Reflection hologram is a monochromatic or
multicolored hologram that must be lit by a frontal and directional
white light (at about 45°) so that the recorded image can be
reconstructed by the selective reflection of certain light frequencies
from the light-sensitive emulsion. It may be a single-step hologram or a
copy hologram. When the recording is made, the object beam and reference
beam are on opposite sides of the recording medium, forming an angle of
nearly 180°.
In a reflection hologram the fringes are packed so closely together
that they constitute layers throughout the thickness of the emulsion.
The spacing between fringes remains constant. The distance between the
fringes is a function of the wavelength of light used in constructing
the hologram and also the angle difference between reference and object
beam. This permits the reflection hologram to absorb the wavelenghts
that are not of the correct length and the wavelengths which matches the
spacing will be reflected.
The playback light or reconstruction beam comes from the same side of
the hologram as the viewer. Depending on the interference pattern some
parts of the incident light are reflected and some are not. Most
reflection holograms are made at a less severe angle, perhaps 160
degrees, so that the light can come in at an angle without being blocked
by the person who is trying to see the hologram.
The image produced by the hologram can either be a real image forming
in front of the holographic film or virtual forming behind the film.
Different film emulsions produce images with different
characteristics:
- Silver Halide- It is an emulsion that is
cheaper, less fragile and easy to handle. This emulsion produces
highest quality images.
- Dichromated Gelatin - It is a chemical gelatin
mix that is coated on glass to produce golden yellow bright images.
- Photo Polymers - These have a plastic backing
to produce bright images with a wider angle view.
Applications of Reflection Hologram
It finds versatile applications in many areas such as reflective label,
webbing, zipper, string, safety vest, wrist band, shoe lace, fancy bags.
It is also the most popular full color method of labelling CDs and DVDs.
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