Patent Publication Number: US-3878811-A

Title: Optical spray control system

Description:
United States Patent Rombach Apr. 22, 1975 [5 OPTICAL SPRAY CONTROL SYSTEM of an automatic spray control system which includes a [75] Inventor: Frank Rombach Kraainem reciprocator for causing the gun to be moved verti- Belgium cally relative to the path of movement of conveyorized articles moving transversely through a spray booth.  
 [731 Assignee: Nordson Corporation, Amherst. The optical control system comprises fixedly mounted Ohi transceivers operable through a system of optical de- [22] Filed: June 4 1973 flectors and reflectors to aim light patterns parallel to [21] Appl. No.: 366,838  
 [52] US. Cl. 118/2; H818; ll8/323 [51] Int. Cl. B05c 5/00 [58] Field of Search ll8/2. 8, 323  
 [56] References Cited UNITED STATES PATENTS [966.093 7/l934 Griffin ll8/2 Primary E.\&#39;aminerDorsey Newton Attorney, Agent, or Firm-Wood, Herron &amp; Evans [57] ABSTRACT An optical detection and spray control system for detecting the presence of articles adjacent the orifice of a spray gun and for causing the gun to be actuated in response to that detection. The spray gun is a portion the spray pattern emitted from the spray gun such that articles passing through those light rays interrupt an electrical control circuit and cause the gun to be oper ated. Because the light rays are aimed parallel to the gun spray pattern, the system is insensitive to movement of the articles toward and away from the spray gun, but it requires the presence of a reflector bar directly in the path of paint or coating material sprayed from the gun, which deflector bar is maintained free of coating material by a novel airflow control housing surrounding and encasing a mirror of the reflector bar. The optical control system also includes optical deflectors vertically movable with the spray gun and op erable to reflect light patterns from stationary transceivers to and from the stationary reflector bars. which movable deflectors are maintained free of paints and contaminants by novel airflow control housings.  
 2] Claims, 7 Drawing Figures OPTICAL SPRAY CONTROL SYSTEM BACKGROUND OF THE INVENTION The invention of this application relates generally to an automatic spray coating system and more specifically to an optical control system for causing a spray gun of the system to be automatically actuated in response to detection of an article adjacent the orifice or orifices of the spray gun or guns of the system.  
  Spray coating systems, and particularly automatic spray coating systems, commonly incorporate a control system operable to cause the spray gun of the system to be automatically turned on and off in response to the movement of objects past the orifice of the guns. Such prior art spray systems are disclosed in Andrews U.S. Pat. No. 2,488,519, Giraudo U.S. Pat. No. 2,714,817 and British Pat. No. 1,205,413.  
  In general, these prior art patents represent two different approaches to the problem of detecting the presence of a workpiece opposite the spray gun and causing the gun to be actuated in response to that detection. The first approach, that depicted in Andrews US. Pat. No. 2,488,519 and Giraudo US. Pat. No. 2,714,8i7 utilizes an optical spray system in which the light pattem is bounced off of a reflector, either a mirror located in back of the workpiece or the workpiece itself, back to a photocell receiver. In this type of system the light ray from the photocell sender is angled relative to the direction of spray from the gun so that the gun does not spray and cover the surface from which the light rays are reflected to the receiver. This type of system, though, is sensitive to movement of the workpiece or article to be sprayed toward and away from the gun. Consequently, this type of control is effective only so long as the workpiece is moved through the spray pattern in a single plane of movement. If the workpiece or article to be sprayed moves toward or away from this plane, the control loses its sensitivity and either sprays too early or too late to effectively cover the article.  
  The second prior art approach to controlling an automatic spray system utilizes a control which is aimed parallel to the direction in which the spray pattern is ejected from the spray gun so that the system is insensitive to movement of the articles or workpieces toward and away from the guns. This is the approach taken in British Pat. No. 1,205,413 and which utilizes magnetic or pneumatic controls for causing actuation of the spray gun or guns. Conventionally this type of control system though has avoided the use of optical receivers or reflectors because of the difficulty of maintaining the receivers or reflectors free of paint or spray ejected from the spray gun. Magnetic and pneumatic controls though are subject to shortcomings which limit their use. Specifically, magnetic control systems are effective only for spraying ferromagnetic articles and pneumatic systems are subject to becoming clogged and short circuited.  
  It has therefore been an objective of this invention to provide an automatic spray system control which is not subject to any of these shortcomings; i.e., not limited to ferromagnetic materials, not subject to frequent clogging and short circuiting or failure, and insensitive to movement of the workpiece or article to be coated toward and away from the spray guns.  
 SUMMARY OF THE INVENTION The control system which accomplishes these objectives is an optical one in which the light pattern is aimed parallel to the direction in which the sprayed material is ejected or emitted from the spray guns. In order to maintain object detection reflectors free of paint or material sprayed from the gun directly toward the surface of the reflectors, the invention of this application incorporates novel reflector bars having airflow channels and configurations operative to maintain the reflectors free of material sprayed from the gun or guns. The contol system also includes novel optical deflectors maintained on the spray gun reciprocating head having airflow channels and patterns which maintain a mirror surface of the deflectors free of contaminants or sprayed materials.  
  The system utilizes fixedly mounted optical transceivers operable to direct a light pattern from the transceivers, through movable deflectors on the crosshead of the spray system, to a stationary reflector bar. Between the deflectors and the reflector the light pattern extends parallel to the gun spray pattern so that the system is insensitive to the movement of objects to be sprayed toward and away from the gun.  
  The novel reflector bars comprise a modular assembly made from boxlike housings on the spray gun side of which there is an opening for admitting light rays into a mirrored chamber contained interiorly of the housings. The boxlike housings also have interconnecting channels for transmitting air from a blower through channels contained interiorly of the boxes to airflow control orifices through which air is directed outwardly from the channel openings of the housings. This modular bar reflector construction ensures that sprayed materials, even though aimed directly into the channel, cannot enter through that channel to contaminate or cover the mirror contained interiorly of the housing.  
  The novel movable deflectors mounted upon the spray gun carrying crosshead of the spray system in clude a novel housing within which there is an airflow channel operative to maintain a scrubbing airflow stream over the surface ofa mirror contained interiorly of the housing. The channel defines an angle at the intersection with the mirror so as to cause a turbulent airflow and consequently scrubbing action of the air as it passes through the channel and over the surface of the mirror, thereby maintaining the mirror free of contaminants.  
  Other objects and advantages of the present invention will become apparent from the following detailed description of the drawings, in which:  
  FIG. 1 is a perspective view of a spray booth to which is applied the invention of this application;  
  FIG. 2 is a diagrammatic cross sectional view taken on line 22 of FIG. 1.  
  FIG. 3 is a cross sectional view taken on line 3-3 of FIG. I and particularly illustrating the reflector bar assembly;  
  FIG. 4 is a cross sectional view through the booth taken on line 4-4 of FIG. 2.  
  FIG. 5 is a cross sectional view through the reflector bar taken on line 55 of FIG. 2;  
  FIG. 6 is a cross sectional view through one of the deflectors of FIG. I; and  
  FIG. 7 is a cross sectional view of a deflector taken on line 7-7 of FIG. 6.  
  Referring to FIG. 1, there is illustrated one preferred embodiment of the invention incorporated into a paint spray system 10. This system utilizes a single automatic electrostatic air spray gun II for spraying liquid paint onto an object 12. It is to be understood that while in one preferred embodiment, the invention is used in this type of spray system, it may also be utilized with other coating systems utilizing other types of spray painting equipment. as for example air or airless spray guns with or without electrostatic charging or deposition of materials other than paints, such as lacquers, varnishes, etc. The invention may also be utilized for applying nonliquid coatings, e.g., polymeric powders.  
  The system illustrated in FIG. 1 comprises a paint spray booth 13 through which a conveyor 14 transports the objects to be coated while the objects are sus pended from conveyorized hooks 15. In passing through the booth 13, the hooks pass through a slot 16 in the top wall and through end wall openings 17. A front opening 18 in the booth l3 accommodates the spray gun l1 and enables it to move vertically while ex tending into the booth from a vertically movable head 19. The head 19 is mounted exteriorally of the booth 13 and is caused to move vertically at a fixed rate over guide rods 22. Movement of the head 19 is effected by a conventional reciprocating apparatus (not shown). Preferrably, the booth 13 includes a suction exhaust port 23 at the bottom through which oversprayed paint or exhaust fumes are exhausted to a paint recovery system.  
  Neither the spray booth 13, nor the spray gun 11, or the reciprocating apparatus forms any part of the invention of this application. These components of the system are all well known in the prior art. One suitable patent gun is disclosed in Us Pat. application Ser. No. 199,] 14. filed Nov. [6, I971 for Electrostatic Spray Gun&#34;, which application is assigned to the assignee of this application. A suitable reciprocator apparatus is disclosed in US. Pat. No. 2,777,419, issued Jan. 15, 1957 to J. A. Paasche and entitled Spraying Apparatus for Successive Articles Carried by a Conveyor.&#34;  
  The invention of this application is concerned with the control system for causing actuation of the spray gun 11 in response to the presence of a workpiece ob ject 12, or a portion of a workpiece, adjacent the outlet orifice of the gun. In FIGS. 1 and 2, the object 12 to be sprayed comprises a window frame having large open areas internally of the framing elements. The control system of this invention is operative to turn on the spray gun in response to the location of a portion of the frame 12 adjacent the outlet orifice of the gun 11, but when an open area internally of the frame is opposite the outlet orifice, the control system causes a valve (not shown) internally of the gun to close, thereby shutting down the spray until such time as a new portion of the frame moves into alignment with the outlet orifice of the gun.  
  This control system comprises a pair of fixedly mounted transceivers 24, 25 operative to transmit light rays 24&#39;, 25 respectively from the transceiver unit 24, 25 to deflectors 26, 27 mounted upon the vertically movable head 19 and subsequently from the deflectors to the object 12. As explained more fully hereinafter, the light rays 24&#39;, 25&#39; from the deflectors cross the space through which the objects 12 move on the conveyor and are reflected by a reflector bar 28. A mirror 29 internally of the reflector bar 28 reflects the light rays back to the deflectors 26, 27 and subsequently back to the transceivers 24, 25. So long as the light rays 24&#39;, 25&#39; between the deflectors 26, 27 and the reflector bar 28, remain unbroken by the passage of objects 12 through the light ray, electrical switches contained internally of the transceivers 24, remain open, thereby maintaining the control circuit to the spray gun open and the valve (not shown) of the gun closed. When an object 12 to be painted, or a portion of the object, breaks either one of the light ray beams 24&#39;, 25&#39;. the electrical switches (not shown) contained internally of the transceivers close and cause an electrical signal to be transmitted via lead 32 to an amplifier circuit 33. The amplified signal from the amplifier 33 is then transmitted via lead 34 to solenoid 35 of a solenoid operated valve 36. When the solenoid 35 is energized it causes the valve 36 to move to a position in which a source 37 of air pressure is connected by pneumatic line 38 to pneumatic line 39 through the valve 36. Connection of this source of air pressure to the spray gun 11 causes a piston 42 of the gun and an attached valve control rod to be moved against the pressure of a spring 43 to a position to open the valve (not shown) of the gun. When the valve of the gun 11 is opened, pressurized paint is free to flow from line 44 through the gun and the outlet orifice of the gun. The paint is then atomized in the conventional manner by impaction of an air stream with the ejected paint as it emerges from the outlet ori- Free of the gun.  
  The transceivers 24, 25 are combination photocell sender and receiver units and are conventional commercially available items of hardware per se. They form no part of the invention of this application and accordingly they have not been illustrated or described in detail herein.  
  Referring now to FIGS. 2, 3, 4, and 5, it will be seen that the reflector bar 28 is fixedly attached to a rear wall 45 of the spray booth l3 and is located in the same vertical plane as that defined by the light rays 24&#39;, 25&#39; from the transceivers 24, 25. As may be seen most clearly in FIG. 4, this vertical plane extends parallel to the axis of the spray gun 11 and is located on the leading or upstream side of the spray gun 11.  
  Referring to FIG. 3, it will be seen that the reflector bar 28 is made up from three identical modular sections 46, 47, and 48. These modules are connected in end to end engagement and are securely fastened together at their ends 49, thereby forming one continuous reflector bar having a continuous passageway 52 for the flow of air through the bar from a fan 53 and a continuous mirrored surface 29 facing toward the front and exposed to light beams 24, 25&#39; from the transceivers 24, 25 and reflected by the deflectors 26., 27.  
  The reflector bar 28 is made of modular sections so that the number of modules maybe varied to accommodate varying size or height spray booths. As best seen in FIG. 5, each modular section 46, 47, and 48 of the bar 28 comprises a generally tubular assembly fabricated from sheetmetal and comprising sidewalls 54, 55, a rear wall 56, and a deflector 57, all sandwiched be tween and welded to top and bottom or end plates 49. These top and bottom plates have a generally rectangular opening 58, which enables the longitudinal passageways 52 of the modular sections of the bar to be interconnected so as to form a continuous airflow passageway from the top of the bar to the bottom which is closed by a flat plate 51.  
  Referring still to FIG. 5, it will be seen that the side walls 54 and 55 of the reflector bar 28 terminate at their forward ends in sections 62 and 63 which are angled inwardly toward each other at an angle of 135 and which terminate in ends 64 which are spaced apart so that they form an open forwardly facing channel 60.  
  The rear wall 56 of the reflector bar is generally U- shaped in cross section and extends for the full height of each modular section. Consequently, the rear wall 56 closes the rear side of the airflow passage 52 of each section of the reflector bar. Similarly, the deflector wall 57 extends for the full height of the modular section of the reflector bar and is welded to the top and bottom plates 49. Each deflector bar 57 is generally Z-shaped in cross section and terminates in an orifice-defining flexible end section 66 of the wall. This flexible end section 66 extends generally parallel to the inwardly turned forward end portion 62 of the side wall 54. At its forwardmost end, the flexible inner wall 66 is turned toward the sidewall, as indicated at 67 so that it defines a narrow adjustable orifice 68. Adjustment of the width of the orifice 68 is controlled by rotation of an adjusting screw 69 having right and left hand screw threads.  
  In order to facilitate the adjustment of the orifice 68, the rear leg 72 of the deflector 57 is welded to and against the side wall 55. The other or opposite leg 73 is welded for approximately one-half its length to the top and bottom walls 49 of the deflector bar modules, but the outer end portion is unwelded to the top and bottom walls so that it is free to be deflected or to move under the control of the adjustment screw 69. Between the leg 73 of the deflector and the side wall 54 there is an airflow channel 74 through which, as explained more fully hereinafter, air may flow from the axial passageway 52 through the channel 74 and out through the orifice 68.  
  The mirror 29 which extends for the full length of the deflector bar 28 is mounted within a generally U- shaped sheetmetal bracket 75 which is attached to the crossbar section 76 of the deflector bar 57. Within this bracket, the mirror is positioned so that its reflective surface extends vertically and faces toward the open channel 60 located between the ends 64, 65 of the sidewalls of the modules 46, 47, 48. Consequently, throughout the vertical movement of the light beam deflectors 26, 27 on the crosshead 19, light rays from the transceivers 24, 25 are passed through the channel 60 of the reflector bar and are reflected from the mirror 29 back to the deflectors 26, 27 and the transceivers 24, 25.  
  in order to maintain the mirror 29 of the reflector bar 28 free of paint or other contaminants which might otherwise enter through the channel 60 in the front of the reflector bar, apertures 77 are provided between the channel 74 and an air dispersion chamber 78 in the front end of the reflector bar. Similarly, orifices 79 are provided through the deflector bar 57 on the opposite side so as to connect the passageway 52 to a channel 82 located between the sidewall 55 of the reflector bar module and the sidewall of the mirror-supporting bracket 75. As a consequence of this construction, air under pressure in chamber 52 flows from the passage 52 through the fixed orifices 79 and channel 82 into the dispersion chamber 78 and out through the forwardly facing channel 60. Additionally, air flows from the passage 52 through the channel 74 through the adjustable orifice 68 defined by the flexible end wall 66 of the deflector bracket and the end portion 62 of the side wall 54. Additionally, air flows from the channel 62 through the fixed orifices 77 into the chamber 78 of the deflector bar. The sum total of all this air flow into, through, and around the deflector chamber 78 is to blow paint or spray away from the channel 60 and to form an effective air curtain which maintains the chamber 78 and the mirror 29 free of paint and any other debris which might otherwise collect there.  
  The two light beam deflectors 26, 27 are identical and consequently only one, 26, is illustrated and described in detail. Referring to FIGS. 6 and 7, it will be seen that each deflector comprises a generally elbowlike fabrication of rear and bottom walls 84, 84 interconnected by a diagonal wall 85 and which are closed by side walls 86, 87. Between the side walls 86, 87 there is a closed triangular air manifold 88 which comprises bottom and end walls 92, 93 and a diagonal wall 94. All three of these walls 92, 93 and 94 extend between sidewalls 86, 87 so that they define an airflow chamber or pocket connected to a source of air pressure by a hose (not shown) which opens into the manifold through a hose port 95 in one side wall 87 of the deflectorv Orifices 96 are provided at the intersection of the walls 92, 93 so that an air stream exiting from the orifices 96 is directed onto the side wall 85. Attached to the inside surface of the side wall 85 there is a mirror 97 which extends at a 135 angle to the two walls 83, 84. Consequently, the walls in combination with the triangular manifold 88 define a generally L-shaped optical passageway at the bottom of which there is a mirror 97 mounted at a 45 angle to the walls of the channel and operatie to reflect light at a anglev In use, air under pressure is supplied to the port and into the triangular manifold 88. From the manifold 88, the air stream is directed via the fixed orifices 96 onto the outer surface of the mirror 97 so that the air stream impringing upon the mirror surface prevents dust or paint from collecting on the surface.  
 OPERATION During operation articles or objects 12 to be painted or sprayed pass into and through the spray booth 13 at a fixed rate of speed while the spray gun 11 is reciprocated vertically at a fixed rate of speed. So long as the gun is reciprocating, air from the fan 53 is directed into the top of the reflector bar and through chamber 52. Simultaneously, air is supplied via a hose (not shown) to the inlet ports 95 of the deflectors 26, 27 from the fan or blower 53. While this relative movement of objects l2 and gun 11 is occurring, light rays 24&#39;, 25&#39; are transmitted from the transceivers 24 and 25 to the deflectors and subsequently to mirror 29 mounted interiorly of the reflector bar 28. So long as the light beams 24, 25&#39; remain uninterrupted, the valve (not shown) of the spray gun 11 remains closed and the gun 11 does not spray paint into the booth. Only when a pair or a portion of a part 12 interrupts the light rays or beams 24, 25 to and from the transceivers 24 and 25 is the gun 11 caused to operate. When the article 12 interrupts the light rays 24&#39;, 25&#39;, switches (not shown) contained interiorly of the receiver portions of the trans ceivers 24 and 25 close to generate an electrical signal which is transmitted to the amplifier 33 and subsequently to the solenoid 35. This solenoid then causes the air valve 36 to direct air under pressure from conduit 38 to conduit 39 and subsequently to the air actuator or air cylinder thus urging the piston 42 against the bias of spring 43 to the right so as to open the paint flow control valve of the gun. When this valve is opened,  
 paint is sprayed from the gun onto the article 12 but the paint is prevented from migrating onto the mirror 29 of the reflector bar 28 by the air stream issuing from the channel 60 and, similarly. the paint is prevented from migrating onto the mirrors 97 of the deflectors 26, 27 by the air streams passing through the deflectors. if, as an example, a window frame is sprayed, the gun ll sprays paint only in response to the passage of solid sec tions of the window frame past the outlet orifice of the gun. The relative positioning ofthe transceivers and deflectors is chosen together with the time delays of the electrical control circuits that a minimal amount of excess paint or over spray results in painting an object such as a window frame. In other words, the timing is such that the spray turns on just as a part or portion of a part which has broken the light beam 24&#39; or 25&#39; arrives in front of the spray gun orifice and the gun turns offjust after the part which has broken the light beam 24&#39; or 25&#39; moves past the outlet orifice of the gun I].  
  The primary advantage of the optical control system of this invention is that it enables greater or closer control to be exercised over actuation of the spray gun 11 so that paint over-spray is minimized, particularly when spraying such difflcult objects as window frames which have a high percentage of hollow space. In other words, prior art paint spraying systems would usually sense the presence of window frames and continuously spray paint onto the framed sections of the window as well as the open spaces in between the frame section; this of course results in a high percentage of paint over-spray which then had to be collected or filtered out of the at mosphere.  
  Another advantage of this control system is that it is insensitive to improper or skewed hanging and/or spacing of articles on the conveyor as well as swinging movement of objects on the conveyor. It is also insensitive to the speed at which the articles are conveyed through the spray booth.  
  While I have described only a single preferred em bodiment of my invention, 1 do not intend to be limited except by the following appended claims.  
 Having described my invention, I claim:  
  1. A system for detecting the presence of articles in a coating zone and for causing a coating material to be applied to said articles in said zone comprising,  
 coating means for ejecting coating material along a spray path to a coating zone,  
 conveyor means for transporting articles through said coating zone,  
 said coating means being adapted to be moved in a path transverse to the direction of movement of said articles on said conveyor,  
 means for transmitting and receiving light remotely positioned and fixedly secured away from coating means.  
 tight deflector means mounted adjacent to and movable with said coating means, reflector means fixedly secured and spaced from said coating means so that articles to be coated pass between said reflector means and said deflector means, said articles in passing between said deflector means and reflector means being operable to interrupt light rays transmitted between said deflector means and said reflector means, and  
 means responsive to interruption of said light rays to actuate said coating means.  
  2. The system of claim 1 wherein said deflector means and reflector means are positioned so as to direct light rays between said deflector means and reflector means in a direction parallel to said spray path of material ejected from said coating means.  
  3. The system of claim 1 wherein said deflector means includes a light reflective surface,  
 a source of air under pressure, and  
 means for directing said air under pressure onto said light reflective surface of said deflector means.  
  4. The system of claim 1 wherein said reflector means includes a light reflective surface encased within an enclosure which is open to said light rays on one side,  
 a source of air under pressure. and  
 means for directing said air under pressure out of said enclosure through said one open side.  
  5. The system of claim 3 wherein said means for directing said air under pressure onto said light reflective surface of said deflector means comprises at least one orifice which is directed onto said light reflective sur face in a direction generally normal to said light reflective surface.  
  6. The system of claim I in which said reflector means comprises a light reflective surface encased within an enclosure, said enclosure comprising a generally longitudinally extending housing, said housing having longitudinally extending walls, a longitudinally extending opening in one of said walls, said light reflective surface being located internally of said housing and facing toward said longitudinally extending opening so that light rays may pass through said opening onto said light reflective surface.  
  7. The system of claim 6 which further includes a source of air pressure, and  
 means for directing air under pressure from said source out of said housing through said longitudi nally extending opening.  
  8. The system of claim 6 in which said enclosure includes a longitudinally extending baffle located inter nally of said housing, said baffle defining a longitudinally extending airflow passage on one side of said baffle and an air diffusion chamber on the other side, said light reflective surface being located in said air diffusion chamber.  
  9. The system of claim 8 which includes a plurality of airflow orifices interconnecting said airflow passage to said air diffusion chamber.  
  10. The system of claim 9 in which at least one of said airflow orifices is adjustable to vary the airflow through said orifice from said airflow passage into said diffusion chamber.  
  11. The system of claim 10 wherein said adjustable orifice is defined in part by a movable end section of said baffle.  
  12. The System of claim 4 wherein said reflector means comprises a plurality of identical housing modules assembled in end-to-end engagement to form said enclosure, said assembled modules having a generally continuous reflective surface and a continuous airflow passageway.  
  13. A system for detecting the presence of articles in a coating zone and for causing a coating material to be applied to said articles in said zone comprising,  
 coating means for ejecting coating material along a spray path to a coating zone,  
 conveyor means for transporting articles through said coating zone,  
 said coating means being adapted to be moved in a path transverse to the direction of movement of said articles on said conveyor,  
 reflector means fixedly secured and spaced from said coating means, said reflector means extending parallel to said path of movement of said coating means,  
 light ray transmitting and receiving means, means for causing light rays from said transmitting and receiving means to pass through said coating zone from said transmitting and receiving means to said reflector means and from said reflector means back to said transmitting and receiving means in a direction parallel to said spray path of material ejected from said coating means, and  
 means responsive to interruption of said light rays to actuate said coating means.  
  14. The system of claim 13 wherein said reflector means includes a light reflective surface encased within an enclosure which is open to said light rays on one side,  
 a source of air under pressure, and  
 means for directing said air under pressure out of said enclosure through said one open side.  
  15. The system of claim 13 in which said reflector means comprises a light reflective surface encased within an enclosure, said enclosure comprising a generally longitudinally extending housing, said housing having longitudinally extending housing, said housing having longitudinally extending walls, a longitudinally extending opening in one of said walls, said light reflective surface being located internally of said housing and facing toward said longitudinally extending opening so that light rays may pass through said opening onto said light reflective surface.  
  16. The system of claim 15 which further includes a source of air pressure, and  
 means for directing air under pressure from said source out of said housing through said longitudinally extending opening.  
  17. The system of claim 15 in which said enclosure includes a longitudinally extending baffle located internally of said housing, said baffle defining a longitudinally extending airflow passage on one side of said baffle and an air diffusion chamber on the other side, said light reflective surface being located in said air diffu sion chamber.  
  18. The system of claim 17 which includes a plurality of airflow orifices interconnecting said airflow passage to said air diffusion chamber.  
  19. The system of claim 18 in which at least one of said airflow orifices is adjustable to vary the airflow through said orifice from said airflow passage into said diffusion chamber.  
  20. The system of claim 19 wherein said adjustable orifice is defined in part by a movable end section of said baffle.  
  21. The system of claim 16 wherein said reflector means comprises a plurality of identical housing modules assembled in end-to-end engagement to form said enclosure, said assembled modules having a generally continuous reflective surface and a continuous airflow passageway.