Holography

Samuel Gravel

(Dated: August 2021)

I. INTRODUCTION

Holography refers to the study and creation of holograms. When the word Hologram is used in popular media itoften refers to something referred to in holography as a false hologram. A good example of these false holograms is inpopular media when a character uses a device which projects a three dimensional image. In reality, a true hologramis quite different. A hologram is an image stored on a two dimensional medium such as glass, film, or many others,which when illuminated reflects back an image which appears to be three dimensional. There are many types ofholograms including reflection holograms, transmission holograms and computationally generated holograms, but inthis experiment only reflection holograms were created.[1]

A. History of Holograms

The man credited with the invention of the holographic technique is Dennis Gabor, a British-Hungarian physicistwho was awarded the Nobel prize in 1971 for his discovery. His work was done in the late 1940s, notably before thedevelopment of the laser, which wouldn’t occur until the 1960s. His initial work would involve what would come tobe known as electron holography, as at the time he was looking into improving the electron microscope, but wouldunexpectedly found this field.

As the laser developed, laser based, or optical, holography emerged in about 1962, when two groups of scientistswould develop the technique apparently independently. In the Soviet Union a physicist named Yuri Denisyuk wasworking on this problem, and simultaneously in the United States at University of Michigan Dennis Leith and JurisUpatniek were working on the same problem, the development of optical holography.

Dennis Leith and Juris Upatniek would develop a technique known as the off axis transmission hologram, and YuriDenisyuk would be credited with developing the reflection hologram. In this experiment, reflection holograms werecreated. [1, 2]

II. THEORY

The pivotal concept to understand in holography is interference of wave forms. Any wave, including light, hasan amplitude, a phase, and a wavelength. When two waves interact, meaning they occupy the same space, theymay interfere in any combination of two ways. They can interfere constructively, causing a bright spot, or interferedestructively resulting in a dark fringe. Holography uses this to create an image which has more of the propertiesobserved from the original image than a traditional photograph.

In order to create interference, we need two different waves to interfere with one another. This is done differentlyin the two techniques previously mentioned, transmission and reflection, however as in this experiment reflectionholograms were created we will discuss primarily reflection holograms. In reflection holography, the laser is shonethrough the holographic medium and reflects off of the subject, meaning the object the hologram will be of. Thisreflected light takes a different path, meaning it can have a different phase when it reaches the plate. This phasedifference causes the reflected light to interfere with the light shining on the plate itself, creating the interferencepattern recorded by the photosensitive chemicals on the plate as a hologram.

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FIG. 1. How a Reflection Hologram is Made [1]

It should be noted that it is very important that the laser light that is used in this process should be coherent,meaning only consisting of a single wavelength. This is because coherent light yields the sharpest image, becauseif the light has different wavelengths than the light at the plate and the light reflected from the subject may notinterfere as expected because their phases would not only depend on distance travelled, but also wavelength, causingthe pattern the hologram records to be blurrier, or not even appear at all.

III. SET UP

For the creation of the holograms we utilized a kit from Integraf, a company known primarily for production ofholography supplies for both hobbyists and professionals. The kit included 30 red sensitive holographic plates, a classIIIa 650 nm (red) 4 mW laser diode, chemicals to mix with distilled water to create the chemicals needed to developthe holograms. [3]

When creating the holograms themselves, the set up pictured below was utilized.

FIG. 2. Experimental Set Up

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The diode was attached to a clothespin which was inserted into sand to try to isolate vibrations affecting the system.The system could probably be improved by isolating any vibrations traveling along the wires from the battery pack.A book was used as a shutter to control exposure time. The subject was placed on a mouse pad which again wasplaced on top of a container of sand which isolated the subject from vibrations. It should be noted that the containerfor the sand was not an optimal one, as it was metal and as such was rather reflective. The reflected light had thecapacity to make the finished hologram blurrier by interfering with the light from the diode, so to decrease roguereflection a black t-shirt was draped over the container in the direction the light shone. Not pictured, a green safetylight was utilized. Since the holographic plate is only sensitive to red light, using a dim green light bulb is possible inorder to increase visibility, as long as the light doesn’t shine directly on the workspace.

IV. PROCEDURE

When creating the hologram it is important that the room is in near total darkness, excluding the safety light if itis utilized. This is to avoid the red component of the white light interfering with the hologram. The laser diode isturned on 5 minutes prior to the creation of the hologram in order to allow the laser to warm up and ensure coherenceof the beam. Once the laser is heated up, the plate is placed behind the shutter on the subject, ensuring that no lightfrom the laser shines on the plate before it is meant to. Next, the shutter is lifted from the table slightly, not enoughto expose the plate in order to give time for any vibrations from moving the shutter time to die down. The shutteris then lifted the rest of the way, exposing the hologram for ten to fifteen seconds. During the experiment it wasfound that generally speaking more than ten seconds but less than fifteen was optimal. After exposure the shutter isreplaced.

After exposure the hologram needs to develop, and still cannot be exposed to any light other than the safety light,so it is recommended that you disconnect the diode between holograms to avoid accidental exposure. The plate is thensubmerged in a mixture of two of the developing agents, in the Integraf kit referred to as developer A and developerB. It is submerged and gently moved around for two minutes or so, in which time the plate should appear to turnblack. Then the hologram is rinsed in distilled water for at least twenty seconds, though a minute spent rinsing ismore optimal. Next the plate is submerged in a bleach from chemicals also included in the kit and again gently movedaround for around two minutes. The plate should now appear to become clear again, and by the end of this step theplate is now able to be exposed to white light or even the diode laser. The plate is again rinsed for a minute, thenoptionally can be rinsed in a mixture of distilled water and Photoflo, a wetting agent included with the kit whichincreases the clarity of the holograms. Now the hologram should be stood up straight and allowed to dry for abouttwo hours, at which point the hologram should become visible under the right conditions.

In order to view the hologram you must shine a non-diffuse light on the plate, like a point source. One of the mainbenefits of creating a reflection hologram is that the light you use does not need to be the diode itself, you can usewhite light such as the flashlight on a phone, or some other source. It can be viewed using ambient light, but usinga flashlight in a darker room tends to be the optimal way to view the image. It should also be noted that the lightsource should shine on the plate at an angle similar to the angle from which the diode shone on the hologram whencapturing the image. This ensures that the waveform reflected off the plate has the same shape as the one which wasrecorded in the interference pattern. Once the light source is at an appropriate angle the viewer is able to move theirhead around and observe some of the three dimensional visual properties of the hologram, such as parallax.

V. CONCLUSION

Several holograms were made successfully, of coins, washers, and a rook from a chess set. The hologram of the rookcame out especially well, and illustrates the key difference between a hologram and a photograph. When viewed fromone angle, you see the front of a rook, and can’t see over the crown, as you would expect from a normal photograph.This changes however when the hologram is viewed from a different angle, allowing you to observe the top of thepiece.

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FIG. 3. Rook From Straight On

FIG. 4. Rook From Above

An interesting thing to note is that despite the use of a coherent laser diode, meaning a single wavelength diode,we observe different colors when observing the holograms from different angles. This is likely due to Bragg’s law,also known as Bragg diffraction. According to Bragg’s law, the wavelength of reflected light is related to the inter-atomic distance and the angle of incidence by the following relation, where n is the order of diffraction, lambda is thewavelength of the reflected light, d is the inter-atomic distance and theta is the incident angle.

nλ = 2dsin(θ);n = 1, 2, 3... (1)

This equation holds true for solids with crystalline structures, meaning solids for which the interatomic distanceis mostly constant. Additionally the original interpretation concerns wavelengths in the X-ray range. It turns outthat you can reinterpret this equation for optical diffraction gratings by taking d to be the width of the diffractiongrating. Since our Hologram is an optical diffraction grating this means that Bragg’s law applies and we see differentwavelengths of reflected light when observing the hologram from different angles. [4]

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[1] G. Saxby, Practical holography. Institute Of Physics Publishing, 3rd ed., 2004.[2] T. H. Jeong, Laser holography: experiments you can do– from Edison. Thomas Alva Edison Foundation, 1987.[3] Integraf, “Standard holokit.” https://www.integraf.com/shop/hk-standard-hologram-kit, 2021.[4] D. Senderakova, “White light reconstructed holograms,” in Holography (E. Mihaylova, ed.), ch. 3, Rijeka: IntechOpen, 2013.