Abstract:
An optical sheet to be used as a screen on which an image is projected from an image projector is provided. The optical sheet is produced by joining a plurality of optical sheet members with end surfaces thereof meeting each other as joint surfaces. The optical sheet members are realized with lenticular lens sheets. Each lenticular lens sheet has lenses, which are elongated in a second direction, juxtaposed in a first direction orthogonal to the second direction. The optical properties of the optical sheet members that are slightly undulated vary cyclically in the first direction. And the optical sheet members to be joined are a selected pair of optical sheet members whose undulations are substantially identical to each other or symmetrical to each other with respect to the joint surfaces.

Description:
This application claims the benefit of Japanese Application No. 2000-395631 filed in Japan on Dec. 26, 2000, the contents of which are incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical sheet having a plurality of optical sheet members joined, an optical sheet manufacturing system for manufacturing the optical sheet, and an optical sheet cutting machine. 
     2. Description of the Related Art 
     Optical sheets including the one realized with a lenticular lens sheet and the one realized with a Fresnel lens sheet are adopted as an optical screen on which an image is projected from an image projector. There is a tendency towards a large optical screen in pursuit of large-screen vision of an image. Accordingly, optical sheets with larger areas are in need. 
     Among the optical sheets, for example, the optical sheet realized with a lenticular lens sheet is structured to have semi-cylindrical projections successively arranged on the surface thereof. A transparent resin material that is heated and softened is rolled out using a roller member that has numerous female semi-cylindrical molds inscribed on the peripheral surface thereof, whereby the lenticular lens sheet is produced. 
     However, considerable pressure must be applied in order to produce a thin lenticular lens sheet. When an attempt is made to increase the width of a lenticular lens sheet with the thinness maintained, required pressure becomes so high that the rigidity of a manufacturing unit must be raised markedly. This leads to the high costs of manufacturing. 
     Therefore, a technology for producing a lenticular lens sheet of a large area at low costs by joining lenticular lens sheets of a predetermined width has been developed. 
     For example, Japanese Unexamined Utility Model Application Publication No. 64-23042 describes a transmissive screen having a plurality of transparent or translucent resin sheets joined. Adjoining of the plurality of resin sheets is made with resin layers, of which optical property is substantially identical to that of the resin sheets, between the resin sheets. More particularly, the resin sheets are realized with lenticular lens sheets, and the lenticular lens sheets are joined at their depressions. 
     However, the lenticular lens sheet is, as mentioned above, produced by rotating a roller member and pressing it against a resin material. In practice, it is unavoidable that minute undulations occur on the surface of the lenticular lens sheet whose surface is realized with the repetition of a depression and an elevation. 
     If lenticular lens sheets each having the undulations are joined as they are, a mismatch caused by the undulations produces an optically adverse effect. Consequently, the joint portions of the lenticular lens sheets are visualized as a streak. 
     Therefore, there is a demand for a technology that takes account the undulations in optical sheet members and prevents them from producing an optically adverse effect. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide an optical sheet in which joint portions of optical sheet members will produce almost no optically adverse effect. 
     Another object of the present invention is to provide an optical sheet manufacturing system optimal for manufacturing of an optical sheet in which joint portion of optical sheet members will produce almost no optically adverse effect. 
     Still another object of the present invention is to provide an optical sheet cutting machine capable of cutting an optical sheet member optimally for joining that will almost not permit joint portions of optical sheet members to produce an optically adverse effect. 
     Briefly, according to the present invention, there is provided an optical sheet to be used as a screen on which an image is projected from an image projector. The optical sheet has a plurality of optical sheet members which are joined together with end surfaces thereof, which are orthogonal to major surfaces thereof. The optical sheet members are substantially identical to each other to such an extent that the optical property of each optical sheet member changes cyclically in a first direction over the major surface of the optical sheet member, and that undulations exist in a second direction orthogonal to the first direction. One optical sheet member and the other optical sheet member that are to be joined with the joint surfaces thereof are optical sheets whose optical properties exhibited over the joint surfaces are substantially identical to each other within a predetermined permissible range. 
     Moreover, according to the present invention, there is provided an optical sheet manufacturing system consisting mainly of an optical sheet cutting machine, an optical sheet joining machine, a reservoir, a conveying machine, and a controller. The optical sheet cutting machine trims an optical sheet member optimally for joining. The optical sheet joining machine joins a plurality of optical sheet members with the edges thereof, which have been cut optimally for joining, met each other. At least one of the optical sheet members cut by the optical sheet cutting machine and the optical sheet produced by the optical sheet joining machine is preserved in the reservoir. The conveying machine conveys optical sheet members among the optical sheet cutting machine, optical sheet joining machine, and reservoir. The controller controls the optical sheet cutting machine, reservoir, optical sheet joining machine, and conveying machine. 
     Furthermore, according to the present invention, there is provided an optical sheet cutting machine consisting mainly of a platform, an investigating device, a cutting blade, a cutting drive source, and a feeding drive source. An optical sheet member to be cut is placed on the platform, and the platform enables adjustment of a slide position and a turn position on the major surface of the optical sheet member. The investigating device investigates the shape of the surface of the optical sheet member placed on the platform so as to determine a cutting band line along which the optical sheet member is cut. The cutting blade is used to trim the optical sheet member. The cutting drive source drives the cutting blade at the same cutting start position. The feeding drive source moves the cutting blade to change the cutting start position. The platform is used to adjust the slide position and turn position on the optical sheet member so that a path along which the cutting blade is moved by the feeding drive source will agree with the cutting band line determined based on the investigation performed by the investigating device. While the cutting drive source is driving the cutting blade, the feeding drive source moves the cutting blade along the cutting band line. The optical sheet member is thus cut. 
     The above and other objects, features and advantages of the invention will become more clearly understood from the following description referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the configuration of an optical sheet manufacturing system in accordance with an embodiment of the present invention; 
     FIG. 2 is a block diagram showing the configuration of the optical sheet manufacturing system in accordance with the embodiment installed in a temperature-and-humidity controlled clean booth; 
     FIG. 3 is a flowchart describing the actions of the optical sheet manufacturing system in accordance with the embodiment; 
     FIG. 4A is a plan view showing the structure of an optical sheet cutting machine included in the embodiment; 
     FIG. 4B is a front view showing the structure of the optical sheet cutting machine included in the embodiment; 
     FIG. 5 is a flowchart describing the actions of the optical sheet cutting machine included in the embodiment; 
     FIG. 6 shows the structure of a coolant feeder employed in the optical sheet cutting machine included in the embodiment; 
     FIG. 7 is a front view, part of which is a sectional view, showing the structure of a base temperature adjuster that retains the temperature of a platform employed in the optical sheet cutting machine included in the embodiment at a predetermined position; 
     FIG. 8 shows the structure of the optical sheet cutting machine included in the embodiment and installed in a temperature-and-humidity controlled clean booth; 
     FIG. 9 is a front view showing an example of an anti-vibration structure for a platform employed in the optical sheet cutting machine included in the embodiment; 
     FIG. 10 is a sectional view showing a structure for the optical sheet cutting machine included in the embodiment which has a suction fixing device incorporated in a turn plate; 
     FIG. 11A is a plan view showing the structure of an optical sheet joining machine included in the embodiment; 
     FIG. 11B is a front view showing the structure of the optical sheet joining machine included in the embodiment; 
     FIG. 11C is an enlarged partial view showing the structure of the optical sheet joining machine included in the embodiment; 
     FIG. 12A, FIG. 12B, FIG. 12C, FIG. 12D, and FIG. 12E are side views showing an operation flow according to which the optical sheet joining machine included in the embodiment joins optical sheet members; 
     FIG. 13 is a flowchart describing the actions of the optical sheet joining machine included in the embodiment; 
     FIG. 14 shows the structure of the optical sheet joining machine included in the embodiment and installed in the temperature-and-humidity controlled clean booth; 
     FIG. 15 is a front view showing an example of an anti-vibration structure for the optical sheet joining machine included in the embodiment; 
     FIG. 16 is a sectional view of a structure for the optical sheet joining machine included in the embodiment which has a suction fixing device incorporated in at least one of a stationary plate and a movable plate; 
     FIG. 17A is a plan view showing an example of a structure for the optical sheet joining machine included in the embodiment which enables sequential joining of elongated optical sheet members; 
     FIG. 17B is a front view showing the example of the structure for the optical sheet joining machine included in the embodiment which enables sequential joining of elongated optical sheet members; 
     FIG.  18 A and FIG. 18B are perspective views showing the appearance of joined optical sheet members employed in the embodiment; 
     FIG. 19 is an enlarged view showing the states of joint portions of optical sheet members employed in the embodiment; 
     FIG.  20 A and FIG. 20B show the state of paired and joined optical sheet members that have undulations extended in substantially the same direction on the surfaces thereof, and that are employed in the embodiment; 
     FIG. 21A, FIG. 21B, FIG. 21C, and FIG. 21D show the states of paired and joined optical sheet members that have undulations extended in substantially symmetrical directions, on the surfaces thereof and that are employed in the embodiment; 
     FIG. 22A is a perspective view showing the state of joined optical sheet members that have depressions thereof met each other to define both ends of a joint line, and that are employed in the embodiment; 
     FIG. 22B is an enlarged view showing part of the end surfaces of the optical sheet members shown in FIG. 22A; 
     FIG. 22C is a sectional view showing the optical sheet members shown in FIG. 22A; 
     FIG. 22D is a perspective view showing the state of joined optical sheet members that have elevations thereof met each other to define both ends of a joint line, and that are employed in the embodiment; 
     FIG. 22E is an enlarged view showing part of the end surfaces of the joined optical sheet members shown in FIG. 22D; 
     FIG. 22F is a sectional view of the joined optical sheet members shown in FIG. 22D; and 
     FIG. 22G is a sectional view showing the state of joined optical sheet members that have elevations or depressions thereof met each other to define both ends of a joint line and that are mismatched in the middle of the joint line. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, various embodiments of the present invention will be described below. 
     FIG. 1 to FIG. 22G show an embodiment of the present invention. FIG. 1 to FIG. 3 are concerned with an optical sheet manufacturing system. FIG. 4A to FIG. 10 are concerned with an optical sheet cutting machine. FIG. 11A to FIG. 17B are concerned with an optical sheet joining machine. FIG. 18A to FIG. 22G are concerned with an optical sheet. 
     To begin with, referring to FIG. 1 to FIG. 3, the overall configuration of an optical sheet manufacturing system and the actions thereof will be described below. FIG. 1 is a block diagram showing the configuration of the optical sheet manufacturing system. FIG. 2 is a block diagram showing the configuration of the optical sheet manufacturing system installed in a temperature-and-humidity controlled clean booth. FIG. 3 is a flowchart describing the actions of the optical sheet manufacturing system. 
     The optical sheet manufacturing system consists mainly of, as shown in FIG. 1, an optical sheet cutting machine  1 , a reservoir  2 , an optical sheet joining machine  3 , and a workstation  4 . The optical sheet cutting machine  1  trims an optical sheet member  6  that is produced by rolling a transparent resin material using a roller member (see FIG.  4 A and FIG.  4 B). At this time, the optical sheet cutting machine  1  trims the optical sheet member optimally for joining performed at a succeeding step. The optical sheet member  6  cut by the optical sheet cutting machine  1  is stored in the reservoir  2  and an optical sheet produced by the optical sheet joining machine  3  as mentioned later is also stored in the reservoir  2 . The optical sheet joining machine  3  joins a plurality of optical sheet members  6 , which is conveyed from the reservoir  2 , so as to produce a large-area optical sheet. The workstation  4  serves as a controller for controlling these units. The optical sheet member  6  or optical sheet is conveyed between the optical sheet cutting machine  1  and the reservoir  2  or between the reservoir  2  and the optical sheet joining machine  3  by means of a conveyor that is a conveying machine. 
     FIG. 2 shows the configuration of the optical sheet manufacturing system. Herein, paths along which the optical sheet member  6  or optical sheet is distributed are enclosed in a temperature-and-humidity controlled clean booth  5 . That is, the optical sheet cutting machine  1 , reservoir  2 , optical sheet joining machine  3 , and the paths of the conveyors are installed in the temperature-and-humidity controlled clean booth  5 . The temperature-and-humidity controlled clean booth  5  provides an environment-controlled zone in which predetermined temperature and humidity are maintained and the number of dust particles per unit volume in the air is kept equal to or less than a predetermined value. 
     The optical sheet member  6  employed in the optical sheet manufacturing system is produced as a thin sheet made of, for example, an acrylic material and may stretch or contract depending on temperature or humidity. Furthermore, since the optical sheet member is readily electrified, dust in the air is likely to adhere to the optical sheet member. Therefore, the optical sheet cutting machine  1 , reservoir  2 , optical sheet joining machine  3 , and conveyors are installed in the temperature-and-humidity controlled clean booth  5  in order to prevent stretch or contraction, or adhesion of dust or the like. Consequently, the optical sheet member  6  can be manufactured highly precisely and maintained to offer high optical performance. 
     Next, referring to FIG. 3, a description will be made of an operation flow for manufacturing an optical sheet using the optical sheet manufacturing system. 
     When a production line starts operating, the workstation  4  obtains information from the optical sheet Cutting machine  1 , reservoir  2 , optical sheet joining machine  3 , and conveyors (step S 1 ). The workstation  4  instructs the optical sheet cutting machine  1  to start cutting (step S 2 ). 
     A standby state is retained until the optical sheet cutting machine  1  transmits a message saying that cutting is completed (step S 3 ). When the completion is confirmed, information on the cut optical sheet member  6  is obtained (step S 4 ). 
     When the optical sheet member  6  is realized with a lenticular lens sheet, the information to be obtained at this time is information indicating whether the optical sheet member is cut by matching the edge of one elevation with the edge of other elevation or the edge of one depression with the edge of other depression. Otherwise, when undulations are observed over the surface of the lenticular lens sheet, the information to be obtained is information indicating the magnitude or direction of the undulations. 
     A serial number is assigned to the cut optical sheet member  6  (step S 5 ). The obtained information is then stored in storage means incorporated in the workstation  4  in association with the serial number. 
     Thereafter, the conveyor is instructed to convey the optical sheet member  6  to the reservoir  2  (step S 6 ). Information concerning a position at which the optical sheet member is stored is then obtained (step S 7 ). 
     When a plurality of cut optical sheet members  6  are stored in the reservoir  2 , the workstation  4  analyzes the shapes of the optical sheet members  6  according to the information stored in association with the serial numbers assigned to the optical sheet members  6  (step S 8 ). A pair of optical sheet members  6  that is a best-matched pair in terms of joining is determined (step S 9 ). At this time, when the optical sheet members are realized with lenticular lens sheets, optical sheet members each of which is cut with both edges of a cut surface formed with an elevation or a depression are selected as a pair to be joined. An optical sheet member cut with both edges of a cut surface thereof formed with an elevation will not be paired with an optical sheet member cut with both edges of a cut surface thereof formed with a depression. Furthermore, undulations on the surface of a lenticular lens sheet are taken account in selecting a best-matched pair. 
     It is then instructed that the optical sheet members  6  of the determined pair should be carried out of the reservoir  2  (step S 10 ). The conveyor is then instructed to convey the carried-out optical sheet members  6  to the optical sheet joining machine  3  (step S 11 ). The optical sheet joining machine  3  is then instructed to start joining (step S 12 ). 
     Thereafter, a standby state is retained until the optical sheet joining machine  3  transmits a message saying that joining is completed (step S 13 ). When the completion is confirmed, information concerning the produced optical sheet is obtained (step S 14 ). A serial number is assigned to the optical sheet (step S 15 ). The information obtained at step S 14  is stored in association with the serial number. 
     The conveyor is instructed to convey the produced optical sheet (step S 16 ). After the optical sheet is stored in the reservoir  2 , information such as a position in the reservoir  2  at which the optical sheet is stored is obtained from the reservoir  2  and stored in the memory (step S 17 ). The operation flow is then terminated. 
     Next, referring to FIG. 4A to FIG. 10, the optical sheet cutting machine will be detailed. FIG. 4A is a plan view showing the structure of the optical sheet cutting machine. FIG. 4B is a front view showing the structure of the optical sheet cutting machine. FIG. 5 is a flowchart describing the actions of the optical sheet cutting machine. FIG. 6 shows the structure of a coolant feeder included in the optical sheet cutting machine. FIG. 7 is a front view part of which is a sectional view and which shows the structure of a base temperature adjuster. The base temperature adjuster helps retain the temperature of a platform included in the optical sheet cutting machine at predetermined temperature. FIG. 8 shows the optical sheet cutting machine installed in a temperature-and-humidity controlled clean booth. FIG. 9 is a front view showing an example of an anti-vibration structure for the platform employed in the optical sheet cutting machine. FIG. 10 is a sectional view showing a structure for the optical sheet cutting machine that has a suction fixing device incorporated in a turn plate. 
     To begin with, the optical sheet member  6  to be cut by the optical sheet cutting machine  1  is produced by pressing a transparent resin material, which is heated and softened, using a roller member that has a lenticular lens female mold inscribed on the periphery thereof. The state of the edges of the optical sheet member is often not optimal for joining. The optical sheet cutting machine  1  is therefore used to trim the optical sheet member  6  so that the optical sheet member  6  will have a joint surface optimal for joining to be performed at a subsequent step. 
     The optical sheet cutting machine  1  has a platform on which the optical sheet member  6  to be cut is placed. The platform consists mainly of a base  11 , a movable plate  12  mounted on the base, and a turn plate  13  that is mounted on the movable plate  12  and on which the optical sheet member  6  is placed. 
     The movable plate  12  is guided by a rail member or the like that is not shown, and thus slid in the longitudinal direction of the optical sheet cutting machine  1  in FIG.  4 A. The movable plate  12  is driven by slide driving means  21 . 
     The slide driving means  21  includes a presser  21   a  capable of freely jutting out or sinking. The presser  21   a  is attached to the center of the upper edge of the movable plate  12  that is constrained to move upward in FIG. 4A. A position at which the presser  21   a  juts out or sinks is changed in order to adjust a slide position. 
     The turn plate  13  can pivot on a turn pin  13   a  which supports the turn plate  13  so that the turn plate can pivot freely and which is fixed to the movable plate  12 . The turn plate  13  is driven to pivot by means of a turn driving means  22 . 
     The turn driving means  22  has a presser  22   a  capable of freely jutting out or sinking. The presser  22   a  is engaged with an engagement arm  13   b  that is projected from the corner of the turn plate  13 , opposite angle to the turn pin  13   a . The turn plate  13  is constrained to pivot clockwise in FIG. 4A on the turn pin  13   a . A position at which the presser  22   a  juts out or sinks is changed in order to adjust a turn position. 
     After the optical sheet member  6  is placed on the turn plate  13 , the optical sheet member  6  is pressed by, for example, a locking member  14  that is a sheet pressure. The optical sheet member  6  is thus locked on the turn plate  13 . 
     The base  11  has, for example, two rail members  23  that extend in a direction of cutting. A slide unit  15  having a wheel cutter  16  that is a cutting blade used to trim the optical sheet member  6  is guided along the rail, members  23  in a direction of thrusting. 
     A rotation motor serving as a cutting drive source that is not shown and a feed motor serving as a feed driving source are incorporated in the slide unit  15 . The rotation motor has the wheel cutter  16 . 
     The wheel cutter  16  has the surface thereof finished with a grinder particulate made of diamond or cubic boron nitride (CBN). The grinder particulate made of diamond or CBN is superior to any other general grinder particulate in terms of hardness and strength. The wheel cutter  16  is rotated by the rotation motor, which is a cutting drive source, at a rotating speed ranging, for example, from 3000 rpm to 30000 rpm. Consequently, the wheel cutter  16  can produce a cut surface of small roughness, optimal for joining, for example, the roughness of Rmax 0.8 S or less. 
     Moreover, the feed motor causes the slide unit  15  itself to move along the rail members  23 . Specifically, a gear rotated by the feed motor is meshed with a feed shaft  24  whose both ends are fixed to fixtures  24   a  secured to the base  11 , and thus moved along the rail members  23 . 
     The positional relationship information between the wheel cutter  16  included in the slide unit  15  and the optical sheet member  6  is image-picked up and obtained using a top observation camera  17  and section observation cameras  18  and  19 . These cameras  17 ,  18  and  19  are investigating devices. Specifically, the top observation camera  17  that is a top image-pickup apparatus is mounted on the slide unit  15  and moved together with the slide unit. The section observation cameras  18  and  19  that are section image-pickup apparatuses are mounted on the base  11  at proximal and distal positions in a direction in which the wheel cutter  16  is advanced. 
     Images picked up by the cameras  17 ,  18 , and  19  are, as shown in FIG. 4B, transferred to a monitor  25  and viewed by an operator. Moreover, the images picked up by the cameras  17 ,  18 , and  19  are processed in order to allow the operator to grasp the shape of the surface of the optical sheet member  6 . Thereafter, the resultant image data is transmitted to the workstation  4  and may be used for analysis of the shape of the surface. 
     Next, referring to FIG. 5, the actions of the optical sheet cutting machine  1  will be described below. 
     When an operation flow is started, the optical sheet member  6  is placed on the turn plate  13  (step S 21 ). An operator may perform this action. For advanced automation, the conveyor or the like should be used to automatically place the optical sheet member on the turn plate. In this case, the workstation  4  gives an instruction and controls a series of associated actions. 
     Thereafter, images picked up by the top observation camera  17  and section observation cameras  18  and  19  are displayed on the monitor  25  or transmitted to the workstation  4 . Information is thus obtained (step S 22 ). In order to grasp the shape condition of the surface of the optical sheet member  6 , the optical sheet cutting machine  1  itself may perform image processing or the like or the workstation  4  may perform it. 
     It is judged from the obtained image information whether the optical sheet member  6  is located at a predetermined position optimal for cutting (step S 23 ). When the optical sheet member  6  is realized with a lenticular lens sheet, the predetermined position is a position at which the ends of the same elevation that is a convex part of the lenticular lens sheet or the same depression that is a concave part thereof lie at the proximal and distal points on a cutting band line. 
     If it is judged at step S 23  that the optical sheet member  6  is not located at the predetermined position, the slide driving means  21  causes the movable plate  12  to slide (step S 24 ). The turn driving means  22  causes the turn plate  13  to pivot (step S 25 ). Thus, the optical sheet member  6  has the position thereof adjusted to lie at the predetermined position. 
     When the optical sheet member  6  is located at the predetermined position, the rotation motor is driven in border to rotate the wheel cutter  16 . The feed motor is driven in order to move the slide unit  15 , which is engaged with the feed shaft  24 , along the rail members  23  (step S 26 ). 
     A standby state is then attained and retained until the optical sheet member  6  is cut up to the distal point on the cutting band line (step S 27 ). When it is confirmed that the cutting is completed, the optical sheet cutting machine  1  is deactivated. 
     When the wheel cutter  16  is rotated in order to trim the optical sheet member  6 , heat is generated due to friction occurring during rotation. The optical sheet member  6  is, as mentioned above, made of, for example, an acrylic. When heat is generated, the optical sheet member  6  may be stretched or contracted, or softened. This hinders accurate cutting. Consequently, the coolant feeder  31  shown in FIG. 6 is used to cool the wheel cutter  16  and a cut region of the optical sheet member  6 . 
     A coolant feed pipe  31   a  is coupled to the coolant feeder  31 , and a discharge port member  31   b  is fixed to the distal end of the coolant feed pipe  31   a . A coolant fed through the discharge port member  31   b  by means of the coolant feeder  31  is dispersed to the wheel cutter  16  and optical sheet member  6  alike. The wheel cutter  16  and optical sheet member  6  are thus cooled. 
     The coolant dispersed and used for cooling flows down a slope  33 . The coolant is then collected along a collection pipe  31   c  and returned to the coolant feeder  31  for reuse. 
     As mentioned above, the optical sheet member  6  stretches or contracts depending on temperature. Therefore, the temperature of the coolant should be neither too high nor too low. A coolant temperature adjusting device  32  is incorporated in the coolant feeder  31  in order to retain the temperature of a coolant at predetermined temperature. 
     The coolant is used to cool generated heat and also proved effective in smoothing a cut surface of the optical sheet member  6  produced by the wheel cutter  16 . 
     Referring to FIG. 7, a structure for keeping the temperature of the base  11  constant will be described below. 
     As mentioned above, unless the optical sheet member  6  is held at certain temperature, it stretches or contracts. The temperature of the platform on which the optical sheet member  6  is placed should therefore be kept constant. 
     A medium for adjusting temperature is therefore circulated through, for example, the base  11  included in the platform. The temperature of the platform is thus kept constant. 
     Specifically, a base temperature adjuster  34  feeds a medium by way of a medium feed pipe  34   a , passes the medium through a medium distribution channel  11   a  formed in the base  11 , and collects the medium through a medium collection pipe  34   b.    
     A medium temperature adjusting device  35  analogous to the coolant temperature adjusting device  32  is incorporated in the base temperature adjuster  34  in order to retain the temperature of the medium at predetermined temperature. 
     Since the temperature of the base  11  is thus kept constant, the temperature of the turn plate  13  with which the optical sheet member  6  comes into direct contact is kept constant. Therefore, the optical sheet member  6  will neither stretch nor contract and can be positioned accurately and cut precisely. 
     FIG. 8 shows the entire optical sheet cutting machine  1  installed in the aforesaid temperature-and-humidity controlled clean booth  5 . The optical sheet cutting machine  1  may be installed solely or may, as shown in FIG. 2, be installed together with the other equipment. 
     Consequently, total temperature control can be achieved on a more stable basis. Moreover, adhesion of dust in the air to the optical sheet member  6  can be prevented effectively. 
     FIG. 9 shows an example of an anti-vibration structure for the optical sheet cutting machine  1 . In this example, an anti-vibration pedestal  36  formed with a rubber or springs is used to bottom the base  11 . 
     Using the structure, even if a highway on which, for example, large trucks run exists in the vicinity of a manufacturing works, the optical sheet member  6  can be cut accurately while being unaffected by vibrations stemming from the running trucks. 
     FIG. 10 shows a structure for the optical sheet cutting machine  1  that has a suction fixing device incorporated in the turn plate  13 . 
     The optical sheet member  6  is locked on the turn plate  13  using the locking member  14 . More preferably, the optical sheet member  6  should be brought into close contact with the turn plate  13  with sufficient flatness ensured. 
     Therefore, a plurality of suction holes  13   c  are, as shown in FIG. 10, bored in the surface of the turn plate  13  on which the optical sheet member  6  is placed. A vent  13   d  with which the suction holes  13  communicate is formed internally. A suction pump  37  is used to suck air through a pipe  37   a.    
     Owing to the structure, cutting can be achieved more accurately. 
     Referring to FIG. 11A to FIG. 17B, the structure of the optical sheet joining machine will be detailed below. FIG. 11A is a plan view showing the structure of the optical sheet joining machine. FIG. 11B is a front view showing the structure of the optical sheet joining machine. FIG. 11C is an enlarged view showing part of the structure of the optical sheet joining machine. FIG. 12A, FIG. 12B, FIG. 12C, FIG. 12D, and FIG. 12E are side views showing a flow operation in which the optical sheet joining machine joins optical sheet members. FIG. 13 is a flowchart describing the actions of the optical sheet joining machine. FIG. 14 shows the optical sheet joining machine installed in the temperature-and-humidity controlled clean booth. FIG. 15 is a front view showing an example of an anti-vibration structure for the optical sheet joining machine. FIG. 16 is a sectional view showing a structure for the optical sheet joining machine that has a suction fixing device incorporated in at least one of a stationary plate and a movable plate. FIG. 17A is a plan view showing an example of a structure for the optical sheet joining machine that enables sequential joining of elongated optical sheet members. FIG. 17B is a front view showing the example of the structure for the optical sheet joining machine that enables sequential joining of elongated optical sheet members. 
     The optical sheet members  6  cut by the aforesaid optical sheet cutting machine  1  are joined by the optical sheet joining machine  3 . 
     Specifically, the optical sheet joining machine  3  consists mainly of a stationary plate  41 , a locking member  43 , a movable plate  42 , a locking member  44 , and a bonding unit  51 . One optical sheet member  6  is placed on the stationary plate  41 . The locking member  43  is realized with a sheet presser or the like and locks the placed optical sheet member  6  on the stationary plate  41 . The other optical sheet member  6  is placed on the movable plate  42 . The locking member  44  is realized with a sheet presser or the like and locks the placed optical sheet member  6  on the movable plate  42 . The bonding unit  51  applies an adhesive  7  to an end surface  6   c  of one optical sheet member  6 , which is a joint surface, from the proximal edge to the distal edge (see FIG. 12A, FIG. 12B, FIG. 12C, FIG. 12D, and FIG. 12E) so that the optical sheet member  6  can be bonded to the other optical sheet member  6 . 
     The movable plate  42  is guided by a guide member  47  and thus movable to approach or recede from the stationary plate  41 . 
     The stationary plate  41  has a rail member  45 . The bonding unit  51  can move along the rail member  45 . A moving mechanism for moving the bonding unit  51  is such that a gear rotated by a feed motor incorporated in the bonding unit  51  is meshed with a feed shaft  46 , which has both end portions thereof fixed to the stationary plate  41 , in order to move the bonding unit  51  along the rail member  45 . 
     Furthermore, section observation cameras  48  and  49  used to observe joint surfaces are located at the proximal and distal positions in a direction, in which the bonding unit  51  advances, on the stationary plate  41 . 
     Moreover, the bonding unit  51  consists mainly of a top observation camera  53 , an electrification preventing device  54 , a dispenser  55 , an adhesive sucking device  56 , a hardening light  57 , and a warm-air fan  58 . The top observation camera  53  that is an imaging device is used to observe joining from above. The electrification preventing device  54  prevents electrification of two optical sheet members  6  to be joined. The dispenser  55  jets an adhesive  7  out to the end surface  6 c of the optical sheet member  6  locked on the stationary plate  41 . Herein, the end surface  6   c  may be referred to as a joint surface. The adhesive sucking device  56  sucks a portion of the adhesive  7  sandwiched between the two optical sheet members  6  which oozes out. The hardening light  57  is used to harden the adhesive  7 . The warm-air fan  58  facilitates joining of the two optical sheet members  6 . 
     Furthermore, images picked up by the cameras  48 ,  49 , and  53  are transferred to a monitor  52  and viewed by an operator. Otherwise, the images picked up by the cameras  48 ,  49 , and  53  are sent to the workstation  4  as information on joining. 
     Next, the actions of the optical sheet joining machine  3  will be described with reference to FIG. 12A, FIG. 12B, FIG. 12C, FIG. 12D, FIG. 12E, and FIG.  13 . 
     When an operation flow is started, two optical sheet members  6  are placed on the stationary plate  41  and movable plate  42  respectively (step S 31 ). As mentioned above, an operator may perform this action. For advanced automation, the conveyor or the like should be used to automatically place the optical sheet members. In this case, the workstation  4  gives an instruction and controls a series of associated actions. 
     Next, images picked up by the top observation camera  53  and the section observation cameras  48  and  49  are displayed on the monitor  52  or transmitted to the workstation  4 . Information is thus obtained (step S 32 ). 
     Based on the information, it is judged whether the optical sheet members  6  are located at predetermined positions optimal for joining on the stationary plate  41  and movable plate  42  respectively (step S 33 ). If the optical sheet members  6  are not located at the predetermined positions, the positions of the optical sheet members are adjusted (step S 34 ). 
     At this time, the end surface  6   c  of the optical sheet member  6  placed on the stationary plate  41  and the end surface  6   c  of the optical sheet member  6  placed on the movable plate  42  are separated from each other by a predetermined distance (see FIG.  12 A). The end surface  6   c  may be referred to as a joint surface. 
     If it is judged at step S 33  that the optical sheet members are located at the predetermined positions, the electrification preventing device  54  is turned on (step S 35 ). Thus, dust in the air is prevented from adhering to the optical sheet members  6 . The bonding unit  51  is then moved along the rail member  45  (step S 36 ). 
     After the bonding unit  51  starts moving, when the dispenser  55  reaches the proximal edge of the end surface  6   c  or joint surface of the optical sheet member  6  placed on the stationary plate  41 , the dispenser  55  starts, as shown in FIG. 12A, jetting out the adhesive  7  (step S 37 ). 
     It is then judged whether the dispenser  55  has reached the terminal edge of the end surface  6   c  of the optical sheet member  6  along with the movement of the bonding unit  51  (step S 38 ). The movement is continued at step S 37  until the dispenser  55  reaches the terminal edge. 
     If it is confirmed at step S 37  that the dispenser  55  has reached the terminal edge of the end surface  6   c  of the optical sheet member  6 , the movable plate  42  is moved along the guide member  47  manually or using a driving mechanism that is not shown. Thus, the movable plate approaches the stationary plate  41  (step S 39 ). 
     Consequently, one optical sheet member  6  and the other optical sheet member  6  approach each other. As shown in FIG. 12B, the adhesive  7  adhering to the end surface  6   c  of one optical sheet member  6  also adheres to the end surface  6   c  of the other optical sheet member  6 . The adhesive  7  is thus sandwiched between the end surfaces  6   c  of the two optical sheet members  6 . 
     At this time, preferably, the two optical sheet members  6  are approached at a constant speed so that joining can be performed uniformly and for fear bubbles may be mixed in the adhesive  7 . 
     Thereafter, the feed motor incorporated in the bonding unit  51  is rotated reversely in order to move the bonding unit  52  in an opposite direction (step S 40 ). The adhesive sucking device  56  sucks an excessive portion of the adhesive  7  oozing out from the joint surfaces of the optical sheet members  6  (step S 41 ). 
     Oozing out of the adhesive  7  is thought to occur on both sides of the optical sheet members  6 . Therefore, the excessive portion of the adhesive should be, as shown in FIG. 12C, sucked from both the sides of the optical sheet members using a plurality of adhesive sucking devices  56 . 
     It is then judged whether the adhesive sucking device  56  has reached the proximal edges of the end surfaces  6   c  of the optical sheet members  6  (step S 42 ). If not, the action of step S 41  is continued until the adhesive sucking device reaches the proximal edges thereof. 
     If it is confirmed at step S 42  that the adhesive sucking device has reached the proximal edges of the end surfaces  6   c  of the optical sheet members  6 , the feed motor incorporated in the bonding unit  51  is rotated forward to move the bonding unit in a forward direction similarly to that at step S 36  (step S 43 ). 
     When the movement is started, the hardening light  57  is, as shown in FIG. 12D, turned on at the same time (step S 44 ). Furthermore, the warm-air fan  58  is actuated in order to start feeding warm air (step S 45 ). The adhesive  7  is thus hardened sequentially along the joint line defined with the joint surfaces. 
     Thereafter, it is judged whether the hardening light  57  and warm-air fan  58  have reached the terminal edges of the end surfaces  6   c  of the optical sheet members  6  (step S 46 ). Otherwise, the actions of step S 44  and step S 45  are continued until the hardening light  57  and warm-air fan  58  reach the terminal edges. 
     If it is confirmed at step S 46  that the hardening light  57  and warm-air fan  58  have reached the terminal edges of the end surfaces  6   c  of the optical sheet members  6 , the plurality of optical sheet members  6  are, as shown in FIG. 12E, joined to produce a large-area optical sheet. The adhesive  7  is made of a transparent material whose optical property such as a refractive index is as close as possible to that of a material made into the optical sheet members  6 . Therefore, the optical property of the optical sheet will not be impaired due to the join surfaces. 
     The electrification preventing device  54  turned on at step S 35  is turned off, whereby the joining operation is terminated. 
     FIG. 14 shows the entire optical sheet joining machine  3  shown in FIG. 11A, FIG. 11B, and FIG.  11 C and installed in the aforesaid temperature-and-humidity controlled clean booth  5 . At this time, the optical sheet joining machine  3  may be installed solely or may be, as shown in FIG. 2, installed therein together with the other equipment. 
     Consequently, total temperature control can be achieved on a more stable basis. Moreover, adhesion of dust in the air to the optical sheet members  6  can be prevented effectively. 
     FIG. 15 shows an example of an anti-vibration structure for the optical sheet joining machine  3 . In this example, an anti-vibration pedestal formed with a rubber or springs similarly to the aforesaid one is used to bottom the stationary plate  41 . Consequently, the optical sheet members  6  can be joined accurately while being unaffected by vibrations. 
     FIG. 16 shows a structure for the optical sheet joining machine  3  that has a suction fixing device incorporated in each of the stationary plate  41  and movable plate  42 . 
     Even at a joining step of a manufacturing process, similarly to at the aforesaid cutting step, the locking members  43  and  44  are used to lock the optical sheet members  6 . For higher flatness, the suction fixing device should be used in addition. 
     Specifically, as shown in FIG. 16, pluralities of suction holes  41   a  and  42   a  are bored in the surfaces of the stationary plate  41  and movable plate  42  respectively on which the optical sheet members  6  are placed. Vents  41   b  and  42   b  with which the suction holes  41   a  and  42   a  communicate are formed internally. A suction pump  61  is then used to suck air through a pipe  61   a.  This enables more accurate joining. 
     Next, referring to FIG.  17 A and FIG. 17B, a description will be made of an example of a structure for the optical sheet joining machine that enables sequential joining of elongated optical sheet members. 
     The optical sheet joining machine  3  sequentially joins optical sheet members  6 A, each of which is shaped like a belt having a predetermined width, with the longitudinal edges of the optical sheet members met each other. 
     Specifically, the optical sheet members  6 A that are not yet joined are, as shown in FIG.  17 A and FIG. 17B, wound about take-up members  64   a  and  64   b  respectively. The optical sheet members  6 A are pulled out onto a plate  63  at a predetermined speed, and joined while being moved. Thereafter, the optical sheet members  6 A are taken up by a take-up member  65 . At this time, when for example, the take-up member  65  is rotated using a driving mechanism such as a motor, an optical sheet moving mechanism is realized for moving the optical sheet members  6 A to the plate  63  or to a bonding unit  51 A that will be described later. 
     To be more specific, two optical sheet members  6 A are placed on the plate  63  so that they can move along the longitudinal direction of the plate  63 . Furthermore, the plate  63  is designed so that the two optical sheet members  6 A separated from each other by a predetermined distance will approach each other in a direction on a moving path in which the optical sheet members advance. 
     The two optical sheet members  6 A placed on the plate  63  are pressed by pressing members  43 A and  44 A respectively so that they can slide. This is intended to move the optical sheet members  6 A with at least the portions thereof to be joined brought into close contact with the plate  63 . 
     The bonding unit  51 A is fixed to the plate  63  located near the pressing members  43 A and  44 A. The bonding unit  51 A enables sequential joining of two optical sheet members  6 A that move over the plate  63 . 
     The portions of the optical sheet members  6 A that are in the opposite direction to advancement direction (near the winding members  64   a  and  64   b ) are separated from each other by a predetermined distance. At this time, the dispenser  55  (see FIG. 11C) incorporated in the bonding unit  51 A sequentially sprays the adhesive  7  to the end surface  6   c  or joint surface of one optical sheet member  6 A. As the optical sheet members  6 A advance, they approach rapidly. When the optical sheet members  6 A come into close contact with each other with the adhesive  7  between them, the adhesive sucking device  56  sucks the excessive portion of the adhesive  7 . Thereafter, the hardening light  57  and warm-air fan  58  are used to harden the adhesive. 
     The structures and abilities of the top observation camera  53  and electrification preventing device  54  incorporated in the bonding unit  51 A are identical to those described in conjunction with FIG.  11 C and others. 
     As mentioned above, the optical sheet members  6 A are joined sequentially by performing an operation flow. The bonding unit  51 A is therefore longer in the direction, in which the optical sheet members  6 A advance, than the one shown in FIG.  11 A and others. Incorporated in the bonding unit  51 A are, for example, in the order shown in FIG. 11C, the observation camera  53 , electrification preventing device  54 , dispenser  55 , adhesive sucking device  56 , hardening light  57 , and warm-air fan  58  which are arranged with a required space between adjoining devices. 
     Owing to the aforesaid structure, two optical sheet members  6 A each shaped like a belt having a predetermined width are joined sequentially. This results in an optical sheet whose width is twice as large as that of the original optical sheet members  6 A. When an optical sheet is used as an optical screen on which an image is projected from an image projector, an optical sheet of a required length is cut out from the belt-like optical sheet produced by joining the optical sheet members. 
     Referring to FIG. 18A to FIG. 22G, the structure of an optical sheet produced according to the aforesaid manufacturing process will be detailed below. FIG.  18 A and FIG. 18B are perspective views showing the structure of an optical sheet produced by joining optical sheet members. FIG. 19 is an enlarged view showing joint portions of optical sheet members. FIG.  20 A and FIG. 20B show paired and joined optical sheet members that have undulations extended substantially in the same direction on the surfaces thereof. FIG. 21A, FIG. 21B, FIG. 21C, and FIG. 21D show paired and joined optical sheet members that have undulations extended in substantially symmetrical directions on the surfaces thereof. FIG. 22A is a perspective view showing optical sheet members joined with depressions of the surfaces thereof met each other to define both end portions of a joint line. FIG. 22B is an enlarged partial view showing the end surfaces of the optical sheet members shown in FIG.  22 A. FIG. 22C is a sectional view showing the optical sheet members shown in FIG.  22 A. FIG. 22D is a perspective view showing paired optical sheet members with elevations of the surfaces thereof met each other to define both end portions of a joint line. FIG. 22E is an enlarged partial view showing the end surfaces of the optical sheet members shown in FIG.  22 D. FIG. 22F is a sectional view showing the optical sheet members shown in FIG.  22 D. FIG. 22G is a sectional view showing a mismatch observed in the middle of a joint line that has both end portions thereof defined with elevations or depressions of the surfaces of optical sheet members which are met to join the optical sheet members. 
     The optical sheet member  6  has a surface thereof formed by cyclically repeating a predetermined shape with a predetermined pitch between adjoining shapes in at least one direction. For example, the optical sheet member  6  is realized with a lenticular lens sheet formed by juxtaposing a plurality of cylindrical lenses unidirectionally, a film whose refractive index varies alternately streakily, a micro-lens array having microscopic lenses arranged two-dimensionally, or a concentric Fresnel lens sheet. 
     When the optical sheet member  6  is realized with, for example, the lenticular lens sheet, the optical sheet member is cut along the axis of any of juxtaposed cylindrical lenses using the optical sheet cutting machine  1 , and joined to the other optical sheet member using the optical sheet joining machine  3 . 
     FIG. 18A shows an optical sheet produced by joining the optical sheet members. When the optical sheet member  6  is realized with a lenticular lens sheet, two major surfaces  6   a  and  6   b  of the optical sheet member  6  have elevations  6   d  and depressions  6   e  extended along the axes of cylindrical lenses thereof. As for the dimensions of the optical sheet member, for example, the maximum thickness H thereof, that is, the length between the elevation  6   d  of one major surface  6   a  and the elevation  6   d  of the other major surface  6   b  is 0.6 mm. A pitch W between adjoining elevations  6   d  or depressions  6   e  is 0.24 mm. 
     In the illustrated example, the optical sheet member is realized with a lenticular lens sheet having the elevations  6   d  and depressions  6   e  formed on the two major surfaces  6   a  and  6   b . Alternatively, a lenticular lens sheet having the elevations and depressions formed on one major surface will be also possible to adopt. 
     Moreover, for example, when an optical sheet is used as an optical screen, holding pieces  8  serving as reinforcement members may be, as shown in FIG. 18B, attached to the edges of the optical sheet outside an optically effective field (outside a field on which an image is projected) to reinforce the joining surface. 
     Furthermore, referring to FIG. 12E, when optical sheet members  6  are joined, the adhesive  7  is sandwiched between the end surfaces  6   c  that may be referred to as joint surfaces. Alternatively, as shown in FIG. 19, the adhesive may also be applied to both surfaces of the optical sheet members over a length corresponding to a range from, for example, 1 pitch to 5 pitches across the joint surfaces of the optical sheet members. Herein, one pitch is a length between adjoining convex or concave parts of one surface of the optical sheet member. The joint portions of the optical sheet members may thus be reinforced. When the transparent adhesive is thus applied to near the joint portions, an optically undesirable effect that may stem from a difference of levels caused by the joint surfaces can be alleviated. 
     By the way, as mentioned above, the optical sheet member  6  cannot help having some undulations over the concavo-convex surface thereof. When the optical sheet members  6  are joined without any consideration taken into the undulations, since the joint surfaces of the optical sheet members are inconsistent with each other, they are optically mismatched. Consequently, the joint surfaces are visualized as, for example, a streak. 
     Consequently, when the optical sheet joining machine  3  is used to join optical sheet members, a best-matched pair of optical sheet members  6  is selected based on information concerning the conditions of the surfaces of the optical sheet members  6  cut by the optical sheet cutting machine  1 . The selected optical sheet members  6  are then joined. 
     The best-matched pair of optical sheet members will be described with reference to FIG. 20A, FIG. 20B, FIG. 21A, FIG. 21B, FIG. 21C, and FIG.  21 D. 
     FIG.  20 A and FIG. 20B show joining of optical sheet members having undulations extended in substantially the same direction on the surfaces thereof. 
     As shown in FIG. 20A, the paired optical sheet members  6  each have undulations whose center portions are angled leftward. Assume that the magnitude of undulations occurring on one optical sheet member  6  near the joint surface thereof is x and the magnitude of undulations occurring on the other optical sheet member  6  near the joint surface thereof is y. A pair of optical sheet members to be selected satisfies a condition of |x−y|&lt;2 pitches. More preferably, a pair of optical sheet members satisfying a condition of |x−y|&lt;1 is selected. In practice, when the pitch W is 0.24 mm as mentioned above, the difference in the magnitude of undulations between optical sheet members should be equal to or smaller than 0.48 mm. 
     Incidentally, the extent of undulations must be, as shown in FIG. 20B, ranged within a length of, for example, about 1 mm from each of the joint surfaces of optical sheet members. 
     FIG. 21A, FIG. 21B, FIG. 21C, and FIG. 21D show a pair of optical sheet members having undulations extended in substantially symmetrical directions on the surfaces thereof. 
     FIG.  21 A and FIG. 21B show a case where undulations take place to cause the center portion of the concavity and convexity on the surface of one optical sheet member to recede from the center portion of the concavity and convexity on the surface of the other optical sheet member. FIG.  21 C and FIG. 21D show a case where undulations take place to cause the center portion of the concavity and convexity on the surface of one optical sheet member to approach the center portion of the concavity and convexity on the surface of the other optical sheet member. 
     In both the cases shown in FIG.  21 A and FIG.  21 B and in FIG.  21 C and FIG. 21D, a pair of optical sheet members satisfying a condition of |x−y|&lt;1.5 pitches is selected. More preferably, a pair of optical sheet members satisfying a condition of |x−y|&lt;0.5 pitches or less should be selected. In practice, when the pitch W is, as mentioned above, 0.24 mm, the difference of the magnitude of undulations between one optical sheet member and the other optical sheet member should be 0.36 mm or less. 
     FIG. 22A, FIG. 22B, FIG. 22C, FIG. 22D, FIG. 22E, FIG. 22F, and FIG. 22G show a case where the optical sheet members  6  having undulations are joined with depressions  6   e , that is, concave parts thereof met each other to define both end portions of a joint line or with elevations, that is, convex parts thereof met each other to define both end portions of a joint line. 
     FIG. 22A, FIG. 22B, and FIG. 22C show a case where the optical sheet members  6  are joined with depressions  6   e , that is, concave parts thereof met each other to define both end portions of a joint line. Because of undulations, the center portion of the joint line is not defined with joining of the depressions  6   e  but at least the both end portions of the joint line are defined with joining of the depressions  6   e.    
     FIG. 22D, FIG. 22E, and FIG. 22F show a case where the optical sheet members  6  are joined with elevations  6   d  thereof, that is, convex parts thereof met each other to define both end portions of a joint line. Similarly to the above case, because of undulations, the center portion of the joint line is not defined with joining of the elevations  6   d  but at least the both end portions of the joint line are defined with joining of the elevations  6   d.    
     The optical sheet members  6  are cut by the optical sheet cutting machine  1  so that the both end surfaces  6   c  or joint surfaces thereof will be sections of elevations or depressions of the concavo-convex surfaces thereof. Thus, elevations  6   d  or depressions  6   e  are met each other to define at least the both end portions of the joint line. Consequently, the optical sheet members to be joined can be optically matched. 
     At this time, elevations or depressions may not be met each other perfectly to define a joint line. In a case shown in FIG. 22G, assume that the amplitude in a height direction of the convex and concave optical sheet members is G and the magnitude of a mismatch adversely affecting an optical property is g. If g is 50% or less of G, the mismatch is permissible. That is to say, the joint portions of the optical sheet members can be relatively optically well matched, and a joint line defined with joining of the joint portions will not substantially be discerned as a streak. 
     As mentioned above, the optical sheet member  6  is cut by rotating the wheel cutter  16 , which is finished with a grinder particulate of diamond or cubic boron nitride, at a high speed. The roughness of the cut surface of the optical sheet member  6  is Rmax 0.8 S or less. An experiment performed by the present applicant demonstrates that the roughness attained ranges from Rmax 0.1 S to Rmax 0.2 S. Thus, an optical effect to be exerted by the cut surface is minimized. 
     According to the aforesaid embodiment, an optical sheet member is cut along a cutting band line optimal for joining so that it will have a smooth cut surface. Appropriate optical sheet members are paired and joined using an adhesive. This results in an optical sheet in which the joint portions of the optical sheet members will almost not exert an optically adverse effect. 
     At this time, when an optical sheet is produced under an environment in which temperature, humidity, and the number of dust particles in the air are controlled, the optical sheet can be produced with higher precision. 
     Occurrence of undulations on the surface of an optical sheet member during a manufacturing process is unavoidable because of the manufacturing process. However, a position on an optical sheet member at which the optical sheet member is cut is determined discreetly or pairing of optical sheet members is achieved carefully. Consequently, an optical mismatch between joint surfaces attributable to undulations can be reduced to the greatest possible extent. 
     Having described the preferred embodiments of the invention by referring to the accompanying drawings, it should be understood that the present invention is not limited to those precise embodiments but various changes and modifications thereof could be made by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.