Method of forming lenticular sheets

Details Method of forming lenticular sheets Method of forming lenticular sheets

2001-06-13

2004-04-27

Vargot, Mathieu D. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Optical article shaping or treating

C264S001700, C264S132000, C359S619000

Reexamination Certificate

active

06726858

ABSTRACT:

FIELD OF INVENTION
The present invention relates generally to the field of lenticular imaging, and in particular, to a method of forming a lenticular sheet for use in forming a substantially ghost-free lenticular image.
BACKGROUND OF THE INVENTION
Lenticular images, which are well known, comprise a plurality of interlaced scenes that are printed directly on or affixed adjacent to a generally planar bottom surface of a lenticular sheet. The opposing top surface of the lenticular sheet typically comprises a plurality of convex lenticles that are parallel to each other and the interlaced scenes printed on the bottom surface of the lenticular sheet. When viewed in cross-section, the top surface of each lenticle in the lenticular sheet typically has either a circular or an elliptical profile.
The individual lenticles act as lenses that are intended to allow an observer to view only a single interlaced scene at a particular viewing angle. As the viewing angle changes, the viewer should view a different interlaced scene. Because the scene observed changes as a function of viewing angle, it is possible to dissect and sequentially print interlaced scenes to form lenticular images that appear to an observer as a motion picture or video clip.
A number of means are known for forming lenticular sheets. These means include machining, platen pressing, injection or compression molding, extrusion embossment, and casting. For reasons of cost and manufacturing ease, lenticular sheets are typically formed from polymeric materials. But other materials such as glass can be used.
Machining is used primarily to manufacture large, one-of-a-kind, lenticular sheets from thick plastic sheets or plates. Milling machines or lathes equipped with diamond tipped cutting tools can be used to fabricate lenticular sheets of this type. It will be appreciated that machining is a slow and costly process, and thus it is not particularly suitable for high volume production of lenticular sheets.
A platen press can be used to stamp or emboss a lenticular pattern into a polymeric material. In such applications, the polymeric material typically comprises a thermosetting material. The temperature of the thermosetting polymeric material is raised to soften the material so that it conforms to the embossed surface. The temperature is then reduced to harden the polymeric material so that it retains the lenticular pattern when removed from the platen press. Like machining, this method is relatively slow and expensive. Furthermore, the size of the lenticular sheets that can be produced by this process is limited by the size of the press. Accordingly, this method is not particularly suitable for high volume production of lenticular sheets. The same disadvantages are present with injection or compression molding techniques, wherein the lenticular sheet is formed by injecting or compressing a molten polymer into a mold that includes a lenticular pattern on one side.
By far, the most common method of fabricating lenticular sheets is by extrusion embossment. Typically, extrusion embossment involves the use of an embossed roll, that may or may not be chilled. A molten polymeric material, which is typically thermoplastic rather than thermosetting, is extruded onto the embossed roll where it conforms to and thus is imprinted with the lenticular relief pattern. As the polymeric material cools, it retains the embossed lenticular pattern and is pulled from the embossed roll.
Although lenticular images have been produced since at least as early as the 1950's, the quality of most lenticular images has not been particularly good. Most known lenticular images suffer from a phenomenon known within the art as “ghosting,” which describes an observer's ability to see more than one interlaced scene at a particular viewing angle. A viewer of a lenticular image that contains “ghosts” will see phantom scenes within the image. For example, if the lenticular image comprises a series of interlaced scenes whereby the letter “C” changes or “morphs” into an image of a cat, a ghost in the form of all or some of the “C” image may be observed in the final cat image.
The current practice in the field of lenticular imaging is to try to eliminate the appearance of ghosting and other undesirable artifacts in lenticular images through trial and error. Lenticular film fabricators usually attempt to address ghosting by randomly varying the lenticular sheet by changing the lens frequency or the thickness of the lenticular sheet. Some printers of lenticular images address ghosting by inserting colored strips between interlaced scenes in an attempt to further separate and differentiate them. These trial and error techniques rarely yield substantially ghost-free images and are implemented at high cost. For example, a lenticular sheet fabricator who decides to vary the lens frequency does so by having a new embossing roll manufactured. Embossing rolls are relatively expensive, and without proper guidance, a new embossing roll may not solve the ghosting problem.
A method is needed whereby fabricators of lenticular sheets and printers of lenticular images can determine the proper geometry and/or thickness of a lenticular sheet suitable for use in forming a substantially ghost-free lenticular image for use in a particular lenticular image forming application. Preferably, the method would permit lenticular sheet fabricators to use existing inventories of embossing rolls and other means for forming lenticular sheets, and thus avoid the expense and time needed to obtain other suitable means.
SUMMARY OF INVENTION
The present invention provides a method of forming a lenticular sheet for use in forming a substantially ghost-free lenticular image. The method according to the invention comprises providing a means for forming a lenticular sheet and forming the lenticular sheet out of a polymeric material using the means such that the lenticular sheet has an actual thickness (d
a
) that is within about ±15% of an optimal thickness (d
o
) as determined according to the formula: d
o
=(ns−1)r/s(n−1), wherein n is the refractive index of the polymeric material, s is the number of interlaced scenes in the lenticular image, and r is the radius of the lenticles formed using the means. Suitable means for forming a lenticular sheet according to the method of the invention include machining, platen pressing, injection molding, compression molding, extrusion embossment, and casting, with extrusion embossment being presently most preferred.
For some lenticular imaging applications, it is desirable to form a lenticular sheet having a predetermined thickness d
a
. In such instances, the method of the present invention comprises providing a means for forming a lenticular sheet and forming the lenticular sheet out of a polymeric material using the means such that the lenticles have an actual radius (r
a
) that is within about ±15% of the optimal radius (r
o
) determined according to the formula: r
o
=(n−1)sd
a
/(ns−1), wherein n is the refractive index of a polymeric material from which the lenticular sheet is to be formed, s is the number of interlaced scenes in the lenticular image, and d
a
is the predetermined actual thickness of the lenticular sheet.
In some lenticular imaging applications it is possible to obtain substantially ghost-free lenticular images by selecting a polymeric material having a proper refractive index. Accordingly, in another embodiment of the method of the present invention, the method comprises providing a means for forming a lenticular sheet and forming the lenticular sheet from a polymeric material using the means, wherein the polymeric material has an actual refractive index (n
a
) that is within ±3% of an optimal refractive index (n
o
) as determined according to the formula: n
o
=(r
a
−sd
a
)/s(r
a
−d
a
), wherein r
a
is the actual radius of each lenticle in the lenticular sheet, s is the number of interlaced scenes in the lenticular image, and d
a
is the predetermined actual t

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