Abstract:
The invention provides a method for plasma coating of optical lenses, particularly lenses made of silicone-containing polymer. The method of the invention comprising selectively depressurizing and pressurizing an entry hold chamber and an exit hold chamber while constantly maintaining a plasma gas in a coating chamber at a process pressure without depressurizing and pressurizing repeatedly the coating chamber, wherein the coating chamber is located between the entry hold chamber and the exit hold chamber.

Description:
This application is a division of U.S. patent application Ser. No. 09/911,221 filed Jul. 23, 2001, now U.S. Pat. No. 6,881,269, which claims benefits under 35 U.S.C. §119(e) of U.S. provisional patent application No. 60/225,940 filed Aug. 17, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a system for coating contact lenses, or other optical lens devices, particularly those made of silicone-containing polymer. Hereinafter the term silicone polymers are used to indicate silicone-containing polymers suitable for ocular uses, including rigid silicone polymers, silicone elastomers and silicone hydrogels. The advantages of silicone polymers as contact lens materials have long been recognized. However, silicone polymers have several disadvantages. For example, certain materials in the eye&#39;s tear film tend to adhere to the lenses and reduce their optical clarity. The silicone lens, especially silicone elastomer or hydrogel lens, may be tacky and this characteristic may render the lens to stick to the cornea, and the material&#39;s hydrophobic nature prevents the lens from wetting. 
     To resolve these problems, it is known to apply a very thin hydrophilic coating using electrical glow discharge polymerization. Generally, the coating process involves placing a silicone lens core in, or moving it through, a plasma cloud so that the material adheres to the core. Although various materials may be used, hydrocarbons such as methane may be used. 
     The polymerized coating provides a highly wettable surface without significantly, if at all, reducing the oxygen and carbon dioxide permeability of the lens. It also provides an effective barrier against tear film material that would otherwise adhere to the lens, thereby preventing the optical clarity degradation that would otherwise occur. 
     Conventional plasma polymerization lens coating techniques employ batch systems in which one or more silicone lens cores are placed in a reactor chamber between opposing electrodes. The chamber is then sealed and depressurized by a vacuum system. Significant time is required to pump the batch system to the operative pressure. When a suitable pressure is achieved in the chamber, a process gas is introduced into the chamber interior, and the electrodes are energized. The resulting plasma cloud may apply a thin polymer coating to the lens. After an appropriate time, the electrodes are de-energized, and the reactor chamber is brought back to atmospheric pressure so that the lenses may be removed. 
     It has been recognized that it is preferable to move the lenses through the plasma cloud. Thus, in certain systems, the silicone lens cores are mounted on a rotating wheel disposed between the electrodes so that the wheel carries the lenses through the cloud. These systems are sometimes described as “continuous” systems to distinguish them from other batch systems. However, all such systems are considered to be batch systems for purposes of the present disclosure in that each requires a reactor chamber that must be repeatedly pressurized and depressurized as one or more groups of silicone lens cores are placed in and removed from the system. 
     One example of a batch system is provided in U.S. Pat. No. 4,312,575 to Peyman et al., the disclosure of which is incorporated by reference herein for all purposes. In “Ultrathin Coating Of Plasma Polymer Of Methane Applied On The Surface Of Silicone Contact Lenses,” Journal of Biomedical Materials Research, Vol. 22, 919–937 (1988), Peng Ho and Yasuda describe a batch system including a bell-shaped vacuum chamber in which opposing aluminum electrodes are disposed. A rotatable aluminum plate sits between the electrodes and is driven by an induction motor within the system. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes and addresses disadvantages of prior art constructions and methods. 
     Accordingly, it is an object of the present invention to provide an improved lens plasma coating system. 
     This and other objects are achieved by a system for treating the surface of an optical lens. The system includes an entry chamber having a first entrance gate and a first exit gate. The first entrance gate and the first exit gate seal the entry chamber when the gates are closed. The entry chamber includes a conveyor extending between the first entrance gate and the first exit gate. A first negative pressure source is in selective communication with the entry chamber. A coating chamber has a second entrance gate and a second exit gate. The second entrance gate and the second exit gate seal the coating chamber when they are closed. The coating chamber includes a pair of spaced apart electrodes disposed therein and a conveyor extending between the second entrance gate and the second exit gate so that the conveyor conveys the lens between the electrodes. A source of plasma gas is in communication with the coating chamber to introduce the gas into the coating chamber. A second negative pressure source is in communication with the coating chamber. An electrical power source is in communication with the electrodes to apply a predetermined electrical potential at each electrode so that, upon establishment of a predetermined pressure in the coating chamber by the second negative pressure source, a plasma cloud of the gas is established between the electrodes. An exit chamber has a third entrance gate and a third exit gate. The third entrance gate and the third exit gate seal the exit chamber when they are closed, and the exit chamber includes a conveyor extending between the third entrance gate and the third exit gate. A third negative pressure source is in selective communication with the exit chamber. The entry chamber communicates with the coating chamber through the first exit gate and the second entrance gate so that the entry chamber conveyor and the coating chamber conveyor communicate to pass the lens from the entry chamber to the coating chamber. The coating chamber communicates with the exit chamber through the second exit gate and the third entrance gate so that the coating chamber conveyor and the exit chamber conveyor communicate to pass the lens from the coating chamber to the exit chamber. 
     A method for treating the surface of an optical lens according to the present invention includes providing first an optical lens and providing a coating chamber including a pair of spaced apart electrodes disposed therein. A plasma gas is maintained in the coating chamber. A first predetermined pressure is maintained in the coating chamber, and a predetermined electric potential is maintained at each electrode so that a plasma cloud of gas is established between the electrodes. An entry chamber is provided upstream from the coating chamber, and the first lens is moved into the entry chamber. Gas is introduced into at least a portion of the entry chamber adjacent the coating chamber, and at least that portion of the entry chamber is brought to the first predetermined pressure. The entry chamber is brought into communication with the coating chamber, and the first lens is moved from the entry chamber into the coating chamber and through the plasma cloud. An exit chamber is provided downstream from the coating chamber. Gas is introduced into at least a portion of the exit chamber adjacent the coating chamber, and at least that portion of the exit chamber is brought to the first predetermined pressure. The first lens is moved from the coating chamber to the exit chamber. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which: 
         FIG. 1A  is a perspective view of a lens holding tray for use in a lens coating system and method according to an embodiment of the present invention; 
         FIG. 1B  is an enlarged perspective view of the holding tray as in  FIG. 1A ; 
         FIG. 1C  is a cross-sectional view taken along the line  1 C— 1 C in  FIG. 1B ; 
         FIG. 2  is a perspective view of a lens tray carrier and a slug (i.e. carrier holding system) for use in a lens coating system and method according to an embodiment of the present invention; 
         FIG. 3 , presented on separate drawing sheets as  FIGS. 3A and 3B , is a schematic illustration of a lens coating system according to an embodiment of the present invention; 
         FIG. 4A  is a partial perspective view of a lens coating system according to an embodiment of the present invention; 
         FIG. 4B  is a cross-sectional view taken along the line  4 B— 4 B in  FIG. 4A ; 
         FIG. 4C  is a partial perspective view of a lens coating system according to an embodiment of the present invention; 
         FIG. 5A  is a partial perspective view of a lens coating system according to an embodiment of the present invention; 
         FIG. 5B  is a cross-sectional view of a chamber and valve as shown in  FIG. 5A ; 
         FIG. 6A  is a cross-sectional view of a lens coating system according to an embodiment of the present invention; and 
         FIG. 6B  is a plan view of the interior magnetic arrangement of a magnetic device for use in a coating chamber of a lens coating system according to an embodiment of the present invention. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     The present invention is directed to an optical lens coating system in which lens cores may enter, pass through and exit the system without requiring the coating zone to be repeatedly pressurized and depressurized. Although the discussion herein describes the use of a methane containing plasma cloud to apply a hydrophilic polymer coating to silicone lens cores, it should be understood that this is for exemplary purposes only and that other plasma and lens materials may be used. For example, the system may employ any suitable plasma, whether generated of hydrocarbon or other appropriate material, that would apply a desirable coating on a lens core. Additionally, the plasma may be comprised of an oxidizing gas so that the lens core surface is oxidized to create a hydrophilic layer. As used herein, a “coating” includes such a layer. Further, the system may be used in conjunction with lens cores made of any material upon which it is desirable to place a coating. Thus, it should be understood by those skilled in this art that the description of silicone lens cores and methane plasma herein is not intended to limit the scope or spirit of the present invention. 
     Furthermore the system may use any suitable apparatus and method for generating plasma to treat lens cores. Such apparatus and methods should be understood by those skilled in the art and are therefore not discussed in detail herein. Thus, it should also be understood that the particular arrangement described below for generating plasma is provided for exemplary purposes only. 
     Prior to entering the system, referring now to  FIG. 1A , the lens cores are placed in a holding tray  10  having an outer frame  12  and vertical intermediate members  14 .  FIG. 1A  illustrates an exemplary holding tray. Seven rows of wire holders  16  extend between each adjacent vertical member. In the two outer columns  18  and  20 , each row contains four holders, while each row in the two inner columns  22  and  24  includes three holders. Thus, the tray includes ninety-eight holders in all. 
     Referring to  FIGS. 1B and 1C , each holder  16  includes an annular wire rim  26  and five wire stems  28  extending radially inward therefrom. A lens core  30  is placed in each holder so that it sits on the stems  28 . Holder  16  is described in more detail in commonly-assigned U.S. Pat. No. 5,874,127, the entire disclosure of which is incorporated by reference herein. 
     Referring again to  FIG. 1A , and also referring to  FIG. 2 , each holding tray  10  includes a pair of hooks  32  on the opposing outer vertical members  14  of frame  12 . Corresponding hooks  34  on a tray carrier  36  receive hooks  32  so that holding tray  10  may be hung on the tray carrier. In the embodiment shown in  FIG. 2 , carrier  36  may hold four holding trays  10  and, therefore, up to three hundred ninety two lens cores. 
     Referring now to  FIGS. 3A and 3B , the tray carrier is placed into a linear plasma coating system  40 . Initially, the trays move through a drying chamber comprised of five subchambers (hereinafter referred to as “zones”)  42 A– 42 E, each approximately five meters long. The tray carrier remains for a total of about twenty minutes in the drying chamber for a desired time, say about twenty minutes or sufficient to meet the necessary vacuum and coating application target. 
     Because the lens cores may contain a hydrophilic material, they can be hygroscopic and therefore can absorb water from the environment. Thus, it may be desirable to allow drying time. The dry zones maintain a constant relative humidity level, e.g., at or below ten percent to permit further drying, if necessary, and also provide a dry buffer area in which to place lens cores prior to entering the coating zones. 
     Referring also to  FIGS. 2 ,  4 A and  4 B, each tray carrier is received in a rectangular slot  44  defined as a “slug”  46 . A pair of bolts secures the carrier in the slug. A bore  48  extends through slug  46  beneath the tray carrier. The drying chamber includes a conveyor to transport the slug and carrier (hereinafter referred to collectively as the “carrier” unless otherwise indicated). The conveyor is comprised of individual conveyors in the zones  42 A– 42 E, each extending between opposing wheels  52  and  54 . A servo motor  56  drives the conveyor and may be controlled by a personal computer, main frame system or other programmable logic circuit (hereinafter referred to generally as “PLC”). Two side members  58  sit on respective sides of the conveyor, and rollers  60  are disposed in gaps  62  in each side member to guide the tray carrier as it passes between the side members. 
     A light source  64  mounted in one side member directs light across to the other side member, where it is detected by a light detector  66 . The light source and light detector are aligned so that light passes between them through bore  48  in slug  46 . Light detector  66  outputs a signal to the PLC which, in turn, controls servo motor  56 . Accordingly, the PLC detects the carrier&#39;s presence as the slug initially breaks the light beam between source  64  and detector  66  upon entering the first dry zone  42 A. Other carrier detection systems may be utilized in lieu of the light detector; for example pressure or contact microswitches may also be employed. When bore  48  reaches the light source/detector pair, detector  66  again detects the light beam, and the PLC stops servo motor  56  for an approximate preprogrammed time say four minutes. At the end of this time, the PLC again activates motor  56  so that the carrier is passed to the second dry zone,  42 B. Dry zone  42 B has a conveyor, motor and side member pair like that of zone  42 A, except that an additional mechanism is included in zone  42 B to rotate the side members and conveyor ninety degrees so that the carrier may be passed to zone  42 C. In each zone, however, a light source/detector pair is provided to detect the presence of a carrier in the zone. The PLC moves the carrier from one dry zone to the next if no carrier is still waiting in the subsequent zone. 
     The entrance to zone  42 A may be open or may have a suitable covering as appropriate for a given system. A respective duct  68  feeds from a suitable air handling system (not shown) and directs the conditioned air or gas to each dry zone. Suitable ventilation ducts may also be provided. The air conditioning system may be independently controlled to continuously provide properly temperature-controlled and humidified air to the ducts, e.g., at approximately 70° F. +/−2°. 
     Referring again to  FIGS. 3A and 3B , the PLC moves the carrier through a slit valve  72  into an entry lock  70  if the carrier has been in dry zone  42 E for a sufficient duration, if no carrier is waiting in entry lock  70 , and if suitable conditions exist in entry lock  70  as described in more detail below. Entry lock  70  includes a conveyor  50  and side members  58  as in the dry zones. A light source/detector pair is also provided so that the PLC senses when the carrier is fully within the entry lock. The PLC then stops the servo motor that drives the conveyor and closes the slit valves at the entry lock&#39;s entrance and exit to seal the entry lock. 
     Referring also to  FIGS. 5A and 5B , the entrance slit valve  72  includes a door  74  having a sealing material  76  that lines the periphery of its inside surface. Door  74  is hinged so that it is movable by a linkage  78  between an open and closed position. The PLC controls linkage  78 . When the door is in its closed position, seal  76  surrounds and seals an entrance passage  80  into entry lock  70 . 
     When the light source/detector in entry lock  70  detects the presence of the carrier through bore  48  ( FIG. 2 ), the PLC closes the slit valves at both ends of entry lock  70 . The entry lock is a stainless steel chamber with which inlets, outlets and sensors may communicate as discussed below. It is a closed chamber except for the slit valves. Thus, when the valves are closed, the entry lock is sealed. 
     When the carrier is in the entry lock, and the chamber is sealed, the PLC activates a valve  82  and a pump  84  to pump out the entry lock and thereby create a vacuum condition therein. Specifically, the pump brings the interior area of entry lock  70  from ambient pressure to a desired preset lower pressure, e.g., at or below one mTorr. The PLC monitors the entry lock&#39;s pressure by a pressure sensor  85  extending through the entry lock&#39;s housing. 
     It should be understood that while the entry lock housing, as well as the housings of the entry hold, entry buffer, coating, exit buffer, exit hold and exit lock chambers discussed below, may all made of stainless steel, the housings may be made of any suitable material and in any suitable construction. Further, the housings for the five dry zones and of the five exits zones discussed below may be made from a rigid polymer such as polymethylmethacrylate (PMMA), but may also be made from steel or other suitable material. 
     When the PLC is notified from pressure sensor  85  that the interior pressure of entry lock  70  is at or below the preset lower pressure, and that a preset minimum time has lapsed, say 290 seconds, since valve  82  was activated, the PLC opens slit valve  86  between entry lock  70  and an entry hold chamber and activates the conveyors in both the entry lock and the entry hold so that the carrier is moved into the entry hold. 
     The entry hold also includes vertical side members and a light source/detector pair. When the slug bore  48  ( FIG. 2 ) aligns with the light detector and thereby indicates to the PLC that the carrier is fully in the entry hold, the PLC closes slit valve  86  and a slit valve  90  to seal the entry hold. After closing valve  90 , the PLC activates a valve  92  that opens a gas line  94  connected to a source (not shown) of dry gas, e.g., nitrogen, to the interior of entry lock  70 . The PLC continues to vent the entry lock with the dry gas until pressure sensor  84  indicates atmospheric pressure in the entry lock. The PLC then opens slit valve  72  so that the entry lock can receive the next carrier. 
     The gas is “dry” in that it has a low water content, for example less than three ppm. A dry vent gas is preferred to prevent undesired water absorption by the lens cores or the chamber walls. In a preferred embodiment, a single source of dry gas is used to provide the vent gas to line  94  as well as to the vent lines for other chambers downstream from the entry lock. Thus, it should be understood that while the chambers are referred to herein as having “respective” vent sources, this includes a construction where all the vent lines may be fed by the same ultimate source of vent gas. Similarly, while individual vacuum pumps are shown in  FIG. 3  and described herein, it should be understood that pumping lines to multiple chambers may communicate with the same source of negative pressure. 
     Before opening valve  86 , the PLC brings entry hold  88  to a pressure less than or equal to the set low pressure by activating a valve  98  that opens the entry hold interior to a vacuum pump  100 . When pressure sensors  85  and  102  indicate that the entry lock pressure and the entry hold pressure are equal, +/−5 mT, the PCL opens slit valve  86  to move the carrier into the entry hold. 
     Despite the prior drying stages, the lens cores may still contain some water. The entry hold therefore acts both as a buffer and a drying stage. Repeated pumping to create vacuum conditions in the entry hold draws water vapor from the chamber&#39;s walls, and potentially from lens cores, thereby creating a dry environment. Dry gas is used to vent the chamber through a valve  104  operated by the PLC to maintain this condition. When the carrier enters the entry hold, and the PLC seals the chamber, valve  98  remains open so that pump  100  draws water vapor from the tray carrier, slug and lens cores. 
     When the PLC determines that when sufficient time has elapsed since closing slit valves  86  and  90 , it closes valve  98  and opens a valve  106  between a mass flow controller  108  and the entry hold interior. The mass flow controller, the construction and operation of which should be understood by those skilled in the art, may be controlled independently of the PLC in this embodiment and introduces process gas from a common line  110  into the entry hold. 
     When pressure sensor  102  indicates that the internal pressure of entry hold  88  is approximately the desired level, the PLC opens slit valve  90  and activates the conveyor motors in the entry hold and in an entry buffer  112  to move the carrier into the entry buffer. Again, the entry buffer includes vertical side members  58  and a light source/detector pair that enables the PLC to determine when the slug bore aligns with the light source and detector, thereby indicating that the carrier is fully within the entry buffer. The PLC then closes slit valve  90  and continues to pump the entry hold through valve  98  until the entry hold reaches the desired preset lower pressure. At that time, provided the other conditions discussed above are also met, the PLC opens the slit valve  86  and moves the next carrier into the entry hold. 
     The entry buffer helps isolate the downstream coating zone from non-process gasses that might otherwise flow to the coating zones from the entry hold. It also acts as a waiting chamber for a carrier waiting to enter the coating zones. It is maintained at the process pressure through a valve  114  that is controlled by the PLC and that opens the entry buffer to a vacuum pump  116 . The PLC monitors pressure in the entry buffer by a pressure sensor  118  and introduces the process gas to the entry buffer through a valve  120  connected to process gas line  110  through a mass flow controller  122 . When necessary, the PLC can vent the entry buffer with dry gas through a valve  124 . 
     From the entry buffer, the carrier moves through tandem coating zones  126  and  128 . The PLC maintains the coating zones at approximately the process pressure by pressure valves  130  and  132  that expose the coating zone interiors to the vacuum pumps  133  and  134 . The PLC provides process gas to the coating zones by valves  136  and  138  that are connected to process gas line  110  through mass flow controllers  140  and  142 . While a single valve/mass flow controller is shown in this embodiment for each coating zone, it should be understood that a respective such pair may be provided for the front half and the back half of each chamber to independently control the flow of gas to each half. If necessary, the PLC can vent the coating zones with dry gas through valves  144  and  146 . The PLC monitors pressure in the coating zones through pressure sensors  148 / 149  and  150 / 151 . 
     An exit buffer  152  follows the second coating zone  128 . As with the entry buffer and the coating zones, it includes a conveyor and servo motor that may be operated by the PLC. It also includes vertical members  58  and a light source and detector pair. The PLC maintains the process pressure level in the exit buffer through a valve  154  opening to a vacuum pump  156 . The PLC monitors pressure in the exit buffer through a pressure sensor  158  and controls the flow of process gas from line  110  into the exit buffer from a mass flow controller  160  by a valve  162 . 
     There are no slit valves between entry buffer  112  and first coating zone  126 , between first coating zone  126  and second coating zone  128 , or between second coating zone  128  and exit buffer  152 . Instead, several steel shoulders  164  extend partially laterally into the system to create a channel extending from the entry buffer through the two coating zones to the exit buffer. Thus, the entry buffer chamber, coating zones and exit buffer chamber define a segmented common chamber. As noted above, the PLC maintains this common chamber at the process pressure, and maintains process gas in the chamber, during the system&#39;s operation through respective valves and mass flow controllers. Because of the selective pressurization and depressurization of the entry hold discussed above, and of the exit hold discussed below, the system may coat lens cores on successive tray carriers without having to pressurize and depressurize the coating zones. 
     The illustrated coating zones  126  and  128  are identically constructed. For ease of explanation, therefore, only the structure of coating zone  126  is described herein. 
     Coating zone  126  includes two tandemly arranged magnetrons, each having a pair of opposing electrodes  166  and  168 . The use of a magnetron is optional, depending on the application. Referring to the schematic cross-sectional view in  FIG. 6A , the coating zone does not include vertical members  58  ( FIGS. 4A–4B ) that would otherwise interfere with the application of the plasma cloud to the lens cores. The cloud is created by electrodes  166  and  168 , which include rectangular titanium plates  170  and  172 . Each titanium plate is separated from a respective magnetic device  174  and  176  by four 2 mm–3 mm ceramic buttons  178 . Each titanium plate is approximately 50 centimeters high, 1/16 inches thick and 18 centimeters long. 
     Each magnetic device  174  and  176  may include an outer metal box, for example made of stainless steel, through which cooling water may be pumped from tubes  180 . Referring also to  FIG. 6B , the interior of each box includes a rectangular central steel core  182  and a surrounding rectangular steel ring  184 . A series of permanent magnets  186  extend between core  182  and ring  184  and are arranged in a north-south pattern as shown in  FIG. 6B  so that central core  182  is a magnetic “south” pole and outer ring  184  is a magnetic “north” pole. Although the exact opposite can also be employed, i.e., the north/south magnets may be totally reversed. Each permanent magnet is separated from adjacent parallel magnets by an approximately two inch gap. Titanium plates  170  and  172  are driven to the same electric potential by an AC power source  188  through a transformer  190 . The strength of the magnets may be varied to control the extent of the plasma by one skilled in the art. 
     A distance of approximately seven to ten centimeters separates titanium plates  172  and  178 . When energized, the plates create a plasma cloud between them as should be understood in the art. The 2 mm–3 mm gap between the titanium plates and their respective magnetic devices is so small, however, that no sufficient plasma occurs there. The magnetic field created by the magnetic devices behind the titanium plates also prevents plasma formation. This creates a predictable, stable and relatively uniform plasma cloud between the plates. While an intensely glowing rectangular plasma area  188  is created immediately in front of each of the titanium plates, a plasma cloud  190  between areas  188  has less plasma definition but more uniformity. Specifically, it is more uniform in the vertical direction. Cloud  190  sits above conveyor  50 , and it is therefore through this cloud that the lens cores are moved. 
     Referring again to  FIGS. 3A and 3B , each electrode pair  166 / 168  includes its own pressure sensor  148 / 149  and vacuum throttle valve  130 . As noted above, each electrode pair may also include its own process gas throttle valve. The PLC constantly monitors the pressure in the area in which each electrode pair is disposed and adjusts valves  130  and  136  accordingly to maintain the processing pressure condition. That is, in one embodiment, the process gas flow rate into the area is constant. Throttle valves  130 , however, are set to the processing pressure and, therefore, control the out flow rate to maintain the desired pressure. Thus, the uniform plasma clouds remain consistent from one electrode pair to the next. Further, the process gas inlet from each valve  136  is placed behind one of the electrodes  166  or  168  so that the flow from the process gas line is blocked by the electrodes and does not disturb the plasma cloud. Other gas diversion schemes may be designed that accomplish the same end, but using the electrode pair is a convenient solution. 
     In one embodiment of the present invention, the process gas is seventy percent methane and thirty percent air (a dry mixture of nitrogen and oxygen). It was been found that including oxygen in the process gas provides highly useful means for maintaining the reaction (plasma) chamber clear of deposits such that the coating zone does not have to be cleaned routinely. As can be appreciated, in a continuous plasma apparatus, it is highly advantageous to utilize a processing gas that prevents or diminishes deposits from accumulating in the coating chamber, especially on the electrodes. 
     As noted above, coating zones  126  and  128  do not include vertical members or light source/detector pairs. Instead, the PLC runs the servo motors in each zone at a constant speed so that the respective conveyors run continuously at preset desired speed, say five m/sec. Thus, once a carrier is driven onto the conveyor in zone  126  from the conveyor in the entry buffer, it moves continuously through the four magnetrons in the two coating zones. 
     The PLC begins a timer when the light detector in the entry buffer indicates that a carrier moves from the entry buffer conveyor to the conveyor in the coating zone  126  and sends a subsequent carrier from the entry buffer into the coating zone only upon expiration of this timer. In one preferred embodiment, the length of the timer is three hundred seconds, which provides enough time for the exit buffer to move a downstream carrier to an exit hold chamber  196 , thereby preventing carriers from stacking up in the coating zones. 
     Exit hold  196  is the mirror of the entry hold. The PLC creates a vacuum by a pump  198  through a valve  200 . It monitors pressure in the exit hold by a pressure sensor  202  and controls the introduction of process gas from line  110  and a mass flow controller  204  by a valve  206 . 
     When pressure sensor  202  indicates that the exit hold pressure is approximately the coating zone pressure say fifty mTorr, and the light detector in exit buffer  152  indicates that a carrier is present in the exit buffer, the PLC opens a slit valve  208  between the exit buffer and the exit hold and activates the conveyors in the exit hold and exit buffer to transfer the carrier to the exit hold. When the light detector in the exit hold-determines that the transfer is complete, the PLC closes slit valve  208  and a slit valve  210  at the exit hold&#39;s downstream end, thereby sealing the exit hold. The PLC then throttles valve  200  to remove the process gas and bring the exit hold to less than or equal to one mTorr, or some other desired vacuum pressure. 
     An exit lock chamber  197  is downstream from the exit hold. Prior to opening slit valve  210 , the PLC pumps the exit lock to a pressure of less than or equal to the set low pressure by throttling a valve  210  controlling the application of a vacuum pump  212  to the exit lock interior. When the PLC determines from pressure sensor  202  and a pressure sensor  214  in the exit lock that the exit hold pressure and the exit lock pressure are approximately equal and at or less than the set low pressure, it opens slit valve  210  and activates the exit hold and exit lock conveyors to move the carrier to the exit lock. At this point, the PLC closes slit valve  210  and a slit valve  216  and vents the exit lock with dry gas by throttling a valve  218  until pressure sensor  214  indicates that the exit lock pressure has reached an ambient level. If the PLC detects an ambient pressure condition in the exit lock and that a carrier is present in the exit lock, it opens slit valve  216  and activates the conveyors in the exit lock and a first exit zone  220 A to move the carrier to the exit zone. When a light detector in the exit zone indicates that the carrier has been transferred, the PLC closes slit valve  216  and pumps exit lock  197  back to the set low pressure to receive the next carrier. 
     The construction of the exit zones  220 A– 220 E is similar to that of dry zones  42 A– 42 E. They may be removed from the final zone  220  manually or by an automatic system so that the now-coated lenses exit in the holders  16  ( FIG. 1 ). 
     It should be understood that the above discussion presents one or more preferred embodiments of the present invention and that various suitable embodiments may fall within the scope and spirit of the present invention. The embodiments depicted are presented by way of example only and are not intended as limitations upon the present invention, and it should be understood by those of ordinary skill in the art that the present invention is not limited to such embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included in the present invention as may fall within the literal or equivalent scope of the appended claims.