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Introduction

Vibrations

Processing

SB Transmission

Exposure

Recording

SB Reflection

MB Transmission

MB Reflection

Lighting

Hardcopy

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 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 15a and 15b. 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 15c and 15d. Arrows indicate beam propagation direction.

Note: The orientation of the beam's polarization is most 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 15a: Horizontal polarization. Reference beam incident from
the side and polarization parallel with the table.

close up horizontal polarization image
Figure 15b: Close-up of horizontal polarization. Reference beam
incident from the side and polarization parallel with the table.

vertical polarization image
Figure 15c: 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 15d: 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 I mentioned in the Laser section of Building a Holography System, your helium neon laser should be linearly polarized. Because linear polarization oscillates at right angles to the propagation direction of the laser beam, you can visualize these oscillations as being a simple plane as shown in Figures 15a-d. 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. See Figure 3a and 3b in the Laser section of 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 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 manufacturers now understand the importance of the orientation.

The least expensive way to find out the orientation is to buy clip-on polarizing eyeglass lenses at a drug store (around $16). These clip-on eyeglasses have their polarizing lines running vertically in each lens. Set the laser on your table with the label facing up, turn it on, 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.

Controlling Polarization Orientation

Let's now 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 pretty easy to do.
  2. If you don't want to mount your laser on its side, you can arrange two mirrors 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 15e, 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, the second mirror should be 9 inches above the table where all the other components downstream in the setup will be. This means that the laser and the first mirror will need to be at a lower height above the table. If you place the first mirror at 7 inches above the table, then the laser aperture should be at 7 inches also and the first mirror should be retro-reflected to insure the beam is parallel with the table surface. As a side note, all optical recording setups will assume a laser with a horizontal polarization since we will always be impinging on the recording plate from the side with the reference beam.

vertical to horizontal polarization change image
Figure 15e: 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 16a. The processed hologram image is reconstructed when the same reference beam light "transmits" through the hologram to the viewer's eyes as shown in Figures 16j in this section and 18a in the section on Recording & Processing.

single-beam transmission hologram setup image
Figure 16a: Single-beam transmission hologram setup.

Figure 16a illustrates what the recording arrangement will look like when it's set up. Refer to this illustration as you set up the arrangement. 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 will be shown in a later illustration.

So let's get started.

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.
top view of plate holder and object scene image
Figure 16b: Top view of plate holder and object scene.
connecting plate holder and object scene image
Figure 16c: Plate holder and object scene locked together.
plate holder/object scene cast shadow image
Figure 16d: Plate holder and object scene casting shadows on screen.
plate holder/object scene illuminated image
Figure 16e: 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 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 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 formulas that you can use based on actual measurements I've made on several types of setups. The first formula shown in Figure 16f is specific to a single-beam transmission hologram. This formula gives you the distance (L) the lens needs to be from the plate holder/object scene based on the focal length (F) of the lens, the total width of the plate and object scene (W (p/o) perpendicular to the incoming beam, a coefficient number (1.274), and the diameter of the laser beam D (b) at the laser's aperture.

lens distance formula image
Figure 16f: Formula to calculate distance of lens from plate/object
for a single-beam transmission hologram.

Figure 16g 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 16g: Illustrated relationship between the focal length of the diverging lens and the distance L.

Let's look at an example. Using Figure 16e, we will determine where a diverging lens should be placed with a focal length of -0.6 inches (-15 mm lens or 10x objective), an object scene/plate holder width of 4.5 inches, and a laser beam diameter of 0.08 inches measured at the laser aperture as shown in Figure 16h:

lens distance formula calculation
Figure 16h: Example calculating the distance of lens from plate/object
scene for a single-beam transmission hologram.

The diverging lens should be placed at least 43 inches away from the plate holder and object scene to have fairly uniform illumination. The location of the lens in Figure 16e is pretty much where it should be. When doing this calculation, ignore the minus sign of the lens.

For a single-beam reflection hologram setup, a slightly different formula 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 of the photographic plate (6.4 inches) 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 16i, I have listed the focal length of various microscope objectives and their equivalent plano-concave or double concave lenses focal lengths and diameters as well as today's costs.

Figure 16i: Available microscope objectives and concave lenses.

Microscope ObjectivePlano or Double Concave Lens
MagnificationFocal Length (mm)CostDiameter (mm)Focal Length (mm)Cost
5x25.5$11025-25$34.50
10x16.5$10512-15(Plano)$31.50
20x8.8$1109-9$30
40x4.5$1156-6$30
60x3$195n/an/an/a
100x2$255n/an/an/a

Microscope objectives are more expensive than plano-concave or double concave lenses, but finding lenses with focal lengths shorter than 9 mm is very difficult but not impossible (you can get lenses with -6 mm focal lengths, but their diameters are 6 mm and can be difficult to mount and get a 2 mm laser beam through, but not impossible). All simple lenses should have MgF2 anti-reflection coatings. Also, if you plan to make holograms larger than 4 inches x 5 inches, you will need to use microscope objectives instead of lenses to achieve the illuminating coverage of the larger plates.

During the exposure of the scene shown in Figure 16e, 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.

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 on the Introduction page of Building a Holography System were made on 4 inch x 5 inch plates. Figure 16j shows how my two geometric shapes will look like in the exposed and processed hologram from two different perspectives showing parallax.

final hologram imagesfinal hologram images
Figure 16j: 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.

 

Revised 5/2/2017