Recording Ultra-Realistic Full-Color Analog Holograms for Use in a Moving Hologram Display

CAUTION: All appropriate safety practices must be followed when using lasers¹² and chemical products, including the use of personal protective equipment such as safety goggles, glasses, gloves, and lab coats.

1. Content creation

1. Model the different elements of the scene (character and background) with 3D computer graphics programs such as Blender, a free and open-source 3D software toolset.
2. Create a 12-frame cyclic animation with the 3D computer graphics program.
3. 3D print and paint the different elements.
   1. Print characters and background at the same scale using a fused deposition modeling (FDM) monochrome 3D printer¹³, with a white polylactic acid (PLA) filament.
   2. Use sandpaper to eliminate printing defects.
   3. Hand-paint the different elements with acrylic paint.
      CAUTION: To avoid unpleasant odors, paint outdoors or use ventilation.
4. Set up the recording box. Fix the background in a rigid wooden box to avoid movement and place the different 3D printed characters inside one after another to allow the recording of the different 4 inch x 5 inch holograms.
   NOTE: To avoid movement during hologram recording, firmly attach the individual elements using glue or plasticine, without applying stress.

2. Hologram recording

1. On an optical table, assemble a Denisyuk single-beam full-color optical setup^9,10 to record the holograms.
   NOTE: To record the different holograms, the three RGB lasers used are a red HeNe, 633 nm, 20 mW; a green diode-pumped solid-state (DPSS), 532 nm, 100 mW adjusted to 20 mW; and a blue DPSS 473 nm 50 mW adjusted to 20 mW. The spatial filter is equipped with a 40x, 0.65 NA achromatic microscope objective and a 10 μm pinhole.
   1. Combine the 3 laser beams (red, green, and blue) with an X-cube prism to get a white laser beam that passes through the same spatial filter.
      NOTE: Use two mirrors for the red and blue lasers to get four degrees of freedom and perfectly align the three beams.
   2. From a distance of 1 m and an angle of 45°, illuminate the recording box with the divergent beam.
   3. Adjust the distance of the three lasers from the cube to get similar beam diameters projected on the object plane.
      NOTE: The recording box has to be illuminated with a wide, clean and homogeneous divergent white beam.
2. Use a power meter to adjust the color balance and determine the exposure time.
   1. Measure the intensity of each laser horizontally, at the position of the holographic plate (see Table of Materials). Since the plate material is isopanchromatic, adjust the color balance equally for the 3 lasers.
      NOTE: The power-meter used allows direct reading of the power of the 633 nm red laser. For the 473 nm blue and 532 nm green lasers, it is necessary to apply a correction coefficient (x0.4 for blue and x0.6 for green).
   2. Determine the exposure time before recording the hologram, according to the following formula:
      (1)
   3. where t is the exposure time (s), H the sensitivity of the material (J/cm²) and E the intensity of the laser (W/cm²). E is measured at the position of the holographic plate with a power meter.
      NOTE: The sensitivity of the materials used here is 200 μJ/cm² per laser for a full-color (RGB) hologram. The intensity of each laser at the position of the holographic plate, measured with the power-meter is 17 μW/cm² per laser, and the exposure time is 12 s according formula (1).
3. Close the laser beam with a shutter.
   NOTE: Use an electronic shutter with a timer to control the exposure time precisely.
4. Prepare plates.
   CAUTION: Handle the plate edges using gloves, and do not allow skin contact with the emulsion at any time.
   1. Remove holographic plates them from the refrigerator to avoid a shift and store them at room temperature (20–25 °C) for 1 h before recording.
      NOTE: The plates used here have to be refrigerated at 4 °C.
   2. Darken the upper edge of the plate with a black marker to avoid internal reflection.
5. Set up the recording plate under a green safelight.
   1. Blow onto the plate to determine the emulsion side. Steam appears only on the glass side.
   2. Place the holographic plate emulsion-side down on the recording box. Allow it to stabilize for 5 min before recording.
6. Open the shutter to expose the recording plate, during the time previously calculated with the formula (1).
7. Keep the recorded plate in a closed box away from light.

3. Hologram development

NOTE: Holograms are developed with a safe and non-staining chemical process developed for the ultimate emulsions.

1. Once the plate has been exposed, prepare 100 mL of developer for a 4 inch x 5 inch plate. Mix the developer at a ratio of 1 part developer to 10 parts distilled or demineralized water (1:10).
   NOTE: The developer is stocked in a concentrated solution in a closed bottle to prevent oxidation and needs to be diluted with distilled or demineralized water just before processing.
2. Heat the developer to 22 °C precisely.
   NOTE: The water temperature must be equal to or greater than 20 °C for the developer to function properly. For repetitions, control the temperature before development with a thermometer.
3. Under a green safelight, place the exposed plate in the tray and submerge it quickly, emulsion-side up, into the developer and agitate slowly for 4 min precisely. At the end of the development, the plate attains a pale yellow/orange color.
   NOTE: The development process becomes visible a few seconds after the plate is fully covered with the developer. Use an insulated tray with a lid to maintain a constant temperature during development. Further development to attempt to get a black density is not required.
4. Remove the developer and wash the plate in its tray under running tap water for 30 s, allowing the water to overflow into a sink.
5. Under normal light, place the developed plate in the tray and submerge it quickly, emulsion-side up, into the bleach without agitation until the plate becomes fully transparent. The bleaching process becomes visible a few seconds after the plate is fully submerged.
   NOTE: The typical bleaching time is 3 to 5 min at room temperature (20–25 °C).
6. Remove the bleach and wash the plate in its tray under running tap water for 2 min, allowing the water to overflow into a sink.
   NOTE: When the plate is still in a wet state after bleaching, a hologram can be observed by transmission with a halogen spot. When the hologram is successful, this image will appear very strong.
7. Place the plate in the tray and submerge it, emulsion-side up, into a demineralized or distilled water solution with some drops of wetting agent without agitation for 1 min.
8. Remove the plate from the tray and dry it vertically for 15-20 min.
9. Repeat these operations for each of the 12 holograms. Before recording, in order to place the different objects in the recording box with great precision, apply a holographic onion-skin method by replacing the previous transparent hologram at its recording position, and observe both images at the same time under laser illumination to check that the new character is well-positioned.
   NOTE: Onion skinning is a procedure usually used in stop-motion animation to see two different frames at the same time.

4. Hologram sealing

NOTE: Holograms are protected by a second clean glass plate sealed to the hologram using optical ultraviolet (UV) glue.

1. Use a scalpel to scrape off 5 mm of the emulsion around the edges.
   NOTE: This operation is easier when the plate is still wet.
2. Laminate the hologram to a clean glass plate of equal size (4 inch x 5 inch), with 1 mL of UV glue.
   NOTE: To facilitate lamination, pre-heat the glue, the hologram and the clean glass plate in an oven at 30 °C for 10 min.
3. Expose the clean glass side of the hologram to sunlight; the UV glue will harden within 5 min.
   NOTE: It is also possible to use a UV lamp, but strong UV exposure should be avoided.
4. Wash the sealed hologram with water and soap, dry it with tissue paper and blacken the back of it with matte black spray paint.

5. Fantatrope assembly and operation

1. Mount the 12 holograms in chronological order in the regularly placed frames of a Fantatrope.
   NOTE: Gentet et al. 2019⁷ describe the manufacture and operation of a Fantatrope.
2. Rotate the Fantatrope at constant speed. An RGB LED strobe light synchronized with the rotation speed successively illuminates the different frames to create a rapid succession of images and produce the illusion of movement.
   NOTE: A rotation of one turn per second is enough to get the sense of a fluid motion.