or so off the bottom of the tray and the outside posts serve to level the tray during pouring of solutions and to center the tray during spinning. The spinning tray and substrate may generate useful turbulence that aids in drying and distributing the solution. Excess solution is caught in the tray and emptied between substrates then is easily soaked clean in hot water after a days activities.

An important component that augments drying and uniformity is the blower-heater. It hangs off center and above the whirling tray. Turbulence and heat combine to make uniform coatings in about five minutes. We recommend the use of a variable temperature 600 watt blower such as might be found in the ceilings of some bathrooms. A little experimenting with angle and position will quickly determine the best place to hang this unit in your clean hood or bench area. Coat and examine uniformity by looking for local fringe patterns under a fluorescent lamp or better yet a fluorescent long wave black light.

BAR COATING APPARATUS

Lab coating bars are available from R.D. Specialty Co. in Webster N.Y. Ph (716)265-0220. A selection of bar types may be purchased for about \$ 50.00. We have used bars of 3/8" diameter wound with # 24 wire as a standard but we have other windings and diameters on hand for special applications and recommend you do the same. These bars are also useful for applying strippable coatings for anti halo backings, and have been used for coating photopolymers and protective epoxy layers etc.

Jigging for bar coating can be as simple as a clipboard with lint free paper placed under the substrate. A better jig is one that holds the plate above a trough that can catch run-off. The whole thing can be plexiglass which is particularly easy to get gelatin off of and it also preferentially over glass attracts dust particles.

Bar or spin coating is done in a class 100 environment and is accomplished by pouring out a line of solution and pulling it down with a uniform pressure and velocity. A little practice will determine correct amount of solution, speed and pressure. The bar is not rotated as it is pulled and a new location or freshly cleaned and dried bar is used on each new substrate. Variations in thickness may be accomplished by changing wire size or viscosity or both. Precautions must be taken to keep the bars clean and undamaged. We place used bars in warm water and rinse and dry them before each use. A rack that holds them suspended above any surfaces is useful for storage, cleaning and serial use. It can be made from plastics or metals. Coated plates need to be placed in a level position where they can air dry in a few minutes. The coating jig should be nominally level.

Cronar, (polyester) substrates are easily coated with these bars. Cronar is a Dupont product. One source is Farrest Chemical & Supply, 680 Toland St., San Francisco, CA (415)8241400. It is available in sheets (C-42) or rolls (C-41) in a variety of sizes.

Exposure of Cronar is done with a thick glass vacuum chuck or by humidifying the gelatin and rolling it against a clean glass plate. It is optically active and you may need to identify its neutral optical axis before exposure. Processing is best done by stretching it in a frame for dipping and agitation or by clipping it flush to a glass substrate fitted with a handle on one side.

FILM PREPARATION

Many factors need to be considered when mixing DCG for holographic film.

Jelly strength

The jelly strength, measured with the Bloom Gelometer, is an important consideration. The current gelatin being used by us for film production is either MCB brand (Mattheson, Coleman, and Bell Manufacturing Chemists, Norwood, OH 45212 # GX-45' OH 45212) # GX-45. Grayslake Type B USP XXIII Box 248 Grayslake,IL . 60030 Phone (312) 223-8141 Contact Bob Buscher. Both gelatins have bloom strengths from 215-235.

Comparable with the jelly strength is the rated solubility of the gelatin, and the mode of manufacturing. (Acid or Alkaline processed.) These can each make a considerable difference in the quality for each lot. It is best to test every specific lot before final acceptance of a gelatin. Perhaps the best rating for gelatin to be used for DCG is the jelly strength-to-viscosity ratio. A ratio of at least 4 or 5 to 1 is considered good. Our current batch has a bloom of 232 grams, a viscosity of 42 mps and a ph of 5.1.

Heating

One important caution when preparing the DCG film mixture is the destabilization of the gelatin at high temperature. When heated for an excessive period of time, the film breaks up, causing what we term as film "pits" in the final emulsion. These "pits" have the appearance of small circles of various sizes and scatter themselves throughout the plate. When the film is processed, the final image has small voids where the "pits" were. So far, the length of the heating time and the peak temperature that cause this have not been determined. In the past, temperature and heating time causing this have fluctuated. But the safe method is to heat the film mixture at the shortest possible heating time to dissolve the gelatin content completely. 130 F to 150 F (60 C.) is usually a high enough temperature to dissolve without cooking. Gelatin "melts" around 40 to 45 C.

The causes of film "pitting" are still unknown to us as well as what the "pits" really are. But their characteristics (and that of gelatin) can give us some ideas. It is important to take all known preventative measures for keeping them off the emulsion. Triple filtering helps and avoiding hot spots while mixing helps. We use a standard mag stir hot plate and glass flasks which are heated slowly while stirring or heated in a water bath, a microwave oven has also worked well using plastic bottles .

Film "pitting", or destabilization, in the past, has seemed to be affected by the solubility of the gelatin. The higher the solubility, the less likely film "pits" occurred. The solubility, of course, is slightly affected by jelly strength and impurities. Literature within the gelatin film industry indicates temperature separation may occur, partly due to the polysaccharide content of the gelatin. There is one speculation of film pits which involves the crystallinity function in drying films. (And this is a function of film temperature.)

The other theory for film "pits" is the presence of insoluble impurities (such as arsenic, grease, etc.) on the surface of the film. These substances probably conglomerate during mixing and heating to make larger hydrophobic areas on the glass. Surfactants would alleviate this but they aggravate adhesion problems as well.

Water

Use deionized water for the DCG film mixture. It eliminates certain salts which have produced inconsistencies in film behavior. Distilled water is also acceptable. Any water should be funnel filtered through a 5 micron or smaller filter and be free of oil, grease, and bacteria that thrive on gelatin.

Storage

Film mixtures may be stored in a refrigerator for a week or two and reheated in water or a microwave oven as needed. When stored longer the become less and less likely to flow when warmed.

Film codes

The film mixtures vary in dichromate and gelatin percentages. The variations depend on the specific use that a DCG film plate has. The film code currently used contains three numbers. The first being the gram-weight of the ammonium dichromate, the second being the gram-weight of the gelatin, and the third being the gram-weight (mls) of the water to be used in the film mixture. (Usually mixed in a 500 ml poly bottle.) The code for film used in broadband image holograms is 8-30-350. Thus, 8 grams dichromate, 30 grams gelatine, and 350 grams (mls) of water are mixed together. The mixture code for "red" holograms is 3-30-200. Most holographic optics are made in 10-30-250 to 8-30-150. Very thick coatings of 30 to 50 microns can be made using a 3-30-125 mixture but special fixturing may have to be made to get the gelatin to flow uniformly and the dried film may come off the substrate unless it is baked on at high humidity. We find adhesion is enhanced by cleaning the substrate in clorox and then baking the coated plate at 130 degree F in the presense of water at saturation.

In using the film code for a variety of mixtures, the 30-gram gelatin weight number always remains constant. Thus, when a thicker emulsion is desired, the water number decreases. And when more absorption is desired, the dichromate number increases, an increase in thickness narrows the bandwidth and an increase in dichromate shifts the color toward the blue.

As a general rule, thicker emulsions require longer process times but are easier to make uniform. The dichromate concentration determines light absorption and the center reconstruction wavelength of the hologram. For higher dichromate concentrations, the increased absorption produces larger gradients of index modulation. Lower the dichromate concentrations produce more uniform index modulations. Larger gradients yield slightly larger bandwidths and the removal of higher percentages of dichromate during processing results in thinner and thus bluer holograms.

When a specific bandwidth is desired, along with a specific reconstruction wavelength; it is best to experiment with various film mixtures. Usually starting with a standard mixture and then adjusting the thickness, and dichromate content to achieve the desired results. The color controllability and uniformity of DCG film improves with thicker film emulsions. Consequently, they are more forgiving in their exposing and developing parameters.

Extremely thick (25 micron or more) emulsions ( X-30-150, a 5 to 1 water-to-gel ratio) are difficult to use. They are prone to excess bubbles, pre-mature jelling, film pits, low viscous flow, increased impurities and during processing sometimes pull up off the substrate if not annealed in a wet oven. Processing of these thick films is often done with room temperature baths, or slightly elavated temperatures, over several minutes in each bath.

Sensitizer

We use ammonium dichromate crystals or for redder reds Potassium dichromate but the most sensitive of the dichromates is Pyridine dichromate. We don't use it because of it's shorter life and difficult preparation. The addition of ammonium nitrate can make the dichromate several times more sensitive, but decreases the useful life and blue shifts the image. Approximate ammonium nitrate concentrations are usually in a ratio of 1 to 5 by weight to ammonium dichromate up to a maximum of 1 to 1. When the additional substance is washed out of the gelatin a net shrinkage occurs which amounts to a blue shift in reflection holograms and lays down Bragg planes in transmission holograms.

Filtering

At a minimum, filter the heated mixture through two coffee filters (Mr. Coffee) for a standard 8-30-350 film. For 6-30-200 and thicker emulsions, use one coffee filter. Run the filtered mix into the pouring container. When necessary, a finer grade lab filter may be used, we have forced warm gelatin through a 1 micron filter using a gear pump and also using a peristaltic pump. The use of a peristaltic pump makes metering and filtering possible at the same time. A simple syringe with a 2 micron filter is very effective and may double as a way to meter out a fixed amount onto a plate.

Applicator

The pouring container (with the film) is kept on an electric warming plate. The temperature of the plate should be carefully controlled to provide only enough warmth to prevent jelling (50-60 degrees C). We like to use a lab hot plate and water bath, the pouring container is a tea pot like bottle modified from a lab wash bottle. Any poly bottle that empties from the bottom will do. Some custom shaping of the "spout" may be necessary to prevent the formation of bubbles.

COATING TECHNIQUES

The coating station consists of a class 100 cleanhood or laminar flow bench, a dryer-heater unit and the turntable. The clean hood should be large enough to fit the turntable and two plate racks inside. (About 2 1/2' x 3 1/2' or larger.) A yellow safelight may also be mounted inside. Air flow should be 200 cfm or higher for this size hood.

Cleaning glass

There is a bit of an art to coating and it takes a little practice to become good at it. The first step is to prepare the plates by soaking over night in a soapy solution that contains some chlorine. The plates also need scrubbing and a rinse in deionized water. The final rinse should be done in or in front of the clean hood used for drying the plates. The chlorine soak has been found to aid in adhesion of the gel to the glass.

The glass may be soda lime plate or float glass or any most any other kind but it has to be thick enough to withstand the shrinking forces generated during exposure. This means that it should be double strength or thicker(3 to 6 mm) for 8 x 10 shots, single strength (2 to 3 mm) for 4 x 5 and 5 x 7, and may be picture glass or as thin as 1 mm for 2 x 2 exposures.

Coating glass

The gelatin is poured over the dried plate in such a way that no gel spills off the edge and no bubbles are formed. This is accomplished by pouring a large puddle and gently rocking the tray till all edges are wet. The turntable is then turned on with the blower/heater for about 5 minutes. If the plate was uniformly wet and had no contaminants then the coating is likely to be uniform using these techniques. The range of RPM we found useful runs from 65 to 100, speeds outside this range failed to be uniform.

Start with a rotation speed of about 80 RPM and position the heater-blower about 6 inches above and to one side of center of the coating tray. For 8 by 10 plates this offset is about 3 inches. The fine tuning of the position of the blower will greatly improve the uniformity of your coatings.

Ageing and thickness

The film is ready for exposure after it has been aged an hour or so for a 350 mixture or a day later for a 150 mixture. The addition of 1 or 2 ml of TMG will extend the useful room temp life of 350 film to a day or two and will make 150 film last for several weeks in a 21 degree C, 50% RH environment. The thicknesses of the commonly used mixtures after spinning at 80 RPM and after processing are as follows: 350 yields 5-6 microns, 250 yields 8-9 microns, 200 yields 10-12 microns, 150 yields 20-24 microns and 125 yields 25 to 50 microns depending on speed.

Bandwidths and color

The relative bandwidths run from 50 to 150 nm for 350 film, depending on processing used. 250 and 200 film make 10 to 50 nm bandwidths depending on processing and 150 film can get down to 8 nm but also runs as high as 30 nm. Very thick film can have bandwidths of less than 8 nm. The color of a film made from a 3-30-200 mixture is around 630 nm when shot at 514 nm. The color of 6- 30 film is around 590 for a 514 shot and a 10-30 mixture will easily be tuned to play back at the same wavelength it was shot at. Methods of planning and controlling color in display holograms are discussed below, similar but more precise methods are used for HOEs.

COLOR DICHROMATE OBJECT PREPARATION

The two color method produces rich red-orange and bright clean blue-green colors that mix to a creamy white. Color coding of the object is optional but helpful in most cases and production is done from two masters in two different films. The three color system requires color coding for red at the mastering stage but no coding for blue or green, which are mastered first. Both systems are part natural, part pseudo color and require only two laser lines and two film formulations. Blue is obtained naturally by using the 458 argon line and green or red are derived from the 514 line.

In production the two color system is identical to current master/copy methods in that batches are shot at 458 or at 514 and later registered and laminated together. The three color system requires blue and green exposures in the same emulsion and red in a second batch. The laser must then be operated multiline or be switched constantly or a second laser introduced. The preferred method is multiline operation with independent shuttering except that max power in each line is reduced because several lines compete for available energy.

The two color, two plate system makes very satisfying flesh tones and color balance is fairly easy to maintain because it can be done by mixing and matching batches and or individual holograms at the laminating stage. The two color single plate method has the obvious advantage of no registration problems but it has a limited color range because there are only 56 nm between 458 and 514nm.

Object preparation

Blue-green areas should be overcoated lightly with a bright blue pigment such as Liquitex Brilliant Blue #20002-381 or Pelikan Deep Blue #39. This will effectively inhibit refection at 514. The red-orange areas must be touched up with yellow pigment such as Liquitex #1002-411 or Pelikan Yellow #10 both of which absorb 458 but reflect 514. At this stage H1 masters or correctly colored copies can be made, the Blue-Green master may be made to reconstruct at 488 so that production copies can be done using only 488 and 514. The 514 exposure is done with the film side facing the reference beam and the 458 exposure is done the other way around with a spacer between the object and film having the same optical thickness as the 514 substrate.

Film preparations

A good blue or green production film can be made by mixing the 8-30-250 formulae with or without a ml of TMG. A good red or yellow film is made by reducing the amount of dichromate to 2 or 3 grams. The plates are ready to use after standing at room temp for an hour and they may be stored in a refrigerator for months on end. Better results may be obtained from some softer gelatins by ageing films for a few days.

Exposure procedures

Blue holograms may be made by exposing in a Denisyuk fashion @ 458, 441, 476 nm or some other line bluer than 488. The energy required is about 20 mj/cm*cm and it helps to do it with the reference at 50 degrees from the normal and with the E vector perpendicular to the plate to reduce noise from mirroring.

Green holograms may be similarly produced by using the 514 line, again near Brewster's angle. This time it may pay to try 55 degrees because absorption is much lower @ 514 so "Newton's wood" type noise is more likely to show up.The energy required is about 90 mj/cm*cm.

Red holograms result from using the red film formula and exposing @ 514 close to Brewster's angle. The fringe structure is expanded to red or yellow reconstruction because less material is washed out during development. If the master has been made in SHG using a HeNe then this copy will be a correct color reproduction.

Processing procedures

The film of gelatin is about 8 or 9 microns thick and requires much longer processing times than 4 or 5 micron broadband films. Development takes 3 to 5 minutes in kodak fixer, followed by a 1 minute rinse in tap water. Dehydration is done in warm isopropyl alcohol (48 to 55 degrees C) using at least 2 baths after the tuning bath and agitating mildly in each for about 30 seconds. Drying is most easily done by removing the plate very slowly from the last and driest bath. If it does not look uniform try soaking in warm water for 10 minutes and then dehydrate with more agitation.

Fine tuning of the color may be done by soaking in the tuning bath. This is the way that we get the center reconstruction frequency to match the copy wavelength. Start with a master that is a little too red and gradually tune it to the correct color by repeated passes through the tuning bath and the last hot dry bath. 350, 250, and even 200 mixtures all respond to this method. A hydrometer is necessary to monitor the specific gravity of the tuning bath and maintain it at or near .86.

Processing 350 film for masters is done this way but the same film can be processed for broadband reconstruction by using a shorter development time and skipping the tuning bath. Experimenting is the only way to get the desired results. Some guides to broadband techniques can be found in the proceedings of the first Lake Forest symposium in 1982.

An alternative to multiple bath processing has been proposed by workers at IBM. They suggest that for thin films, on the order of our 350 or 400 mixtures, spinning the plate while spraying a series of fluids works best. Thin films are not easy to process in baths because of the fast diffusion of solvents in and out of the rather porous gelatin. In the IBM method, all of the regular baths are sprayed progressively for only a few seconds each onto the spinning plate. They felt that the spray system would be a superior way to automate processing techniques, we experimented with spraying many years ago but did not have the success that IBM has had.

HAZARDS

Dichromate powder is dangerous if inhaled and the liquid mixture may irritate some people if left on the skin. Dust masks and rubber gloves are therefore recommended whenever film is being made. Isopropyl alcohol has low toxicity but is quite flammable and must always be heated in a safe manner such as in a water bath. Alcohol fires may be extinguished with water, dry chemical, Halon or CO2. Glass must be handled carefully and whenever possible the edges should be ground before handling.

REFERENCES

These references are all by the same author and may be useful to the holographer that tries to apply the methods detailed in this paper. A design guide and brochure for HOE's is available on request. A video tape demonstrating this technology is also conditionally available from the author.

• "Devices, Tuning and Quality Control in Dichromated Gelatin (DCG)." S.P.I.E. Proceedings, Volume 212, pp. 22, 1979
• "Notes and Considerations for the Dichromated Gelatin (DCG) Holographer." Lake Forest College Holography Workshop and First International Symposium on Display Holography, July 1982. Lake Forest, IL.
• "Practical Polymers for Holography", Second International Symposium on Display Holography, Lake Forest College, IL.
• "Materials for Volume Phase Holographic Notch Filters" SBIR #A 86-68 Final Report, U.S. Army CECOM, Ft. Monmouth, N.J. Aug. 1987.
• "Alternative Volume Recording Media, A Qualitative Comparison" Third International Symposium on Display Holography, Lake Forest College, IL 1988
• "Survey of properties of volume holographic materials", SPIE vol. 1051, Practicle Holography III, 1989 p. 68 - LA, CA.
• "Novel Enhancement of Photopolymers", SPIE vol 1212, Practical Holography IV, 1990 LA, CA.