FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for vacuum coating plastic parts, and especially, for reactive sputter coating of plastic ophthalmic lens elements. As used herein, lens elements include, according to context, edged lenses, semi-finished lenses and lens blanks. Also included are wafers for forming laminate lenses or wafer blanks therefor. Ophthalmic uses of the lens elements include uses in eyeglasses, goggles and sunglasses. 
     BACKGROUND AND OBJECTS OF THE INVENTION 
     Ophthalmic lens elements are frequently coated to achieve special properties. Anti-reflection coatings improve the transmittance of visible light and the cosmetic appearance of the lenses. Reflective coatings may be employed in sun lenses to reduce light transmittance to the eye, to protect the eye from UV radiation and/or to impart cosmetic colorations to the lens. Coatings may also provide other beneficial properties such as increased hardness and scratch resistance and anti-static properties. 
     Desirable lens coatings may be created by applying single or multiple layers of metal or semi-metal oxides to surfaces of the lens element. Such materials include oxides of silicon, zirconium, titanium, neobium and tantalum. Examples of such multilayer coatings are given, for example, in U.S. Pat. No. 5,719,705 to Machol entitled “Anti-static Anti-reflection Coatings”, assigned to applicant. Interference filter coatings for sunglasses are disclosed, for example, in U.S. Pat. No. 2,758,510 to Auwarter. 
     Various methods are disclosed in the prior art for applying metal and semi-metal oxide coatings to ophthalmic lenses. Ritter et al. U.S. Pat. No. 4,172,156 discloses vacuum evaporation in an oxygen atmosphere of Cr and Si to form coating layers on a plastic lens. Reactive sputter deposition of various oxide layers on lens elements is disclosed in the above-mentioned &#39;705 patent to Machol. 
     Reactive sputtering in general is a conventional technique often used, for example, in providing thin oxide coatings for such items as semi-conductor wafers or glass lamp reflectors. Examples of various conventional vacuum deposition systems for the formation of coatings by reactive sputtering are disclosed in the following patents: U.S. Pat. Nos. 5,616,224 to Boling; U.S. Pat. No. 4,851,095 to Scobey et al.; U.S. Pat. No. 4,591,418 to Snyder; U.S. Pat. No. 4,420,385 to Hartsough; British Patent Application GB 2,180,262 to Wort et al.; Japanese Kokai No. 62-284076 to Ito; and German Patent No. 123,714 to Heisig et al. 
     Most ophthalmic lenses produced today are made from a single plastic body or laminated plastic wafers. The plastic material may include thermoplastic material such as polycarbonate or thermoset material such as diallyl glycol carbonate types, e.g. CR-39 (PPG Industries). The material may also be a cross linkable polymeric casting composition such as described in U.S. Pat. No. 5,502,139 to Toh et al and assigned to applicant. The challenge is to adapt conventional vacuum deposition systems to high volume production of plastic lens elements, while ensuing a high degree of control over the thickness and composition of the coating. 
     Accordingly, it is an object of the present invention to improve the degree of control over the thickness and composition of thin metal and semi-metal oxide coatings deposited on plastic lenses. 
     Anti-reflection coating of plastic ophthalmic lenses by physical vapor deposition has traditionally been performed by means of thermal evaporation, or more recently, e-beam evaporation of metal and semi-metal oxides in a vacuum of typically significantly better than 10E-5 Torr. Anti-reflection coating of plastic lenses in spinning drum coaters by means of sputter technology is a relatively recent development. A conventional vacuum system used for this purpose is shown in FIG.  1 . The system includes a vacuum coating chamber  11 , which contains a hollow workpiece holder or drum  12 . Lens elements, such as lens  13  are arranged in columns on an external surface of the drum  12 . A coating applicator  14  is located in a wall of the vacuum chamber adjacent the drum  12 . In a preferred embodiment, the coating applicator may be a combination of magnetron sputtering targets, microwave plasma generator, reactive gas supply, and reversing power supply such as disclosed in U.S. Pat. No. 5,616,224 to Boling, which is hereby incorporated by reference. 
     A pumping plenum  15  is attached to vacuum pumps (not shown) which evacuate the vacuum chamber  12 . A cryopumping surface is provided in the form of cryocoils  16  in the plenum  15 . The cryopumping surface is also known as a “Meissner trap”. Conventionally the Meissner trap takes the form of a coiled or serpentine metal tube through which a coolant passes. Advantageously, the coolant is maintained at a temperature well below the freezing point of water. The Meissner trap is used to remove water vapor from the system. 
     In most such drum coaters the placement of the cryocoils in the pumping chamber plenum is favored. The prime reason for this particular placement is that it is done with a view to protecting the subsequent pumps, especially large roughing pumps, from excessive water contamination which can reduce the life and efficacy of such pumps. A secondary reason for such placement appears to be the desire to keep the cryocoils away from the rotating drum to avoid somewhat greater mechanical complexity, the danger of the parts held on the drum colliding with the cryocoils and to provide greater ease of maintenance of both the coils and the drum. However, Meissner traps have been located in the vacuum chamber rather than the plenum in systems for vacuum coating work pieces on holders other than plastic lenses on drums as disclosed in U.S. Pat. No. 4,647,361 to Bauer and U.S. Pat. No. 5,121,707 to Kanoo. 
     Plastic lenses are usually thoroughly baked at temperatures in excess of 90° C. for one to several hours prior to introduction to the vacuum system to reduce water vapor outgassing. Load sizes of plastic lenses have been limited to about 200. Pump-down times to base vacuums in the 10E-6 Torr regime are typically in the order of 30 minutes or more. 
     It is another object of the present invention to reduce the processing time required to deposit vacuum coatings on plastic parts. 
     It is another object of the present invention to provide an apparatus for depositing a high quality vacuum coating on large numbers of plastic lens elements in a system which is relatively inexpensive to construct and operate. 
     These and other objects and features of the present invention will be apparent from the written description and drawings presented herein. 
     SUMMARY OF THE INVENTION 
     One apparently unrecognized problem with the vacuum coating of plastics is the ongoing effect of large amounts of water vapor (or other gas or solvent) outgassing from the plastic in the presence of plasmas, even after a base pressure is reached which would be considered satisfactory for beginning to process low outgassing materials (e.g. glass). For instance, a particular problem has been found in the deposition of multi-layer anti-reflection (AR) coatings of metal and semi-metal oxides on plastic ophthalmic lenses by means of sputter deposition in a drum coater. The problem is that conventionally designed coaters do not provide sufficient process control in the presence of the outgassing from plastics whereas the same coater design is found to be perfectly acceptable for coating glass components. The problem arises from the breakdown of water molecules into their constituent atoms in the presence of sputter plasmas. The problem may well be exacerbated by the presence of specialized plasmas such as those in processes such as described in U.S. Pat. No. 5,616,224 to Boling, in which a microwave excited plasma is used to increase the rate of oxidation of freshly deposited metal surfaces and to overcome some problems which arise with sputter magnetrons utilizing polarity reversing power supplies. 
     Applicant has determined that the conventional placement of the cryocoil in the plenum ignores, to a large extent, the different requirement that drum coaters and plastic workpieces place on such systems compared with conventional evaporative box coaters. In the latter the vast majority of molecules in the vacuum chamber have unobstructed access to (i.e. can “see”) the cryocoil in the plenum. In a drum coater this is not true. Applicant has observed that, in the reactive sputtering drum coater design previously used by applicant to provide coated lenses in the prior art, the vast majority of molecules in the chambers were obstructed from direct access to the plenum—they could not “see” the cryocoil because the drum wall runs close (within a few inches) of the mouth of the plenum. 
     As noted above, the original use of the drum coating system was in a process to deposit multi-layer coatings on glass objects, especially lamps and reflectors. The amount of water vapor outgassed by glass components in a vacuum, especially if they have been preheated or outgassed in a heated oven, is believed to be considerably less than that outgassed by a large load of plastic lens elements (typically about 400 lenses, each 3″ diameter). These lens elements absorb water throughout the material of which they are composed in contrast to the situation with glass components where water is merely adsorbed on the surface. Some plastic lenses absorb up to several percent by weight of water. 
     Conventional wisdom has it that once a vacuum system has been pumped down to a satisfactory base pressure for a particular process then that is sufficient. However, applicant has determined that process instability results from the very substantially greater water out-gassing from plastics (as compared to glass components). Even after reaching a base pressure which had been shown to be perfectly satisfactory for glass coating, continued out-gassing and instability are believed to be present. The increased stability and improved pump down speed provided by the present invention were surprising. 
     A preferred embodiment of the present invention is a method and system for sputter coating plastic ophthalmic lens elements. The system includes a vacuum chamber containing a hollow, apertured drum with a substantially hollow interior. Large numbers of plastic ophthalmic lens elements (for example 200 to 400) are located in a two dimensional array on a radially outwardly facing surface of the drum so that radially inwardly facing surfaces of the lens elements are exposed through apertures in the drum, to the hollow interior of the drum. Conduits for circulating coolant are located in at least one end wall of the vacuum chamber adjacent the hollow interior of the drum. A majority (i.e. at least 50%) of the water vapor outgassed by the plastic lens elements when placed under vacuum condenses on the conduits, whereby it is removed from active areas of the vacuum system. The system is configured so that at least one face of substantially all of the plural lens elements lies on an unobstructed line of sight with the at least one coolant conduit. The drum and at least one sputtering station are moved relatively to one another to apply various sputter coatings to the radially outward surfaces of the plastic lens elements. 
     Advantageously, in such a system the sputter coating is performed by a reactive DC or mid frequency magnetron sputter process in which sputter material reacts with a reactant gas to form an insulating layer on the radially outward surfaces of the lens elements and on portions of a sputter target. Oxides may be formed on electrode surfaces of the sputtering apparatus and may require arc suppression. The sputter coating may be performed using a microwave plasma generator and at least one applicator or sputter target located adjacent one another and radially outwardly from the drum. 
     In preferred embodiments of the present invention at least 200 lens elements are loaded onto the work holder before drawing a vacuum in the system and pump down is achieved in less than 10 minutes. 
     The present invention also includes apparatus for reactive sputtering of a thin oxide coating onto surfaces of plural plastic lens elements. The apparatus may include a vacuum chamber and a lens element holder located in the vacuum chamber and rotatable about an axis intersecting at least one wall of the vacuum chamber. The holder rotates the plural plastic lens elements past an elongated sputtering electrode. A source of oxygen is provided to facilitate formation of oxide layer(s) on the lens element. An elongated microwave plasma generator may be located adjacent to the sputtering electrode. The holder rotates the plural plastic lens element past the elongated plasma generator which produces a plasma to facilitate the reaction of the oxygen with material sputtered from the sputtering electrode to thereby provide an oxide coating on surfaces of the plural plastic lens elements. 
     At least one cooled surface is located in at least one wall of the vacuum chamber intersected by the axis of rotation of the holder. The cooled surface condenses substantially all the water vapor released into the vacuum chamber by exposed surfaces of the plural plastic lens elements. In a more preferred embodiment, the axis of rotation of the holder intersects two end walls of the vacuum chamber. Cryocoils may extend through at least about half of the portions of the two end walls facing the open ends of the rotating holder. 
     The lens holder may be a hollow drum rotated about its central axis, for example the drum may be generally cylindrical in shape. The plural lens elements may be arranged in columns on an outside surface of the drum. Advantageously, the drum is formed with apertures through which water vapor passes from an uncoated back surface of each lens element. The cryocoils on the end walls of the vacuum chamber may extend adjacent to edges of the external surface of the drum to facilitate condensing water vapor which would otherwise pass into the sputtering and reaction zones adjacent the external surface of the drum. 
     The cooled conduits employed in the present invention are arranged in a coil in each of the end walls of the vacuum chamber. The coils may be in serpentine form or in the form of loops, spirals or helices. The apparatus may also employ a second sputtering electrode located outside the holder and adjacent to at least one of the microwave plasma generator or first sputtering electrode. The second sputtering electrode may sputter a different metal or semi-metal than the first sputtering electrode to produce alternating coating layers of different oxides. The magnetron and sputtering electrode(s) may be located on a door through which lens elements are loaded onto the holders. 
     The foregoing has been provided as a convenient summary of aspects of the invention. The invention intended to be protected is, however, defined by the claims and equivalents thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view in partial phantom of a system known in the prior art for vacuum coating plural plastic lens elements. 
     FIG. 2 is a pictorial view in partial phantom of drum vacuum coating system in which the cryocoils do not lie on a direct line of sight with surfaces of the workpieces. 
     FIG. 3 is a cross-sectional view of a vacuum system in which the cryocoils do not lie on a direct line of sight with surfaces of the workpieces. 
     FIG. 4 is a pictorial view in partial phantom of a system for vacuum coating plastic lens elements employing cryocoils in the upper and lower walls of the vacuum chamber in accordance with the present invention. 
     FIGS.  5 ( a ) and ( b ) are top and bottom views of a vacuum system in accordance with the present invention for coating plural plastic lens elements. 
     FIG. 6 is a pictorial view of the cryocoils employed in the system of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     The disclosed embodiments address the need for effective cryopumping to handle large and continuing outgassing for plastic substrates, particularly in systems containing drum workpiece holders. 
     The conventional placement of the cryocoils, either in the pumping chamber plenum or on the inside vertical walls of the chamber is ineffective because the cold surface cannot be seen by the majority of molecules inside the chamber, i.e. inside the hollow spinning drum which holds the plastic parts as discussed above in connection with FIG.  1 . 
     The effects of large water vapor outgassing loads throughout the process are especially deleterious when sputter deposition is employed. The plasma employed dissociates water vapor (and carbon dioxide) to create uncontrolled sources especially of oxygen but also of hydrogen. Both oxygen and hydrogen take part in the physical and/or chemical interactions of sputtering and oxidization in the growing metal or semi-metal oxide film. It should be noted that the uncontrolled source of hydrogen has deleterious effects on the process in that every hydrogen ion accelerated toward the sputter target contributes to the sputter ion current but not measurably to the sputter yield. It has also been conjectured that hydrogen may be incorporated in the growing film in a manner which may cause undesirable changes to its optical properties. 
     FIG. 2 represents a cryocoil configuration suggested to applicant by a third party supplier. In the system serpentine cryocoils  21  are located in side walls of the vacuum chamber  22 . Coolant for the coils enters and exits the coils through lines  23  which pass through the plenum  24 . 
     When used with a drum workpiece holder, the drum blocks most line-of-sight paths between the coils and the plastic parts mounted on the drums. Furthermore, the cryocoils cannot obstruct the sputter applicator  25 . It is difficult to locate a substantial amount of cooled surface in the vicinity of the sputter applicator  25 . 
     FIG. 3 represents a cryocoil configuration similar to that shown in U.S. Pat. No. 4,647,361 to Bauer, in which a coil of coolant conduit  31  is located adjacent to a bottom wall  32  of a vacuum chamber  33 . If a workpiece holder drum  34  were used in such a system (not shown in Bauer) and mounted for rotation on shaft  35 , it will be seen that the line-of-sight access from plastic parts mounted on the drum would be oblique and quite limited. 
     In preferred embodiments of the present invention cryocoils of appropriate size are placed in a drum coater in locations whereby the majority of vapor molecules have direct and unobstructed access to the cryocoil. Such locations and coils at the top and bottom of the hollow rotating drum are illustrated in FIG.  4 . 
     In FIG. 4, a first serpentine cryocoil  41  is located in an upper wall  42  of a drum coater vacuum chamber  43  and covers a substantial area of the upper wall including a central portion thereof. Similarly, a second serpentine cryocoil  44  is located in a lower wall  45  of the vacuum chamber and covers a substantial area of the lower wall including a central portion. 
     A flow of refrigerant is provided to the cryocoil through inlets  46  passing through the plenum  47  of the vacuum chamber. The refrigerant may be hydrofluorocarbon, liquid nitrogen, liquid air or other coolant having sufficient capacity to cool the surface of the conduits to facilitate vapor condensation. 
     A hollow, cylindrical workpiece holder or drum, such as shown in FIG. 1, is located in the vacuum chamber. A portion of the drum is indicated at  48 . The drum is mounted for rotation about an axis A—A which passes generally perpendicularly through the upper and lower walls  42  and  45  of the vacuum chamber. 
     Plural plastic lens elements such as elements  49  (for example 400 such lens elements) are mounted on the drum. Radially outwardly facing optical surfaces of the lens element may be coated by repeatedly rotating the elements past various sputtering applicators. The lens elements are mounted such that radially inwardly facing surface of the lens elements is exposed to the hollow interior of the drum through apertures in the drum wall. A direct line of sight path exists between these exposed surfaces and substantial portions of the cryocoils  41  and  44 . Several such lines of sight for lens elements  49  are indicated by the dotted lines  50 . 
     Approximately the same total length of cryocoil (as compared to that in the plenum in FIG. 1) is placed in the system illustrated in FIG.  4 . The effect on pumpdown time and, more particularly for process stability, for reactive AR sputter coating of plastic lenses is dramatic. Furthermore, the stabilization of the process, due to continued removal of water vapor and thus of the uncontrolled evolution of oxygen, allows clear, fully oxidized films to be produced with a full load of lenses. This had not been achieved with the cryocoil placement of the prior art. Plastic lenses of higher refractive index materials often have significantly more water uptake than CR39 and thus the advantages of the invention are even more significant in that case. 
     Vacuum Flow Regimes, Placement of Cryocoils 
     The following is from Leybold&#39;s Vacuum Notes: 
     Vacuum Flow Regimes 
     
       
         
               
               
               
             
           
               
                   
               
             
             
               
                 Mean Free Path (L) 
                 L × P 
                 = 5.07E−3 (Torr cm) 
               
               
                 is given by 
                 ┘ 
                 = 50.7 (m Torr mm) (for air at 20° C.) 
               
               
                   
                 where p 
                 = pressure. 
               
               
                   
               
             
          
         
       
     
     Viscous Flow 
     
       
         Pd&gt;4600 mTorr mm L&lt;d/100 
       
     
     where d=the shortest distance across a conducting member. 
     Intermediate (Transitional, Knudsen) Flow 
     
       
         100&lt;P d &lt;4600 mTorr mm d/100&lt;L&lt;d/2 
       
     
     Molecular Flow 
     
       
         Pd&lt;100 mTorr mm L&gt;d/2 
       
     
     Sputter Drum Coater Flow Regimes 
     Argon and air have very similar mean free paths (=to within 5% at 1 Torr). The Mean Free Path L for water vapor is almost exactly ⅔ that of air at 1 Torr). 
     Table I sets out the various flow regimes during deposition for a typical sputter drum coater at various working pressures in the mTorr regime and for two characteristic distances is the radial spacing from drum to chamber wall and from drum to sputtering target. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 Flow Regimes 
               
             
          
           
               
                   
                   
                 Characteristic 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Pressure (P) 
                 MFP (L) 
                 Distance (d) 
                 Where is d 
                 Pd (mTorr 
                   
                 d/100 
                   
                 d/2 
               
               
                 (mTorr) 
                 (mm) 
                 (mm) 
                 measured 
                 mm) 
                 Pd Regime 
                 (mm) 
                 L (mm) 
                 (mm) 
                 L Regime 
               
               
                   
               
               
                 3 
                 17 
                 25 
                 drum to chamber 
                  75 
                 Molecular Flow 
                 0.25 
                 17 
                 12.5 
                 Molecular 
               
               
                   
                   
                   
                 wall 
                   
                   
                   
                   
                   
                 Flow 
               
               
                 4 
                 10 
                 25 
                 drum to chamber 
                 125 
                 Transitional 
                 0.25 
                 10 
                 12.5 
                 Transitional 
               
               
                   
                   
                   
                 wall 
                   
                 Flow 
                   
                   
                   
                 Flow 
               
               
                 8 
                  6 
                 25 
                 drum to chamber 
                 200 
                 Transitional 
                 0.25 
                  6 
                 12.5 
                 Transitional 
               
               
                   
                   
                   
                 wall 
                   
                 Flow 
                   
                   
                   
                 Flow 
               
               
                 12  
                  4 
                 25 
                 drum to chamber 
                 300 
                 Transitional 
                 0.25 
                  4 
                 12.5 
                 Transitional 
               
               
                   
                   
                   
                 wall 
                   
                 Flow 
                   
                   
                   
                 Flow 
               
               
                 3 
                 17 
                 67 
                 drum to target 
                 201 
                 Transitional 
                 0.67 
                 17 
                 33.5 
                 Transitional 
               
               
                   
                   
                   
                   
                   
                 Flow 
                   
                   
                   
                 Flow 
               
               
                 5 
                 10 
                 67 
                 drum to target 
                 335 
                 Transitional 
                 0.67 
                 10 
                 33.5 
                 Transitional 
               
               
                   
                   
                   
                   
                   
                 Flow 
                   
                   
                   
                 Flow 
               
               
                 8 
                  6 
                 67 
                 drum to target 
                 536 
                 Transitional 
                 0.67 
                  6 
                 33.5 
                 Transitional 
               
               
                   
                   
                   
                   
                   
                 Flow 
                   
                   
                   
                 Flow 
               
               
                 12  
                  4 
                 67 
                 drum to target 
                 804 
                 Transitional 
                 0.67 
                  4 
                 33.5 
                 Transitional 
               
               
                   
                   
                   
                   
                   
                 Flow 
                   
                   
                   
                 Flow 
               
               
                   
               
             
          
           
               
                   
                 Viscous Flow 
                   
                 Pd 
                 &gt;4600 mTorr 
                 d/100&lt; 
                 L 
                 &lt;d/100 
               
               
                   
                   
                   
                   
                 mm 
               
               
                   
                 Transitional 
                 4600&gt; 
                 Pd 
                 &gt;100 mTorr 
                   
                 L 
                 &lt;d/2 
               
               
                   
                 Flow 
                   
                   
                 mm 
               
               
                   
                 Molecular 
                   
                 Pd 
                 &lt;100 mTorr 
                   
                 L 
                 &gt;d/2 
               
               
                   
                 Flow 
                   
                   
                 mm 
               
               
                   
                   
               
             
          
         
       
     
     Note that the process does not operate very close to the Viscous Flow regime and in fact is usually bordering on the Molecular Flow regime or occasionally in it. Whether the drum to chamber wall distance is the typical 25 mm (1″) or 30 mm (1¼) (or somewhere near that figure) will make no difference to the conclusions regarding operating flow regime. 
     Placement of Cryocoils 
     Advantageously, the cryocoils are placed so that they are at the top and bottom of the drum. They should also be placed as near as possible to the portion of the chamber where the sputter applicators and plasmas are located. 
     As shown above, the process is usually operating in the transitional flow regime and closer to the molecular flow regime than the viscous flow regime. The gas conductance on both sides of the drum, from a vertical midpoint on the drum, to the top and bottom of the drum, is demonstrably greater on average than that to the first available position for cryocoil tubes on the inside walls of the chamber beyond the edges of the coating applicator. Conventionally, the coating applicators are located in the vacuum chamber door. Placing cryocoils in the door is fraught with mechanical difficulties of placement, design and manufacture. Their total effective area (near the operational plasma zones) will be very limited. 
     In contrast, standard ⅝″ OD cryocoils of some 30 feet in length (or more) at the top and bottom of the drum are feasible and relatively straightforward to install with copper tubing in positions which are at less risk to mechanical damage. In a 45 inch diameter vacuum system, four coils of ⅝″ tubing starting at 36″ diameter and spaced 2.125″ center to center [1.5″ from OD to OD] will provide about 30 ft of tubing at the top of the drum—a similar arrangement may be placed at the bottom of the drum. 
     In the first few seconds of sputtering silica on the lenses there is very good reason to believe that the outgassing from the coated (outwardly facing) lens surfaces will decrease markedly, probably to negligible proportions, due to the excellent moisture barrier properties of silica. This being the case, the remaining major source of outgassing during most of the deposition cycle will be the rear surfaces of the plastic lenses. 
     Cryocoils placed top and bottom of the drum will deal with this outgassing very effectively and can act as a trap to stop water vapor diffusing from the inside of the drum over the top and bottom of the drum to the outside where it is difficult to provide an effective Meissner trap in the operational region near the plasmas. 
     Another preferred embodiment of the present invention is illustrated in FIGS. 5 and 6. FIGS.  5 ( a ) and ( b ) are, respectively, top and bottom views of a vacuum system  100  for coating plastic lens elements. The system employs an arrangement of cryocoils  102 , which are shown in isolation in perspective view in FIG.  6 . 
     The vacuum chamber has an outer wall  104  in the shape of a twelve-sided prism. A cylindrical drum  105  is located inside the vacuum chamber. A chamber door  106  is hinged at  108  and provides access to the outer cylindrical surface of the drum  105  for loading lens elements onto the drum. The drum  105  is mounted for rotation about an axis passing through points B. Lens elements (not shown) may be mounted in registration with apertures on the drum so that one side is exposed to system coating applicators and the other side is exposed to direct lines of sight with the cryocoils in the top and bottom walls of the system. 
     The vacuum coating applicators may be located in the door  106 . In a preferred embodiment the coating applicators may include a first sputter magnetron  112 , a microwave plasma generator  114  and a second sputter magnetron  116 . Alternatively the positions of the first sputter magnetron  112  and the microwave plasma generator  114  may be reversed. Advantageously, the first and second sputter magnetrons may include targets of different metal and/or semi-metal materials to form sequential coatings of diverse oxides on the lens elements, the coatings having different indices of refractions. Layers are built up by repeatedly rotating the lens elements on the drum past the vacuum coating applicators. For example, the system may be used to apply a multi-layer oxide coating to a lens element whose radially outwardly facing optical surface has been treated with a hard coat. A five layer coating may comprise alternating layers of silicon oxide and zirconium oxide, silicon oxide layers being outermost and innermost. 
     The outer cylindrical face of the drum  105  is typically 1 to 2 inches from the inner wall of the vacuum chamber and typically 2 to 3 inches from the target surface of the coating applicators. The drum itself may be on the order of 40 inches in diameter and 40 inches high and carry hundreds of lens elements on its outer surface. Initially the lens elements may present on the order of 5600 square inches of exposed surface, approximately half of which (one side of each lens element) is coated during a coating run. Using the system depicted in FIGS. 5 and 6 pump down has been achieved in less than 10 minutes with a full load of 400 baked-out, uncoated 3″ lenses. This represents an approximately three fold reduction in pump down time in comparison to a system with cryocoils located in the plenum. Smaller loads would present smaller uncoated surface area or the order of 1000 square inches (about 1400 square inches for a load of 200 3″ lenses). 
     FIG. 6 is a perspective view of the cryocoils used in the system of FIG.  5 . The cryocoils on the upper wall and the cryocoils on the lower wall are indicated at  118  and  120 , respectively. Conduits running along the side walls are indicated at  122 . Cryocoils in the plenum are indicated at  124 . 
     The instant invention has been described with respect to particular preferred embodiments. The invention to be protected, however, is intended to be defined by the literal language of the claims and equivalents thereof.
A method and apparatus for vacuum coating plastic lens elements employs Meissner traps and a drum work holder configuration for effectively condensing water vapor in the system.