Off-axis and off-bisector reflection-type holographic visual display elements have been recorded in dichromated gelatin deposited on planar or spherical shell substrates of glass or Plexiglas. A procedure for bonding gelatin to Plexiglas is given. Holographic elements are recorded at the argon wavelength of 514.5 nm and reconstructed with spectral lines from a low pressure mercury arc lamp. Measured image characteristics for a flat substrate hologram agree with ray-tracing calculations. A swelling of the gelatin by approximately 6.6% after processing does not perceptibly affect the dispersion, astigmatism, or distortion in the image, that is, the grating equation depends on the spacing between the fringes on the surface of the gelatin and is not affected by the swelling or shrinking. However, the Bragg equation depends on the distance normal to the fringe planes and is affected by thickness changes of the gelatin. Therefore, this thickness change is taken as an independent parameter and used to adjust the wavelength for maximum diffraction efficiency, without affecting the image angle. Data reveal a near linear relationship between the dichromate concentration of 0.5–10% used to photosensitive the gelatin and the display wavelength of maximum diffraction efficiency. Lateral dispersion is 0.12 ± 0.01°/nanometer for both planar and spherical shell substrate elements recorded in quite similar geometry, but their astigmatisms are not alike.
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Prepare a 4% by weight aqueous solution of J. T. Baker Chemical Co. U.S.P. gelatin at 50°C.
Add 0.5% ammonium dichromate based on the weight of dry gelatin and filter through Whatman No. 31 filter paper in a heated funnel.
Thermostat the filtrate at 50°C.
Attach clean glass plates to a level glass platform with a drop of water.
Pour an excess is of warm gelatin solution across one end of a plate and wipe across the entire plate with a doctor blade fixed at 0.8-mm clearance.
Dry the liquid film overnight under dust-free conditions and harden by baking in a convection oven at 150°C for 2h.
By gel casting:
Prepare a 6% solution of gelatin containing 0.9% ammonium dichromate as described in 1(a) through 1(c).
Arrange a clean glass plate at a fixed distance above a flat level Plexiglas sheet. If 0.5-mm spacers are used the dry film will have a thickness of approx mately 12 μm.
Fill a warm pipette with the gelatin solution and allow capillarity to completely fill the volume between the flat sheets.
Place the mold in a refrigerator at 5°C for 2–3 h and then separate the glass and gelatin gel from the Plexiglas support.
Dry the gel in the dark overnight at ambient temperature and then light-harden as described in footnote a.
By film transfer:
Attach a Plexiglas sheet to a level glass platform. Clean the surface with isopropanol applied clean with a soft clean towel to remove any adhesive residue.
With a doctor blade set at 0.8-mm clearance, coat the sheet with a 4% gelatin solution containing 0.9% ammonium dichromate.
Dry overnight in a light-tight cabinet containing Drierite.
Cut the dry film to shape, and release each disk from the temporary support by applying a few drops of isopropanol at each cut.
Harden the gelatin disks as described in footnote a.
Pool distilled water on the substrate to which the gelatin is to be transferred and drop disk a gelatin onto the surface of the pool.
After volume swelling has reached equilibrium (5–10 min) slowly withdraw water from the pool with a hypodermic syringe and needle.
When one edge of the disk becomes attached to the substrate, carefully pour off the remaining water and allow the assembly to dry in air.
The procedure is essentially that of Brandes et al.6 An equivalent gelatin film can be obtained by incorporating 0.7–0.9% ammonium dichromate into the film and exposing the dry film to ambient room light for at least 4 h.
Table III
Preparation of a Preholographic Element on a Poly(Methyl Methacrylate) Substrate
Formulating the subbing solution:
Type I
Hercules Powder Co. nitrated carboxymethyl cellulose (NCMC; D.S. of −CH2COOH, 0.06; 12.5% N) is dried in a vacuum oven at 50°C for 48 h.
Dissolve 1.5 parts of methyl vinyl ether–maleic anhydride copolymer (Gantrez AN-169-GAF Corporation) and 0.5 part of dry NCMC in 98.0 parts of 2-mechoxyethanol (methyl cellosolve).
Gravity filter 1 through a coarse porosity fritted funnel.
Dry du Pont “pyro” grade nitrocellulose (NC; 12.6% N) in a vacuum oven at 50°C for 48 h.
Dissolve 2.0 parts of dry NC in 98.0 parts of 2-methoxyethanol.
Gravity filter through a coarse porosity fritted funnel.
Subbing the plastic support:
Immerse a Plexiglas substrate in a pool of type I or II subbing for solution for 1 min.
Drain and dry in air for at least 4 h.
Hydrolyze the polyanhydride (Type I) in water for 30 min; denitrate Type II undercoats for 1 h in a 1:1 ethanol-ammonium sulfide bath; keep the surface wet until a gelatin layer is applied.
Affixing the gelatin layer:
Type I undercoats—gel casting
For flat surfaces follow the procedures outlined in Table II.
For nonplanar substrates, hold the piece with a vacuum chuck and carefully position a matching die, e.g., one fabricated from Sylgard 184 Encapsulating Resin Dow Corning, at a fixed distance above the substrate. Then proceed as outlined in Table II.
Type II undercoats—film transfer
Follow the procedure described in Table II, 3(f) through 3 (h).
An equivalent subbing solution can be prepared using NCMC.
Table IV
Processing Procedure for Hardened Gelatin Films
1. Kodak rapid fixer (with hardener)
5 min with continuous agitation
2. Wash
25 min in running water at 16–20°C
3. Wash
5 min in methanol bath No. 1 and 3 min in fresh methanol bath with continuous agitation to remove sensitizing dye
Prepare a 4% by weight aqueous solution of J. T. Baker Chemical Co. U.S.P. gelatin at 50°C.
Add 0.5% ammonium dichromate based on the weight of dry gelatin and filter through Whatman No. 31 filter paper in a heated funnel.
Thermostat the filtrate at 50°C.
Attach clean glass plates to a level glass platform with a drop of water.
Pour an excess is of warm gelatin solution across one end of a plate and wipe across the entire plate with a doctor blade fixed at 0.8-mm clearance.
Dry the liquid film overnight under dust-free conditions and harden by baking in a convection oven at 150°C for 2h.
By gel casting:
Prepare a 6% solution of gelatin containing 0.9% ammonium dichromate as described in 1(a) through 1(c).
Arrange a clean glass plate at a fixed distance above a flat level Plexiglas sheet. If 0.5-mm spacers are used the dry film will have a thickness of approx mately 12 μm.
Fill a warm pipette with the gelatin solution and allow capillarity to completely fill the volume between the flat sheets.
Place the mold in a refrigerator at 5°C for 2–3 h and then separate the glass and gelatin gel from the Plexiglas support.
Dry the gel in the dark overnight at ambient temperature and then light-harden as described in footnote a.
By film transfer:
Attach a Plexiglas sheet to a level glass platform. Clean the surface with isopropanol applied clean with a soft clean towel to remove any adhesive residue.
With a doctor blade set at 0.8-mm clearance, coat the sheet with a 4% gelatin solution containing 0.9% ammonium dichromate.
Dry overnight in a light-tight cabinet containing Drierite.
Cut the dry film to shape, and release each disk from the temporary support by applying a few drops of isopropanol at each cut.
Harden the gelatin disks as described in footnote a.
Pool distilled water on the substrate to which the gelatin is to be transferred and drop disk a gelatin onto the surface of the pool.
After volume swelling has reached equilibrium (5–10 min) slowly withdraw water from the pool with a hypodermic syringe and needle.
When one edge of the disk becomes attached to the substrate, carefully pour off the remaining water and allow the assembly to dry in air.
The procedure is essentially that of Brandes et al.6 An equivalent gelatin film can be obtained by incorporating 0.7–0.9% ammonium dichromate into the film and exposing the dry film to ambient room light for at least 4 h.
Table III
Preparation of a Preholographic Element on a Poly(Methyl Methacrylate) Substrate
Formulating the subbing solution:
Type I
Hercules Powder Co. nitrated carboxymethyl cellulose (NCMC; D.S. of −CH2COOH, 0.06; 12.5% N) is dried in a vacuum oven at 50°C for 48 h.
Dissolve 1.5 parts of methyl vinyl ether–maleic anhydride copolymer (Gantrez AN-169-GAF Corporation) and 0.5 part of dry NCMC in 98.0 parts of 2-mechoxyethanol (methyl cellosolve).
Gravity filter 1 through a coarse porosity fritted funnel.
Dry du Pont “pyro” grade nitrocellulose (NC; 12.6% N) in a vacuum oven at 50°C for 48 h.
Dissolve 2.0 parts of dry NC in 98.0 parts of 2-methoxyethanol.
Gravity filter through a coarse porosity fritted funnel.
Subbing the plastic support:
Immerse a Plexiglas substrate in a pool of type I or II subbing for solution for 1 min.
Drain and dry in air for at least 4 h.
Hydrolyze the polyanhydride (Type I) in water for 30 min; denitrate Type II undercoats for 1 h in a 1:1 ethanol-ammonium sulfide bath; keep the surface wet until a gelatin layer is applied.
Affixing the gelatin layer:
Type I undercoats—gel casting
For flat surfaces follow the procedures outlined in Table II.
For nonplanar substrates, hold the piece with a vacuum chuck and carefully position a matching die, e.g., one fabricated from Sylgard 184 Encapsulating Resin Dow Corning, at a fixed distance above the substrate. Then proceed as outlined in Table II.
Type II undercoats—film transfer
Follow the procedure described in Table II, 3(f) through 3 (h).
An equivalent subbing solution can be prepared using NCMC.
Table IV
Processing Procedure for Hardened Gelatin Films
1. Kodak rapid fixer (with hardener)
5 min with continuous agitation
2. Wash
25 min in running water at 16–20°C
3. Wash
5 min in methanol bath No. 1 and 3 min in fresh methanol bath with continuous agitation to remove sensitizing dye
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