Abstract:
A method of mass producing a micropolarizer including the steps exposing films of predetermined polarization states to electromagnetic radiation through masks of predetermined patterns, etching away exposed parts of each film and aligning and laminating the films to one another to provide a micropolarizer comprising alternating sets of microscopic polarizers with different polarization states.

Description:
RELATED CASES 
     This is a Continuation application of application Ser. No. 08/527,094, filed Sep. 12, 1995 now U.S. Pat. No. 5,844,717, entitled “Method And System For Producing Micropolarization Panels For Use In Micropolarizing Spatially Multiplexed Images Of 3-D Objects During Stereoscopic Display Processes (As Amended)”; which is a continuation of application Ser. No. 07/536,419, filed Jun. 11, 1990, entitled “METHODS FOR MANUFACTURING POLARIZERS”, now U.S. Pat. No. 5,327,285, issued on Jul. 5, 1994. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of polarizers and the high throughput mass manufacturing of a new class of polarizars called micropolarizers. Micropolarizers have been developed for use in spatial multiplexing and demultiplexing image elements in a 3-D stereo imaging and display system. 
     2. Description of Related Art 
     This invention is related to my co-pending application Ser. No. 07/536,190 entitled “A System For Producing 3-D Stereo Images” filed on even date herewith incorporated herein by reference in its entirety, which introduces a fundamentally new optical element called a micropolarizer. The function of the micropolarizer is to spatially multiplex and spatially demultiplex image elements in the 3-D stereo imaging and displaying system of the aforementioned co-pending application. As shown in FIG. 1, the micropolarizer (μPol)  1 ,  2  is a regular array of cells  3  each of which comprises a set of microscopic polarizers with polarization states P 1  and P 2 . The array has a period p which is the cell size and is also the pixel size of the imaging or displaying devices. 
     It is possible to turn unpolarized light into linearly polarized light by one of three well known means: 1) Nicol prisms; 2) Brewster Angle (condition of total internal reflection in dielectric materials); and 3) Polaroid film. These are called linear polarizers. The Polaroids are special plastic films which are inexpensive and come in very large sheets. They are made of polyvinyl alcohol (PVA) sheets stretched between 3 to 5 times their original length and treated with iodine/potassium iodide mixture to produce the dichroic effect. This effect is responsible for heavily attenuating (absorbing) the electric field components along the stretching direction while transmitting the perpendicular electric field components. Therefore, if P 1  is along the stretching direction of the PVA sheets, it is not transmitted, where as only P 2  is transmitted, producing polarized light. By simply rotating the PVA sheet 90 degrees, P 1  state will now be transmitted and P 2  will be absorbed. 
     The aforementioned three known means for producing polarized light have always been used in situations where the polarizer elements have large areas, in excess of 1 cm 2 . However, for 3-D imaging with μPols using 35 mm film, to preserve the high resolution, the μPol array period p may be as small as 10 micron. Therefore, there is no prior art anticipating the use of or teaching how to mass produce μPols having such small dimensions. 
     SUMMARY OF THE INVENTION 
     The present invention provides a means for high through put mass manufacturing of micropolarizer arrays. To use the μPols in consumer 3-D photography, and printing applications, the economics dictate that the cost of μPols be in the range of 1 to 5 cents per square inch. For this reason, the low cost PVA is the basis for the manufacturing process. 
     The present invention also provides a flexible μPols manufacturing process which can be adapted to low and high resolution situations as well as alternative manufacturing methods, each of which may be advantageous in certain applications and adaptable to processing different polarizer materials. The present invention further provides an electronically controllable μPol. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of a micropolarizer array according to the present invention. 
     FIGS. 2 and 3 illustrate fabrication processes of linear micropolarizers using, respectively, bleaching and selective application of iodine. 
     FIG. 4 shows final alignment and lamination processes for making linear micropolarizers. 
     FIG. 5 illustrates a process for fabricating linear micropolarizers by means of etching. 
     FIG. 6 illustrates a method for patterning micropolarizer by mechanical means. 
     FIG. 7 shows final alignment and lamination processes for making linear micropolarizers by the etching method. 
     FIG. 8 shows final alignment and lam ion processes for making circular micropolarizers by the etching method. 
     FIGS. 9 and 10 illustrate processes for making linear and circular polarizers eliminating an alignment step. 
     FIGS. 11 and 12 illustrate photo-lithographic patterning steps. 
     FIG. 13 illustrates an automated high through-put process for continuous production of micropolarizer sheets by photo-lithographic means. 
     FIG. 14 illustrates an automated high through-put process for continuous production of micropolarizer sheets by direct application of bleaching ink or iodine-based ink. 
     FIG. 15 illustrates an active electronically controllable micropolarizer based on electro-optical effect of liquid crystals. 
    
    
     DETAILED DESCRIPTION 
     Since its invention by E. Land in the 1930&#39;s, polyvinyl alcohol (PVA) has been the polarizer material of choice. It is available from several manufacturers including the Polaroid Corporation. It comes as rolls 19 inches wide and thousands of feet long. The PVA, which is 10 to 20 micron thick, is stretched 2 to 5 times original length and treated with iodine to give it its dichroic (polarizing) property. The PVA treated in this manner crystallizes and becomes brittle. The processes below employ certain chemical properties of the PVA. These are: i) resistance to organic solvents and oils; ii) water solubility, 30% water and 70% ethyl alcohol; iii) bleaching of the dichroic effect in hot humid atmosphere and by means of caustic solutions; iv) manifestation of dichroic effect by painting the PVA in iodine/potassium iodide solution; and v) the stabilization of the dichroic effect in boric acid solution. The starting PVA material comes laminated to a clear plastic substrate which protects the brittle PVA and facilitates handling and processing. The substrate is made either of cellulose aceto bytyrate (CAB) or cellulose triacetate (CTA), and is typically 50 to 125 micron thick. CAB and CTA are ultra-clear plastics and at the same time they are good barriers against humidity. For some applications, large glass plates are also used as substrates. Although other polymers, when stretched and treated by dichroic dyes, exhibit similar optical activity to that of PVA and may be fabricated into micropolarizers following the methods taught here, only PVA is considered in the manufacturing processes described in the present invention. 
     The physical principles on which the polarization of light and other electromagnetic waves, and the optical activity which produces phase retardation (quarter wave and half wave retarders) are described in books on optics, such as: M. Born and E. Wolf, Principles of Optics, Pergamon Press, London, fourth edition, 1970; F. S. Crawford, Jr., Waves, McGraw-Hill, New York, 1968; and M. V. Klein, Optics, Wiley, N. Y., 1970. There are several important facts used in this invention: 
     1. Two linear polarizers with their optical axis 90 degrees from each other extinguish light. 
     2. A linear polarization which is 45 degrees from the optical axis of a quarter wave retarder is converted into a circular polarization. 
     3. A linear polarization which is 45 degrees from the optical axis of a half wave retarder is converted into a linear polarization rotated 90 degrees. 
     4. Two linear polarization states, P 1  and P 2 , 90 degrees from each other, are converted into clockwise and counter-clockwise circular polarization states by means of a quarter waver retarder. 
     5. A circular polarization is converted into a linear polarization by means of a linear polarizer. 
     6. A clockwise circular polarization is converted into a counter-clockwise polarization by means of a half-wave retarder. 
     The process for producing the micropolarizers, μPols,  1 ,  2  in FIG. 1 is described in FIG. 2 which starts with a sheet of linear polarizer  5  laminated onto a clear substrate  4 . The laminate is coated with photosensitive material  6  called photoresist. This can be one of several well known liquid photoresists marketed by Eastman Kodak and Shipley, or in the form of a dry photoresist sheet called Riston from the Du Pont Company. The latter is preferred because complete laminated rolls of the three materials  3 ,  5 ,  6  can be produced and used to start the μPols process. The photoresist is subsequently exposed and developed using a mask having the desired pattern of the μPols cell  3  producing a pattern with polarization parts protected with the photoresist  6  and unprotected parts  7  exposed for further treatment. These exposed parts  7  are treated for several seconds with a caustic solution e.g., a solution of potassium hydroxide. This bleaching solution removes the dichroic effect from the PVA so that the exposed parts  8  are no longer able to polarize light. The photoresist is removed by known strippers, which have no bleaching effect, thus the first part  9  of the μPols fabrication is produced. Alternatively, FIG. 3 shows a method for making linear μPols by starting with a laminate of PVA  10  which is stretched but does not yet have the dichroic effect, i.e., it has not yet been treated with iodine, and the substrate  4 . Following identical steps as above, windows  7  are opened in the photoresist revealing part of the PVA. The next step is to treat the exposed parts with a solution of iodine/potassium iodide and subsequently with a boric acid stabilizing solution. The exposed parts  11  of the PVA become polarizers while those protected with the photoresist remain unpolarizers. Stripping the photoresist completes the first part of the process. 
     As illustrated in FIG. 4, a complete μPol is made using two parts  13 ,  14  produced by either the process of FIG. 2 or FIG. 3 except that part  13  has polarization axis oriented 90 degrees from that of part  14 . The two parts are aligned  15  so that the patterned polarizer areas do not over lap, and then laminated together to from the final product  16 . The μPol  16  is laminated with the PVA surfaces facing and in contact with each other. The μPol  17  is laminated with the PVA of part  13  is in contact with the substrate of part  14 . The μPol  18  is laminated with the substrates of both parts are in contact with each other. Finally, it is possible to produce the μPol  19  with only one substrate onto which two PVA films are laminated and patterned according to the process described above. The above process leaves the patterned PVA film in place and achieves the desired result by either bleaching it or treating it with iodine solution. The processes described in FIGS. 5 and 6 achieve the desired result by the complete removal of parts of the PVA. In FIG. 5, the starting material is any PVA film  20  (linear polarizer, quarter wave retarder, or half wave retarder) or any non-PVA optically active material laminated to a substrate. As described above, windows  7  in the photoresist are opened. The exposed PVA  7  is removed  21  by means of chemical etching (30% water/70% ethyl alcohol solution), photochemical etching, eximer laser etching or reactive ion etching. Stripping the photoresist, the first part  22  of the μPols process is completed. 
     The removal of PVA can also be accomplished by mechanical cutting or milling means. FIG. 6 illustrates the process which uses a diamond cutter  66  mounted on a motor driven shaft  74 . In one embodiment, the PVA  68  is sandwiched between two polymers, such as poly-methyl methacrylate, PMMA, film  67 , and the sandwich is laminated onto a substrate  69 . The diamond saw is used to cut channels. The channel width and the distance between the channels are identical. The PMMA serves to protect the top PVA surface from abrasion and protects the substrate from being cut by the saw. Next the PMMA on top of the PVA and in the channel is dissolved away, leaving the part  71  with clean substrate surface  70 . This part can be used as is to complete the μPol fabrication or the original substrate  69  is removed by dissolving away the rest of the PMMA, after having attached a second substrate  72 . This part which consists of the patterned PVA  68  laminated to the substrate  72  is used in a subsequent step to complete the μPol. 
     Even though this process is mechanical in nature, it has been shown in Electronic Business, May 14, 1990, page 125, that channels and spacings as small as 5 micron can be made using diamond discs manufactured by Disco HI-TEC America Inc., of Santa Clara, Calif. Realizing that using only one disc makes the process slow and costly, the arrangement in FIG. 6 is used where many discs  73  in parallel  75  is preferred. Each disc has its center punched out in the shape of a hexagonal so that it can be mounted on a shaft  74  with a hexagonal cross section. Hundreds of such discs are mounted on the same shaft and are spaced apart by means of spacers  76  whose diameters are smaller than those of the discs. The diameter difference is used to control the cutting depth. The spacers also have hexagonal centers. The cutting discs and the spacers have the same thickness in order to obtain identical channel width and channel spacing. The discs and spacers are mounted on the shaft tightly to prevent lateral motion, while the hexagonal shaft prevents slipping. The discs are made to rotate between 20,000 and 50,000 RPM and the laminate is cut in continuous fashion, thus achieving high through put. 
     To complete making a whole μPol the parts  22 ,  71 ,  72  prepared by the PVA removal methods are used as in FIG.  7 . If the PVA is a linear polarizer, then, parts  23 ,  24  have patterned polarizers which are oriented 90 degrees from each other, and when aligned  25 , and laminated together, complete linear μPols  26 ,  27 ,  28 ,  29  result. If the PVA is quarter wave retarder, then the parts  30 ,  31  of FIG. 8 have patterned retarders with optical axes oriented 90 degrees from each other, and when aligned  32  and laminated to a sheet of linear polarizer  33 , complete circular μPols  34 ,  35 ,  36  result. 
     Up until now all μPols have been made using two patterned parts aligned to each other and then laminated as in FIGS. 4,  7 , and  8 . It possible make μPols with a single patterned part  38  or  40  in FIGS. 9 and 10, and without the alignment step. In FIG. 9, the single patterned part  38  consists of a patterned half-wave retarder on a substrate  4 . It is mounted simply on a sheet of polarizer  39  with no alignment necessary and a complete μPols results. If a linear polarizer sheet  39  is used, the result is a linear μPols. If a circular polarizer sheet  39  is used, the result is a circular μPols. In FIG. 10 the single patterned part  40  has a linear polarizer which is simply mounted on a circular polarizer sheet  41  to produce a complete μPols. 
     FIG. 11 shows the apparatus  42  used for contact printing of the laminate  46  made of photoresist, PVA, and its substrate. The apparatus consists of a vacuum box  47 , and a vacuum pump  48  attached thereto. The top of box is flat surface with vacuum holes which hold the laminate flat during exposure. The mask  45  with its emulsion facing down, makes direct contact with the photoresist surface with the aid of the top glass cover  44 . The very high intensity UV lamp  43  is then turned on for 30 to 60 seconds to expose the photoresist. The laminate is subsequently removed for development and the rest of the μPols fabrications processes as described in FIGS. 2,  3 , and  5 . This printing process using apparatus  46  is automated for large area μPols production as shown in FIG.  12 . The laminate  46  is furnished in a large roll, is fed to apparatus  42  when the vacuum pump  48  is off and the mask and cover  44  are open. By means of an electronic controller, the following automatic sequences are carried out: (1) the vacuum is turned on; (2) the cover and mask are lowered; (3) the lamp is turned on for certain period of time; (4) the lamp is turned off; (5) the mask and cover are, lifted; (6) the vacuum is turned off; and (7) the laminate is advanced. 
     These steps are repeated until the whole roll is finished. The exposed roll  49  is then processed further. This exposure apparatus is simple and has no critical alignment requirements. 
     The fully automated embodiment in FIG. 13 is used for continuous mass production. The raw roll of laminate  46  enters from the right and the finished roll  56  of μPols exists from the left. As one laminate segment is exposed, it is advanced to the left, developed and rinsed in station  50 . Said segment is then further advanced to the left to be dried in station  51 , and advanced further to section  52 . This station carries out the most critical μPols process by one of three methods discussed above in connection with FIGS. 2,  3 , and  5 . These are: 
     1. Bleaching by means of potassium hydroxide then rinsing. 
     2. Polarizing by means of iodine/potassium iodide solution, boric acid stabilizing solution, then water/methyl alcohol rinse. 
     3. Dry or wet etching of the PVA. 
     After the rinsing step in station  52 , the segment is advanced to station  53  for drying and heat treatment. The photoresist stripping and rinsing is done in  54  and the final drying step in  55 . The finished roll  56  is laminated with a polarizer sheet according to FIGS. 9 and 10 complete the μPols. 
     The photolithographic printing used above involves several steps: 
     1. Application of the photoresist 
     2. Baking 
     3. Making contact with the mask 
     4. Exposure 
     5. Development 
     6. Rinsing 
     7. Drying 
     8. Post baking 
     9. Stripping 
     10. Rinsing 
     11. Drying 
     These steps have been eliminated by using the mechanical method described in FIG.  6 . They are also completely eliminated by using the embodiment illustrated in FIG.  14 . This apparatus  57  promises to be the least expensive high volume manufacturing process for μPols. It consists of a plate drum  58  to which a plate a fixed, a blanket drum  59  which has a rubber surface, and an impression drum  60 . The inks from ink fountains  62 ,  65 , are transferred to the plate by means of rollers  63 ,  64 . The pattern is transferred from the plate to the blanket drum which in turn it transfers to the PVA laminate  61 . The rotation of the blanket drum and the impression drums draws in the laminate, and blanket rubber surface pressing on the laminate causes proper printing. Although the hardware is similar to that used in offset printing press, the process is different from offset printing. The principal difference is in the ink formulation. In offset printing slightly acidic water is used in fountain  65 , and an oil-based paint (linseed oil, pigments, binder, and other additives) is used in fountain  62 . These are not intended to interact w the paper. The pigments in the oil based solution will remain bonded to the paper, and the water evaporates. In the μPols printing process, on the other hand, the oil based solution is clear and is not intended to remain, while the water based solution is intended to interact with the PVA and permanently modify it, by bleaching it or by endowing it with the dichroic property. Another difference is the use of the negative image on the plate to print a positive image of the pattern on the PVA laminate, whereas in the offset printing, the opposite occurs. The plates are made by means which are well known in the offset printing industry. 
     The μPols process using apparatus  57  has three embodiments which depend on the content of the water based solutions in fountain  65 , while fountain  62  contains a fast drying clear oil solution: 
     1. Selective Bleaching: The water based solution contains a bleaching agent such as potassium hydroxide or sodium hydroxide which applied selectively as pattern on the polarized PVA. Where applied, the solution removes the iodine and its polarizing effect. Rinsing and drying steps follow this bleaching step. 
     2. Selective Dichroism: The water based solution contains a iodine/potassium iodide which is applied selectively as a pattern on the unpolarized PVA. Where applied, the solution turns the PVA into a polarizer. This step is followed by a stabilizing step using a boric acid solution and subsequently rinsing using a methyl alcohol solution and drying steps. 
     3. Selective Etching: The water based solution contains a clear polymer which is applied selectively as a pattern on the polarized or unpolarized PVA. Where applied, the solution leaves a protective polymer layer. This step is followed by an etching step to remove the unprotected PVA, by rinsing and drying steps. 
     Electrically Controllable Micropolarizers 
     There are applications in which a variable μPols are needed, and in particular, μPols which are electronically alterable. This can be accomplished by using electro-optical materials such as liquid crystals or organic nonlinear optical polymers, see C. C. Teng and H. T. Man, Applied Physics Letters, 30, 1734 (1990), or magneto-optical materials which have large Faraday rotation. All these materials rotate the polarization of incident radiations by applying voltages or magnetic fields. The preferred embodiment  77  in FIG. 15 uses a twisted nematic liquid crystal  78  which rotates the polarization 90 degrees by applying a voltage alternating at 10 to 20 KHz and having an RMS value of about 10 volts. This voltage is applied between the checker-board patterned transparent electrode made of indium-tin oxide ITO  82  on a glass substrate  80  and an unpatterned ground ITO layer  81  deposited on a second glass substrate  79 . The patterned ITO  82  are connected to a common voltage bus  85 . Each connection  86  is made of aluminum film whose area is a small percentage of the ITO area, in the order of 10%. Thus we created two types of cells: One type which has liquid crystal and ITO  81 ,  82  on both sides, will be affected by the applied electric field; and the other type which has liquid crystal but has ITO  81  on one side only and hence will not be affected by the applied electric field. The polarizer sheet  83  with polarization state P 1  is laminated to the glass substrate  80  completes the electronic μPols. 
     The operating principles of electronically switchable μPols is as follows: When the voltage  84  is zero, the polarization P 1  of the incident light will not change. When a voltage is applied, the cells with ITO on both sides will rotate the polarization to a state P 2 , while the cells with ITO on one side only leave the polarization P 1  unchanged. The end result is a regular periodic array of cells with two polarization states P 1  and P 2 . This is a μPol that can be turned off and on.