When you take a picture with a film camera, four basic steps happen in an instant:
- A shutter opens.
- Light passes through a lens and hits the photographic emulsion on a piece of film.
- A light-sensitive compound called silver halide reacts with the light, recording its amplitude, or intensity, as it reflects off of the scene in front of you.
- The shutter closes.
You can make lots of changes to this process, like how far the shutter opens, how much the lens magnifies the scene and how much extra light you add to the mix. But no matter what changes you make, the four basic steps are still the same. In addition, regardless of changes to the setup, the resulting picture is still simply a recording of the intensity of reflected light. When you develop the film and make a print of the picture, your eyes and brain interpret the light that reflects from the picture as a representation of the original image. You can learn more about the process in How Vision Works, How Cameras Work and How Film Works.
Like photographs, holograms are recordings of reflected light. Making them requires steps that are similar to what it takes to make a photograph:
- A shutter opens or moves out of the path of a laser. (In some setups, a pulsed laser fires a single pulse of light, eliminating the need for a shutter.)
- The light from the object beam reflects off of an object. The light from the reference beam bypasses the object entirely.
- The light from both beams comes into contact with the photographic emulsion, where light-sensitive compounds react to it.
- The shutter closes, blocking the light.
Just like with a photograph, the result of this process is a piece of film that has recorded the incoming light. However, when you develop the holographic plate and look at it, what you see is a little unusual. Developed film from a camera shows you a negative view of the original scene -- areas that were light are dark, and vice versa. When you look at the negative, you can still get a sense of what the original scene looked like.
But when you look at a developed piece of film used to make a hologram, you don't see anything that looks like the original scene. Instead, you might see a dark frame of film or a random pattern of lines and swirls. Turning this frame of film into an image requires the right illumination. In a transmission hologram, monochromatic light shines through the hologram to make an image. In a reflection hologram, monochromatic or white light reflects off of the surface of the hologram to make an image. Your eyes and brain interpret the light shining through or reflecting off of the hologram as a representation of a three-dimensional object. The holograms you see on credit cards and stickers are reflection holograms.
You need the right light source to see a hologram because it records the light's phase and amplitude like a code. Rather than recording a simple pattern of reflected light from a scene, it records the interference between the reference beam and the object beam. It does this as a pattern of tiny interference fringes. Each fringe can be smaller than one wavelength of the light used to create them. Decoding these interference fringes requires a key -- that key is the right kind of light.
Next, we'll explore exactly how light makes interference fringes.