Holograms and Light

To understand how interference fringes form on film, you need to know a little bit about light. Light is part of the electromagnetic spectrum -- it's made of high-frequency electrical and magnetic waves. These waves are fairly complex, but you can imagine them as similar to waves on water. They have peaks and troughs, and they travel in a straight line until they encounter an obstacle. Obstacles can absorb or reflect light, and most objects do some of both. Reflections from completely smooth surfaces are specular, or mirror-like, while reflections from rough surfaces are diffuse, or scattered.

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

Light reflection can be specular, mirror-like (left), diffuse or scattered.
Light reflection can be specular, mirror-like (left), diffuse or scattered.

White light, like sunlight, contains all of the different frequencies of light traveling in all directions, including ones that are beyond the visible spectrum. Although this light allows you to see everything around you, it's relatively chaotic. It contains lots of different wavelengths traveling in lots of different directions. Even waves of the same wavelength can be in a different phase, or alignment between the peaks and troughs.

Laser light, on the other hand, is orderly. Lasers produce monochromatic light -- it has one wavelength and one color. The light that emerges from a laser is also coherent. All of the peaks and troughs of the waves are lined up, or in phase. The waves line up spatially, or across the wave of the beam, as well as temporally, or along the length of the beam. You can check out How Lasers Work to see precisely how a laser does this.

White light vs. laser light

You can make and view a photograph using unorganized white light, but to make a hologram, you need the organized light of a laser. This is because photographs record only the amplitude of the light that hits the film, while holograms record differences in both amplitude and the phase. In order for the film to record these differences, the light has to start out with one wavelength and one phase across the entire beam. All the waves have to be identical when they leave the laser.

Here's what happens when you turn on a laser to expose a holographic plate:

  1. A column of light leaves the laser and passes through the beam splitter.
  2. The two columns reflect off of their respective mirrors and pass through their respective diverging lenses.
  3. The object reflects off of the object and combines with the reference beam at the holographic film.

There are a couple of things to keep in mind about the object beam. One is that the object is not 100 percent reflective -- it absorbs some of the laser light that reaches it, changing the intensity of the object wave. The darker portions of the object absorb more light, and the lighter portions absorb less light.

light waves reflecting­
When light waves reflect,
they follow the law of reflection.
The angle at which they strike
the surface is the same as the angle
at which they leave it.

On top of that, the surface of the object is rough on a microscopic level, even if it looks smooth to the human eye, so it causes a diffuse reflection. It scatters light in every direction following the law of reflection. In other words, the angle of incidence, or the angle at which the light hits the surface, is the same as its angle of reflection, or the light at which it leaves the surface. This diffuse reflection causes light reflected from every part of the object to reach every part of the holographic plate. This is why a hologram is redundant -- each portion of the plate holds information about each portion of the object.

The holographic plate captures the interaction between the object and reference beams. We'll look at how this happens next.

Redundancy
If you tore a hologram of a mask in half, you could still see the whole mask in each half. But by removing half of the hologram, you also remove half of the information required to recreate the scene. For this reason, the resolution of the image you see in half a hologram isn’t as good. In addition, the holographic plate doesn’t get information about areas that are out of its line of sight, or physically blocked by the surface of the object.