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Introduction

Vibrations

Processing

SB Transmission

Exposure

Recording

SB Reflection

MB Transmission

MB Reflection

Lighting

Resources

Creating a Single-Beam Transmission Hologram

Producing a single-beam transmission hologram will help you achieve four goals:

Note: All of the optical arrangements discussed from this point forward are again based on a 4 foot x 6 foot optical table and the holograms produced will be 4 inch x 5 inch in size. All of the optical arrangements are scalable. All components in an illustrated optical arrangement are not necessarily to scale for the sake of clarity. All table mounts and optical mounts are not included in the illustrations of optical setups for the sake of clarity.

Determining and Controlling Polarization

Determining and controlling the orientation of the laser's beam polarization is important for two reasons:

The optical arrangements for a single-beam transmission hologram, single-beam reflection hologram, and a multi-beam transmission hologram will have the reference beam incident on the recording plate from the side and parallel with the table surface. A multi-beam reflection display hologram will have the reference beam incident on the recording plate from overhead or underneath. If your reference beam is incident from the side, your beam's polarization needs to be horizontal to the table surface as shown in Figures 58 and 59. If your reference beam is incident from overhead or underneath, your beam's polarization needs to be vertical to the table surface as shown in Figures 60 and 61. Arrows indicate beam propagation direction.

Note: The orientation of the beam's polarization is very important when using recording film sandwiched between two glass plates. The laser light can reflect internally between the two glass plates, causing extraneous interference fringes on the film which degrade your holographic image because you have to look through these fringes to see the image. This can even happen with a recording plate where there can be internal reflections between the inside sides of the glass plate supporting the emulsion. The correct polarization orientation will help minimize or eliminate these extraneous fringes in combination with using a reference beam incident angle of 56°. This is called Brewster's angle and will discussed later.

horizontal polarization image
   Figure 58: Horizontal polarization. Reference beam incident from
                  the side and polarization parallel with the table.

close up horizontal polarization image
      Figure 59: Close-up of horizontal polarization. Reference beam
       incident from the side and polarization parallel with the table.

vertical polarization image
 Figure 60: Vertical polarization. Reference beam incident from above the plate and polarization
                        perpendicular with the table. Plate is semi-transparent to show cone.

close up of vertical polarization image
Figure 61: Close-up of vertical polarization. Reference beam incident
from above the plate and polarization perpendicular with the table.
                        Plate is semi-transparent to show cone.

As was mentioned in the sub-section Laser in the section on Building a Holography System, your helium neon laser should be linearly polarized, usually 500:1 for a He-Ne laser. Because linear polarization oscillates at right angles to the propagation direction of the laser beam (an electric field at right angles to a magnetic field), you can visualize these oscillations as being a simple plane as shown in Figures 58 thru 61 (the electric field, not the magnetic field, actually exposes the plate and creates the plate's processed density). The very first thing you want to do before you start setting up your first optical arrangement is to find out the orientation of your laser's polarized beam.

Determining the Laser's Polarization Orientation

Linear polarized helium neon lasers come encased in either rectangular shaped metal casings or cylindrical metal casings. Usually the top of the laser casing has a label with the company's name and other information and on the bottom of the casing is a 1/4-20 threaded hole for mounting and rubber footers. See Figures 4 and 5 in the sub-section Laser in the section on Building a Holography System. If the manufacturer has orientated the laser tube correctly inside the casing, the laser's beam polarization orientation will be either vertical or horizontal when the label is on top. With my first 5 mW helium neon laser, built 8 years (1970) after lasers were invented, the tube was installed so the polarization orientation was at 10/4 o'clock instead of noon/six for vertical or 3/9 for horizontal. Since that time, I've had two 35 mW He-Ne lasers, one with horizontal polarization and one with vertical polarization. It looks like some laser manufacturers now understand the importance of the orientation for certain applications.

The least expensive way to find out the orientation of your laser's beam is to buy clip-on polarizing eyeglass lenses at a drug store. These clip-on eyeglasses have their polarizing lines running vertically in each lens when worn horizontally on your face. Set the laser on your table with the label facing up, turn it on, let it warm-up for 60 minutes, and point the beam at a piece of white mounting board. Insert one of the lenses of the eyeglasses into the beam so the beam passes through the lens perpendicularly and so the lens is orientated as you would wear it, that is, with the polarizing lines running vertically. Now rotate the lens through 90 degrees and observe the beam on the white card to see if the beam gets darker or brighter. If the laser's polarization is vertical, the beam will be brightest when you first insert the lens and get darker as you rotate the lens towards 90 degrees. If the laser's polarization is horizontal, the beam will be dark when you first insert the lens and get brighter as you rotate the lens towards 90 degrees. Continue rotating back and forth until you find the brightest beam and mark that angle on the output aperture of the laser with a permanent marker. If your beam is other than vertical or horizontal, you'll need to rotate the laser along its length to make it's beam vertical or horizontal and mount the laser to its table mounts accordingly. A photographic polarizing filter also works for this testing. A white triangle on its edge indicates where the polarizing lines run across the filter's diameter.

Controlling Polarization Orientation

Let's say that your laser was built correctly and it's polarization was chosen to be vertical instead of horizontal, with the laser's label facing straight up. With this first single-beam transmission hologram arrangement, you'll be bringing the reference beam in from the side, so your polarization orientation needs to be horizontal. There are two ways you can change the beam's polarization orientation from vertical to horizontal:

  1. You can mount the laser on its side which is not easy but doable, or
  2. You can arrange two mirrors as shown in Figure 62 close together so that the reflected beam's polarization orientation from the second mirror (mirror 2) is rotated 90 degrees from the incident beam's orientation on the first mirror (mirror 1). This mirror combination setup, shown in Figure 62, shows a vertically orientated beam being converted to a horizontally orientated beam. The setup is exactly the same for converting a horizontal beam to a vertical beam. I will refer to this two-mirror setup as a polarization rotator.

Make sure you use separate table mounts for each mirror because it's much easier to align and adjust each mirror separately. It's impossible to mount both mirrors on the same table mount pole and align them properly. Additionally, mirror 2 should be 9 inches above the table where all the other components will be downstream in the setup. This means that the laser and the mirror 1 will need to be at a lower height above the table. If you place the center of mirror M1 at about 7 inches or less above the table, then the laser aperture should be at the same height and mirror M1 should be retro-reflected to insure the beam is parallel with the table surface. As a side note, all optical recording setups will use horizontally orientated polarization since we will always be impinging on the recording plate from the side with the reference beam except for the single-beam and multi-beam reflection hologram setups where the reference beam will be impinging on the plate from overhead or underneath. Since in the above example we assumed the laser had vertically orientated polarization, in the reflection hologram setups the laser beam aperture and mirror M1 should be 9 inches above the table because you don't need to change the beam's vertically orientated polarization to horizontally orientated polarization.

vertical to horizontal polarization change image
Figure 62: Converting vertical orientated polarization to horizontally
                                          orientated polarization.

Setting Up A Single-Beam Transmission Hologram Arrangement

A single-beam transmission hologram is a very simple optical arrangement and requires less system stability than multi-beam arrangements. It's called a transmission hologram because the reference beam exposes the photographic plate on the same side as the object scene's reflected light exposes the plate, as shown in Figure 63. The processed hologram image is reconstructed when the same reference beam light "transmits" through the hologram to the viewer's eyes as shown in Figure 72 in the sub-section Determining Where to Put the Diverging Lens in the sub-section Creating a Single-Beam Transmission Hologram under Creating Transmission & Reflection Holograms and Figure 75 in the sub-section View Your First Hologram under the sub-section Recording and Processing under Creating Transmission & Reflection Holograms.

single-beam transmission hologram setup image
                                        Figure 63: Single-beam transmission hologram setup.

Figure 63 illustrates what the recording arrangement will look like when it's set up. Refer to this illustration as you set up the arrangement. Ignore mirror 2 and mirror 3 at this time. I will discuss the purpose of these mirrors later and whether you will need them.

As a quick overview description of this setup, the laser's beam travels to a mirror which reflects the beam through a diverging lens and illuminates the photographic plate holder and object scene. A diverging beam from the lens is not shown in this illustration for the sake of clarity but is shown in Figure 67.

So let's get started.

connecting plate holder and object scene image
           Figure 64: Plate holder and object scene locked together.
top view of plate holder and object scene image
              Figure 65: Top view of plate holder and object scene.

Note: Why a 56 degrees incident angle on the plate? This angle is called Brewster's angle. If you're using film sandwiched between two glass plates instead of a photographic plate, the laser light can reflect internally between the two glass plates, causing extraneous interference fringes on the film, degrading your image. Brewster's angle eliminates this problem. A plate decreases this problem but can still possibly develop these extra fringes without using Brewster's angle.
plate holder/object scene cast shadow image
       Figure 66: Illumination is uniform on the back screen and the
    object scene is not casting shadows on the plate holder screen.
plate holder/object scene illuminated image
             Figure 67: Plate holder and object scene illuminated properly with diverging beam.

Determining Where to Put the Diverging Lens

No matter what optical setup your doing, at some point during the set up you have to decided what focal length microscope objective or simple lens to use and where to place it in the beam whether it be a single-beam or multi-beam setup. Usually you want to know how far away the diverging lens needs to be to cover the recording plate and object scene uniformly such as in the setup you're now arranging.

I have come up with a couple equations that you can use based on actual measurements I've made on several types of setups. The first formula shown in Figure 68 is specific to a single-beam transmission hologram. This equation gives you the distance (L) the diverging lens needs to be from the plate holder/object scene based on the focal length (F) of the diverging lens, the diagonal width (W[p,o]) of the plate/object scene or just plate perpendicular to the incoming beam, a coefficient number (4.412), and the diameter of the laser beam D (b) at the laser's aperture

lens distance formula image
           Figure 68: Formula to calculate distance of lens from plate/object
                               for a single-beam transmission hologram.

Figure 69 illustrates this relationship graphically from a top-view perspective. The Rule: the shorter the focal length F of the lens, the shorter the distance L needs to be.

lens distance formula diagram
                                 Figure 69: Illustrated relationship between the focal length
                                                   of the diverging lens and the distance L.

Let's look at an example. Using Figure 67, we will determine where a 40x microscope objective should be placed with a focal length of 0.17 inches, an object scene/plate holder diagonal length of 6.4 inches, and a laser beam diameter of 0.08 inches measured at the laser aperture as shown in Figure 70:

lens distance formula calculation
          Figure 70: Example calculating the distance of a 40x
          microscope objective from a plate/object scene for
                      a single-beam transmission hologram.

The objective should be placed at least 60 inches away from the plate holder and object scene to have excellent uniform illumination. The location of the objective in Figure 67 is pretty much where it should be. When doing this calculation, ignore any minus signs of a simple lenses.

For a single-beam reflection hologram setup, a slightly different equation is used because the object scene is not included in the W factor of the formula. Here, the W factor is just the diagonal length (6.4 inches) of the 4 inch x 5 inch photographic plate since the object scene is directly behind the plate. This is also true in a multi-beam setup since the plate's incoming beam is completely separate from the beam for the object scene. Use the diagonal length of the plate for W.

In Figure 71, I have listed the focal length of various microscope objectives plus the focal lengths and diameters of various plano-concave lenses at year 2020 costs.

Figure 71: Available microscope objectives and plano-concave lenses.

Microscope Objective DIN AchromaticPlano-Concave Lens MgF2 coating
MagnificationFocal Length (mm)CostDiameter (mm)Focal Length (mm)Cost
4x29.27 (1.15")$8025 -25 (0.98")$29.70
10x17.02 (0.67")$11012-12 (0.47")$31.50
20x8.33 (0.33")$1159-9 (0.35")$30.50
40x4.35 (0.17")$1206-6 (0.24")$29.50
60x3.21 (0.31")$195n/an/an/a
100x1.88 (0.074")$260n/an/an/a

Microscope objectives are more expensive than plano-concave lenses, but finding lenses with focal lengths shorter than -6 mm is not possible at this time. 6 mm diameter lenses can be difficult to mount but it can be done using the optic mount discussed in the sub-section Optic Mounts under the section Building a Holography System. All simple lenses should have MgF2 anti-reflection coatings. Additionally, if you plan to make holograms larger than 4 inch x 5 inches , you will need to use microscope objectives instead of simple lenses to achieve the uniform coverage of the larger plates. I strongly recommend going completely with microscope objectives and start with 20x, 40x, and 60x powers.

During the exposure of the scene shown in Figure 67, the hologram is created by the light interference between the light rays reflected from the objects to the plate and the light rays passing by the objects directly to the plate. The light rays reflected from the objects are considered the object beam, as in a multi-beam arrangement, and the light rays passing the object and going directly to the plate are considered the reference beam. Path lengths, discussed in multi-beam setups, are not a requirement in single-beam setups.

In this setup, I want you to be successful, so choose objects for the scene that are white and rigid. A small white figurine, small white car, or small white chess pieces are good examples. Having two objects will enhance the parallax effect but not required. The castle and unicorn images in the sub-section Introduction page of Building a Holography System were made on 4 inch x 5 inch plates. Figure 72 shows how two geometric shapes will look like in the exposed and processed hologram from two different perspectives showing parallax.

final hologram imagesfinal hologram images
                  Figure 72: Processed single-beam transmission hologram showing parallax.

Now that you have your first optical arrangement setup and ready to record a hologram, I'll next show you how to calculate your exposure time needed during the recording process.