What is a Hologram?
Holograms are images formed by microscopic interference patterns on a surface which cause rays of light to look like they are coming from 3D objects in front of or behind the display surface. These microscopic interference patterns are called diffraction gratings.
How Do Diffraction Gratings Work?
Diffraction gratings work because light behaves like a wave. If two peaks line up, the light is amplified, and if a peak and a valley line up, the light is cancelled. Where the light is in this peak/valley cycle is called the light's phase. If we can control the phase of the light, we can control where the peaks and valleys line up, and use this to steer the light to produce 3D images.
There are many different types of holograms which control the phase of light in different ways. You can control it by making some of the light reflect off a surface slightly before or after the rest of the light using a bumpy shiny surface, you can use a bumpy clear surface to cause some of the light to be slowed down for longer than the surrounding light, advancing its phase more, or you can use a speckley surface where some of the light is opaque and the rest is clear to just get rid of the light where it has the wrong phase. Visible light has a wavelength of only a few hundred nanometers, so these bumps are usually too small to see without an electron microscope.
How Can You Create a Video-Hologram?
The problem with normal holograms, is they are like photographs. You can't change the way they look after they have been captured, so you can't use them to show holographic movies. In order to create holographic video, we need to be able to create changeable microscopic bumps in a surface. The perfect candidate for this is sound. Sound is really just compressed air moving around, making the air bumpy. By creating ultrasonic sound waves (SAWs) in a crystal we can create an updatable hologram.
In the image below, the RF input is like the TV signal on old TVs, but holographic. This creates sound bumps on the surface of the crystals. These bumps change the phase of the incoming light, which creates a 1 dimensional hologram on the output.
How Does the Rest of the Holomonitor Work?
The output of the crystal makes light that is being moved really fast to the side really fast because sound moves really fast. We counteract this by attaching a spinning mirror (Horizontal Scanner) to the output. We then move the image up and down with a twisting mirror (Vertical Scanner) so that the hologram can reach every part of the screen. Finally, we view the hologram through a parabolic mirror to give ourselves a larger field of view, with the trade off of making the image smaller.
What Does the Output Look Like?
You can view some of the output by visiting this playlist on YouTube.
1. Ultrasonic Haptic Feedback Group
2. Andrew Henrie, Hardware and Software Improvements to a Low-Cost Holographic Video Monitor