I was invited to the Holocenter's week long Holography Workshop at Ohio State University.
This was a (to my perspective) an intensive workshop led by Sam Moree (Holocenter- pictured below Right) http://holocenter.org/artists/sam-moree
and Harris Kagan (Ohio State - pictured below Left) http://www.physics.ohio-state.edu/~kagan/
Holograms contend with a few different principals: light, optics, chemistry (to be general).
Below is an image of one of the most fundamental (and in my opinion) the most interesting aspects of holography - this is an image of an interference pattern produced by splitting one coherent beam of light into two. Wikipedia states this better than I ever could:
Interferometry makes use of the principle of superposition to combine waves in a way that will cause the result of their combination to have some meaningful property that is diagnostic of the original state of the waves. This works because when two waves with the same frequency combine, the resulting pattern is determined by the phase difference between the two waves—waves that are in phase will undergo constructive interference while waves that are out of phase will undergo destructive interference. Most interferometers use light or some other form of electromagnetic wave.
So what is coherent light?
What is light? A wave or a particle? For this purpose, I am defining light as both: wave comprised of photons, and as I understand it, there is some discussion about which is correct (wave vs. particle), but as I am merely a lay person, I will defer to you opinion on this matter.
In order to define light, it is imperative that there is a general understanding of the properties of the waves.
Waves have a peak and a trough, those peaks and troughs happen a certain amount in time, and those peaks and troughs have a particular height (+/-).
AMPLITUDE : How High the peaks are from zero. How much difference from 0 to peak.
FREQUENCY : How often the peaks and troughs happen in time.
WAVELENGTH : How much space is between the peaks.
Frequency and Wavelength differ in measurement:
We already know that when we talk about wavelength, we are talking about the distance between two peaks of a wave. Wavelength is usually measured in meters (m). Frequency is the number of cycles of a wave to pass some point in a second. The units of frequency are thus cycles per second, or Hertz (Hz).
So now that the properties of light are defined. What makes one light a particular color? Each color has a different wavelength.
So: Because wavelengths are measured in meters, and the varying difference in color is on the nano scale between light within the visible spectrum, we measure light waves in nanometers.
Green is the most brilliant to humans, as it falls right in the middle of our visible spectrum.
So what makes light coherent?
There are two attributes/measurements of coherence: Spatial and Temporal.
Temporal refers to the wavelengths traveling in phase and nanometers.
Spatial refers to - the waves come from a point and end at a point.
The figure to the left in reference with the Temporal Coherence--:
The amplitude of a single frequency wave as a function of time t (red) and a copy of the same wave delayed by τ(green). The coherence time of the wave is infinite since it is perfectly correlated with itself for all delays τ
The spatial coherence is the cross-correlation between two points in a wave for all times.
Figure below (from wikipedia)
Wikipedia article on coherence: https://en.wikipedia.org/wiki/Coherence_%28physics%29
Helium Neon lasers are gas lasers which produces a 690nm coherent beam.
(OSU's holography lab's HeNe pictured below)
All of this talk of light and waves and wavelength and and are a means to understanding the most fundamental aspect of holography: Interference.
So what is interference?
"the combination of two or more electromagnetic waveforms to form a resultant wave in which the displacement is either reinforced or canceled".
What does this mean?
Here's a nice description of interference: http://www.daviddarling.info/encyclopedia/I/interference.html
Hence the two *absolutley* required minimums for making a hologram are met - Coherent Light and Interference ....
Here is a link to a flickr set of the workshop: