Ulllted States Patent [19]
`Cobb, Jr. et al.
`
`US005919551A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,919,551
`Jul. 6, 1999
`
`[54] VARIABLE PITCH STRUCTUREI) OPTICAL
`FILM
`
`[75] Inventors: Sanford Cobb’ Jr” st Nlary!S Point,
`Minn‘; Mark E- Gardiner, Santa Rosa,
`Calif; Keith M. Kotchick, Woodbury,
`Mm“; Kf‘zPhlko TOYOOka’ Yamagata’
`141ml;WllllamA-Hlbbardawoodbllrya
`Mlnn-
`
`[73] Assignee: 3M Innovative Properties Company,
`St, Paul, Minn,
`
`_
`[21] Appl' NO" 08/631’073
`[22] Filed?
`APY- 12, 1996
`
`Int. Cl.6 ............................... .. G02B 5/04; B32B 3/30
`[51]
`[52] US. Cl. ........................ .. 428/156; 428/141; 428/156;
`428/913; 428/179; 428/167; 428/163; 359/530;
`359/831; 359/834; 359/833; 359/837; 362/337;
`362/339; 362/26; 385/146; 385/901; 385/36
`[58] Field Of Search ................................... .. 428/141, 156,
`428/913, 179, 167, 163; 359/530, 831,
`834, 833, 837; 362/337, 339, 26; 385/146,
`901’ 36
`
`[56]
`
`References Cited
`
`U'S' PATENT DOCUMENTS
`
`8/1934 Lehman ................................ .. 362/337
`1,969,982
`1/1935 Maillet .................................. .. 362/339
`1,986,065
`375417606 11/1970 Heenan et al'
`359/530
`gansen """" "
`' ' ' ' "428/156
`2/1985 Breaecrllanh ' ' ' ' '
`4’497’86O
`4’775’219 10/1988 appli’aa'n£5ail"IIIIIIIIIIIIIIIIIIIII 350/103
`4:799:137
`1/1989
`362509
`4,823,246
`4/1989
`362/339
`4,874,228 10/1989
`350/345
`4,906,070
`3/1990
`.. 350/286
`
`7/1990 Nelson et a1. ........................ .. 350/103
`4,938,563
`8/1991 Cobb, Jr.
`350/452
`5,040,883
`5,056,892 10/1991 Cobb, Jr. ..
`. 359/831
`5,175,030 12/1992 Lu et a1 ................................. .. 428/30
`IS_u
`....................................... .. 3256942153
`572067746 42993 o‘gilet g1
`35/9/4O
`5,237,641
`8/1993 Jacobson et al.
`385/146
`5,245,454
`9/1993 Blondev ........ ..
`359/70
`5,303,322
`4/1994 Winston et al
`385/146
`5,349,503
`9/1994 Blondev .... ..
`359/530
`5,363,470 11/1994 Wortman
`385/147
`5,467,208 11/1995 Kokawa et a1.
`. 359/49
`5,471,348 11/1995 Miller et a1.
`359/837
`5,506,929
`4/1996 Tai et a1.
`385/146
`5,585,164 12/1996 Smith et a1. .......................... .. 428/156
`FOREIGN PATENT DOCUMENTS
`
`573 268 A2 12/1983 European Pat Off~ _
`44 11 206 A1 10/1995 Germany .
`6-82635
`3/1994 Japan .
`8313710 11/1996 19p?“ ~
`664581??? 3/1982 SW1tZer1and~
`36g”;
`6 $533
`'
`OTHER PUBLICATIONS
`Olof Bryngdahl, “Moiré: Formation and Interpretation”,
`Optica Acta, 24 (1), 70—77 (1977).
`Primary Examiner—William P. Watkins, III
`Attorney, Agent, or Firm—Stephen W. Buckingham
`
`ABSTRACT
`[57]
`The present invention includes a structured optical ?lm With
`variable pitch peaks and/or grooves to reduce the visibility
`of moiré interference patterns and optical displays incorpo
`rating one Or more layers of the film The Pitch Variations
`can be over groups of adjacent peaks and/or valleys or
`between adjacent pairs of peaks and/or valleys.
`
`35 Claims, 4 Drawing Sheets
`
`LGD_001082
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`LG Display Ex. 1030
`
`

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`LGD_001083
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`

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`U.S. Patent
`
`Jul. 6, 1999
`
`Sheet 2 of4
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`5,919,551
`
`Fig. 3A
`
`LGD_001084
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`

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`U.S. Patent
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`Jul. 6, 1999
`
`Sheet 3 of 4
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`5,919,551
`
`LGD 001085
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`LGD_001085
`
`

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`U.S. Patent
`
`Jul. 6, 1999
`
`Sheet 4 0f 4
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`5,919,551
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`w
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`oo
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`mm .wE
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`2.
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`LGD_001086
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`

`
`1
`VARIABLE PITCH STRUCTURED OPTICAL
`FILM
`
`FIELD OF THE INVENTION
`
`The present invention relates to the ?eld of structured
`optical ?lms and optical displays incorporating the struc
`tured optical ?lms. More particularly, the present invention
`relates to optical ?lms having a structured surface in Which
`the pitch of the valleys and/or peaks vary.
`
`BACKGROUND OF THE INVENTION
`
`Structured optical ?lms are used in optical display sys
`tems and in other applications Where control over the
`direction of light, transmitted and/or re?ected, is desired to
`increase brightness, reduce glare, etc. Structured optical
`?lms are described generally in US. Pat. No. 4,906,070
`(Cobb). Essentially, they comprise ?lms of light transmis
`sible materials in Which a series of prisms are located such
`that the ?lms can be used to redirect light through re?ection
`and refraction. When used in an optical display such as that
`found in laptop computers, Watches, etc., the structured
`optical ?lm can increase brightness of an optical display by
`limiting light escaping from the display to Within a pair of
`planes disposed at desired angles from a normal aXis running
`through the optical display. As a result, light that Would eXit
`the display outside of the alloWable range is re?ected back
`into the display Where a portion of it can be “recycled” and
`returned back to the structured ?lm at an angle that alloWs
`it to escape from the display. That recycling is useful
`because it can reduce poWer consumption needed to provide
`a display With a desired level of brightness.
`An undesirable effect of using a structured optical ?lm in
`an optical display is the appearance of re?ected moiré
`caused by the interference of tWo periodic patterns. Moire
`effects are discussed in O. Bryngdahl, “Moire: Formation
`and Interpretation,” Optica Acta, Vol. 24(1), pp. 1—13
`(1977). In an optical display incorporating a single layer of
`structured optical ?lm, the periodic patterns causing moiré
`are the pattern in the ?lm itself and the re?ected image of the
`?lm pattern (as re?ected by other surfaces in the optical
`display).
`Some optical displays incorporate a second structured
`optical ?lm in Which the prisms are oriented at an angle With
`the prisms in the ?rst optical ?lm. That angle can be
`anyWhere from greater than Zero to 90°, although it is
`typically about 90°. Although using tWo structured optical
`?lms can increase the brightness of the display Within a
`narroWed vieWing range, it can increase the effects of moiré
`by providing a second plano surface (on the loWer structured
`?lm) that re?ects more light back through the periodic
`pattern in the ?rst, or upper, structured ?lm.
`In addition, the second structured optical ?lm may also
`lead to optical coupling that may result in uneven light
`transmission from the display, i.e., visible bright spots,
`streaks, and/or lines in the display. Optical coupling is
`caused by contacting, or very nearly contacting, a plano
`surface With the structured surface of a structured optical
`?lm.
`
`SUMMARY OF THE INVENTION
`The present invention includes a structured optical ?lm
`With variable pitch peaks and/or grooves to reduce the
`visibility of moiré interference patterns and optical displays
`incorporating one or more layers of the ?lm.
`In one embodiment, the present invention includes a
`structured optical ?lm having a structured surface that
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`includes a plurality of generally parallel peaks, each pair of
`adjacent peaks being separated by a valley; a ?rst group of
`adjacent peaks having a ?rst peak pitch; and a second group
`of adjacent peaks having a second peak pitch, the second
`group of adjacent peaks being located adjacent to the ?rst
`group of adjacent peaks, Wherein the ?rst peak pitch is
`different than the second peak pitch. The ?rst group prefer
`ably includes 20 or feWer adjacent peaks, more preferably 10
`or feWer and even more preferably 3 or feWer peaks.
`Alternatively, the ?rst group can be de?ned in terms of
`Width, With one preferred Width being about 0.5 millimeters
`or less, more preferably about 200 micrometers or less. It is
`also preferable that the valley pitch Within the ?rst group
`varies over any three adjacent valleys.
`In another embodiment, the present invention includes a
`structured optical ?lm having a structured surface that
`includes a plurality of generally parallel valleys, each pair of
`adjacent valleys being separated by a peak; a ?rst group of
`adjacent valleys having a ?rst valley pitch; and a second
`group of adjacent valleys having a second valley pitch, the
`second group of adjacent valleys being located adjacent to
`the ?rst group of adjacent valleys, Wherein the ?rst valley
`pitch is different than the second valley pitch. The ?rst group
`preferably includes 20 or feWer adjacent valleys, more
`preferably 10 or feWer, and even more preferably 3 or feWer
`valleys. Alternatively, the ?rst group can be de?ned in terms
`of Width, With one preferred Width being about 0.5 milli
`meters or less, more preferably about 200 micrometers or
`less. It is also preferable that the peak pitch Within the ?rst
`group varies over any three adjacent peaks.
`In yet another embodiment, the present invention includes
`a structured optical ?lm having a structured surface, Wherein
`the structured surface comprises a plurality of generally
`parallel valleys, each pair of adjacent valleys being sepa
`rated by a peak, Wherein the peak pitch is substantially
`constant, and further Wherein the valley pitch varies Within
`a group of three or more successive adjacent valleys.
`In still another embodiment, the present invention
`includes a structured optical ?lm having a structured
`surface, Wherein the structured surface comprises a plurality
`of generally parallel peaks, each pair of adjacent peaks being
`separated by a valley, Wherein the valley pitch is substan
`tially constant, and further Wherein the peak pitch varies
`Within a group of three or more successive adjacent peaks.
`The above and other features of the invention are more
`fully shoWn and described in the draWings and detailed
`description of this invention, Where like reference numerals
`are used to represent similar parts. It is to be understood,
`hoWever, that the description and draWings (Which are not to
`scale) are for the purposes of illustration only and should not
`be read in a manner that Would unduly limit the scope of this
`invention.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a perspective vieW of a prior art structured
`optical ?lm.
`FIG. 2 is a perspective vieW is an exploded perspective
`vieW of a pair of structured ?lms according to FIG. 1 in
`Which the prisms are crossed at an angle of about 90°.
`FIG. 3A is a schematic diagram of one section of a
`structured ?lm according to the present invention With a
`constant peak pitch and a varying valley pitch.
`FIG. 3B is a schematic diagram of one section of a
`structured ?lm according to the present invention With a
`constant valley pitch and a varying peak pitch.
`FIG. 4A is a schematic diagram of one section of an
`alternative structured ?lm according to the present invention
`With a varying peak pitch.
`
`LGD_001087
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`5,919,551
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`3
`FIG. 4B is a schematic diagram of one section of an
`alternative structured ?lm according to the present invention
`With a varying valley pitch.
`FIG. 5A is a schematic diagram of one section of an
`alternative structured ?lm according to the present invention
`With a varying peak pitch and a varying valley pitch.
`FIG. 5B is a schematic diagram of one section of an
`alternative structured ?lm according to the present invention
`With a varying peak pitch and a varying valley pitch.
`FIG. 6 is a schematic diagram of an optical display
`assembly incorporating at least one layer of structured
`optical ?lm according to the present invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`The present invention, described in connection With the
`illustrative embodiments depicted in FIG. 3A and the fol
`loWing ?gures, provides a structured optical ?lm in Which
`the peak pitch and/or valley pitch vary to reduce the vis
`ibility of the moire interference patterns When using one or
`more layers of the structured ?lms in, for example, an optical
`display.
`FIGS. 1 and 2 generally illustrate the concept of struc
`tured optical ?lms. FIG. 1 depicts a section of a regular,
`periodic structured optical ?lm 10 including a structured
`surface 12 and a plano surface 14. The structured surface
`includes a series of regularly spaced valleys 16 and peaks 18
`that de?ne prisms 20. The prisms 20 are de?ned by facets
`formed betWeen the valleys 16 and peaks 18. The geometry
`of the structured surface 12 and the material used to manu
`facture the ?lm 10 foster total internal re?ection and refrac
`tion of light entering the plano side 14 of ?lm 10 to minimiZe
`the escape of light through the structured surface outside of
`the desired range of angles.
`FIG. 2 illustrates a pair of structured optical ?lms 22 and
`24 in Which the prisms 26 and 28, respectively, are oriented
`at approximately 90° angle With respect to each other. In use,
`it is preferred that the structured surface 28 be in contact
`With, or nearly in contact With, the plano surface 27 of the
`upper ?lm 22.
`Although the prisms/facets generally depicted in connec
`tion With the present invention are shoWn as having a
`dihedral angle of about 90° betWeen generally planar facets,
`it Will be understood that the present invention includes
`structured optical ?lms having prisms/facets formed in any
`optically useful shape, including lenticular arrays, prisms
`With rounded peaks and/or valleys, curved facets, etc. In
`other Words, the present invention is useful With any struc
`tured optical ?lm displaying a periodic pattern that could
`result in visible moiré interference patterns in the absence of
`variations in pitch as described herein. Furthermore,
`although the embodiments discussed beloW include a plano
`surface, it Will be understood that the opposing surface of the
`structured optical ?lms manufactured according to the
`present invention, i.e., the surface opposite the structured
`surface, could be substantially planar or it could be provided
`With a structure, texture, as a smooth surface, or With any
`other ?nish as desired.
`It should also be understood that the embodiments
`depicted in FIGS. 3A and folloWing are generally planar
`cross-sections of structured optical ?lms constructed accord
`ing to the present invention taken generally perpendicular to
`the length of the grooves/valleys. Given the variable nature
`of the optical ?lms according to the present invention (to
`reduce the visibility of moiré interference patterns), it Will be
`understood that the cross-sections of a given ?lm may or
`
`4
`may not remain constant along the length of the grooves/
`valleys. This may be particularly true in the structured
`optical ?lms manufactured according to the present inven
`tion using tooling constructed by thread cutting a cylindrical
`roll.
`FIG. 3A schematically depicts a cross-section, normal to
`the plano surface 32, of one structured optical ?lm 30
`according to the present invention. The ?lm 30 includes a set
`of prisms de?ned by peaks 36 and valleys 38. The peaks 36
`and valleys 38 de?ning the prisms are preferably substan
`tially parallel to each other although slight variations Would
`be acceptable. The spacing betWeen adjacent peaks 36, i.e.,
`the peak pitch, of the optical ?lm 30 is substantially con
`stant. The spacing betWeen adjacent valleys 38, hoWever,
`varies over any group of three successive valleys 38. That
`spacing betWeen valleys 38 can also be referred to as valley
`pitch, Pv. By varying the valley pitch, the visibility of moiré
`interference patterns can be reduced When using ?lm 30 in
`an optical display.
`The peak pitch in a ?lm 30 manufactured according to the
`present invention Will preferably be about 1 millimeter or
`less, more preferably the peak pitch Will be about 100
`micrometers or less When the structured ?lm 30 is used in
`optical displays incorporating liquid crystal display panels
`and similar devices. More preferably, the peak pitch for
`those applications Will lie Within about 20 to about 60
`micrometers.
`An optical ?lm 30 can be produced using a tool manu
`factured by any knoWn method. If the tool used to produce
`the ?lm 30 is a roll, it can be manufactured by thread cutting
`at a constant thread pitch, plunge cutting using a constant
`spacing betWeen grooves, or any other useful method. It is
`preferred to form each groove in the tool to a constant, but
`differing, depth When forming the tool used to manufacture
`the ?lm 30.
`If the tool used to form the ?lm 30 is a cylindrical roll
`formed using thread cutting, it is preferred to constantly vary
`the depth of the groove formed in the roll by a cutting tool.
`That variation could include varying the depth of the groove
`at a constant or changing rate betWeen a minimum and
`maximum, although it may also be helpful to have interim
`targeted depths betWeen the minimum and maximums that
`are interspersed about the circumference of the roll to avoid
`adding periodicity into the grooves and, thus, the ?lm.
`When thread cutting, it may also be desirable to vary the
`number of revolutions, or “Wraps,” over Which the cutting
`tool is moved betWeen different targeted groove depths and,
`also, to use a number of roll revolutions betWeen targeted
`depths that is not an integer. Even more preferably, it is
`desirable to use a number of revolutions including a frac
`tional portion that is not easily multiplied to equal an integer.
`Examples of useful numbers of revolutions over Which
`groove depth Would be varied include, for example, 0.85,
`1.15, 1.3, or 2.15. The targeted depth of the groove Would
`then vary betWeen the starting and ending point of each
`desired number of revolutions of the roll.
`After the tool is manufactured, the ?lm 30 can be manu
`factured using the tool according to any suitable method.
`Examples of methods and materials for forming structured
`optical ?lms are discussed in US. Pat. Nos. 5,175,030 (Lu
`et al.) and 5,183,597 (Lu). It Will be understood that the
`chosen manufacturing process is at least someWhat depen
`dent on the material used for the ?lms.
`In the ?lm 30 depicted in FIG. 3A, peak pitch is held
`constant While the valley pitch varies. The tooling used to
`manufacture the ?lm 30 can, hoWever, be replicated by
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`electroforming or other suitable processes, thus forming a
`“negative” of the pattern formed on the master tool. When
`that replicated tool is then used to form a ?lm, the result,
`depicted in FIG. 3B, is a ?lm 130 that is a “negative” of the
`?lm 30 depicted in FIG. 3A. As a result, ?lm 130 has a
`constant valley pitch, Pv, betWeen valleys 136 While the
`peak pitch, Pp, betWeen peaks 138 varies across the ?lm
`130. That is exactly the opposite of the pitch characteristics
`of the ?lm 30. Like ?lm 30, the ?lm 130 in FIG. 3B is also
`useful for reducing the visibility of moiré interference
`patterns When used in an optical display.
`The valley pitch in a ?lm 130 manufactured according to
`the present invention Will preferably be about 1 millimeter
`or less, more preferably the valley pitch Will be about 100
`micrometers or less When the structured ?lm 30 is used in
`optical displays incorporating liquid crystal display panels
`and similar devices. More preferably, the valley pitch for
`those applications Will lie Within about 20 to about 60
`micrometers.
`FIG. 4A is a schematic diagram of an alternative struc
`tured optical ?lm 40 that includes a plano surface 42 and
`structured surface 44. Structured surface 44 includes a
`plurality of generally parallel prisms de?ned by peaks 46
`and valleys 48. The peaks 46 are all preferably formed With
`substantially the same height, Hp, above the plano side 42 of
`the ?lm.
`In the ?lm 40, the peak pitch, Ppl, remains constant over
`a ?rst group 50 of peaks 46. A second group 52 of peaks 46
`is located immediately adjacent to the ?rst group 50. The
`second group 52 of peaks 46 has a constant peak pitch, Pp2,
`that is different from the peak pitch of the ?rst group 50. It
`is the variation in peak pitch that contributes to reducing the
`visibility of moiré interference patterns When using ?lm 40.
`The peak pitch in a ?lm 40 manufactured according to the
`present invention Will preferably be about 1 millimeter or
`less, more preferably the peak pitch Will be about 100
`micrometers or less When the structured ?lm 140 is used in
`optical displays incorporating liquid crystal display panels
`and similar devices. More preferably, the peak pitch for
`those applications Will lie Within about 20 to about 60
`micrometers. Typical peak pitches used in connection With
`the present invention include groups of peaks spaced at 50,
`40, 30 and 20 micrometers. It may be helpful to provide
`maximum peak pitch to minimum peak pitch ratios of about
`1.25 or greater, more preferably about 1.5 or greater and
`even more preferably about 2.0 or greater to reduce the
`visibility of moiré interference patterns.
`The number of peaks 46 in each of the groups can be
`varied to improve moiré interference reduction. The ?lm 40
`includes groups 50 and 52 in Which groups of three adjacent
`peaks 46 have a constant peak pitch. In some structured
`?lms according to the present invention, it may be helpful
`Where at least one of the groups has about 20 or feWer peaks;
`preferably about 10 or feWer; more preferably about 5 or
`feWer; and even more preferably about 3 or feWer peaks. In
`some structured ?lms, it may also be helpful to include only
`tWo adjacent peaks 46 in a group, i.e., provide a pattern in
`Which the peak pitch varies betWeen successive pairs of
`peaks.
`Although the ?lm 40 includes only tWo groups 50 and 52,
`it Will be understood that the present invention includes
`?lms having at least tWo or more groups of peaks, i.e., the
`?lm 40 could include any number of groups, not just tWo
`groups. Also, although the ?lm 40 is shoWn as having tWo
`groups With equal numbers of peaks 46, it Will be understood
`that each group may include the same or different number of
`peaks 46.
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`An alternative measure of the group siZe can be based on
`the Width of the groups as measured generally perpendicular
`to the peaks and valleys. Preferably, the Width of each group
`is about 1 millimeter or less, more preferably about 0.5
`millimeters or less, more preferably about 200 micrometers,
`more preferably about 100 micrometers or less, and even
`more preferably about 50 micrometers or less. It Will be
`understood that, in part, the desired group Widths are based
`on the pitch of the peaks and valleys in the ?lm 40.
`An optical ?lm 40 can be produced using a tool manu
`factured by any suitable method. It Will be understood that
`the height of the peaks, Hp, in the ?nished ?lm 40 is a
`function of the depth of the grooves cut into the tool. If the
`tool used to produce the ?lm 40 is a cylindrical roll, it can
`be manufactured by thread cutting the roll to a constant
`depth at a constant thread pitch over the grooves used to
`form each group of peaks having a constant peak pitch. If
`thread cutting is used to form a roll, it is desirable to hold the
`thread pitch constant for a number of roll revolutions that is
`not an integer. Even more preferably, it is desirable to hold
`the thread pitch constant for a fractional number that is not
`easily multiplied to equal an integer. Example of useful
`numbers of revolutions over Which thread pitch could be
`held constant include, for example, 0.85, 1.15, 1.3, or 2.15.
`It Will be understood that the integer portion of the number
`of revolutions over Which thread pitch is held constant
`determines the number of peaks in each of the groups.
`After the tool is manufactured, the ?lm 40 can be manu
`factured using the tool according to any suitable method.
`Examples of methods and materials for forming structured
`optical ?lms are discussed in US. Pat. Nos. 5,175,030 (Lu
`et al.) and 5,183,597 (Lu). It Will be understood that the
`chosen manufacturing process is at least someWhat depen
`dent on the material used for the ?lms.
`The tooling used to manufacture the ?lm 40 can be
`replicated by electroforming or other suitable processes,
`thus forming a “negative” of the pattern formed on the
`master tool. When that replicated tool is then used to form
`a ?lm, the result, depicted in FIG. 4B, is a ?lm 140 that is
`a “negative” of the ?lm 40 depicted in FIG. 4A. As a result,
`the valley pitch, Pv, in ?lm 140 remains constant over a ?rst
`group 150 of valleys 146. Asecond group 152 of valleys 146
`is located immediately adjacent to the ?rst group 150. The
`second group 152 of valleys 146 has a constant valley pitch,
`Pv2, that is different from the valley pitch of the ?rst group
`150. It is the variation in valley pitch that contributes to
`reducing the visibility of moiré interference patterns When
`using ?lm 140.
`The valley pitch in a ?lm 140 manufactured according to
`the present invention Will preferably be about 1 millimeter
`or less, more preferably the valley pitch Will be about 100
`micrometers or less When the structured ?lm 140 is used in
`optical displays incorporating liquid crystal display panels
`and similar devices. More preferably, the valley pitch for
`those applications Will lie Within about 20 to about 60
`micrometers. Typical valley pitches used in connection With
`the present invention include groups of peaks spaced at 50,
`40, 30 and 20 micrometers. It may be helpful to provide
`maximum valley pitch to minimum valley pitch ratios of
`about 1.25 or greater, more preferably about 1.5 or greater
`and even more preferably about 2.0 or greater to reduce the
`visibility of moiré interference patterns.
`The number of valleys 146 in each of the groups can be
`varied to reduce the visibility of moiré interference patterns.
`The ?lm 140 includes groups 150 and 152 in Which groups
`of three adjacent valleys 146 have a constant valley pitch. In
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`5,919,551
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`7
`some structured ?lms according to the present invention, it
`may be helpful Where at least one of the groups has about 20
`or feWer valleys; preferably about 10 or feWer; more pref
`erably about 5 or feWer; and even more preferably about 3
`or feWer valleys. In some structured ?lms, it may also be
`helpful to include only tWo adjacent valleys 146 in a group,
`i.e., provide a pattern in Which the valley pitch varies
`betWeen successive pairs of valleys.
`Although the ?lm 140 includes only tWo groups 150 and
`152, it Will be understood that the present invention includes
`?lms having at least tWo or more groups of evenly-spaced
`valleys, i.e., the ?lm 140 could include any number of
`groups, not just tWo groups. Also, although the ?lm 140 is
`shoWn as having tWo groups With equal numbers of evenly
`spaced valleys 146, it Will be understood that each group
`may include the same or different number of valleys 146.
`An alternative measure of the group siZe can be based on
`the Width of the groups as measured generally perpendicular
`to the peaks and valleys. Preferably, the Width of each group
`is about 1 millimeter or less, more preferably about 0.5
`millimeters or less, more preferably about 200 micrometers,
`more preferably about 100 micrometers or less, and even
`more preferably about 50 micrometers or less. It Will be
`understood that, in part, the desired group Widths are based
`on the pitch of the peaks and valleys in the ?lm 140.
`After the replicated tool is manufactured, the ?lm 140 can
`be manufactured according to any suitable method.
`Examples of methods and materials for forming structured
`optical ?lms are discussed in US. Pat. Nos. 5,175,030 (Lu
`et al.) and 5,183,597 (Lu). It Will be understood that the
`chosen manufacturing process is at least someWhat depen
`dent on the material used for the ?lms.
`Turning noW to FIG. 5A, a cross-section of another
`alternative structured optical ?lm 60 according to the present
`invention is shoWn in a schematic diagram as including a
`plano surface 62 and a structured surface 64. Structured
`surface 64 includes a plurality of generally parallel prisms
`de?ned by peaks 66 and valleys 68.
`Structured ?lm 60 includes groups of peaks 66 With
`constant peak pitch. The peak pitch, Ppl, remains constant
`over a ?rst group 70 of peaks 66. Asecond group 72 of peaks
`66 is located immediately adjacent to the ?rst group 70. The
`second group 72 of peaks 66 has a constant peak pitch, PP2,
`that is different from the peak pitch of the ?rst group 70.
`The number of peaks 66 in each of the groups can be
`varied to reduce the visibility of moiré interference patterns
`caused by the ?lm 60. The ?lm 60 includes groups 70 and
`72 in Which groups of three adjacent peaks 66 have a
`constant peak pitch. In some structured ?lms according to
`the present invention, it may be helpful Where at least one of
`the groups has about 20 or feWer peaks; preferably about 10
`or feWer; more preferably about 5 or feWer; and even more
`preferably about 3 or feWer peaks. In some structured ?lms,
`it may also be helpful to include only tWo adjacent peaks 66
`in a group, i.e., provide a pattern in Which the peak pitch
`varies betWeen successive pairs of peaks.
`An alternative measure of the group siZe in ?lm 60 can be
`based on the Width of the groups as measured generally
`perpendicular to the peaks and valleys. Preferably, the Width
`of each group for many applications is about 1 millimeter or
`less, more preferably about 0.5 millimeters or less, more
`preferably about 200 micrometers, more preferably about
`100 micrometers or less, and even more preferably about 50
`micrometers or less. It Will be understood that, in part, the
`desired group Widths are based on the pitch of the peaks and
`valleys in the structured ?lm 60.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`Although the ?lm 60 is depicted With only tWo groups 70
`and 72, it Will be understood that the present invention
`includes ?lms having at least tWo or more groups of peaks,
`i.e., the ?lm could include any number of groups, not just
`tWo groups. Also, although the ?lm 70 is shoWn as having
`tWo groups With equal numbers of peaks 66, it Will be
`understood that each group may include the same or differ
`ent number of peaks 66.
`An optical ?lm 60 can be produced using a tool manu
`factured by any knoWn method. It Will be understood that the
`height of the peaks in the ?nished ?lm is a function of the
`depth of the grooves cut into the tool. If the tool used to
`produce the ?lm 60 is a cylindrical roll, it can be manufac
`tured by thread cutting the roll at a constant thread pitch over
`the grooves used to form each group of peaks having a
`constant peak pitch, i.e., the number of revolutions at any
`given thread pitch Will de?ne the number of grooves formed
`at that thread pitch (Which also corresponds to the number of
`peaks With the given peak pitch).
`If thread cutting is used to form a roll, it is desirable to
`hold the thread pitch constant for a number of roll revolu
`tions that is not an integer. Even more preferably, it is
`desirable to hold the thread pitch constant for a fractional
`number that is not easily multiplied to equal an integer.
`Examples of useful numbers of revolutions over Which
`thread pitch could be held constant include, for example,
`0.85, 1.15, 1.3, or 2.15. It Will be understood that the integer
`portion of the number of revolutions over Which thread pitch
`is held constant determines the number of peaks in each of
`the groups.
`While thread pitch (and, therefore, peak pitch) are varied
`as discussed above, the depth of the grooves on the tool used
`to manufacture the ?lm 60 can also be varied to change the
`valley pitch as Well. If the tool is formed by thread cutting,
`it is preferred to constantly vary the depth of the groove
`formed in the cylindrical roll. That variation could include
`varying the depth at a constant or changing rate betWeen a
`minimum and maximum, although it may also be helpful to
`have interim targeted depths betWeen the minimum and
`maximums that are interspersed about the circumference of
`the roll to avoid adding periodicity to the grooves and, thus,
`the ?lm 60 formed using the tool.
`It may also be desirable to vary the number of revolutions
`it takes to move betWeen different targeted groove depths
`and, also, to use a number of roll revolutions betWeen
`targeted depths that is not an integer. Even more preferably,
`it is desirable to use a number of revolutions including a
`fractional portion that is not easily multiplied to equal an
`integer. Examples of useful numbers of revolutions over
`Which groove depth Would be varied include, for example,
`0.85, 1.15, 1.3, or 2.15. The targeted depth of the groove
`Would then vary betWeen the starting and ending point of
`each desired number of revolutions of the roll.
`The changes betWeen targeted cutting tool depths, i.e.,
`groove depths, may correspond to the changes in thread
`pitch about the roll, or alternatively, the changes in targ