TECHNICAL
Dichromated Gelatin (DCG) for Hologram Material
Dichromated gelatin material for hologram recording and dcg processing technique to get different color in holographic images are discussed below.
Dichromated gelatin (DCG) is the best for volume phase holograms capable of producing very high diffraction efficiency and low scattering noise. Unfortunately because of poor self life of DCG plate, it cannot be commercialized. The dichromated gelatin has the following unique characteristics which make it an ideal material for hologram recording.
DCG has resolution capability extending beyond 5000 lines/mm. Its response is uniform over a broad range of spatial frequencies from 100 lines/mm to 5000 lines/mm.
The refractive index modulation capacity of DCG is very high.
DCG has low absorption over a wide range of wavelengths.
DCG can give reconstructions even without development.
DCG hologram can be redeveloped to get any desired refractive index modulation and peak diffraction wavelength.
The thickness of DCG can be increased or decreased by controlling the exposure and processing conditions. This property is unique because other materials normally shrink after development.
DCG hologram has high SNR.
Hologram Formation in DCG
Dichromated gelatin was first used as hologram recording material by Shankoff who found that very large index modulation can be obtained in relatively thin films. When a gelatin film, sensitized with ammonium dichromate, is exposed to actinic radiation, the hexavalent chromium ion is photoinduced to trivalent chromium ion.
A widely used method of preparing a DCG film is to dissolve out the silver halide in a silver halide photographic plate by soaking the unexposed plate in fixer. Kodak 649F spectroscopic plates give good results. However, since these plates are not manufactured for recording of DCG holograms, they require lengthy procedures for the recording of high quality holograms.
It is also possible to coat glass plates with gelatin films. A 10 micrometer film can be made by mixing 1 g ammonium dichromate with 3 g of gelatin and 25 g of water. It is stirred together, heated, filtered and coated on a 8x10 inch glass plate. The coating can be done by spin method (75 rpm) or doctor's blade method.
The plates should be hardened initially so that the film will not dissolve in water during processing. It should be soft enough so that photochemical reaction produces a significant difference in hardness. The degree of prehardening is a critical parameter for obtaining good results. If the hardening is too high, the refractive index modulation decreases. If it is too low, the hologram appears milky.
The sensitized plates should be kept under relative humidity below 40%. The sensitized emulsion can be placed in a liquid gate containing xylene. This would avoid the need for keeping the relative humidity of the exposure laboratory below 40%.
Hologram Optimization
A wide range of holographic properties can be achieved by suitable control of various parameters in the film preparation, exposure and development process.
Gelatin film hardness: Decreasing the film hardness will increase the level of scattering.
Gelatin film thickness: Decreasing the film thickness will increase the spectral bandwidth.
Concentration of sensitize: Decreasing the concentration of the sensitize will increase the fringe spacing. The spectral tuning position will move towards the red end of the visible spectrum.
Exposure energy: Decreasing the exposure energy will decrease the diffraction efficiency. The spectral tuning position will move towards the red end of the spectrum.
Exposure wavelength: Decreasing the exposure wavelength will shift the spectral tuning position towards the blue end of the spectrum.
Exposure geometry: Decreasing the angle of incidence decreases the fringe spacing. The spectral tuning position will move towards the blue end of the spectrum.
Temperature of the developer bath: Decreasing the process temperature will decrease the diffraction efficiency and the spectral bandwidth.
The factors which influence the reconstruction wavelength are listed below in order of decreasing importance.
Concentration of dichromate sensitize
Exposure wavelength
Exposure energy
Initial hardness
Processing temperature
pH of baths in processing
Concentration of water/alcohol baths
Processing rate
Depending on the temperatures and times spent in each bath, a variety of effects can be obtained.
(a) Cold baths produce better uniformity and lower noise.
(b) Hot baths can yield very high index modulation but often with increased noise.
(c) Processing and reprocessing can vary the index modulation from nearly nothing to a very high value.
(d) Repeated processing can produce variations of colors and bandwidth. A red shifted hologram can be converted into a blue shifted hologram. Also a 100 nm bandwidth HOE can be shifted to a 20 nm bandwidth HOE by reprocessing.
The direction of shift can be controlled by the ratio of water and alcohol in the bath preceding 100% alcohol. A 30% water-70% alcohol bath will give different results than a 70% water-30% alcohol bath. The amount of shift can be varied by the time in this bath.
The Russian long-life Slavich Dichromated Gelatine emulsion for recording in the blue and green spectral region is commercially available Lithuania.
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