Single Beam Transmission Hologram


Creating a single beam transmission hologram will help you achieve four goals:
  1. You will become familiar with setting up an optical arrangement for recording a hologram.
  2. You will learn the step-by-step recording procedure for obtaining a successful, high-quality hologram.
  3. You will become familiar with the processing chemistry and the steps involved.
  4. You will be successful in producing a hologram that will build confidence in your abilities to create more complicated multibeam holograms for display purposes.
   
A single beam transmission hologram is a very simple optical arrangement and requires less system stability than multibeam arrangements. This hologram is called a transmission hologram because during the viewing of the hologram (technically called reconstruction), the light source is on the opposite side of the hologram from the viewer and the reconstructing light is "transmitted" through the hologram.
   
single beam tramsmission hologram setup
Figure 15: Single beam transmission hologram setup
   
Figure 15 illustrates the recording arrangement on a small 3-foot by 4-foot optical table. The laser L should be located on the left corner of the table. The output beam from the laser should be 9 inches above the table surface (use your tape measure to check this). The beam is directed to the center of mirror M1 which reflects the beam to mirror M2 which reflects the beam toward the object scene OS and the plate holder PH. The beam should always be 9 inches above the table as the beam moves from mirror to mirror. To insure this, use mirror M1 to reflect the beam back to the laser. Adjust mirror M1 so the beam is seen hitting the laser on either the right or left side of the laser's output aperture (we'll call this method, which will be used later in other setups, "retro-reflection"). Now, direct the beam with M1 toward M2. The beam should be about 9 inches above the table surface just in front of M2. Next, reflect the beam back to mirror M1 using mirror M2, aligning the beam to hit the beam dot on M1. Next, direct the beam with mirror M2 back towards the object scene and plate holder. If you use this method each time a mirror is placed in an optical arrangement, the beam should remain around 9 inches above the table. Mirrors M1 and M2 should be 4" x 5" in size to achieve uniform illumination of the object scene and plate.
   
The object scene OS and plate holder PH are placed far enough from M2 so they are both illuminated uniformly by the diverging lens DL. The center of the object and the center of the plate holder should be about 9 inches above the table and the object should be placed as close to the plate as possible without casting a shadow on it. A 4" x 5" piece of white matte board should be used as a temporary plate in the plate holder to check for shadows and uniform illumination. The object scene table mount should be connected to the plate holder table mount using an aluminum bar and clamps. This will ensure a successful image since both the scene and plate holder will move together if vibrations occur during an exposure. The platform for the objects is a 4" x 5" x 1/4" piece of black Plexiglas attached to a table mount using a short rod. 8-32 threads will need to be tapped into the edge of the plastic. Actually, any platform can be used as long as it is heavy and has a flat top surface for placing the object(s) on, like a brick for example.
   
A diverging lens DL should be placed somewhere between the laser and the object scene/plate area so the object scene and plate are uniformly illuminated. There is a mathematical relationship between the focal length of the diverging lens and the size of the beam diameter after the laser beam passes through the lens DL that can be used to determine how far the diverging lens must be placed from the object scene. This divergence equation is:
   
equation of focal length vs beam diameter
   
visual relationship of focal length and beam diameter
Figure 16: Graphical relationship between focal length of diverging lens and laser beam diameter
   
Figure 16 illustrates this relationship graphically from a top-view perspective. The focal length of the lens [F] is inversely proportional to the diameter of the beam [D(o/p)]: the shorter the focal length, the larger the diameter of the beam as it moves away from the lens. Let's look at an example. Using Figure 15, we will determine where a diverging lens should be placed with a focal length of -0.32 inch (-8 mm or 20x objective), an object scene/plate width of 10 inches, and a laser beam diameter of 0.08 inch (2mm):
   
mathematical results of equation
   
The diverging lens should be placed at least 80 inches away from the object scene/plate area to have uniform illumination. The location of the lens in Figure 15 is strictly diagrammatic and does not necessarily represent the correct location. Actually, with a -8 mm lens and a 3 foot x 4 foot table, an 80" distance will place the lens close to the laser.
   
This is the basic optical arrangement. During the exposure, the hologram is created by the interference between the light rays reflected from the object to the plate and the light rays passing by the object directly to the plate. In a multibeam arrangement, the object light is called the object beam and the beam passing the object and going directly to the plate is called the reference beam.
   
The object selected as a subject for this optical arrangement should be three dimensional. I suggest chess pieces or geometric wooden shapes--both are rigid and have the desired characteristics discussed next. The object's reflectivity and color are both important. Since red laser light is being used, the color of these objects should be preferably white, but can also be red, orange, or yellow (blue or green objects will absorb the red light and create dark images). Proper coloration of the object will allow the object to reflect a great deal of laser light. Two to four objects should be used so the effects of parallax can be seen in your image (parallax in holography means the ability to look around one object to see another object hidden behind). Look through the plate holder and align your objects so that you need to look around one object to see another behind to achieve the parallax effect. It will also be apparent that the object scene will be illuminated only from the side when seen from the plate's position. With a multibeam transmission hologram, discussed further on, the scene can be fully illuminated. The objects should be rigidly attached to the object platform (glue, clay, or double-sided tape works well).
   
The plate holder should face the object scene squarely with the reference portion of the beam hitting the plate (white matte board) at an angle alpha (Figure 15) of about 45-60 degrees from the plate's normal (the normal is a line drawn perpendicular to the plate and you can use a protractor to measure this angle). This angle is necessary so that when viewing the hologram image, the reconstructing beam is reflected away from the viewer and does not show up as part of the image. If you plan to use film sandwiched between two glass plates, the angle alpha should be 56 degrees. This is called Brewster's angle and will eliminate any internal reflections between the glass plates which causes interference patterns on the film that degrade the image quality. The plate holder should have two table mounts with the second mount attached to the primary mount with an aluminum bar and clamps. This will keep the plate holder in position when you load it with film or a plate.


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