Friday, December 14, 2012

Your Questions: Holograms

  • Can you break down holograms for me? (Submitted by Jaysin on the Facebook page)
I remember being fascinated with holographic cards when I was young. I had no idea how they worked, though. I'd like to say that I knew exactly how holograms worked when this question was submitted, but that would be a lie. I did have fun researching them, though, and here's what I've learned:

There are several types of holograms and often things that are called holograms are not actually holograms at all. One of the most famous holograms (according to a Google image search) is this one of Tupac Shakur brought back to life at the Coachella music festival earlier this year. 

This image looks pretty life-like, but it is not a hologram. It's actually an optical illusion known as Pepper's Ghost, and it's been understood since 1584 and has been used on stage since 1862 when John Pepper created a ghost-like image for Charles Dicken's The Haunted Man. The illusion works by shining light on an object an angle from a glass plate. The light from this object reflects off the glass plate (which the audience cannot see) and into the audience. The audience will "see" the object as being behind the glass.


The Tupac illusion works by projecting an image to the ground. This image reflects back to the glass plate and out into the audience. Snoop Dogg (I think we're supposed to call him Snoop Lion now) stands behind the glass plate giving the illusion that he and Tupac are standing right next to each other.

So back to holograms, then. 

Holograms are 2 dimensional images that are made in a way that they appear 3 dimensional. For example, this mouse is printed on a 2 dimensional card. As you turn it the mouse appears 3 dimensional because the image changes depending on which angle you view it from. This is hopefully no surprise to you (after all, you asked how holograms work, so maybe I should stop explaining what a hologram is and get busy explaining).

The most interesting thing about these types of holograms is how they are made. Holograms are very similar to a sound recorder - except instead of recording sound waves to hear later we are recording light waves to see later. Before we can continue let's make sure a couple of things are clear. First, light is an electromagnetic wave. Second, waves can interfere with themselves and cause a diffraction pattern. If you've ever watched two water waves crash into each other you know what I'm talking about.

Holograms are made by firing a laser beam at a beamsplitter. Half of the light passes through the mirror and hits the object you want to make a hologram of. The light bounces off the object and heads toward a photographic plate. Meanwhile, the other half of the light bounces off another mirror so that it arrives at the photographic plate at the exact same time the first half does. When these two beams of light meet they form a diffraction pattern that encodes the image onto the photographic plate. This pattern can then be seen later when any other light source reflects off of it. 







Another type of hologram is the type seen in this video:


This just uses a concave mirror to create an image that appears to float. They track hand movement using the IR sensors in a Wiimote (which I've explained in a different post) and add the sensation of touch using ultrasonic waves. The hologram itself is nothing too impressive, but the combination of sight, touch, and interaction is.

If you have questions or something you'd like me to write about send an e-mail to chad@thecollapsedwavefunction.com. Or, you can always ask via Facebook or Twitter.