Holograms and How Holograms Are Made

What Are Holograms?
What Makes Holograms Different To Normal Photographs?
How Are Holograms Made?
Holographic Image Recording Media
Types of Hologram
Reflection Holograms
Transmission Holograms
Embossed Holograms

What Are Holograms?

Holograms are no longer a mysterious part of science fiction. They are now an integral part of numerous consumer goods including credit cards, drivers licences, software CDs and even the cash currency of some countries. The reason they are used in these applications is to identify the product as a genuine article rather than counterfeit goods. This is because forgeries are extremely difficult to produce. Holograms are essentially a subset of photography but instead of producing a two dimensional image, holograms are a three dimensional photograph.

What Makes Holograms Different To Normal Photographs?

A normal photograph records the amount and wavelength (or colour) of light reflected from an object. The first difference between a photograph and a hologram is that a hologram requires a coherent energy source, normally a laser, to produce it rather than white light. The holographic imaging process results in an image that rather than being a two dimensional representation of the object, is three dimensional and contains all the information about the object.

If you tear a photograph into pieces, the photograph is destroyed. If a hologram is broken into pieces, each of the pieces still contains all the information about the subject and the image can be reconstructed.

How Are Holograms Made?

A hologram is basically a laser generated interference pattern. Starting with a laser, the beam is split into two parts. Some of the beam illuminates the subject for the hologram before being reflected onto the image recording medium. This is known as the object beam. The rest of the beam is diverted directly to the imaging medium. This is the reference beam.

The result is that the two beams interfere with each other to create dark and light spots corresponding to negative and positive interference. These spots combine to make the entire image.

In order to see the hologram, light like the reference beam must be shone on or through the image.

Figure 1. Schematic of how holograms are made

Holographic Image Recording Media

Holograms can be recorded on a range of different image recording media. These include photographic emulsions, photoresists, photopolymers, photothermoplastics, photochromics and photorefractives.

Types of Hologram

Although many different types of hologram can be produced through a number of methods the three most common types currently produced are reflection holograms, transmission holograms and embossed holograms.

Reflection Holograms

If you’ve seen a hologram in a gallery, it was probably a reflection hologram. Reflection holograms are illuminated by a white light source on the viewing side of the hologram. The light is reflected from the hologram to show the image. Reflection holograms can be produced in colour and the images are virtually indistinguishable from the real object.

Transmission Holograms

Transmission holograms are the type shown in Figure 1. They are viewed by illuminating from the rear with a laser. The advantage of transmission holograms is that they can be very sharp, loaded with detail and the image can have great depth. For instance a small hologram can show an entire room and it’s contents as if it were being viewed through a window.

Embossed Holograms

Embossed holograms are the type commonly found on consumer goods and credit cards. Although illuminated from the front they are actually transmission holograms. The hologram is embedded on a thin plastic film backed with reflective aluminium. The aluminium works like a mirror reflecting incident light back through the hologram activating it.

Embosssed holograms are mass produced by first creating a master on a photoresist. The master image is then transferred to a the holographic material and backed with the reflective layer.

Source: AZoOptics