WO2000004360A1

WO2000004360A1 - Method for determining quality of lenticular sheet

Info

Publication number: WO2000004360A1
Application number: PCT/US1999/013379
Authority: WO
Inventors: James Samuel Nelson Dalton; Jon Thomas Moore
Original assignee: Eastman Chemical Company
Priority date: 1998-07-15
Filing date: 1999-06-14
Publication date: 2000-01-27

Abstract

A method is provided for evaluating the quality of a lenticular sheet prior to its utilization. The lenticular sheet having an undetermined number of lenticules per unit of measurement is positioned over a grating having a known number of lines per unit of measurement. The resulting moiré fringe pattern is evaluated to determine the number of lenticules the lenticular sheet has per unit of length.

Description

METHOD FOR DETERMINING QUALITY OF LENTICULAR SHEET

The present invention relates to a method for determining the quality of a lenticular sheet material and more particularly, to a method for determining the quality of a lenticular sheet prior to its utilization in the imaging arts.

It is known in the art to fabricate an image having an appearance of three dimensionality. This generally requires overlaying an image produced using a special three dimensional or stereoscopic camera with a lenticular lens. The object is photographed from a plurality of angularly displaced positions. A master image may consist of from 7 to about 24 photo views. Setting up the photo is quite difficult and different from a conventional two-dimensional shoot. The photo requires precise positioning of the visual elements to maximize the dimensional effect. The relationships among such factors as the number of exposures, distance between track positions for the camera, and front or rear locations of the objects and backgrounds must be computed mathematically in order to simulate a natural parallax for the eyes. The photograph can then be scanned into a computer and subsequently split apart into a multitude of image lines interleaved from the plurality of angularly displaced images also know as "bundles". Each bundle contains an image line from each of the original images in sequence. These images are then combined or "laid up" onto a single compressed print consisting of a pattern of very narrow bands. When the image bundles are equal in width and abut one another, the width of a bundle is the "pitch". When image bundles of equal width are equally spaced apart the pitch is the sum of the bundle width and the width of the space between the bundles.

Lenticular lenses used to produce the three-dimensionality are well known and commercially available from a variety of manufacturers. These lenses typically consist of an array of identical spherically-curved surfaces embossed or otherwise formed on the front surface of a plastic sheet. Other geometric patterns for the lens may also be used such as pyramidical. In the case where the lens is spherically-curved, each individual lens or lenticule resembles a semi-cylinder extending the full length of the underlying image over which it is superposed. The back surface of the lens is typically substantially flat or planar and is the surface in contact with or adjacent to the underlying flat image.

To form a dimensional effect, ideally the lenticular sheet overlay has the same pitch as the bundles in the compressed print so when the lenticular sheet is placed over the image, and in proper alignment with the image lines, the compressed print projects the plurality of images at different viewing angles corresponding to the viewing angles of the original scene. Preferably, image changes are small and gradual to simulate what a viewer would perceive as he would move past the image. It is important that the compressed print be properly centered to the lenticular array and that the scan lines be parallel to the axes of the lenticules forming the lenticular sheet. If the scan lines of the lenticular image are not parallel to the lenticule axes, the image break is seen as an objectionable, angled moire pattern superposed over the image area. Moire interference results from the overlapping of two or more grid patterns which detracts from the clarity and viewer's enjoyment of the image. Moire interference may also be produced when the lenticular sheet does not have the desired number of lenticules per unit of measurement or the lenticular sheet has an irregular number of lenticules per unit of measurement causing the final image to have an inexact registration and be of less than optimal quality. In the past, the lenticules and image elements or bundles were relatively wide and each lenticule had to be perfectly aligned over each image element to obtain a clearly defined three dimensional image. It has been difficult to obtain precise alignment and virtually impossible to mass produce three dimensional images efficiently and economically. The lenticular sheet had to be manually positioned over the image sheet and then carefully adjusted to eliminate the moire interference. This is time consuming, inefficient and expensive. To achieve uniformity, U.S. Patent No. 5,113,213 issued to Sandor et al. on May

12, 1992 discloses using a computer for making autosterographic images of an object. A predetermined number of planar images of the object are taken with each image being a view of the object from the respective viewpoint. These viewpoint images are then input into the computer from which they are interleaved using a high-resolution output imaging device. U.S. Patent No. 5,161,979 issued to Sekuguchi on November 10, 1992, discloses a process where the images are scanned into a computer and then subsequently altered or modified so that at least one and preferably all the images are masked and striped by electronically removing, erasing, canceling or otherwise deleting a symmetrical pattern of spaces on the images.

U.S. Patent No. 5,633,719 issued to Oehlbeck et al. on May 27, 1997 discloses a method for aligning a lenticular overlay with an image print having lenticular print bundles. The method uses two-dimensional fiducial indicia on the image surface. The lenticular sheet is superposed over the image and moved until at least two coaxial indicia on the print margin coincide with a single lenticule juncture.

U.S. Patent No. 5,457,515 issued to Quadracci et al. on October 10, 1995 discloses a method of forming a three-dimensional graphical image using a lenticular sheet. The image elements are printed onto the flat side of the lenticular sheet at an angle which corresponds to the pitch of the lenticules of the lenticular sheet. European Patent Application 0 619 513 discloses a method of correcting alignment of a preprinted film to a lenticular material to compensate for affects such as shrinking and curvature. The method includes placing registration images on the outer edges of the film so that alignment between the lenticules and the registration areas is achieved. European Patent Application 0 743 552 discloses a method for producing lenticular images by projecting the images directly onto a photosensitive emulsion which is coated on the back surface of the lenticular sheet prior to exposure.

Although the above methods describe how a composite image may be aligned with a lenticular sheet to eliminate any moire fringe patterns produced, it will be appreciated that the prior art is silent as to means or methods by which the quality of the lenticular sheet prior to its utilization may be determined. It will be further appreciated that the quality of the lenticular sheet is of importance in the quality of the resulting image. Accordingly, there is a need by which the quality of the lenticular sheet may be evaluated and particularly its quality prior to the sheet's utilization in a lenticular composite image.

SUMMARY OF THE INVENTION

This invention relates to a method of judging the quality or fitness for use of a lenticular lens, and particularly to a method of determining a sheet's quality or fitness to produce a three-dimensional like image with reduced moire and improved clarity, prior to positioning the lenticular lens over a printed or photographic image. Briefly described, the invention provides a method comprising the steps of superposing a lenticular lens over a grating having a known number of spaced apart lines per unit of measurement; and evaluating a resulting moire fringe pattern to determine the number of lenticules per unit of measurement in the lenticular sheet. By evaluating the fringe pattern, the quality of the lenticular sheet may also be determined. In a preferred embodiment of the invention, the lenticular sheet is superposed over the line grating in a non-registration relationship. That is, the rotational alignment of the lenticular sheet and the line grating are purposefully obliquely angled so that a moire fringe pattern is produced.

It is an object of the invention to provide a method for determining the quality of a lenticular sheet, and more specifically, to evaluate its quality prior to the lenticular sheet being utilized in a composite image.

It is another object of the invention to provide a method for determining the quality of a lenticular sheet using a moire fringe pattern.

It is another object of the invention to provide a method for assessing the quality of a lenticular sheet at the extrusion line as the sheet is produced.

These and other objects and advantages of the invention will become readily apparent to those skilled in the art with reference to the following specification and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded, isometric view of a lenticular overlay and a grating having a known number of lines per unit length. FIG. 2 is a depiction of a lenticular lens having two different side-by-side pitches

FIG. 3 illustrates the moire fringe pattern produced from the lenticular sheet of FIG. 2 having two different pitches, 94 and 106 lenticules per inch, when superposed over a grating having 100 lines per inch, (lpi) and skewed at an angle of less than about 15 degrees. FIG. 4 shows an angle indicia which may be placed on the line grating as a means for obliquely aligning the lenticular lens and the lines of the grating.

FIG. 5 illustrates the moire fringe pattern produced from a lenticular sheet superposed over a grating having 30 lpi.

FIG. 6 illustrates the fringe pattern produced from a lenticular sheet superposed over a grating having 34 lpi.

FIG. 7 illustrates the moire fringe pattern produced when a 32.4 lpi lenticular sheet is superposed over a grating having 30.0 lpi.

FIG. 8 illustrates the moire fringe pattern observed on a Lunometer Type F.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of drawings wherein like parts or objects in the various views have similar reference numerals, a method for determining the quality of a lenticular lens is described and includes the step of superposing a lenticular lens 10 over a grating 12 having a known number of spaced apart lines 14 per unit of measurement; and evaluating a resulting moire fringe pattern 16, seen in FIG. 3, to determine the number of lenticules 18 per unit of measurement. As used herein, the term "per unit of measurement" is a unit arbitrarily chosen by the person skilled in the art, but generally is understood to be a means of measuring distance. For instance, the unit of measurement can include, but is not limited to, inches, feet, yard, millimeters, centimeters and meters. As shown, the lenticular lens 10 is provided in a sheet format having a planar surface 20 which, when the lenticular lens 10 is utilized for making a three-dimensional composite image is positioned adjacent to and superposed over the image. Positioned distally from the planar surface 20 is an array or plurality of adjacently positioned curvilinear surfaces of the individual lenticules 18. As illustrated, each lenticule 18 is a semi-cylindrical member having a longitudinal axis, X, with substantially identical curved surface embossed or otherwise formed on the plastic sheet. The radius of curvature of the lenticule 18 determines its width and focal length. The shape of the lenticule 18 can be adjusted to facilitate shorter focal lengths without having as great a multiplicity of lenticules 18 per unit of measure by forming each lenticule with a greater curvature at its apex than throughout the remainder of the lenticule to provide a shorter focal length with a broader overall lenticule cross-section. As is generally understood by those skilled in the art, when the lenticules 18 have a large cross-section, a better three- dimensional effect is achieved by the lateral alignment of each lenticule 18 over each element of the composite image.

The number of lenticules 18 per unit of measurement comprising the lenticular lens 10 can vary from about 3 per inch to about 250 per inch. Preferably, the number of lenticules 18 per unit of measurement ranges from about 15 per inch to about 150 per inch, and more preferably, from about 25 per inch to about 75 per inch. Generally, as the number of lenticules 18 increases and the corresponding image elements or bundles are made narrower, the need for precise alignment of the lenticular sheet 10 over the image is reduced.

The preferred materials useful in forming the lenticular lens 10 are those capable of being thermoformed using techniques known to those skilled in the thermoplastic art. Non-limiting examples of such materials include: polyesters, such as polyethylene terephthalate (PET), polybutylene terephthalate (PBT) and copolyesters of terephthalic acid and aliphatic glycols such as PETG 6763 (available from Eastman Chemical Company, Kingsport, Tennessee); polycarbonates; polystyrene; acrylics and polyacrylates, such as, poly(methyl methacrylate), methyl methacrylate and copolymers thereof with other vinyl monomers such as other alkyl methacrylates or alkyl acrylates and the like; polyolefins such as polyethylene; vinyl chloride polymers such as polyvinyl chloride (PVC) and its copolymers with other vinyl monomers such as vinylidene chloride, styrene and the like including heavily plasticized PVC compositions or rigid vinyl formulations; polyurethanes including polymers having residues from 2,4- toluenediisocyanate, 1 ,4-diisocyanatobenzene and the like with diols such as 4,4'- bis(omega-hydroxyalkoxy)biphenyls, poly(propylene glycol), poly(tetramethylene) glycol, poly(l,4- butyl ene adipate), poly(ethylene-co-l,4-butylene adipate), polycaprolactone, poly(l,6-hexamethylene) carbonate and the like.

Particularly preferred materials suitable for forming the lenticular lens 10 are copolyesters of terephthalic acid and aliphatic glycols. Desirably, the acid component of the copolyester is at least 75 mole percent terephthalic acid. The acid component may be modified with up to about 25 mole percent of dibasic acids containing about 4 to about 40 carbon atoms such as naphthalenedicarboxylic, succinic, glutaric, azelaic, adipic, suberic, sebacic, isophthalic, sulfoisophthalic, 1 ,4-cyclohexanedicarboxylic acid and mixtures thereof. Any of the naphthalenedicarboxylic acid isomers or mixtures of isomers may be used such as the cis-, trans-, or cis/trans mixtures of 1 ,4-cyclohexanedicarboxylic acid, but the 1,4-, 1,5-, 2,6-, and 2,7-isomers are preferred.

The aliphatic glycol component of the copolyester has from about 97 mole percent to about 35 mole percent ethylene glycol and from about 3 mole percent to about 65 mole percent 1,4-cyclohexanedimethanol. More preferably, the aliphatic glycol component of the copolyester has from about 75 mole percent to about 65 mole percent ethylene glycol and from about 25 mole percent to about 35 mole percent 1,4-cyclohexanedimethanol. The glycol component used in the esterification of the copolyester may be further modified with up to 20 mole percent with glycols such as diethylene glycol, propylene glycol, neopentyl glycol, 1 ,4-butanediol, 1,6-hexanediol, 2,2,4,4-tetramethyl-l,3-cyclobutanediol, 1,3-propanediol, cis/trans mixtures of 1 ,4-cyclohexanedimethanol and mixtures thereof.

Suitable materials for forming the lenticular lens 10 are commercially available from BASF Corporation under the name ELASTOLLAN®, B.F. Goodrich under the name ESTANE®, Bayer Corporation under the names TEXIN® and DESMOPAN®, and Dow Chemical Company under the name PELLETHANE®.

The lenticules 18 can be formed using techniques that are known to those skilled in the thermoplastic art. For Example, a thermoplastic resin, such as those discussed above, can be extruded by means of a conventional melt extrusion process with the curvilinear surfaces of the lenticules 18 being formed by subjecting the molten sheet to a chilled grooved cylindrical roll. The lenticular lens 10 can also be produced by application of a UV-curable thermosetting resin to a flat thermoplastic sheet. The coated flat sheet is then subjected to a grooved roll and subsequent UV curing. The thickness of the thermoplastic sheet used in forming the lenticular lens 10 can range from about 0.04 of an inch to about 0.5 of an inch.

The grating 12 generally has a height (H) and a width (W) that are substantially equal to the respective dimensions of the lenticular lens 10. Desirably, the grating 12 has a height (H) and width (W) dimension of not less than 75 percent of the lenticular lens 10. The grating 12 can be made from any material that is capable of having the plurality of precisely spaced apart lines 14 placed thereon. Suitable materials include rigid and semi-rigid, crystalline or amorphous materials. Desirably, the materials have glass transition, Tg, greater than about 5 and preferably greater than about 15. For instance, suitable materials include glass; thermoplastics such as those described above, UV curable acrylics and thermosetting epoxies. Flexible materials, such as paper and thermoplastic films may further be utilized for producing the grating 12, provided the flexible material has a suitably rigid backing material or substrate for substantially preventing pitch distortion of the lines attributed to flexing, shrinking or stretching of the flexible material. To prevent movement, the flexible material may be suitably affixed to the rigid backing using mechanical fasteners such as screws, bolts, rivets, and the like or laminated thereto using compatible adhesives for the selected materials. The spaced apart lines 14 may be placed on or in the grating substrate using methods well known to those skilled in the art, such as, etching, engraving and preferably, printing using such methods as web offset, flexographic, gravure, stochastic or electronic deposition using laser printing, video jet or ink jet. Other methods of fabricating a grating 12 having spaced apart lines 14 include using a photosensitive material, such as a photographic film, and a laser to precisely define and position the lines 14.

Another method includes inserting a solid material, such as fine wire or strands of a natural or synthetic material, in machined grooves or channels of a transparent material. The groves can be formed in one material then overlaid by, and preferably laminated to, another grooved or nongrooved layer. Alternatively, the grooves or channels may be formed through the substrate. These as well as other inter ferometric techniques known to those skilled in the grating fabrication art may be used to fabricate the grating 12. Irrespective of the method used for creating the spaced apart lines 14, it is desirable that a high degree of precision be employed.

The grating's plurality of spaced apart lines 14 have a predetermined pitch or number of lines 14 per unit of measurement. The pitch of the lines 14 may differ from the targeted pitch of the lenticular lens by less than about 15 percent, preferably, less than about 10 percent and more preferably, less than about 5 percent. Most preferably, the pitch of the lines 14 is substantially equal to the targeted pitch of the lenticules 18 on the lenticular lens 10.

In accordance with the invention, the lenticular lens 10 is superposed over the grating 12 of a known line pitch in a manner whereby a moire fringe pattern 16 is generated. The moire fringe pattern 16 is produced when: the lenticular lens 10 does not have the desired number of lenticules 18 per unit of measurement relative to the grating 12, the lenticular lens 10 has an irregular number of lenticules 18 per unit of measurement, or when the parallel lines 14 of the grating 12 and the axis, X, of the lenticules 18 parallel to the lines 14 are superposed at a slight angle to each other, i.e., rotationally misaligned. The pitch of the resultant moire fringe pattern 16 is used to determine the number of lenticules 18 per unit of measure. Advantageously, if the resultant moire fringe pattern 16 is irregular one can further determine if the lenticular lens 10 is irregular. The frequency, F, of the moire fringe pattern 16 is related to the pitch of the grating, f,, and the pitch of the lenticular lens 10, f, by the following equation: F = f, - f₂ when f, > f₂ -loin a preferred embodiment of the method, the axis, X, of the lenticules 18 and the spaced apart lines 14 of the grating 12 are positioned in a non-registration alignment or relationship to produce the moire fringe pattern 16. Desirably, the longitudinal axis, X, of the lenticules 18 is positioned at an acute angle of less than about 15 degrees out of registration, preferably, less than about 10 degrees, and more preferably less than about 5 degrees out of registration. As used herein, the term "registration" means that the parallel spaced apart lines 14 of the grating 12 are substantially parallel, rotationally, to the axis, X, of the lenticules 18, and desirably having no more than about 3 minutes to about 1 degree of rotational misalignment. To assist in positioning the lenticular lens 10 and the grating 12 in a nonregistration relationship, the grating 12 may further include an angled indicia 24, as seen in FIG. 4. Although positioning the lenticular lens 10 at an acute angle relative to the grating 12 is possible using only one angled indicia 24, it is preferred to use at least two to make sure the rotational alignment of the lenticular lens 10 and grating 12 is easy and accurate. The angled indicia 24 is desirably printed at the desired angle on the grating 12. The angled indicia 24 can be located anywhere within the grating, but preferably is spaced from the outer edge of the grating 12 at any distance which is a multiple of the pitch of the lines 14.

The angled indicia 24 can be any shape having at least one acute angle such as a triangle, diamond, arrow, chevron, or other polygon. As the lenticular lens 10 or grating 12 is rotated with respect to each other, the image of the angled indicia 24 changes as viewed through the lenticular lens 10. When the image of the angled indicia 24 is accurately projected, the lenticular lens 10 is at the desired angled relationship with respect to the grating 12. The following examples are meant for exemplification and illustrate further the applicability of the invention for determining the quality of a lenticular sheet. EXAMPLE 1

A thermoplastic lenticular sheet having two different side-by-side pitches (or lenticules per inch) of 94 and 106 lenticules per inch was extruded using EASTMAN PETG 6763 copolyester. The lenticular sheet was superposed over 100 lpi line grating (line width of 0.005 inches on glass) prepared for the Shirley Institute by Schmidt Manufacturing Company. The lenticular sheet was rotationally misaligned at a slight angle of about 10 degrees. A moire fringe pattern was observed having an abrupt change in the fringe direction (as shown in FIG. 3). This example demonstrates the ability to visually determine that the pitch of the lenticular sheet changed without actually counting the number of lenticules per unit length of sheet by observing the difference in the direction of the moire fringes.

EXAMPLE 2

A line grating having a size of 8.5 x 11 inches and 30 lines per inch (0.003-inch line width) was prepared using a Pascal line-generating program. The line grating was printed on a Hewlett Packard Laser Jet 4 printer. The line grating was measured at selected points using an image analysis technique. The line grating was scanned with Hewlett Packard Scan Jet 4C scanner using sharp black and white photograph mode at four different areas of the sheet ranging from 5 to 7 lenticules in length. The image was cropped to reduce file size with Micrografx Picture Publisher software and analyzed with Image Tool software. An image analysis micrometer (10-mm scale, 0.005-mm divisions) was used for spatial calibration in the analysis. The mean line pitch was 30.10 lines per inch and the standard deviation was 0.0071 lines per inch.

A 60 mil thick thermoplastic lenticular sheet extruded from EASTMAN PETG 6763 copolyester using a lenticular grooved roll having a targeted 32.5 lenticular grooves per inch was superimposed over the above 30.10 lpi line grating. Four measurements were made using a 7X magnification loupe and a C Thru brand plastic ruler with 1 mm graduations of unknown calibration to determine the mean fringe pitch (2.65 fpi) and the fringe pitch standard deviation (0.006 fpi) of the moire fringe pattern (see FIG. 5). Based on Equation 1, the mean lenticular pitch of the sheet was calculated to be 32.75 lenticules per inch. The decision to subtract or add the mean fringe pitch of 2.65 fpi to 30.10 lpi in Equation 1 is made by examining the direction of the fringes as the lenticular sheet is rotated clockwise over the line grating. If the fringes move counter clockwise, the lenticular sheet lpi is less than the line grating lpi. In this case, f, is the line grating and f₂ is the lenticular sheet pitch. If the fringes move clockwise, then the number of lenticules per in on the lenticular sheet is greater than the line grating lpi. In this example, f, is the lenticular sheet and f₂ is the line grating.

EXAMPLE 3

The lenticular sheet from Example 2 was superimposed over a glass line grating having 34 lpi (prepared for the Shirley Institute by Schmidt Manufacturing Co.). The resultant moire fringe pattern, as seen in FIG. 6, was measured visually with a scale and found to have a pitch of about 1.6 fringes per inch. Using Equation 1, and observing the direction of rotation of the moire fringe pattern, the number of lenticules per inch in the sheet was calculated to be 32.4.

EXAMPLE 4

As in Example 2 above, a thermoplastic lenticular sheet was extruded from EASTMAN PETG 6763 copolyester having a targeted 32.5 lenticules per inch. A Lunometer brand Type F line grating with increments ranging from 25 to 60 lines per inch in increments of 1 line per inch was superimposed over the above lenticular sheet. A series of parabolic moire fringe patterns was produced having an apex between the 32 to 33 readings on the Lunometer scale (see FIG. 8). As estimated, the sheet had 32.5 lenticules per inch. H.P. Luhn Associates produced the Lunometers. The Lunometer was designed and marketed for use in measuring the number of threads or courses per inch in woven textiles. In this range, this Lunometer was useful for discriminating the number of lenticules to one-half lenticule per inch.

EXAMPLE 5

This example demonstrates the use of moire fringe patterns to visually examine large lenticular sheets for distortion. A 30 lpi, 3 x 4 feet, line grating with a dark line-to- space ratio of R = 0.6 was generated using Imagineering software and printed using an OCE 9400 electrostatic printer. The printed line grating had no obvious visual defects. A 2 x 3 feet lenticular sheet as described in Example 2 was extruded using EASTMAN PETG 6763 and superimposed over the line grating. The sheet was pressed flat to remove any air between the grating and the lenticular sheet. The moire fringe patterns were 2.6 per inch across the sheet. No obvious visual distortion of the fringe pattern was observed over the entire lenticular sheet. A portion of the pattern is shown in FIG. 6.

EXAMPLE 6

A portion of the lenticular sheet in Example 2 was placed, lenticule side down, on a Hewlett Packard Scan Jet 4C scanner with a Shirley 34 lpi glass line grating superimposed on the lenticular sheet. They were scanned at a 2400 dpi using the sharp black and white photo setting. The moire image, FIG. 6, was cropped using Micrografx Picture Publisher software to reduce the file size. Next, the file was transferred to Image Tool, an image analysis program. The distance between fringes was measured and the fringe pitch was calculated to be about 1.6. Using Equation 1, and observing the direction of rotation of the moire fringe pattern, the number of lenticules per inch in the sheet was calculated to be 32.4. EXAMPLE 7

A line grating having 32.50 lpi and a line width of 0.00770 was prepared using Free Hand software and output to an Agfa Select Set 7000 image setter and onto a 20 x 25 inch clear film. A PETG lenticular sheet (22 x 27 inches) as described in Example 2 (with a target 32.5 lenticules per inch) was superimposed over the line grating. The measured fringe pitch was 1 fringe per 14 inches or 0.071 fringes per inch. Using Equation 1, and observing the direction of rotation of the moire fringe pattern, the number of lenticules per inch in the sheet, i.e., the pitch of the sheet, was calculated to be 32.57.

EXAMPLE 8

This example demonstrates the preferred use of moire patterns (or the lack thereof) to align the lenticular sheet with a line grating to check for registration. A line grating having 20 opaque lines per inch, a line width of 0.0250 of an inch, and a space width of 0.0250 of an inch was prepared using Free Hand software. The output was sent to an Agfa Select Set 7000 image setter and onto a clear film measuring 20 x 25 inches. A lenticular sheet measuring 22 x 27 inches and having a target of 40 lenticules per inch was superimposed over the line grating. Using a 7X loupe to register with the line grating, the lenticular sheet was rotated until the fringe pattern disappeared. Sharp black lines and clear spaces were observed on each alternate lenticule of the lenticular sheet with no fuzzy or gray lines over the entire lenticular sheet.

EXAMPLE 9

This example demonstrates the use of the method on a lenticular sheet having a different cross-sectional shape from the previous examples. A Lunometer brand Type R line grating having from 50 to 120 lines per inch in increments of 1 line per inch is superposed over a sample prismatic film manufactured by 3M. This film has lenticules with a triangular cross-sectional shape as compared to the circular cross-sectional shapes of the previous examples. A series of parabolic moire fringe patterns, similar to the type depicted in FIG. 8, was produced. The pattern had an apex at 71 on the Lunometer scale. This Lunometer was useful for discriminating the number of lenticules to one lenticule per inch.

Moire fringe patterns provide a means for visually detecting minute differences between the line grating 14 and the lenticular lens 10. The precision and accuracy of moire fringe test method for determining the number of lenticules 18 per unit length of sheet is partly limited by the precision and accuracy of the scale or device used to measure the fringes 16, by the accuracy and precision of the line grating 14 used to generate the fringes 16, and by the measurement technique. The ability to discriminate between lenticular sheets of varying pitch depends on the accuracy of the instruments used to make the measurements. A rule that allows one to measure 1/16 of an inch will not discriminate as well as an image analysis technique that allows one to measure to a few microns. The precision of the moire fringe test method is linked to how precisely the grating 14 is calibrated.

If one desires to detect a difference of 1 lenticule per inch between a sheet with a target pitch of 31.0 lenticules per inch using a line grating of 30.0 lpi, then a minimum of only one inch would be required to make the measurement as the difference in the two pitches would be 1.0 fringes per inch. If one desires to detect a difference of 0.1 lenticules per inch between a sheet with a target pitch of 30.1 lenticules per inch using a line grating of 30.0 lpi, then a minimum of 10 inches (10 inches x 0.1 fringe per inch) would be required to make the measurement. Similarly, if one desires to detect a smaller difference about the same level, the minimum required length to observe the fringe pattern becomes larger and may not be practical. Alternately, if one does not observe a discernable fringe pattern between a 30.0 lpi grating and the lenticular sheet in a given length of one inch, then the difference would be less than 1 lenticule per inch. Likewise, if no fringe is observed in 10 inches, then the difference would be less than 0.1 lenticules per inch. Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be inteφreted as limiting the invention described herein. No doubt that after reading the disclosure, various alterations and modifications will become apparent to those skilled in the art to which the invention pertains. It is intended that the appended claims be inteφreted as covering all such alterations and modifications as fall within the spirit and scope of the invention.

Claims

We claim:

1. A method for determining the quality of a lenticular sheet comprising the steps of: a. supe╧åosing a lenticular sheet over a line grating having a known number of spaced apart lines per unit of measurement; and b. evaluating a resulting moire fringe pattern to determine the number of lenticules per unit of length of said lenticular sheet.

2. The method of claim 1 wherein said lenticular sheet includes a substantially planar surface and said substantially planar surface is positioned adjacent to said grating.

3. The method of claim 1 wherein said lenticular sheet is positioned in a nonregistration relationship relative to said grating.

4. The method of claim 3 wherein said lenticular sheet is positioned out of registration with said grating at an angle of less than about 15 degrees.

5. The method of claim 3 wherein said lenticular sheet is positioned out of registration with said grating at an angle of less than about 10 degrees.

6. The method of claim 3 wherein said lenticular sheet is positioned out of registration with said grating at an angle of less than about 5 degrees.

7. The method of claim 1 wherein said lenticular sheet comprises a thermoplastic.

8. The method of claim 7 wherein said thermoplastic is selected from the group consisting of polyesters and copolyesters of terephthalic acid and aliphatic glycols, polycarbonates, polystyrene, acrylics, polyacrylates, polyolefms, vinyl chloride polymers, styrene, polyurethanes, and mixtures thereof.

9. The method of claim 8 wherein said copolyesters of terephthalic acid and aliphatic glycols comprise an acid component of at least 75 mole percent terephthalic acid and a glycol component of about 3 to about 65 mole percent 1,4-cyclohexanedimethanol and of about 97 to about 35 mole percent ethylene glycol.

10. The method of claim 9 wherein said copolyester has a glycol component comprising from about 25 mole percent to about 35 mole percent 1,4-cyclohexanedimethanol and from about 75 mole percent to about 65 mole percent ethylene glycol.

11. A method for determining the quality of a lenticular sheet comprising the steps of: a. providing a lenticular sheet having a substantially planar surface and an undetermined number of lenticules per unit of measurement; b positioning said substantially planar surface adjacent to a grating having a known number of spaced apart lines per unit of measurement; and c. evaluating a resulting moire fringe pattern to determine the number of lenticules per unit of length.

12. The method of claim 11 wherein said lenticular sheet is positioned out of registration with said grating at an angle of less than about 15 degrees.

13. The method of claim 11 wherein said lenticular sheet is positioned out of registration with said grating at an angle of less than about 10 degrees.

14. The method of claim 11 wherein said lenticular sheet is positioned out of registration with said grating at an angle of less than about 5 degrees.

15. The method of claim 1 1 wherein said lenticular sheet comprises a thermoplastic selected from the group consisting of polyesters and copolyesters of terephthalic acid and aliphatic glycols, polycarbonates, polystyrene, acrylics, polyacrylates, polyolefms, vinyl chloride polymers, styrene, polyurethanes, and mixtures thereof.

16. The method of claim 15 wherein said copolyesters of terephthalic acid and aliphatic glycols comprise an acid component of at least 75 mole percent terephthalic acid and a glycol component of about 3 to about 65 mole percent 1,4-cyclohexanedimethanol and of about 97 to about 35 mole percent ethylene glycol.

17. The method of claim 9 wherein said copolyester has a glycol component comprising from about 25 mole percent to about 35 mole percent 1,4-cyclohexanedimethanol and from about 75 mole percent to about 65 mole percent ethylene glycol.

18. A method of determining the quality of a lenticular sheet comprising the steps of: a. providing a lenticular sheet having a substantially planar surface and an undetermined number of lenticules per unit of measurement; b positioning said substantially planar surface in a non-registration relationship adjacent to a grating having a known number of spaced apart lines per unit of measurement; and c. evaluating a resulting moire fringe pattern to determine the number of lenticules per unit of length.

19. The method of claim 18 wherein said lenticular sheet is supe╧åosed over said line grating.

20. The method of claim 18 wherein said number of lines in said grating differs from said undetermined number of lenticules in said lenticular sheet by less than about 15 percent.

21. The method of claim 18 wherein said number of lines in said grating differs from said undetermined number of lenticules in said lenticular sheet by less than about 10 percent.

22. The method of claim 18 wherein said number of lines in said grating differs from said undetermined number of lenticules in said lenticular sheet by less than about 5 percent.

23. The method of claim 18 wherein said number of lines in said grating is substantially equal to said undetermined number of lenticules in said lenticular sheet.