DCG Thickness

[Dutchelm05]

I found this on the web regarding a means for measuring DCG film. I have always been interested in these kind of processes. It appears in the diagram, they use a CCD camera and a diode laser. They seem to shine the light through the plate and the camera is on the other side. I imaging they determine the intensity of light using a gray scale and some program that can quantify thickness. They also seem to use a beaker of emulsion as a baseline.

Can anyone comment of this? I would be interested in how others think this works.

Thanks,
Tony


http://cslin.auto.fcu.edu.tw/~cslin/oe/II.htm

II. DCG HOLOGRAM FABRICATING PROCESS



There are many reports about the use of silver halide materials to produce gelatin holograms for laboratory preparation. This involves processing photographic plates in Kodak rapid fixer and sensitizing the plate with an ammonium dichromate solution. The subsequent silver halide removal leaves voids in the gelatin and causes increased scatter in the diffraction field of the hologram. Furthermore, this process is expensive and troublesome. Here we present a simplified DCG hologram fabricating process:



1. A 5 % w/v by weight solution of gelatin in distilled water is prepared and filtered. Then ammonium dichromate is slowly added to a 5 weight proportion of gelatin in the mixer with rapid stirring . An electronic image system is used to record the image of the solution. Many people insist that it is imperative to keep the relative humidity below about 40 % after sensitization. In our procedures the humidity control is not necessary.

2. Keep the temperature of the solution at 50° C to preserve the liquid flow characteristics. Prepare a large area clean glass plate and adjust its horizontal position. Pour 30-50 ml solution on to a 300mm´300mm glass plate and wait for the emulsion to dry. The image system is used to monitor the uniformity of the emulsion on the glass plate and a steel rod of 5mm diameter is used to drive the denseness of the emulsion. The curing time is almost 12 hours.

3. Drop blue ink into the gelatin layer in order to analyze hardness. The initial hardness of the prepared gelatin layer, as well as its thickness, will significantly affect the quality of the hologram.

4. Record the thickness of the emulsion, then expose the plate to 100-400 mJ/cm2 diode laser energy at a wavelength of 532 nm.

5. Develop the plate in a 50 % isopropanol solution for 0.2 to 3 minutes and then rapidly remove and place in a 100 % solution of isopropanol. Then remove the plate and place in hot dry air.

6. Bake the plate for 1 to 2 hours at a temperature of 90 ° C and monitor the variations in diffraction efficiency.



In Fig.2, we can see that the image system plays an important role in our DCG hologram fabrication technology. At first we obtain the color data of the DCG solution for further calibration. However the gray levels of the image vary according to the level of the illumination. To solve this problem, an auto-calibration algorithm has been added to our system and the variable R1 is the calibration parameter. In the dye spot diffusion test, the image system can obtain the hardness of the emulsion and determine if enough curing time has elapsed. After curing, the image system can calculate the uniformity and thickness of the holographic plate [9]. Finally, the image system can monitor the variations in the diffraction efficiency, scattering, transparency and spectral response in real time.