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:
- A column of light leaves the laser and passes through the beam splitter.
- The two columns reflect off of their respective mirrors and pass through their respective diverging lenses.
- 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.
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.