There are various types of holograms. The most common ones are the reflection hologram and the transmission hologram. Reflection holograms are those holograms which are viewed with the light source on the same side as the viewer. This is a reflected light as the light is reflected off the hologram. This hologram is the most common type shown in galleries. In such a hologram, a truly three-dimensional image is seen near its surface. The reflection hologram is illuminated by a ray of white incandescent light. The light is held at a specific angle, distance and located on the viewer's side of the hologram. The image thus formed consists of light reflected by the hologram. Nowadays, these holograms are also displayed in various colors and their images are indistinguishable from the original objects. For example, the holographic image of the mirror reflects white light, the holographic image of the diamond is seen to dazzle or sparkle etc.

Why are reflection holograms important?

The various reasons for making and using a reflection hologram are as follows:

• Compared to the transmission hologram, the exposure time is generally shorter which leads to a less stringent condition on the mechanical stability required.
  
  
• The reflection hologram can be easily be viewed with a point source of white light or the sun, which is not the case with other holograms.
  
  
• The reflection holograms are used widely because they are viewed with reflected light.

Properties of Reflection Hologram

Some common properties of reflection hologram are as follows:

• These holograms can be viewed in regular light.
  
  
• The finished reflection hologram is monochromatic , i. e., there is a single color for each laser color used.
  
  
• Colors of reflection hologram can be shifted by pre- or post-shrink/expanding recording material.

Recording Process of Reflection Hologram

1-step Reflection Hologram:
In comparison to a transmission hologram, the recording of a reflection hologram requires 10 to 100 times more power. This leads to a longer exposure time. During the process of recording the hologram, the two beams-the reference beam and the object beam-lights the film plate on opposite sides which results in high resolution of film emulsion. The two beams of coherent light traveling in two directions interact. The fringes are formed in layers and are more or less parallel to the emulsion's surface. The reflection hologram selects the appropriate band of wavelengths to reconstruct the image if it is illuminated by a highly directed beam of white light like a spotlight or sun light. This is because a reflection hologram reflects light within a narrow band of wavelength and the remainder of the light passes straight through. The entire process is illustrated in the figure below.


2-step Reflection Hologram:
The 1-step reflection hologram is not always practical. Another method that is used is called 2-step reflection hologram. In such a method, first we make a transmission hologram called H1. This is the master hologram or the first hologram. We make multiple copies from the master hologram. Transfer copies are made in quantity from the master. Transfer copy means making another hologram using the image on the master as the subject. These transfer holograms are either laser-visible transmission holograms or reflection holograms H2. Suppose we want any object in the final hologram just to appear half in front and half behind the recording plate. In such circumstances, the 2-step hologram is of great use. Now, since we can make a hologram or a copy of the H1's image, we can move the image around to wherever we want it positioned. In other words, we adjust the H2 recording plate so that the image of the object is half in front and half behind the plate and then make our H2.

Sources of Illumination of Reflection Holograms

The different holograms are differentiated by the way in which they are lighted or illuminated. In case of reflection holograms, there are different film emulsions which make images with different characteristics. These are as follows:

• Silver Halide: Silver halide glass plates are used to achieve the highest quality images. This is the most common emulsion of choice for most artists and holographers. Silver halide film, usually cheap, less fragile and easy to handle. The drawback is that it does not produce the depth, resolution and projection which are possible with glass plates. However, such kind of film is used successfully for longer production runs.
  
  
• Dichromated Gelatin (DCG): This is a type of chemical-gelatin mix. The mixture is coated onto a piece of glass. It produces very bright images in a golden-yellow color. The images formed using DCG have the least range of depth, but they are easily viewable in normal room light without using any special spotlights. This makes DCG a suitable option for small gift products used in both consumer gift market or the premium & incentive industry.
  
  
• Photo Polymer: This is one of the newest of the recording materials. Photo polymers are suitable for long production runs and have a plastic backing. Compared to silver halide, the image depth of photo polymers is slightly less. The images are brighter, with a wider angle view.

Applications of Reflection Hologram

Reflection hologram finds its applications in many areas like as follows:

Reflective label Zipper Sunglasses Webbing String

Wrist band Safety vest Fancy bags Labeling CDs and DVDs