This invention relates to thin films with micro-structured surfaces. In particular this invention relates to large area micro-lens array optical films for improving the out- coupling of light from light emission devices, and methods for producing such films using moulding drums.

The increasing popularity of low energy solid state lights based on inorganic light emitting diodes (LEDs) and organic light emitting devices (OLEDs) presents new challenges for light luminaire designers. In order to create luminaires incorporating inorganic LEDs, which are generally high intensity point sources of illumination, the light needs to be sculpted into tailored beams of light, removing glare and creating light with the required light distribution properties and cosmetic appearance. This needs to be done at low cost and with the light being transmitted efficiently, and as such, plastic or glass with volumetric scattering properties and light transmission efficiencies between 60% and 80% is not suitable to perform this task. Structured optical films have been found to be a beneficial way of increasing the transmission efficiency while still providing the required light distribution properties.

For OLEDs, large areas of a substrate are coated with light emitting polymers to create flat, large area light sources. However, a significant problem with such devices is that a considerable proportion of the light which is generated within the device is not emitted to the outside from the face of the transparent substrate, but instead is trapped due to total internal reflection within the substrate in waveguiding modes and is emitted from the edges of the substrate. Therefore in order to improve the efficiency of these devices, light needs to be out-coupled from the substrate to prevent its total internal reflection. A low cost efficient way of doing this is to add micro-lens structures to the surface of the substrate, e.g. by laminating an optical film onto it.

Such optical films are typically manufactured using a moulding drum mould with a structured surface and roll to roll embossing processes, e.g. UV cure embossing. Large width (>1 m) films can be produced using this technique. To achieve high quality optical performance from the films, this manufacturing technique relies almost completely on the nature and quality of the structures on the moulding drum mould. However a number of limitations exist with the current methods for the manufacture of moulding drum moulds to produce high quality, tailored micro-lens optical films.

The masters for the moulding drum moulds are generally created on flat squares known as master tiles, which have limited size. Once the structure has been created on the master tile, for example by using laser ablation, photolithography or micro-milling, replicates are created and then tiled out into so-called wallpaper shims. These tiled shims are then wrapped around a drum to produce the moulding drum. Using tiled shims results in seams at the borders between the tiles used to create the shim as well as a seam where the two ends of the shim sheet meet once it has been wrapped around the drum. As will be appreciated, this results in seams on the embossed optical film, i.e. a defect which reduces the optical quality and hence light transmission efficiency of the optical film, as well as loss of control over the light distribution. Consequently if a high quality optical film is required, i.e. without defects, this can only be produced on an area whose size is limited by the size of the master tile used to produce the moulding drum. The structure for the whole area of the drum could be created, e.g. by micro-milling using an appropriately shaped diamond drill, but the time scale involved would be completely impractical. For example, for a moulding drum with a surface area of approximately 0.5 m ² , micro-milling a structure of micro-lenses with a pitch of 50 microns across the area of the drum would take over 6 years.

Single point diamond turning is able to create two-dimensional structures by rotating a drum while a cutting tool is advanced into the moulding drum, e.g.

extruded lenticular or prismatic structures, but this process is not able to produce three-dimensional structures.

It is an aim of the present invention to provide an improved method for producing large area, high quality optical films.

When viewed from a first aspect the invention provides a method of manufacturing a moulding drum for producing an optical film, the method comprising: rotating a moulding drum blank,

forming a plurality of micro-structures on said mould drum blank using depth-modulated single-point diamond turning. When viewed from a second aspect the invention provides a method of

manufacturing a seamless optical film, the method comprising:

passing a continuous film of transparent material over a moulding drum, wherein the moulding drum comprises discrete three-dimensional micro-structures on its surface which are adapted to impart corresponding features to the film passing over the moulding drum, wherein the micro-structures on the moulding drum have been formed using depth-modulated single-point diamond turning.

The invention also extends to a moulding drum made according to the method of the first aspect and an optical film manufactured according to the method of the second aspect. The film material could comprise a base substrate made from one of polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), polycarbonate (PC), polyethylene (PE), or a fluorinated polymer (ETFE).

In accordance with the invention, the optical film is conveniently manufactured using a low cost roll-to-roll manufacturing technique, such as UV casting or hot embossing.

As will be appreciated by those skilled in the art, the present invention provides a method for manufacturing moulding drums and large area, seamless optical films with 3D structures formed on the surface of the moulding drum and therefore subsequently the optical film. Using the technique of depth-modulated single-point diamond turning, the moulding drum can be manufactured within a timescale of hours rather than years as would have been the case with previous techniques, i.e. too long a timescale to ever consider contemplating.

Depth-modulated single-point diamond turning is a technique in which the depth of the cutting tool can be varied while the moulding drum is being engraved. This therefore enables 3D structures to be created as the depth of the tool can be varied as the moulding drum is rotated. Furthermore, the position of the cutting tool can be synchronised with the rotational position of the moulding drum. This allows regular, repeatable structures to be cut into the surface of the moulding drum. In a preferred set of embodiments the depth of the cutting tool is synchronised with the rotational position of the moulding drum. In another, not necessarily mutually exclusive, set of embodiments, the lateral position of the cutting tool, i.e. the direction parallel to the axis of the rotating moulding drum, is synchronised with the rotational position of the moulding drum.

In one set of embodiments the method uses a position signal to control the position of the cutting tool while it is engraving the moulding drum. The position of the tool can be synchronised with the rotational position of the moulding drum by phase locking the position signal to the rotational position. This phase locking of the position signal ensures that the desired structures will be accurately engraved in a regular and repeatable manner in the moulding drum. The cutting tool could be controlled by a number of different actuation technologies. In one set of embodiments the actuation of the cutting tool is controlled by a piezoelectric actuator. This enables the position of the cutting tool to be moved very rapidly, e.g. along a defined axis, which could be the depth of the tool. The speed at which the cutting tool is moved and the speed at which the moulding drum is rotated determines the size of the micro-structures cut into the moulding drum. In the set of embodiments in which a position signal is used, the speed at which the cutting tool is moved determines the frequency of the position signal. For example, the minimum size of micro-structure able to be created is determined by the maximum speed of the cutting tool and the minimum speed of rotation of the moulding drum.

In one set of embodiments the micro-structures have a period along or across the moulding drum of less than 100 microns, e.g. less than 50 microns. In one set of embodiments the depth of the micro-structures is less than 50 microns, e.g. less than 30 microns.

The micro-structures could comprise a number of different structures, e.g. lenticular and/or prismatic structures, such that the desired overall micro-structure across the film can be created. The shape of the micro-structures could be determined by the shape of the cutting tool and/or its varying position, i.e. the sideways profile of the cutting tool at different depths determines the shape of the structure. More than one cutting tool could be used during the process of manufacturing the moulding drum, e.g. a prismatic cutting tool or a lenticular cutting tool.

Thus a number of different possibilities exist for the types of micro-structures that can be created. The profile across the film, i.e. parallel to the axis of the moulding drum, could comprise a circular segment profile, an elliptical segment profile, a triangular facet profile, a sine wave profile, a rectified sine wave profile, or indeed any combination of these features. The profile along the film, i.e. in the

circumferential direction of the moulding drum, could also comprise a circular segment profile, an elliptical segment profile, a triangular facet profile, a sine wave profile, a rectified sine wave profile, or indeed any combination of these features. The profiles across and along the film can be combined to create different 3D profiles, e.g. a circular segment profile across the film and a rectified sine wave profile along the film, an elliptical segment profile across the film and a rectified sine wave profile along the film, a triangular facet profile across the film and a rectified sine wave profile along the film (this can be either a symmetric or an asymmetric prismatic profile). The profile of the micro-structures could change along and/or across the film, but in one set of preferred embodiments the profile along the film repeats itself, and in another set of preferred embodiments the profile across the film repeats itself. Therefore one set of embodiments exists in which both the profile across the film and the profile along the film repeats itself.

In the embodiments in which a triangular facet is provided, preferably the apex angle of the facet is between about 80° and 100°, e.g. 90°.

In one set of embodiments the period of the profile along the film is greater than the period of the profile across the film (where the period is the distance between adjacent micro-structures). The aspect ratio (the ratio of the period along the film to the period across the film) could be between 1 and 20, e.g. between 1 and 2, or between 2 and 20. The micro-structures could be arranged in any way as has been outlined above, and in the embodiments in which repeating profiles are used, adjacent structures could be in phase with each other across the film, in anti-phase with each other, or the phase could be generated using a random statistical method. Alternatively the phase of adjacent structures could change by a fixed amount, e.g. between 1 ° and 90°, across the film.

The structures need not necessarily be engraved into the moulding drum in a single pass of the cutting tool. Two or more passes may be used to engrave a structure into the moulding drum, e.g. with the cutting tool in each pass being controlled by a different position signal. In one set of embodiments the position signal could be the same for two passes, except that for being in anti-phase during the second pass as compared to the first pass. As has been outlined, preferred embodiments of the present invention are particularly suitable for creating large area, seamless optical films. In one set of embodiments the width of the optical film is greater than 1 m, and therefore in this set of embodiments the width of the moulding drum is greater than 1 m. A preferred method of passing the continuous film of transparent material over the moulding drum comprises using a roll-to-roll production process, and therefore in this set of embodiments the length of the optical film able to be produced is limited only by the length of transparent material on the roll.

Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Fig. 1 is a schematic view of apparatus used to cut a moulding drum in accordance with the invention;

Fig. 2 is a schematic illustration of different prism shapes that can be formed with a common apex angle;

Figs. 3a, 3b, 3c and 3d show the paths of a cutting tool engraving a moulding drum and the resultant cross sectional profiles cut into the moulding drum; and

Figs. 4a and 4b show plan views of profiles cut into a moulding drum. Fig. 1 shows, schematically, an arrangement for cutting a moulding drum 2 for use in the production of optical film in accordance with an embodiment of the invention. The drum 2 is made from a suitable metal, such as electro-deposited copper or nickel, and is mounted to rotate in a direction C around its longitudinal axis. A diamond-tipped cutting tool 4 is mounted on a carriage 6 which can be translated parallel to the axis of the drum 2 in a direction X, and which can also be rotated in a direction B about an axis parallel to a tangent to the drum. The carriage can also be advanced towards and away from the drum. In an exemplary embodiment the carriage comprises a fast tool servo, e.g. actuated by a piezo-actuator.

In general the point angle of the tool (and thus the internal angle of the ultimate micro-structure formed by the mould cut using the tool) is preferably greater than around 30 degrees, so that the tool is not too long and thin and fragile. This is factored into the design of the micro-structured optical film in accordance with the preferred embodiments of the invention.

Ideally, but not essentially, the cutting angle used for a given tool is set so that angle of each facet from the vertical is greater than around 3 degrees. This helps to ensure that the prism can be released from the mould during the high speed optical film manufacture process - non-shrinking optical UV curable lacquers need to be used to ensure that the cast shape closely replicates the shape of the mould and this then makes it harder to release 'difficult' structures from the mould without damaging them. To produce a moulding drum for producing optical films, first a suitable diamond point cutting tool 4 is mounted on the carriage 6. The drum 2 is then rotated as indicated by the arrow 6, at a suitable speed, for example around 600 revolutions per minute. The cutting angle of the tool 4 is set by rotating the carriage 6 in the direction B under the control of a pre-programmed controller. The effect of altering the cutting angle of the tool can be seen in Fig. 2. This show three exemplary prism shapes 10, 12, 14 which can be achieved through varying the cutting angle of a single tool 4.

Once the cutting angle has been set, the carriage is translated along the X axis by the controller to position the apex of the tool 4 at the desired position along the X axis. The tool 4 is then moved towards the drum 2, cutting the surface of the drum, with the depth of the tool being controlled by a position signal from the preprogrammed controller. Once the desired profile has been cut around the surface of the moulding drum, the tool is then retracted and a new cutting angle can be set by rotating the tool about B. The carriage 6 is then moved along to a new position on the X axis and the cutting operation is repeated. This way it is seen that a part of the mould is formed which will produce a segment of the ultimate micro- structured arrangement on the optical film comprising a set of prismatic features. The position signal for the cutting tool 4 is phase locked to the rotational position of the moulding drum 2, thus ensuring accurate synchronisation of the depth of the tool tip with the position on the moulding drum 2. The speed of rotation of the moulding drum 2 and the speed at which the depth of the cutting tool 4 can be varied impose a maximum frequency on the position signal. This then translates into a minimum feature size on the moulding drum surface.

The tip of the cutting tool 4 is a 3D structure with the actual shaped tool tip sitting on top of an inclined surface. It is important that the angle of the inclined surface is chosen such that movement of the tool tip does not result in the support structure hitting the surface of the moulding drum 2 when the depth of the cutting tool 4 is varied. As has been explained, to produce a structured surface the depth of the tool 4 is modulated with the phase locked control position signal and a variable controlled depth profile is cut around the drum 2 at that position. The sideways shape of the tool 4 determines the shape of the structure at the relevant different depths.

There are therefore three constraints on the micro-structures that can be created using this technique:

1 ) the minimum phase length, i.e. distance between adjacent structures along the moulding drum 2, which can be achieved, determined by the speed of rotation of the moulding drum 2 and the speed at which the depth of the tool 4 can be varied;

2) the tool profile included angle, determined by the shape of the tool 4;

3) the tool clearance angle, determined by the shape of the support surface for the tool 4. ln general, standard cutting tools do not have suitable incline included angles for the micro-structures that are required for the optical films manufactured using this technique and therefore special cutting tools need to be manufactured to enable higher aspect ratio micro-structures to be constructed.

As an example of these design constraints, consider a cutting tool which has a sideways profile radius of 30 microns and an included angle of 40 degrees. In simulations, this has been shown to enable a structure with width (size of the structure across the moulding drum) of 56.4 microns and depth (size of the structure into the moulding drum) of 19.74 microns to be created, giving a maximum aspect ratio (the ratio of the structure's depth to width) that can be achieved of 0.35. This should be noted to be a different aspect ratio to that discussed earlier. The structure can be extended in depth at its maximum beyond 19.74 microns but in that case the profile at the very edge of the lens will have a constant slope rather than a circular profile. A similar analysis applies to the tool incline included angle to the shapes profile around the drum. Once a series of features has been cut with the first tool, the tool can be changed, e.g. to give a different shape or apex angle. The cutting procedure outlined above is then repeated to produce another segment of the ultimate micro-structured arrangement. Figs. 3a, 3b, 3c and 3d show a series of example paths for the depth of the cutting tool and the resultant cross sectional profile created in the surface of the moulding drum. Fig. 3a shows the path 20 for a first pass of the cutting tool engraving the moulding drum, i.e. a repeated sinusoidal depth. Fig. 3b shows the cross sectional profile 22 created in the surface of the moulding drum by a cutting tool which takes the path 20 illustrated in Fig. 3a, i.e. the surface of the moulding drum is positioned half way down the depth profile of the path.

Fig. 3c shows the path 24 for a second pass of the cutting tool engraving the moulding drum, i.e. the same repeated sinusoidal depth profile as in Fig. 3a, but in anti-phase. Therefore when the cutting tool follows the path in Fig. 3c, the resultant cross sectional profile 26 is that shown in Fig. 3d, i.e. a rectified sine wave.

Figs. 4a and 4b show plan views of different micro-structured arrangements that can be engraved on the surface of a moulding drum and therefore created on an optical film. Axis X is the direction across the moulding drum and optical film, i.e. parallel to the axis of the moulding drum; axis Y is the direction around the circumference of the moulding drum and along the optical film. In Fig. 4a the individual micro-structures 30 are in phase with each other across the moulding drum and therefore the optical film; in Fig. 4b the individual micro-structures 32 are in anti-phase with each other across the moulding drum and therefore the optical film.

Once the moulding drum has been created as explained above, it can then be used in the manufacturing of micro-structured optical films.

In embodiments of the invention, flat micro-structured optical film may be manufactured using a roll-to roll process in which a base film, such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), ETFE, acrylic or a similar optically transparent polymer film, is coated with a transparent UV curable lacquer (resin) and the film exposed to UV light while compressed against a moulding drum on which a reverse of the desired structure has been engraved (as outlined above). The film should be transparent, resistant to weathering but have a high adhesion to the cured lacquer. An example of a suitable resin is Rad-Kote X-6JA-68-A which is commercially available from Rad-Cure Corporation, 9 Audrey Place, Fairfield, New Jersey 07004. This lacquer has been formulated to cure through visible light and its viscosity is 500 cP.

The coated film is impressed onto the drum, with the coating side face on to the drum. In this way the coating is forced into the inverse features of the mould. At this point a UV light is used partially or completely to cure the coating, thus making it take up the form of the inverse of the mould. The resulting film is pulled of the drum resulting in a base film on whose surface are the desired micro-structured features. This is implemented as a continuous process which can manufacture optical film at high production rates, for example 15 metres per minute. It will be appreciated by those skilled in the art that many variations and

modifications to the embodiments described above may be made within the scope of the various aspects of the invention set out herein. For example the profile of the tool tip can be varied, e.g. chosen to be spherical or prismatic. The depth of the micro-structures can be varied to a maximum determined by the cutting tool. Also variable is the relationship between the modulation depth around the moulding drum and the profile shape across the moulding drum which therefore produces micro-structures with different aspect ratios. In the embodiments discussed only examples of structures which are in phase and in anti-phase with each other have been shown, however it is possible to have structures for which the phase is randomised, i.e. no relationship between the phase of adjacent structures, or the phase varies with a slow modulation.