So You Think You Can Be Princess Leia?

“Help me, Obi-Wan Kenobi; you’re my only hope,” might have crossed your mind if you thought about going to see the MIT Museum’s holography collection through the Cambridge Science Festival’s “Explore Holography” event. Perhaps you decided to go because you were excited at the prospect of being able to don your Princess Leia costume and reenact that famous scene. And maybe, you were completely disappointed when you finally saw what the Museum meant by “holograms.” The truth is, most of what you see in the news and media claiming to be holograms actually aren’t real holograms. Sure, they might provide some cool 3D visuals, but they’re not done using the holographic technology that I discussed in my previous two blog posts.

Most of what we think we know about holography comes from television and movies. If you’re like me, you probably watched the original Star Wars Episode IV: A New Hope and saw the scene where R2D2 plays back a “holographic” video of Princess Leia’s plea for Obi-Wan Kenobi’s help. The robot projected the video in the middle of thin air, which was marvelous – but the result of computerized special effects post-production, not holography. Fast forward a few years, and we’re watching CNN’s coverage of the 2008 US presidential election. Wolf Blitzer “beamed” Jessica Yellin, a news correspondent who was in Chicago at the time, into the New York studios to have a “face-to-face” chat via “hologram.” However, it wasn’t a real hologram. A hologram needs to be projected from a medium; it can’t just float in air. Yellin even revealed the magic behind the illusion: she was standing in a tent with 35 cameras arranged in a circle pointing at her. Each camera captured one perspective of her body and fed the video to a cluster of 20 computers to crunch the data and send to the studios in New York to project onto blue screens. Blitzer couldn’t actually see Yellin in real time; he was actually talking to a blue screen and watching monitors that had the video of Yellin inserted into the video of him. Hardly holography. The same thing is true of the movies Minority Report and Iron Man, where Tom Cruise and Robert Downey Jr. didn’t need computer screens because they were able to interact with the information in a hologram that floated in midair. Also not real holograms.

After seeing what real holograms look like, you might be reminded of those 3D movies that you have to wear a pair of red and blue glasses to watch. They seem pretty similar, right? The images seem like they’re literally popping out of the screen, but they don’t use the same technology at all. To make those movies, each frame is rendered in red and blue, each of which is a slightly different viewing perspective of the same object. Why the different perspectives? Well, it’s based on how the brain processes visual information. Your eyes are spaced about two to three inches apart, so each eye captures a slightly different perspective of an object. Your brain combines the two images so that you can see the object in three dimensions. 3D movies do the same thing and overlay the two images. When the light hits your red and blue glasses, each lens filters out one of the colors (red lens filters out red light so that your eye receives only the blue light, and vice versa). As a result, your eyes and brain get two images from different angles, so the film looks 3D. This method is actually an illusion and can’t be considered holographic technology.

Lastly, you may have seen or heard of “touchable holography,” which is a technology that some researchers at the University of Tokyo developed in the fall of 2009. This technology adds a tactile dimension to holograms. The researchers claim that viewers cannot interact with traditional holography. To change this, they use Wii remotes to sense the motions of a viewer and move the hologram accordingly. To make the viewer feel the hologram, a beam of ultrasonic waves is directed at the viewer, which generates pressure on the surface of the viewer’s body. While all this is exciting technology, it cannot be called true holography. What the researchers call a “hologram” is actually an illusion made with some curved mirrors. An LCD projector projects an image onto the mirrors, which reflect the light to a point close to the viewer, so the image appears as if it were 3D and floating in air.

Hopefully, I’ve dispelled some common misconceptions about what a hologram is and isn’t. Holography has great promise for the future (e.g. data storage and true holographic video), but it won’t be quite like the things we’ve seen in sci-fi movies or even TV news shows. If you have any questions about holography in general, feel free to leave a comment!

Holograms: More than just pretty pictures?

At this point, you may be wondering, “Why does MIT, a university known for driving innovation, care about and collect pretty 3D pictures?” Well, it turns out that holography has some pretty important real-world applications, such as data storage.

As you probably know because you’re reading this blog, we live in an age of information. Of course, this explosion of information wouldn’t be possible without ways to quickly store and transfer large quantities of data. So far, we’ve been relying on conventional optical storage technologies (e.g. CD’s, DVD’s, Blu-Ray) to handle this need. While current storage needs are being met, storage technologies must continue to improve in order to keep pace with the rapidly increasing demand.

This is where holography comes in. Although optical storage technologies have improved by leaps and bounds since the advent of CD’s (DVD’s can hold 15 times more information than CD’s and Blu-Ray discs can hold 10 times more than DVD’s), they are still limited to recording information on the surface of a disc. They all use lasers to etch a spiral of individual bits (1’s and 0’s) onto the surface of a recording medium, but holographic data storage uses lasers to etch pages of bits throughout the entire volume of a recording medium. Theoretically, holographic discs can store 40 times more data than Blu-Ray discs can. How exactly is this done?

In my first blog post, I went over the process of using holography to make images of physical objects but how would we go about using holography to store information that lives inside of a computer and that we can’t actually touch or see? Suppose that you want to store a movie onto a holographic disc. Your computer converts the data into a sequence of bits and sends this to a spatial light modulator (SLM). The SLM is a screen that arranges this sequence into a page of bits. Each bit is represented by either a black or white square (0 is black; 1 is white), so that the LCD screen looks like a checkerboard of black and white squares. Now that the data is rendered into a visible form, we can make a hologram of it. Once again, a laser beam is split into a reference beam and an object beam. The object beam goes through the SLM and comes out carrying the pattern of the image that was displayed on the SLM. The two beams eventually meet at some place within the recording medium (e.g. at the surface or in the middle), which records the interference pattern of the two beams. To read a page of data, you recreate the object beam by illuminating the recording medium with the reference beam. The recreated object beam hits a charged coupled device (CCD), which is a sensor that is connected to a computer. The computer takes a page of black and white boxes and converts them back into a string of bits and back into data.

Note that the reference beam must approach the SLM from the exact same angle that was used to write a page of data; otherwise, the phase of the beam will be different and you will recreate the wrong object beam. Because you can recreate different object beams by varying the phase and wavelength of the reference beam, it is possible to store multiple holograms in the same volume of recording medium. You can also stack holograms on top of each other by using mirrors and lenses to specify the location of the interference pattern within the medium. So far, the company InPhase Technologies was able to cram 500 Gigabytes into one square inch of medium that was as thick as a CD. Based on this figure, a disk the size of a CD could hold up to about four Terabytes (one Terabyte = 1000 Gigabytes) of information. With this technology, we could fit the entire printed collection of the US Library of Congress onto three discs.

Not only are holographic discs superior to CD’s, DVD’s, and Blu-Ray discs in terms of storage capacity, they also permit high data transfer rates. One page of data (equivalent to 125 Megabytes) can be read with a single flash of light, whereas CD players are limited to reading one bit (1/8th of a byte) per flash of light. Since multiple holograms stored in the same volume of medium can be read by altering the angle of the reading laser, high transfer rates are possible since the angle of a laser can be manipulated quickly without inertia, unlike a CD player which relies on a mechanical part to move the laser back and forth. Another advantage of holographic storage is that it can be searched extremely quickly. If you illuminate a disc with an object beam, you will reproduce the corresponding reference beam and its angle, which immediately identifies the page on which the information is stored. This means that database searches can be done using physics rather than software.

So if scientists already know how holographic data storage works and have even built a few prototypes, why haven’t we seen any of these systems on the market today? In this technology, lasers have to be directed and aligned very precisely. Any slight deviation would make it near impossible to store and retrieve error-free data. For this reason, the components of the system are extremely expensive to manufacture quickly and in large quantities. Another issue arises out of the ability to stack multiple holograms. When a computer attempts to read a disc’s data, the reference beam will reproduce its corresponding object beam, but will also produce a lot of noise from the other holograms that are stacked on top of it. The more holograms there are, the more noise there will be. Noise makes it difficult to retrieve correct data. It’s like trying to watch TV with poor picture quality. You might be able to discern a few shapes here and there, but you can’t be absolutely sure of what you’re seeing. Theoretically, thousands of holograms can be stacked in a disc as wide as a CD, but it doesn’t make sense to do that if you can’t retrieve the data. The developers of this technology need to figure out how to reduce the noise as much as possible before the product can be commercialized.

Hello Holography

Are you tired of your boring old two-dimensional photographs? Ever gone through your old family photo albums and wished that you could relive some moments? Don't despair; holograms are here!

Holograms are commonly described as "3D photographs," though the processes involved in making the two different types of photographs are quite different. Both conventional photographs and holograms are made on a flat piece of photographic film that reacts to different intensities of light, but holograms render information about the depth of the object: the object appears to literally pop out of the page. How do holograms manage to do that?

When you take a conventional photograph, your camera opens the shutter to let light through to hit the film. The light that enters your camera has already hit and reflected off the object that you're capturing. The object reflects light with different intensities (brightness) depending on the physical characteristics of the object. The chemicals on the film (usually a light-sensitive compound called silver halide) react with the light. How much the film reacts depends on how intense the light is. The regions that react more are darker in the resulting photograph. So, a photograph is merely a record of the intensity distribution of the object. However, it does not record any information about the phase of the light waves (see Figure 1), which we need if we want to know anything about the depth and dimensions of the object (a point that is further from the camera will have a phase different from the phase of a point closer to the camera).

Figure 1

How, then, do holographers capture—and then render—information about depth? They do it by making use of a standard or reference. This is similar to when you measure something with a ruler. You could just lay your ruler down on the surface that you're measuring and record a number, but that number is useless if you don't know what that number is relative to. You need to designate a specific point as zero (i.e. the reference). In holography, this reference is called the reference beam. The reference beam will combine with the light from the object, creating an interference pattern (see Figure 2). The film records the interference pattern. Since the intensity at any point in the interference pattern also depends on the phase of the light from the object, the hologram contains information about the phase as well as the intensity of the light waves.