As described in the History of Holograms transmission and reflection holograms have different fringe structures so look different.
In the reflection hologram only selected wavelengths (colors) are reconstructed while the transmission hologram diffracts all the wavelengths of light so can it can have a rainbow appearance. The hologram does not change the wavelength (color) of light but controls how light is re-directed.
There are a number of distinct types of art holograms that can be defined by their optical-geometry and the recording medium.
The Denisyuk Reflection Hologram
A Denisyuk hologram is created with a ‘single’ beam’ of laser light that is shone through the hologram surface and then bounces off object back into the hologram surface.
Unless the physical structure is chemically manipulated the reflection hologram will appear the color that it is recorded. To create a true color refection hologram a multiple laser colors are combined – usually a red, green and blue.
Pseudo-color Reflection Hologram
Hologram are images of light, so colors are controlled by selecting and combining spectral colors.
is By manipulating the chemical processing of , the holographic fringe structure can be expanded or shrunk changing the color of the reconstructed image. Pseudo-color reflection holograms can be created through multiple exposures between which the emulsion is swollen or shrunk to shift the recorded fringe spacing and therefore color, a technique that has been used extensively by John Kaufman and Iñaki Beguiristain.
Pseudo-colour Reflection Hologram, 30 x 40 cm
(image courtesy of the artist)
Pseudo-colour reflection hologram, 30 x 40 cm
(via: Inaki Beguiristain)
Laser viewable transmission holograms
The laser viewable transmission hologram allows for a near perfect reconstruction of the optical field. This means that the recorded scene appears behind the film, and when replayed by a laser this scene can be very deep and sharp. These holograms are also used as a master recording that can then be transferred into a reflection or transmission holographic print.
Artist Paula Dawson who has worked extensively with laser viewable transmission holograms describes these recordings as ‘concrete’ holographic images because they create a sense of physical presence.
Right: Two perspectives showing the details of the scene from small holograms 4 x 5” (10.2 x 12.7 cm) The Jonathan Ross Hologram Collection
Rainbow Holograms
Transmission holograms have a different a visual quality and the color is controlled by geometry rather than chemistry. When illuminated with a white (broad spectrum) light source the transmission hologram will diffract all the wavelengths of light into the image. However as red-wavelengths are longer, they are deflected more than blue-wavelengths and so the image will have some color smear – the light spreads into a rainbow image.