Patent Publication Number: US-2002012248-A1

Title: Optical film with defect-reducing surface and method for making same

Description:
BACKGROUND  
       [0001] The present invention is directed generally to a light transmissive film, and particularly to a film that reduces the incidence of defects in a display.  
       [0002] The use of films for displays is well known. For example, in backlit displays, brightness enhancement films use a prismatic structure to direct light along the viewing axis, thus enhancing the brightness of the light perceived by the viewer. As another example, a backlit computer display screen may use a number of different films in order to produce a screen with high contrast and high overall brightness, while simultaneously maintaining high, uniform brightness in certain selected directions and lower brightness in other directions. Such screens may use several types of films, such as diffusing films, in combination with a prismatic film or a lenticular film.  
       [0003] One problem with using films in a display is that the cosmetic requirements for a display intended for close viewing, such as a computer display, are very high. This is because such displays are viewed closely for long periods of time, and so even very small defects may be detected by the naked eye, and cause distraction to the viewer. The elimination of such defects can be costly in both inspection time and in materials.  
       [0004] Defects are manifested in several different ways. There are physical defects such as specks, lint, scratches, inclusions etc., and also defects that are optical phenomena. Among the most common optical phenomena are “wet-out” and Newton&#39;s rings. “Wet-out” occurs when two surfaces optically contact each other, thus effectively removing the change in refractive index for light propagating from one film to the next. This is particularly problematic for films that use a structured surface for their optical effect, since the refractive properties of the structured surface are nullified. The effect of “wet-out” is to create a mottled and varying appearance to the screen. Newton&#39;s rings are the result of a slowly varying air gap between two films, as may be created by a particle of dust between two films. Newton&#39;s rings may be formed in transmission or in reflection. The result of Newton&#39;s rings is that the viewer perceives a contour pattern on the screen that may be distracting.  
       [0005] Several approaches have been followed to overcome the problem of defects in multiple-film display assemblies. One is simply to accept a low yield of acceptable display assemblies produced by the conventional manufacturing process. This is obviously unacceptable in a competitive market. A second approach is to adopt very clean and careful manufacturing procedures, and impose rigid quality control standards. While this may improve the yield, the cost of production is increased to cover the cost of clean facilities and inspection.  
       [0006] Another approach to reducing defects is to introduce a diffuser to the display, either a surface diffuser or a bulk diffuser. Such diffusers may mask many defects, and increase the manufacturing yield at low additional cost. However, the diffuser scatters light and decreases the on-axis brightness of light perceived by the viewer, thus reducing the performance.  
       [0007] There continues to be a need to reduce the occurrence of defects in displays, so that the manufacturing yield may be improved with little additional cost while, at the same time, maintaining performance.  
       SUMMARY OF THE INVENTION  
       [0008] Generally, the present invention relates to a surface on a film that may be used to reduce the occurrence of optical defects in a display that uses the film. In particular, the surface has randomized characteristics that reduce such defects as wet-out, Newton&#39;s rings, and Moiré effects. The invention also relates to a method for making the film, a tool used for making the film, and a method of making the tool.  
       [0009] In one embodiment the film has a first surface free of regular structure, the first surface having a plurality of local height maxima, a characteristic measure of the first surface having a random value within a predetermined range. The film also has a second surface opposing the first surface. A method of making an optical film includes embossing a pattern lacking regular structure onto a first surface of a film, wherein the first surface has a plurality of local height maxima, a characteristic measure of the first surface having a random value within a predetermined range. The characteristic measure may be a difference between the actual heights and a nominal height of the film surface, average separation between the local height maxima on the surface, or slope angle of the first surface proximate a local height maximum.  
       [0010] In another embodiment, the invention includes a film having an anti-wet-out surface on a first surface having no regularly refractive structure, and a second surface opposing the first surface. In another embodiment, the film includes a first surface, and wet-out reducing means disposed on the first surface for reducing wet-out between the first surface and another optical surface.  
       [0011] In another embodiment of the invention, an optical device includes a light source, and a film having an anti-wet-out surface on a first surface having no regularly refractive structure. A second optical component has a second surface opposing the first surface, and light from the light source passes through the film and the second optical component.  
       [0012] In another embodiment of the invention, a method of making an optical film includes forming an anti-wet-out surface on a first surface of the optical film.  
       [0013] In another embodiment of the invention, a method of making a drum for embossing a surface on a film includes rotating a drum around a rotation axis relative to a cutting tool, cutting the surface of the drum with the cutting tool, and randomly varying a cutting characteristic of the cutting tool to produce characteristic variations lying randomly within a predetermined range. A drum for embossing a surface on a film includes a surface free of regular structure and having a plurality of local height minima, a characteristic measure of the surface having a random value within a predetermined range.  
       [0014] The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:  
     [0016]FIGS. 1A and 1B illustrate the problem of “wet-out” occurring between adjacent films;  
     [0017]FIGS. 2A and 2B illustrate the problem of Newton&#39;s rings formed between adjacent films;  
     [0018]FIGS. 3A and 3B illustrate a film structure according to an embodiment of the present invention;  
     [0019] FIGS.  4 A- 4  illustrate surface-structured films with wet-out reducing surfaces according to the present invention;  
     [0020]FIG. 4D illustrates a flat film having a wet-out-reducing surface according to the present invention;  
     [0021]FIGS. 5A and 5B illustrate embodiments of illumination devices incorporating a film having a wet-out-reducing surface according to the present invention;  
     [0022]FIG. 5C illustrates light passing through the light redirecting film of the illumination device shown in FIG. 5B;  
     [0023]FIG. 6 schematically illustrates one process for fabricating films according to the present invention;  
     [0024]FIG. 7 schematically illustrates a first method of fabricating a drum for manufacturing films according to the present invention;  
     [0025]FIG. 8 illustrates some random control signals for controlling a cutting tool according to the present invention;  
     [0026]FIG. 9 schematically illustrates a second method of fabricating a drum for manufacturing films according to the present invention;  
     [0027]FIG. 10 illustrates a tool for use in the method of fabricating the drum illustrated in FIGS. 7 and 9; and  
     [0028]FIGS. 11A and 11B illustrate an example of a film having a surface formed according to the present invention.  
     [0029] While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
    
    
     DETAILED DESCRIPTION  
     [0030] The present invention is applicable to light management films, and is believed to be particularly suited to light management films for use with liquid crystal displays used as computer displays or monitors. The invention is also useful in other areas where multiple light management films are used, such as in rear projection screens and in overhead projection displays. One of the advantages of the invention is that the intensity of defects on the viewing area is reduced, thereby increasing the manufacturing yield.  
     [0031] The invention is described below, for the purposes of illustration, in the particular application of multiple-film liquid crystal computer displays. It will be appreciated that the use of the invention is not so limited and that there is a wide range of applications having multiple light management films where the invention may be useful.  
     [0032] Wet-out and Newton&#39;s rings are optical phenomena that are sources of defects for multi-layer displays. FIGS. 1A and 1B illustrate the problem of wet-out in a multilayer film  100 . The multi-layer film  100  is shown to have at least two layers  102  and  104 . The upper surface  103  of the second film  104  is optically contacted at optical contacting portions  106  to the upper film  102 . When optical contacting occurs, any light that passes through an optically contacted region  106  passes from one film into the next with a reduced refractive effect. Where the refractive indices of the upper and lower films  102  and  104  are identical, there is no refractive effect. In contrast, where light passes from one film to the other where there is no optical contacting, for example as shown for ray  110 , the light is refracted at each film-air interface. The result is that the viewer detects the wet-out regions  106  as being regions where the transmissive properties are different from the surrounding area and are therefore viewed as an abnormality or a defect.  
     [0033] Another source of defects is the formation of Newton&#39;s rings, which may be viewed either in transmission or reflection. Newton&#39;s rings may be formed in a multi-layer display  200 , having at least two films  202  and  204 . A particle of dust  206  may be trapped between the two films  202  and  204 , resulting in an air gap  208  between respective lower and upper surfaces  203  and  205  of the upper and lower films  202  and  204 . The separation between the two surfaces  203  and  205  varies with distance from the dust particle  206 . Interference rings are formed by light passing through the display  200  where the air gap  208  is a multiple of half wave lengths of light, as is commonly understood for the formation of Newton&#39;s rings. The effect is particularly pronounced where the separation between the two surfaces  203  and  205  is less than approximately 1.5 μm, since white light fringes may be formed. If the separation between the surfaces  203  and  205  is more than approximately 1.5 μm, the effect is less pronounced, since the fringes are color-specific, and not white-light fringes, and are not perceived by the viewer as readily as white light fringes.  
     [0034] It has been observed that several defects, including wet-out and Newton&#39;s rings, occur when the adjacent surfaces of different films are smooth and flat. An approach to reducing such defects is to make the height of at least one of the film surfaces vary in a random fashion. This is illustrated, for example, in FIG. 3A where a multi-film display  300  has two films  302  and  304 . The upper film  302  has upper and lower surfaces  306  and  308 .  
     [0035] The lower film  304  has an upper surface  310  which has a randomly varying height across the plane of the film  304 . The surface  310  has certain local maxima  312  that are sufficiently high as to contact the upper film  302 . There may also be other local maxima  314  that are not sufficiently high to contact the film  302 .  
     [0036] The random nature of the height prevents the formation of wet-out regions like those illustrated in FIG. 1. The lower film  304  contacts the upper film  302  only at a number of very small points invisible to the viewer, rather than over a large area as happens with wet-out. In addition, the appearance of Newton&#39;s rings may be reduced by the surface with random height variations where the local maxima are spaced sufficiently close to one another that a Newton&#39;s ring pattern is too small to be perceived by the viewer. For example, consider a surface where adjacent local maxima are separated by an average distance of approximately 200 microns. A change in the separation distance between two film surfaces of around 1.5 microns is a distance of approximately three wavelengths, which is the equivalent of six fringes. Therefore, there are six fringes in, on average, a distance of 100 microns (half the inter-maximum separation) from the local maximum. The resultant feature size is around 16 μm, which is too small to be perceived by the viewer.  
     [0037] The random variation in the height of the film surface produces a number of other unexpected and favorable results, in addition to the reduction of wet-out and Newton&#39;s rings. First, films fabricated according to the present invention do not have the appearance of high performance, transparent optical films. Instead, they have deceptive, almost hazy, appearance that may mask some of the small defects in any structure on the film. This can considerably improve the yield of the manufacturing process. Second, films of the invention may also help to eliminate or hide Moiré patterns that result from interference between structures on different films in the display. Another important result is that the optical effects of different films in a multi-layer display, such as brightness enhancing, diffusion, or collimation, may be essentially unaffected where the surface of one or more of the films in the film stack has randomized heights.  
     [0038] For the purposes of this description, a surface having portions of randomized height is referred to as an “anti-wet-out surface”, although it is understood that such a surface does more than reduce wet-out, for example reducing Newton&#39;s rings. The anti-wet-out surface is substantially flat on a large scale typically perceivable by a viewer. On a smaller scale, typically not perceivable by a viewer, there are many peaks, or local surface height maxima, on the surface. The average difference in height between these local surface height maxima and intervening local surface height minima is typically small, of the order of one or two microns. Consequently, if the anti-wet-out surface is placed against another smooth surface, the vast majority of the area on the anti-wet-out surface is held out of contact with the second surface, at a distance of a few microns or less. The peaks contact the second surface, and the area of each point of contact is sufficiently small to avoid any perceptible wet-out at any one peak. The peaks may be irregularly spaced, with an average separation, for example, in the range 50 μm to 500 μm, and preferably in the range 100 μm to 250 μm. The peaks may have different heights, within a specific range, with an average difference in height between the local maxima and the local minima less than about 5 μm, and preferably less than about 2 μm. The average separation in height between the local minima and maxima may be approximately 1.5 μm.  
     [0039] There may be, therefore, an irregularity, or degree of randomness, in the separation between the peaks. There may also be another degree of randomness in the height of the peaks. There is not true randomness in the peak separation or peak height, however. The values of peak height and/or the peak separation may each take on a value, randomly, or pseudo-randomly, that lies within a preselected range. By placing upper and lower bounds on the peak separation and/or the peak height, certain types of defects arising from extreme, but statistically possible, peak separations or peak heights may be reduced.  
     [0040] Another characteristic of the anti-wet-out surface that may be permitted to vary in a random fashion, within limits, is the slope angle of the surface leading up to a local maximum. Where the slope angle is high, light passing through that portion of the surface having the high slope angle is refracted at a larger angle to the film normal than light passing through a portion of the surface whose slope angle is small. This may affect the dispersion qualities of the film, so that a film having a surface with a higher slope angle disperses light through a greater angle. Furthermore, the slope of a peak on an anti-wet-out surface need not be symmetrical around that peak, but may, for example, have a large slope, and therefore large dispersion, relative to a first axis, and a small slope, and therefore little dispersion, relative to a second axis orthogonal to the first axis. The slope of the surface in a given direction on the film surface may be random within prescribed limits.  
     [0041] Films fabricated according to the present invention may be made of any substantially transparent material. Bulk diffusing material may be incorporated in a film according to the invention, although in many cases this may degrade the performance of the optical film. In addition, multiple layers of film and material may be included in a single film in order to produce a specific optical effect, such as reflection polarization. Unitary, extruded films of acrylics and polycarbonates are good candidates for film materials. Also, the film may be a two-part construction where the structured surface is cast and cured on a substrate having the smooth, randomized height surface according to the present invention. For example, ultraviolet-cured acrylics cast on polyester substrates may be used. Films of polyethylene terphthalate (“PET”) have been shown to work well as substrates on which structures may be cured. By axially oriented PET is often preferred for its mechanical and optical properties. A polyester film that may be used as a substrate is commercially available from duPont ICI America&#39;s Inc., Hopewell, Va. under the trade name MELINEX™617. Polyethylene naphthalate (PEN) has also been shown to work well as a polymeric material for manufacturing optical films, both alone and as a co-polymer with PET (known as CoPEN).  
     [0042] An anti-wet-out surface may be provided on many different kinds of optical films. A prismatic brightness enhancement film  400 , for example such as is described in U.S. Pat. No. 5,056,892, may be provided with a lower surface  402  that is an anti-wet-out surface, as illustrated in FIG. 4A. It should be noted that the drawing, like the following drawings in FIGS.  4 B- 4 D, is not drawn to scale.  
     [0043] The brightness enhancing film  400  has a series of prismatic structures  404  disposed parallel on one surface, which is referred to as a structured surface. The prismatic structures  404  enhance the brightness of the image viewed through the display by the viewer. Light, for example rays  410  or  412 , pass into the brightness enhancing film  400  through the lower surface  402 . The light rays are directed towards the normal to the film surface on exiting from the film  400 , and so light that would otherwise have been transmitted at a larger angle to the film normal, is directed towards the film normal and hence the display appears brighter to a viewer. Some light, for example, ray  414 , is totally internally reflected by a prismatic structure  404  and it is returned to the light source. If the light source is contained within a suitably reflecting enclosure, the reflected light ray  414  is recycled for propagation through the brightness enhancing film  400 .  
     [0044] The film  400  typically has a thickness, t, that lies in the range of 100 μm to 250 μm. The variation in height, δ, between the local maxima and local minima on the anti-wet-out surface  402  is typically in the range 0 μm to 5 μm, and is preferably in the range 0 μm to 2 μm. Most preferably, the value of δ is approximately 1.5 μm. At this value, color separation reduces the visibility of the Newton&#39;s&#39; rings. The heights of the peaks, the local maxima, may also be described in terms of difference between an actual height and a nominal height. For example, the peaks may have actual heights that range by up to 5 μm in difference from the nominal height. Preferably, the difference between actual heights and nominal heights is less than 1.5 μm. The average separation between local maxima is typically in the range from 20 μm to 400 μm, and is preferably in the range from 100 μm to  250 μm.    
     [0045] As indicated above, the size of the Newton&#39;s ring pattern is dependent on the average separation between local maxima. The larger the separation becomes, the greater the probability that the Newton&#39;s ring pattern will become visible. Therefore, it is advantageous to reduce the average inter-maximum separation. On the other hand, where the average height of the local maxima is constant, a reduction in the average inter-maximum separation results in a larger surface slope angle. A larger surface slope angle produces greater diffusion of the light passing through the film surface. While a degree of surface diffusion may be acceptable in some applications, there are other applications where surface diffusion should be minimized. Accordingly, a value of δ of approximately 1.5 μm, and an average separation between local maxima in the range 150 μm and 250 μm may be used to reduce wet-out and Newton&#39;s rings problems, while also reducing surface diffusion. These values are provided only as suggested operating points and are not intended to limit the invention.  
     [0046] Another advantage of the invention is the ability to preferentially orient the anti-wet-out surface relative to a structure on the other side of the film. For example, the anti-wet-out surface  402  may be selected to have a surface slope that is small in a direction perpendicular to the prisms of the prismatic structure  404  while, at the same time, having a larger slope on the surface  402  in a direction parallel to the prisms. Such an arrangement may advantageously diffuse light in a direction parallel to the prismatic structures, without increasing the diffusion of light in a direction across the prismatic structures.  
     [0047] Another example of a structured film that may use an anti-wet-out surface is illustrated in FIG. 4B. The film  420  has a lenticular pattern  423  on an upper surface  424 . The lenticular pattern  423  may be used, for example, for diffusing light in one dimension, in a direction at right angles to the grooves of the lenticular pattern  423 . For example, light rays  426  and  428  are refracted in a direction within the plane of the figure upon exiting the lenticular surface  424 . Therefore, the light that passes through this film  420  is distributed in the x-direction. The lower surface  422  is an anti-wet-out surface provided to reduce optical defects. The lenticular film  420  may have an average thickness, t, that typically lies in the range from 100 μm to 500 μm.  
     [0048] Another type of structured film that may have an anti-wet-out surface is illustrated in FIG. 4C. The film  440  is a Fresnel lens, having a Fresnel structure  444  on the upper surface  445 . The lower surface  442  is an anti-wet-out surface.  
     [0049] It will be appreciated that an anti-wet-out surface may be used in other structured films than those illustrated in FIGS. 4A to  4 C. For example, other structures may be present on the structured surface of the film, such as lenses, or other light refracting or diffracting structures.  
     [0050] In addition, an anti-wet-out surface may be used on a film that has essentially no surface structure, such as a polarizer, bulk diffuser, scattering film, retardation film or the like. The optical effect of such films is typically based on an optical interaction that takes place within the bulk of the film, rather than relying on a refractive effect at a structured surface. A polarizing film  460  is illustrated in FIG. 4D, where the lower surface  464  is an anti-wet-out surface. It will be appreciated that a film having no refracting structure may have an anti-wet-out surface on each surface  462  and  464 .  
     [0051] The film  460  is illustrated having an imaginary plane  466  passing through the center of the film  460 , substantially parallel to the surfaces  462  and  464 . Each surface  462  and  464  is shown with an overall variation in height from the plane  466  that is within respective ranges δ 1  and δ 2 . Typically, δ 1  and δ 2  are within the range 0 μm to 5 μm, and more typically are approximately 1.5 μm.  
     [0052] It will be appreciated, therefore, that the provision of anti-wet-out features to otherwise smooth film surface may be used for the reduction of defects in displays. This is applicable to many different types of films. Such films may include refractive structure on a surface, or may be based on a bulk optical interaction effects that depend on the propagation of light through the film. The films with the anti-wet-out surface may also be based on a combination of bulk and surface effects.  
     [0053] One particular example of an apparatus using a film having an anti-wet-out surface is illustrated in an exploded view in FIG. 5A. A liquid crystal display (LCD) illumination module  500  uses a fluorescent lamp  502  and reflector  503  as a light source to direct light into a light guide  504 . The light guide is provided with diffusely reflecting extraction dots  506  on the lower surface  507 . A broadband, diffuse reflector  508  is positioned below the light guide  504  to reflect any light recirculated from any components above the guide  504 . Light from the fluorescent lamp  502  enters the side of the light guide  504  and is guided along the light guide  504  via internal reflection at the surfaces of the guide  504 . A light ray  510  incident on one of the extraction dots  506  is diffusely reflected to produce a number of diffusive rays  512 .  
     [0054] Light propagating upwards from the extraction dots  506  passes through the upper surface  513  of the guide  504 . A diffuser  514  may be positioned above the light guide to further diffuse light extracted from the guide  504 , and thus make the subsequent illumination of an LCD display  524  more uniform.  
     [0055] Light continuing in an upwards direction may then pass through a lower brightness enhancement film (BEF)  516 , having a prismatic structure on an upper surface similar to the prismatic structure illustrated for the brightness enhancing film in FIG. 4A. The lower BEF  516  reduces the divergence of light along one dimension (for example, out of the plane of the figure). An upper BEF  518 , placed above the lower BEF  516 , has its prismatic structure oriented at about 90° relative to the prismatic structure of the lower BEF  516  to reduce divergence of the light along a second dimension (for example, in the plane of the figure). Light reflected by either the upper or lower BEF  518  and  516  may be recirculated by the reflector  508 . The pair of crossed BEF films  516  and  518  may be effective in reducing the overall divergence of light extracted from the light guide  504 .  
     [0056] A reflective polarizing film  520  is positioned above the upper BEF  518 . A reflective polarizer transmits light of one polarization and reflects light of an orthogonal polarization. Therefore, the light passing through the polarizing film  520  is polarized. The light reflected by the polarizing film  520  may be recirculated by the reflector  508 . The polarizing film  520  is provided with an anti-wet-out upper surface  522 .  
     [0057] An LCD matrix  524  is positioned above the polarizing film  520 . Polarized light passing through the LCD matrix is spatially modulated with information, for example an image, which is then transmitted. The anti-wet-out surface  522  reduces the formation of wet-out and Newton&#39;s rings between the polarizing film  520  and the LCD matrix  524 , thus enhancing the quality of the image seen by the viewer. The use of an anti-wet-out surface  522  avoids the need to include a wet-out-reducing cover sheet between the polarizing film  520  and the LCD matrix  524 .  
     [0058] Other components may be included in the module  500 , such as a cover sheet between the upper BEF  518  and the polarizing film  520 .  
     [0059]FIG. 5B shows another type of display  550 , that uses a light redirecting film according to the present invention. Light from a lighting element  552  is directed by a reflector  554  into a light guide  556 . The lighting element  552  is typically a fluorescent tube, although other types of elements may be used. The light guide  556  is wedged, but other shapes, such as a pseudo-wedge could be used. The light guide  556  may be transparent or may include a bulk diffuser.  
     [0060] Light emerging from the light guide  556  at a low or grazing angle enters the light redirecting film  558 . The light redirecting film  558  has a structured surface side  560  having a plurality of linear prisms such as linear prisms  562 . A linear prism  562  has a first side  564  and a second side  566 . Light from the light guide  556  typically enters the redirecting film  558  through the first sides  564  of the linear prisms  562  and is totally internally reflected by the second sides  564 , as shown for light ray  565  in FIG. 5C. After total internal reflection, the light emerges from the redirecting film  558  through the output surface  568 . The light may then pass through a light gating device  570 , such as a liquid crystal display. The output surface  568  of the redirecting film  568  may be an anti-wet-out surface to prevent optical defects from occurring between the redirecting film  568  and the light gating device  570 .  
     [0061] It will be appreciated that the description of modules  500  and  550  is given only for illustrative purposes and is not intended to limit the use of a film having an anti-wet-out surface in any way. A film with an anti-wet-out surface may be employed in many different types of optical system where two optical surfaces may otherwise contact one another and produce wet-out or Newton&#39;s rings.  
     [0062] Optical films are commonly manufactured by various different methods, including embossing, extrusion, casting and curing, compression molding and injection molding. These methods are suitable for forming an anti-wet-out surface on the film. For example, a film may be cast between a pair of rollers that are spaced apart by a specific dimension, as is illustrated in FIG. 6, where a film  602  is pulled from a reservoir  601 , through a die  600  by an extrusion roller  604 . The film  602  is nipped between the extrusion roller  604  and a second roller  606 . Where the film  602  has a surface structure, the second roller  606  may be a pattern roller, provided with a prescribed surface for embossing a pattern onto the film  602 . For example, where the film  602  is being manufactured as a brightness enhancement film, as illustrated in FIG. 4A, the second roller  606  is provided with a plurality of prismatic structures  608  around its surface, which create complementary impressions in the upper surface  612  of the film  602 . The pattern roller may have a diameter whose value lies in the range 15 cm-60 cm. The extrusion roller  604  may also be provided with an embossing pattern that is used to emboss a pattern onto the lower surface  618  of the film. After passing between the rollers  604  and  606 , the film  602  cools, for example in a cooler  620 , and maintains the patterns embossed on it by the rollers  604  and  606 . In the particular embodiment shown, the extrusion roller  604  has a surface  616  that has random variations in height, for embossing an anti-wet-out surface on the lower surface of the film  618 .  
     [0063] The upper roller  606  may be provided with many different types of embossing patterns. Examples of embossing patterns that may be used on the upper roller  606  include a prismatic pattern for a brightness enhancement film, a lenticular pattern for a lenticular film, and a Fresnel pattern for a Fresnel lens. In addition, the prismatic structures on the upper roller  606  may be arranged in a direction perpendicular to the direction of rotation, around the circumference of the roller  606 , rather than in a direction parallel to the direction of rotation, as shown in FIG. 6. The upper roller  606  may also be smooth to provide a flat film surface, or may be provided with a surface for embossing an anti-wet-out pattern on the upper surface  612  of the film  602 . The surface of the extrusion roller  604  may include an irregular embossing pattern to produce an anti-wet-out surface, although this is not a requirement. Where both rollers  604  and  606  have an irregular embossing pattern, the resulting film has two anti-wet-out surfaces.  
     [0064] Other approaches may be followed to producing a film having one or more anti-wet-out surfaces, including embossing a sheet, injection molding and compression molding. In one particular approach, a film of embossable material, applied to a web, is compressively held against a pattern surface to emboss the complement of the pattern surface onto the film. The embossable material may be a thermoplastic material, and so the film may be film may be cooled while being held against the pattern surface in order to solidify the material with the pattern embossed thereon. In a variation of this approach, the embossable material may be a curable polymer that is cured, or partially cured, in place against the pattern surface or after the pattern surface is removed.  
     [0065] In another approach to forming an anti-wet-out surface, the film may be injection molded using a mold having an irregular pattern thereon. The resulting injection molded film has an anti-wet-out surface that is the complement of the irregular pattern in the mold. In another approach, the film may be compression molded. The molding tool may be provided with an irregular surface that forms the anti-wet-out surface on the molded part.  
     [0066] After a film has been embossed, it may undergo additional post processing procedures, such as coating, for example to produce an anti-reflection coating, or the like.  
     [0067] The film may also be stretched after fabrication, using one or more of a number of different methods. For example, one possible method is length orientation, where the web is nipped between two sets of rollers and the downstream set of rollers rotates faster than the upstream set. Another method is tentering, which involves gripping the edges of a film with, for example, continuous clamps arranged in a conveyor belt-like manner on either side of the web. As the clamps move forward, the continuous clamps move apart, stretching the film in between over a prescribed distance. Tentering is typically performed so as to stretch the film only in one direction, e.g. across the web, while not stretching in the down-web direction. The web is typically stretched to an extent sufficient to achieve a desired thickness or a desired molecular orientation in the film. The width of a film may be increased by a factor in the range 2-10 times in the tentering process, more typically by a factor in the range 3-8 times. A film may also be stretched as a sheet, rather than as a web, in a stretching frame where the edges of the sheet are attached to the sides of a frame, and the sides of the frame pulled apart.  
     [0068] Since the volume of the film material remains essentially constant when stretching, the cross-sectional shape of the film changes in a reasonably well-defined manner. Where the film is laterally stretched by a factor of X times, the height of the film reduces by a factor of X, since the cross-sectional area remains the same. Therefore, if an unstretched film has an anti-wet-out pattern embossed on one surface, and the average difference in height between local maxima and local minima is Y, then the average difference in height between local maxima and minima for the stretched film is approximately Y/X. For example, if an unstretched film has an anti-wet-out surface with average difference in height between local maxima and minima of 8 μm, and the film is stretched by a factor of 4, then the average height difference between local maxima and minima is approximately 2 μm after the film is stretched.  
     [0069] Therefore, the anti-wet-out surface is formed on the film having dimensions, e.g. average peak height, average peak separation, that are selected depending on whether or not the film is to be stretched. If no stretching takes place, then the surface formed on the film has the desired peak height and average separation. However, if the film is to be stretched, say in one dimension, then the peak height formed on the surface is X times the desired final peak height, where X is the stretching factor. Additionally, the average peak separation in the stretching direction formed on the film surface is 1/X times the desired final peak separation in the stretching direction. If stretching takes place in one dimension only, then the average peak separation in the direction orthogonal to the stretching direction does not change. Therefore, the average peak separation in the orthogonal direction is formed on the film to have the same value as the desired value after stretching. This is further explained in the description of the example presented below.  
     [0070] Where the film is stretched in two dimensions, e.g. across and along the web, the dimensions of the surface formed on the film are selected so that the dimensions on the anti-wet-out surface after stretching are within the desired limits.  
     [0071] Masters for the tools used for manufacturing structured films, such as prismatic films, lenticular films and films having Fresnel lenses, may be made by diamond turning techniques. These masters may be used for manufacturing a film by an extrusion or by a cast and cure process. Typically, the tools for linear patterns are made by diamond turning on a cylindrical blank known as a roll. The surface of the roll is typically of hard copper, although other materials may also be used. The structures are formed in continuous patterns around the circumference of the roll. In one specific embodiment, the structures may be machined by a technique known as thread cutting, in which a single, continuous cut is made on the roll while the diamond tool is moved in a direction transverse to the turning roll. If the structures to be produced have a constant pitch, then the tool moves at a constant velocity along the roll. A typical diamond turning machine may provide independent control of the depth that the tool penetrates into the roll, the horizontal and vertical angles that the tool makes to the roll and to the transverse velocity of the tool. Additionally, the diamond turning machine may control the rotational speed of the roll. Similar techniques may be adapted for manufacturing an anti-wet-out embossing roll.  
     [0072] A method for manufacturing the roll for embossing an anti-wet-out surface is illustrated in FIG. 7. A drum  700  is rotated around an axis  702  by a drum drive  704 . A computer  706  controls the drum drive  704  and may also monitor the current angular position, ψ, of the drum  700 . The controlling computer  706  also controls the movement and operation of a diamond cutting tool  708 . The computer  706  directs control signals to the cutting tool holder for movement in the z-direction, parallel with the axis  702 , the x direction which is radially directed towards the axis  702 , and may also direct control signals for the angle, Π, between the tool  708  and the surface of the drum  700 . The size and shape of the cutting tool are selected depending on the particular type of film for which the roll  700  is to be used for manufacturing.  
     [0073] Typically, the computer  706  drives the cutting tool in the z-direction to move the tool  708  along the rotating drum  700 . Control of the cutting tool  708  in the x-direction controls the depth of cut into the surface of the drum  700 . The diamond cutting tool  708  may be held in a fast servo unit  710  which is attached to a mount  712 . The mount  712  is typically translatable in the x and z directions under control from the computer  706 . The fast servo unit  710  also translates the cutting tool  708  in the x-direction. However, the cutting tool  708  operates at frequencies not normally obtainable with regular machine tool mounts. The upper frequency limit of the fast servo unit&#39;s response may lie in the range from kilohertz to tens of kilohertz, whereas the frequency response of a regular machine mount is typically not greater than 5 Hz. The length of the stroke that the fast servo unit  710  produces in the x-direction is typically short, less than 50 μm, and may be less than 20 μm. It will be appreciated that there may be a trade-off between length of stroke and upper frequency response. In general, the fast servo unit  710  is used to produce short, fast excursions of the cutting tool  708  in the x-direction, while the mount  712  is used to produce longer, but slower excursions of the cutting tool  708  in the x-direction. An anti-wet-out surface pattern is cut on the drum by thread cutting a shallow groove on the drum, i.e. translating the cutting tool  708  in the z-direction while cutting into the surface of the drum  700 . The mount  712  may be a second servo unit operating in a lower frequency band than the fast servo unit  710 .  
     [0074] Several different approaches may be employed for controlling the surface cut on the drum  700 . In one approach, illustrated in FIG. 7, the computer  706  generates control signals for z and, optionally, Π. The computer  706  also generates a control signal x for the cutting tool  708  that has two components. The first component, x 0 , is a slowly varying function that is directed to the mount  712  to translate the fast servo unit  710  in the x-direction. The slowly varying function may be, for example, sinusoidal or random. The second component, x 1  is a computer-generated noise function, that controls the fast servo unit  708  to make small, rapid, independent, randomly-timed movements in the x-direction.  
     [0075] The resulting random cut on the surface of the roll  700  has an average spacing between local minima around the roll circumference that is dependent on the surface speed of the roll relative to the cutter  708 , and the average period of time between randomly timed excursions of the cutting tool  708  in the x-direction. Because the surface of the drum  700  forms the complementary surface on the film, local minima on the roll surface correspond to local maxima on the film surface. Where the average separation between local minima in the height of the roll surface is selected to be approximately 150 μm, and the average time between excursions of the roll  708  is 200 μs (corresponding to an average cutting tool operation frequency of 5 kHz), the surface speed of the roll is selected to be approximately 0.75 ms −1 .  
     [0076] In addition to varying the x-position of the cutting tool  708  by a fast-x translation by the fast servo unit  710 , the x-position of the cutting tool  708  may be varied by a slow-x translation of the mount  712 , under the control of the x 0  signal component. Where the x 0  signal is varied at a rate that is not an harmonic or subharmonic of the rotation rate of the roll  700 , the appearance of a regularity in the periodicity of the cut surface may be avoided. The slow-x translation of the mount  712  may be used to vary the height of the surface cut into the roll  700  by an amount greater than the stroke length of the fast servo unit  710 .  
     [0077] FIGS.  8 A- 8 C illustrate examples of different types of control signal, as a function of voltage plotted against time, that the computer  706  may transmit to the fast servo unit  710  for cutting a random, or pseudo random, pattern on the surface of the roll  700 .  
     [0078] In FIG. 8A, the control signal  800  includes a series of pulses of fixed amplitude and width, with randomly varying inter-pulse separation. In FIG. 8B, the control signal  802  includes a series of pulses having fixed amplitude and separation, but randomly varying width. In FIG. 8C, the control signal  804  includes a series of pulses having fixed width and separation, but randomly varying amplitude.  
     [0079] The pattern of randomly appearing pulses appearing in the signals  800 ,  802  and  804  are translated by the fast servo unit  710  to respectively equivalent patterns on randomly distributed minima cut onto the surface of the roll  700 . The series of pulses in the control signal supplied to the fast servo unit  710  may include any combination of randomly varying pulse amplitude, pulse width and inter-pulse separation. An advantage of supplying a randomly, or pseudo randomly, varying control signal to the fast servo unit  710  from the computer  706  is that a randomly generated pattern cut onto one roll may be programmed into the computer  706  and repeated on another roll.  
     [0080] Amongst other advantages of the pulsed control signal, the heat generated in the fast servo unit  710  may be reduced, it may be easier to increase roll cutting speed and it may be easier for the designer to understand and model the cutting process.  
     [0081] Other types of signals may be transmitted to the fast servo unit  710  to provide the appropriate translation of the cutting tool  708  to cut the roll  700 , and the presentation of the examples in FIGS.  8 A- 8 C is not intended to limit the invention.  
     [0082] It was mentioned above that an advantage of the invention is the ability to select a characteristic of the anti-wet-out surface to be a specific value relative to a certain direction, for example the surface slope in a direction parallel to the direction of the prisms on a prismatic film. In another example of orienting an anti-wet-out surface, it may be possible to select a range of “down-thread” separation between surface minima while cutting a drum, for example between 150 μm and 200 μm, while not imposing any particular restraint on the “across-thread” separation between local minima. Such orientation selection of a random characteristic may be useful, for example, for avoiding linear effects, such as a linear Moire-pattern.  
     [0083] Another approach to cutting an anti-wet-out surface onto a drum is illustrated in FIG. 9. Components that are similar to the components of FIG. 7 have similar identifying numbers. The computer  900  that controls the cutting operation is connected to the drum drive  704  to control and monitor the angle, ψ, of the drum  700  as it rotates around the axis  702 . The computer  900  transmits a control signal to the mount  712  for controlling motion of the cutting tool  708  in the y direction and a slow-x signal for relatively slow motion of the cutting tool  708  in the x-direction. The computer  900  may also provide control signals to control the angle, θ, between the cutting tool  708  and the surface of the roll  700 .  
     [0084] A noise source  904  generates a noise signal, I n , that typically passes through a bandpass filter  906 . The filtered noise signal is amplified, if necessary, in an amplifier  907 , and applied to the fast servo unit  710  as a fast-x signal to produce irregular, fast motion of the cutting tool  708  in the x-direction.  
     [0085] The pass band of the filter  906  may be adjusted to pass frequencies that are within a desired range of noise frequency. For example, the bandpass may extend over a window having a width of a few kilohertz, and may be centered over a range from one or two kilohertz to tens of kilohertz. The center of the frequency band is selected depending on the desired average spacing between local minima on the cut surface of the drum, and the width of the selected passband depends the desired spread of inter-minima distances. The selection of the center of the frequency band may also depend on the stroke length required of the fast servo unit  710 . The stroke length of a fast servo unit typically decreases with increased operating frequency.  
     [0086] It will be understood that, since the signal from the noise source  904  is filtered, the signal applied to the fast servo unit  710  is not truly random, but is pseudo-random. However, limiting the frequency of the noise signal avoids the occurrence of statistically extreme frequency excursions in the fast-x signal. Such excursions may result in the roll having a portion of relatively short inter-minimum spacing that is adjacent a portions of relatively long inter-minimum spacing, which may lead to a defect in the display that is visible to the viewer.  
     [0087] In one particular example of a system as illustrated in FIG. 8, a cutting tool  708  having a 64 μm radius diamond is used at a cutting pitch of 40 μm. The cutting pitch is the distance traveled by the cutting tool  708  in the z-direction during one revolution of the drum  700 . The depth of the cutting tool was varied using a slow-x translation of approximately 6 μm every 1.69 revolutions of the drum, a ratio chosen to provide a constant pattern of varying depths without cyclical problems seen in using a cycle of close to, and especially slightly less than one revolution. On top of the slow-x translation is added a 3 micron random signal generated using a filtered, random noise generator. The filter passed noise in the pass band of 4 kHz to 5.6 kHz. The drum&#39;s surface speed was selected so that the nominal, or average spacing between peaks on the drum surface was approximately 170 microns.  
     [0088] One particular embodiment of a fast servo unit for holding a cutting tool is illustrated in FIG. 10. The fast servo unit  1000  includes a cutting tool  1002  that extends from a case  1004  having walls  1006  and back  1008 . The cutting tool  1002  is supported by a stack of piezo-electric elements  1010  on either side. When the piezo electric stack  1010  is stimulated by a rapidly varying electrical signal, the cutting tool  1002  is caused to move such that the distance it extends from the case  1004  changes by a small amount. It is possible for the piezo electric stack  1010  to be stimulated by a signal of constant, programmed, or irregular frequencies. However, to generate a surface on the roll  800  having a randomly varying height, the signal applied to the piezo electric stack  1010  is typically random or pseudo-random. As used herein, the term random is understood to include pseudo-random.  
     EXAMPLE  
     [0089] A reflective polarizer film having an anti-wet-out surface was fabricated using a roll whose surface was prepared in the following manner. The roll, was rotated with a surface speed of approximately 0.8 ms −1 . The fast servo unit was operated with a signal from a noise source having a frequency in the range 4 kHz to 5.6 kHz. The stroke length of the fast servo unit was about 7 μm. A slow-x translation of about 3 μm, with a period equal to about 1.69 rotations of the roll was imposed on the fast servo unit. The diamond tool had a radius of about 50 μm, and cut a groove into the surface of the roll with a pitch of about 22-28 μm. The average sag of the cut was about 2 μm. The maximum peak to valley difference in depth, therefore, was about 12 μm (7+3+2 μm). The z-dimension of the peak between adjacent grooves did not vary smoothly around the roll, but had some fast z-variation, depending on the relative cut depth of the valleys on either side of the peak. however, the average separation between minima in the z-direction was equal to the z-pitch, i.e. 22-28 μm. The average separation between minima was approximately 175 μm in the circumferential direction along the groove.  
     [0090] The roll was used to form a surface on a film in an extrusion process, similar to the process illustrated in FIG. 6. The film was a multi-layer, reflective polarizer film, formed from alternating layers of PEN and COPEN as described, for example, in published PCT Patent Applications WO95/17303, WO96/19347, WO95/17699, WO95/17692 and WO95/17691, the contents of which are incorporated herein by reference. After the film was formed, the film was stretched laterally by a factor of about 6. Accordingly, the maximum peak to valley height in the stretched film was about 2 μm (12÷6) and the average peak separation in the stretched direction was 132 μm-168 μm. The average peak separation in the direction along the groove was not affected by the stretching and remained at about 175 μm.  
     [0091] An example of an anti-wet-out surface on a film is illustrated in FIGS. 11A and 11B. The film was formed on a drum cut with a 50 μm radius diamond tool using a 25 μm pitch. The cutting tool had a 7 μm high frequency random throw, and a 3 μm slow-x throw. The film was stretched by a factor of about 5 after fabrication. A three-dimensional surface plot of the anti-wet-out surface is illustrated in FIG. 11A for a section of the film 1.81 mm (z-axis)×1.36 mm (y-axis). The stretching direction was along the y-axis. The figure shows a variation in height of approximately 1.2 μm over the sampled surface area. FIG. 11B illustrates a surface profile for a line along the surface, parallel to the z-axis. The variation in height along the length of the line profile is approximately 0.3 μm.  
     [0092] As noted above, the present invention is applicable to display systems and is believed to be particularly useful in reducing cosmetic defects in displays and screens having multiple light management films, such as backlighted displays and rear projection screens. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent processes, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art to which the present invention is directed upon review of the present specification. The claims are intended to cover such modifications and devices.