Optical: systems and elements – Holographic system or element – Hardware for producing a hologram
Reexamination Certificate
2001-02-13
2003-01-21
Chang, Audrey (Department: 2872)
Optical: systems and elements
Holographic system or element
Hardware for producing a hologram
C359S001000, C359S249000, C359S030000, C369S112020
Reexamination Certificate
active
06509983
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to the field of hologram production and, more particularly, to hologram production where a recording laser beam polarization is adjusted.
2. Description of the Related Art
One-step hologram (including holographic stereogram) production technology has been used to satisfactorily record holograms in holographic recording materials without the traditional step of creating preliminary holograms. Both computer image holograms and non-computer image holograms may be produced by such one-step technology. In some one-step systems, computer processed images of objects or computer models of objects allow the respective system to build a hologram from a number of contiguous, small, elemental pieces known as elemental holograms or hogels. To record each hogel on holographic recording material, an object beam is conditioned through the rendered image and interfered with by a reference beam. Examples of techniques for one-step hologram production can be found in the U.S. Patent Application entitled “Method and Apparatus for Recording One-Step, Full-Color, Full-Parallax, Holographic Stereograms,” Ser. No. 09/098,581, naming Michael A. Klug, Mark E. Holzbach, and Alejandro J. Ferdman as inventors, and filed on Jun. 17, 1998, which is hereby incorporated by reference herein in its entirety.
In general, the holographic recording materials used in the fabrication of holograms include photopolymerizable compositions, dichromated gelatin, and silver halide emulsions. These holographic recording materials are typically placed on a glass or plastic substrate before being used in hologram production equipment (e.g., a “holographic printer”). Glass is a particularly useful substrate because of its good optical properties (e.g., high transmission, low distortion, low birefringence) and because of other beneficial mechanical properties including flatness, dimensional stability, scratch resistance, and chemical inertness.
FIG. 1A
illustrates a typical holographic film
100
based on a holographic recording material layer
120
such as a photopolymerizable composition. Although a variety of different types of holographic recording material can be used with the techniques discussed herein, including photopolymerizable compositions, dichromated gelatin, and silver halide emulsions, holographic recording material layer
120
is preferably formed from a photopolymer. Photopolymers include a wide range of materials that undergo physical, chemical, or optical changes through selective polymerization when exposed to light. Changes in the photopolymer's refractive index, transparency, adhesion, and/or solubility differentiate light and dark regions when these materials are exposed to an activating light source. Photopolymers capable of recording volume phase holograms include those developed by Canon Incorporated (based on polyvinyl carbazole), Polaroid Corporation (based on polyethylene amine/acrylate), and E. I. du Pont de Nemours and Company (based on polyvinyl acetate and polymethyl methacrylate). Those having ordinary skill in the art will readily recognize that a variety of different photopolymer compositions can be used in the practice of the inventions described herein. Nevertheless, preferred photopolymer films are provided by E. I. du Pont de Nemours and Company under the trade designations, for example, OmniDex™ 706, OmniDex™ 801, HRF-800X001-15, HRF-750X, HRF-700X, HRF-600X, and the like.
FIG. 1A
illustrates a typical photopolymer holographic film
100
as it is delivered from the film's manufacturer. Holographic film
100
includes a holographic recording material layer
120
, a base sheet
110
, and a cover sheet
130
. Base sheet
110
and cover sheet
130
provide protection to holographic recording material layer
120
, as well as dimensional stability to assist in the handling of the holographic film. Because of its protective and/or dimensional stability functions, base sheet
110
(and/or cover sheet
130
) can be referred to as a “film substrate.” As will be seen below, this film substrate is distinct from substrate
140
as shown in FIG.
1
C. Base sheet
110
and cover sheet
130
are typically formed from polymer films, such as polyethylene, polypropylene, cellulose, polyvinyl chloride (PVC), and polyethylene terephthalate (PET). Although not shown, holographic film
100
can include additional layers, such as a barrier layer used, for example, to prevent interlayer diffusion of sensitizing dyes, and to provide protection from oxygen during exposure.
In preparation for placement of the holographic recording material layer
120
on a substrate, cover sheet
130
is removed from holographic film
100
as shown in FIG.
1
B. The remaining portions of holographic film
100
(i.e., a holographic recording material layer
120
, and a base sheet
110
) are then placed on glass or plastic substrate
140
, as illustrated in FIG.
1
C. The natural tackiness of recording material layer
120
usually is sufficient to bind recording material layer
120
to substrate
140
. Because at least some of the light used to record a hologram in holographic recording material layer
120
typically passes through base sheet
110
, base sheet
110
preferably has good optical and material qualities including, for example, low scatter, flatness, low or no birefringence, mar-resistance, strength, and suitable thickness.
However, typical steps in the manufacturing process (and variations in the manufacturing process generally) for materials used for base sheet
110
can lead to at least one undesirable optical property, changes in birefringence from one portion of the material to another. For example, in the manufacturing of PET (e.g. Mylar® from E. I. du Pont de Nemours and Company) a common material used for base sheet
110
, molten PET is extruded onto a chill roll drum to form the initial film. The film is first stretched in the direction of the extruded film path (i.e., the “machine direction” or the “down-web direction”) using a series of rollers running at increasingly faster speeds. The film is then stretched in a transverse direction using, for example, a tenter, that pulls the film at right angles to the machine direction. Stretching rearranges the PET molecules into an orderly structure in order to improve the film's mechanical properties. Nevertheless, minor variations in this process, or the operation of the equipment used in this process, can lead to variation of the orientation of the film's molecules, which in turn can cause changes in birefringence from one portion of the film to another.
The birefringence of base sheet
110
affects the quality of polarized light (e.g., the polarized laser light from a reference or object beam) used in holographic recording. In general, birefringent materials have different indices of refraction for different directions of light transmitted therethrough. Materials typically used for base sheet
110
, such as PET) can be classified as uniaxial or biaxial materials. Uniaxial films usually have two indices of refraction, one in the direction of stretch and the other which is generally perpendicular to the stretched direction. Biaxial materials typically have three indices of refraction: one in the direction of stretch or linear extent of the film material and generally in the plane of the material; a second perpendicular to the first and also in the generally in the plane of the material; and a third index of refraction looking through the material at an edge view of it. In these materials there are one or more axes along which there is no change in the index of refraction exhibited by the material. Those axes typically are referred to as the optical axes or optic axes, and generally define at least one dominant polarization direction.
If the polarization of the laser light transmitted through the film substrate is aligned with the dominant polarization direction of the film substrate, the modulation depth of the recorded hologram is maximized. Correct
Ascolese Marc R.
Campbell Stephenson Ascolese LLP
Chang Audrey
Zebra Imaging, Inc.
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