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SB Transmission



SB Reflection

MB Transmission

MB Reflection



Creating a Single-Beam White Light Reflection Hologram

Figure 77 illustrates the optical arrangement for a single beam reflection hologram. This setup is identical to the single-beam transmission hologram arrangement except for the positioning of the plate holder and object scene. The plate holder should be perpendicular to the incident beam and tilted at an angle of 56 degrees towards the incident beam creating an overhead reconstruction angle for viewing the finished hologram. The object scene should be placed directly behind the plate holder on the opposite side from the incident reference beam and almost touching the plate. The plate emulsion should face the object scene.

Because the reference beam is incident on the plate from overhead along the 5 inch width of the plate, your beam polarization needs to be vertical. Since we assumed the polarization was horizontal in the single-beam transmission hologram setup, you will need to use the two-mirror polarization rotator in this setup. This means that the mirror and diverging lens shown in Figure 77 will need to be moved to the right somewhat so you can insert the rotator to the left of the mirror. You will also need to lower the laser so its beam aperture is at about 7 inches or less above the table as previously discussed in the sub-section Controlling Polarization Orientation under the sub-section Creating a Single-Beam Transmission hologram.

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                                         Figure 77: Single beam reflection hologram setup.

Figure 78 shows the diverging laser beam from the lens to the plate holder and Figure 79 shows a close-up view of the plate holder and object scene positioning, and reference beam incident angle.

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             Figure 78: Plate holder and object scene illuminated properly with diverging beam.

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                          Figure 79: Plate holder and object scene
                    positioning and reference beam incident angle.

During the exposure, the laser beam will pass through the hologram plate, illuminate the object scene, and the scene will reflect its light back to the plate. At the same time, the beam passing through the plate serves as the reference beam. The elimination of vibrations and component movement during the exposure is even more critical during a reflection hologram recording. Because the angle between the reflected light from the object scene to the plate and the incident angle of the reference beam totals 124 degrees, the spacing between fringes in the interference pattern in the emulsion is much narrower (finer) than a transmission hologram and must not move during the exposure. To help prevent any movement between the object scene and plate holder, you must once again connect the table mounts together as you did for your single-beam transmission hologram.

The recording and processing procedure for this type of hologram is the same as for the transmission hologram except that you want to end up with an exposure density of 2. So again, you want to check the hologram's density after you've developed the plate through the developer and stop bath, washing, photoflo, and drying. Once you've found the right exposure time for a density of 2, you can continue the development process with a one minute soaking of the plate, bleaching, washing, photoflo, and drying. Again, make sure the emulsion faces the object scene.

A reflection hologram is viewed differently from a transmission hologram as shown in Figure 80. The person viewing the hologram is now on the same side of the hologram that the reconstructing light source is on. This is called a reflection hologram because the reconstructing light is reflected from the plate to the viewer. This is a white-light hologram and can be viewed with sunlight (best), a clear light bulb with a vertical filament, or a flashlight. Because the fringes are more narrowly spaced, they filter out all but one wavelength of the white light, giving you a monochromatic image. At a 56 degree reconstructing angle, its color will be golden or reddish since you're using a red laser. You can even change the angle of reconstruction to get other monochromatic colors.

You'll remember that the plate was titled towards the beam at a 56 degree angle, but the object scene was level with the beam. This means that when you view the hologram, the plate will not be tilted towards you but vertical and you'll need to look downward into the plate to see the image. This is an overhead reconstructed hologram because when you hold the plate vertical in front of your eyes, the reconstructing light will be incident on the plate from overhead or over your shoulder.

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Figure 80: Reconstructing reflection hologram virtual image.

You could have placed the object scene up higher and centered and perpendicular to the recording plate during the exposure as shown in Figure 81. Then the image would be centered in the middle of the finished hologram and you wouldn't have to look downward into the hologram to see the image. Since that would have been a slightly more difficult object scene setup, I went with an easier setup for your first time reflection hologram.

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               Figure 81: Plate holder with object scene positioned
                     high and perpendicular to the recording plate.

The final hologram will be clear and you will be able to see background objects through the hologram as well as the image. The image will be less bright under these conditions. To increase the brightness of the image, place the plate, emulsion up, on a piece of paper towel that is on a large area of newspaper outdoors and spray the plate with 2 to 3 thin coats of quick-drying flat black enamel paint. Do not over spray during each coat. It's better to use multiple thin coats than one or two thick coats. Use a left to right, right to left sweeping motion when spraying. Let the paint dry for an hour or so between sprayings. Let the last spraying dry for at least 6 hours before viewing. Now, when you reconstruct the image in white light, it will be much brighter. Painting the emulsion side of the hologram using the above spraying technique should not affect the fringe spacing in the hologram. If it does and shifts the color, then face the emulsion away from the object scene during the recording so you can paint the glass side of the plate or acetate side of the film. I've never seen this happen though.