Understanding the Magic of Holograms

To see a hologram, you don’t need to go far—check your wallet. Most driver’s licenses, ID cards, and credit cards feature holograms. If you're not yet old enough to drive or use credit, you can spot holograms in various household items, including CD and DVD packaging, and many things labeled as "official merchandise."

Sadly, the holograms you encounter daily—designed to prevent fraud—aren’t particularly breathtaking. While you might notice shifting colors or shapes when you move them, they often appear as nothing more than glimmering images or blurry patches of color. Even popular holograms, such as those featuring movie or comic characters, can resemble dull green photographs rather than stunning 3D visuals.

Large-scale holograms, often illuminated with lasers or set in dimly lit rooms with strategically placed lighting, are astonishing. These holograms are two-dimensional surfaces that perfectly display three-dimensional representations of real-world objects. What’s amazing is that you don’t need to wear special glasses or use a View-Master to experience the 3-D effect.

When you view these holograms from various angles, the objects shift in perspective, much like how you would observe a real object from different positions. Some holograms even appear to animate as you move around them, while others change color or reveal entirely different views based on your perspective.

Holograms possess other remarkable qualities as well. If you slice one in half, both pieces retain the full image, as each fragment still contains the entire holographic picture. Even a tiny piece will capture the complete image. Additionally, if you create a hologram of a magnifying glass, it will magnify the other objects in the hologram, just like the real thing.

Once you grasp the fundamental concepts behind holograms, understanding how they achieve all these effects becomes straightforward. This article will break down how a hologram, light, and the human brain collaborate to produce clear 3-D images. Every property of a hologram originates from the process used to create it, so we’ll begin by outlining what’s involved in making one.

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Creating a Hologram

To create a hologram, you don’t need many tools. Here’s what you’ll need:

There are various ways to arrange these components, but for now, we’ll focus on a simple setup for a transmission hologram.

In the following section, we will explore the requirements for your workspace.

Workspace Setup

Achieving a high-quality image requires an ideal workspace. In many ways, the space requirements are more critical than the equipment itself. A darker room yields better results. For minimal light without disturbing the hologram, a safelight, like those used in darkrooms, works well. Since darkroom safelights are typically red, and red light is often used in holography, there are also specialized green and blue-green safelights designed specifically for holography.

A stable, vibration-free surface is essential for holography. The workspace must be impervious to movement, whether from walking across the room or traffic outside. Professional holography labs and studios use specially engineered tables with honeycomb support layers and pneumatic legs to dampen vibrations, ensuring the equipment remains perfectly still.

You can build your own holography table by setting up inflated inner tubes on a low table and placing a box filled with a thick layer of sand on top. The inner tubes and sand will mimic the role of honeycomb and pneumatic supports found in professional tables. If space is limited, you can improvise by using cups filled with sand or sugar to support each piece of equipment, though this won't provide as much stability as a larger table.

For clear holograms, it's essential to minimize air vibrations. Air currents from heating and air conditioning systems, as well as movement from your body, breathing, and even heat dissipation, can affect the image. To reduce these disturbances, turn off the heating or cooling systems and wait a few minutes after setting up the equipment to allow the air to stabilize.

These steps might sound like photography advice taken to the extreme — in photography, you must keep the lens clean, control light levels, and hold the camera perfectly still. The process of making a hologram is similar, but it requires capturing microscopic details. We will explore the similarities between holograms and photographs in the next section.

Holograms vs. Photographs

When you capture an image with a film camera, four essential steps happen in an instant:

There are many variables you can adjust in this process, like the degree to which the shutter opens, the magnification power of the lens, and the additional light you introduce. However, no matter the adjustments, the core process remains the same. Furthermore, the resulting image is still merely a recording of light intensity. When the film is developed and a print is made, your eyes and brain interpret the reflected light as a representation of the original scene. For more details, explore How Vision Works, How Cameras Work, and How Film Works.

Holograms, much like photographs, are recordings of reflected light. The process of creating them shares similarities with capturing a photograph:

Much like a photograph, this process results in film capturing the incoming light. However, once you develop the holographic plate and view it, the result appears quite strange. Traditional film from a camera shows a negative version of the scene – areas where light fell are dark, and vice versa. Even in its negative form, the original scene is still recognizable.

However, a developed holographic film doesn't reveal anything resembling the original scene. Instead, you might see a dark frame or an abstract array of lines and swirls. To transform this film into a recognizable image, it requires the appropriate lighting.

In a transmission hologram, monochromatic light passes through the hologram to create an image. In contrast, a reflection hologram uses monochromatic or white light that reflects off the surface of the hologram to form an image. Your eyes and brain perceive the light either passing through or reflecting off the hologram as a representation of a three-dimensional object. The holograms found on credit cards and stickers are examples of reflection holograms.

To view a hologram, the right light source is essential, as it records the phase and amplitude of light like a code. Rather than merely capturing a simple pattern of reflected light from a scene, it records the interference between the reference beam and the object beam. This results in a pattern of minuscule interference fringes, with each fringe potentially smaller than one wavelength of the light used to generate it. Decoding these fringes requires a key — which is the correct type of light.

Now, let's delve into how light creates interference fringes.

Holograms and Light

To grasp how interference fringes appear on film, it's helpful to understand a bit about light. Light is a part of the electromagnetic spectrum and consists of high-frequency electrical and magnetic waves. These waves are complex, but can be compared to waves on water. They have peaks and troughs, traveling in straight lines until they hit an obstacle. These obstacles can either absorb or reflect light, with most objects doing both. A smooth surface causes specular, or mirror-like reflections, while rough surfaces produce diffuse or scattered reflections.

The wavelength of light refers to the distance from one peak of the wave to the next. This is linked to the wave's frequency, which is the number of waves that pass a point within a given time. The frequency of light determines its color and is measured in cycles per second (Hertz, Hz). Colors like red have lower frequencies, while those at the violet end of the spectrum have higher frequencies. The amplitude of the light waves, or their height, is related to the intensity of the light.

White light, like sunlight, consists of a range of light frequencies moving in all directions, including those beyond the visible spectrum. While this light enables us to see our surroundings, it is relatively disorganized. It contains many different wavelengths traveling in various directions. Even waves with the same wavelength may not align, meaning their peaks and troughs may be out of phase.

In contrast, laser light is highly structured. Lasers generate monochromatic light, meaning it has one specific wavelength and color. The light emitted from a laser is also coherent. All of its waves' peaks and troughs are aligned, or in phase. The waves align spatially across the beam's width and temporally along the beam's length. To explore this phenomenon in detail, check out How Lasers Work.

Light Reflection

You can capture and view a photograph using random white light, but to create a hologram, you need the structured light from a laser. This is because photographs only capture the amplitude of light that hits the film, whereas holograms capture both the amplitude and phase differences. For the film to record these differences, the light must begin with a single wavelength and phase across the entire beam. All waves need to be identical as they exit the laser.

Here’s what occurs when you activate a laser to expose a holographic plate:

A few things are important to note about the object beam. For one, the object is not fully reflective — it absorbs some of the laser light that reaches it, thus altering the intensity of the object wave. Darker areas of the object absorb more light, while lighter areas absorb less.

Moreover, the object’s surface is microscopically rough, even if it appears smooth to the naked eye, causing a diffuse reflection. It scatters light in all directions, adhering to the law of reflection. In simple terms, the angle at which the light strikes the surface is the same as the angle at which it reflects. This diffuse reflection results in light from every part of the object reaching every section of the holographic plate. This redundancy is why a hologram is unique — each part of the plate contains information about every part of the object.

The holographic plate records the interaction between the object and reference beams. Let’s now explore how this process occurs.

Capturing the Fringes

The light-sensitive layer used to produce holograms captures the interference between the light waves of the reference and object beams. When two wave peaks coincide, they reinforce each other, known as constructive interference. On the other hand, when a peak aligns with a trough, they cancel each other out, which is called destructive interference.

You can visualize the peak of a wave as a positive value and the trough as a negative one. Whenever these two beams intersect, these values combine, either canceling out or strengthening that part of the wave.

This concept is similar to how information is transmitted through radio waves. In amplitude modulation (AM) radio, a sine wave is combined with a wave that has varying amplitudes. In frequency modulation (FM) radio, a sine wave is combined with a wave that has fluctuating frequencies. In both cases, the sine wave serves as the carrier wave that is combined with another wave to convey the information.

In a hologram, when two light waves intersect, they form a pattern of hyperboloids — three-dimensional figures that resemble hyperbolas spun around one or more focal points. For more information on hyperboloidal shapes, visit Wolfram MathWorld.

The holographic plate, positioned where the two wave fronts meet, captures a thin slice or cross-section of these 3D shapes. To better understand this, imagine peering through the side of a transparent aquarium filled with water. If you drop two stones at opposite ends of the aquarium, the resulting waves will radiate toward the center in concentric circles. As these waves collide, they interfere with each other constructively and destructively. If you take a snapshot of the aquarium, covering all but a thin central slice, you'll get a cross-section showing how the two wave sets interfere at a specific point.

The light that reaches the holographic emulsion behaves like the waves in the aquarium, with peaks and troughs of varying heights. Some of the waves are taller, while others are shorter. The silver halide in the emulsion reacts to these light waves similarly to how it responds to light in a regular photograph. After developing the emulsion, areas that receive stronger light become darker, while areas exposed to weaker light stay lighter. These variations in light intensity create the interference fringes.

In the upcoming section, we'll explore the process of bleaching the emulsion.

Bleaching the Emulsion

The amplitude of the waves is related to the contrast between the interference fringes. The wavelength of these waves defines the shape of each fringe. Both the spatial coherence and the contrast of the fringes directly result from the laser beam reflecting off the object.

To convert these interference fringes into a visible image requires light. However, the dense, overlapping fringes can cause the hologram to become so dark that it absorbs most of the light, allowing only a small amount to pass through for the image reconstruction. To address this, the holographic emulsion often undergoes a bleaching process using a bleach bath. Another method is to use a light-sensitive material other than silver halide, such as dichromated gelatin, to record the interference fringes.

After bleaching, a hologram becomes clear rather than dark. The interference fringes are still present, but now they exhibit a different refractive index instead of a darker color. The refractive index is the difference between the speed of light in a medium and the speed of light in a vacuum. For instance, the speed of light can vary depending on whether it is traveling through air, water, glass, various gases, or different types of film.

At times, this results in noticeable distortions, like the illusion of a spoon bending when placed in a half-filled glass of water. The variations in the refractive index are also responsible for phenomena such as rainbows on soap bubbles and oil spots on parking lots. In a bleached hologram, differences in the refractive index alter how light waves interact with and reflect off the interference fringes.

These fringes act like a coded message. It takes your eyes, your brain, and the proper lighting to interpret them into a coherent image. We will explore how this decoding process works in the next section.

Decoding the Fringes

The microscopic interference fringes on a hologram are not easily perceptible to the human eye. In fact, since the overlapping fringes are both dark and minuscule, if you observe the developed film of a transmission hologram, all you’ll likely see is a dark square. However, when monochromatic light passes through it, the situation changes, and suddenly, a 3D image appears in the same location where the original object was when the hologram was captured.

Several processes occur simultaneously to enable this effect. First, the light travels through a diverging lens, which spreads the monochromatic light — light composed of a single wavelength color — so that it strikes every part of the hologram at once. Since the hologram is transparent, it allows much of this light to pass through without alteration.

No matter if the interference fringes are dark or light, they reflect some of the light. Here's where it gets fascinating: each interference fringe behaves like a tiny, curved mirror. The light that strikes it adheres to the law of reflection, just as it did when it reflected off the object to form the hologram in the first place. The angle of incidence equals the angle of reflection, and the light then scatters in many directions.

However, this is just one aspect of the process. When light encounters an obstacle or passes through a slit, it undergoes diffraction and spreads out. The more a light beam diverges from its original course, the dimmer it appears at the edges.

You can visualize this by using an aquarium with a slotted panel across its width. Dropping a pebble into one end sends waves that expand into concentric rings. Only small portions of these rings pass through the slits, continuing to spread on the other side.

This phenomenon is a direct consequence of light behaving as a wave. As a wave passes an obstacle or slit, its front expands on the far side. The numerous slits within the interference fringes of a hologram act like a diffraction grating, generating many intersecting wave fronts in a compact space.

Recreating the Object Beam

Inside the hologram, diffraction gratings and reflective surfaces recreate the original object beam. This beam is a perfect replica of the one before it was combined with the reference wave. It's similar to how a radio works: the receiver extracts the sine wave carrying the modulated information, restoring the original wave before it was mixed for transmission.

The beam travels in the same direction as the original object beam, expanding as it moves. Since the object was placed on the opposite side of the holographic plate, the beam now heads toward you. Your eyes focus the light, and your brain interprets it as a three-dimensional image located behind the transparent hologram.

Though it may seem unbelievable, you experience this effect daily. Whenever you look into a mirror, you see your reflection and the surroundings as though they exist beyond the surface. However, the light rays forming this image aren't from behind the mirror; they bounce off the surface and reach your eyes. Many holograms also serve as color filters, making the object appear in the laser's color rather than its natural hue.

This virtual image results from light striking the interference fringes and spreading toward your eyes. Conversely, light hitting the reverse side of each fringe behaves oppositely. Instead of spreading, it converges and moves downward, forming a focused reproduction of the object — a real image that appears if you place a screen in its path. The real image is pseudoscopic, flipped back-to-front, and appears opposite to the virtual image seen without a screen.

When illuminated properly, holograms can display both images simultaneously. However, in some cases, whether you see the real or the virtual image depends on which side of the hologram faces you.

Your brain plays a crucial role in how you perceive both of these images. When your eyes detect light from the virtual image, your brain interprets it as light reflected from a real object. It relies on several cues, including shadows, the relative positions of objects, distances, and parallax (the difference in angles), to correctly interpret the scene. These same cues help your brain understand the pseudoscopic real image as well.

This explanation pertains to transmission holograms made using silver halide emulsion. Next, we will explore other types of holograms.

Other Hologram Types

The holograms you find on novelties or your driver's license are reflection holograms. These are typically produced in large quantities using a stamping process. When a holographic emulsion is developed, its surface collapses as silver halide grains are converted into pure silver, which alters the texture of the surface.

One way to mass-produce holograms is by coating the surface with metal to strengthen it, then using it to stamp the interference pattern onto metallic foil. These holograms can usually be viewed under normal white light. Another method is to print holograms from a master hologram, similar to how multiple photographic prints can be made from a single negative.

Reflection holograms can be just as intricate as the transmission holograms we've previously discussed. Many different object and laser configurations can produce these holograms. One common setup is the inline configuration, where the laser, emulsion, and object are aligned in a straight line. The laser beam first acts as the reference beam, passing through the emulsion, reflecting off the object, and returning as the object beam, creating an interference pattern.

You observe this hologram when white or monochrome light reflects off its surface. What you're viewing is still a virtual image — your brain's interpretation of light waves that appear to be originating from a real object located behind the hologram.

Reflection holograms are typically thicker than transmission holograms. The extra thickness allows for more space to record interference fringes, which in turn means there are additional reflective layers for the light to pass through. You can imagine these holograms as having several layers, each only about half the wavelength deep. When light enters the first layer, some reflects back towards the light source, while the rest travels to the next layer, continuing the process.

The light from each layer interacts with the light in the layers above, a phenomenon known as the Bragg effect. This effect is crucial for the reconstruction of the object beam in reflection holograms. Furthermore, holograms with a pronounced Bragg effect are categorized as thick holograms, whereas those with a less noticeable effect are considered thin.

The Bragg effect can alter the way a hologram reflects light, particularly in holograms that can be viewed under white light. At different viewing angles, the Bragg effect varies for different wavelengths of light. As a result, you might see a hologram as one color from one angle and a different color from another. This effect also explains why most novelty holograms appear green, despite being created with a red laser.

Multiple Images

In films, holograms can seem to come alive and generate entire animated scenes in thin air, but the holograms of today can only simulate movement. The illusion of motion is achieved by exposing the same holographic emulsion multiple times at various angles with objects in different positions. The hologram renders each image only when light strikes it at the correct angle.

As you observe this hologram from various perspectives, your brain interprets the differences in the images as motion. It’s akin to flipping through a holographic animation book. Additionally, a pulsed laser can be employed, firing for a brief moment to capture still images of moving objects.

By exposing the same plate multiple times, you can create interesting effects. For instance, using two different images from two angles can result in a hologram that shows different images depending on the viewing angle. If you use red, green, and blue lasers to expose the same scene, you can produce a full-color hologram.

However, this technique can be challenging, which is why it’s not typically used in mass-produced holograms. Another method is exposing the same scene before and after the object has undergone a change, such as being subjected to a gust of wind or a vibration. This allows researchers to observe how the stimulus affected the object.

While making 3D images with lasers may seem like an artistic or novelty process, holography has an expanding list of practical applications. For example, scientists use holograms to examine objects in three dimensions, and acoustical holography is used to reconstruct sound waves in 3D.

Holographic memory is becoming an increasingly popular method for storing vast amounts of data in compact spaces. Some researchers even theorize that the human brain may store information in a way that mirrors holograms. Although holograms don’t yet move like in movies, studies are underway to project fully 3D holograms into the air. In the future, holograms could allow you to do everything from watching television to choosing the best hairstyle for you.

To explore more about holograms, follow the links below.