Patent Publication Number: US-2005115498-A1

Title: Reflector for UV curing systems

Description:
CROSS REFERENCE OF RELATED APPLICATIONS  
      This application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 60/505,681 entitled IMPROVED REFLECTOR FOR MICROWAVE LAMPS, filed on Sep. 23, 2003, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention generally relates to an ultraviolet (UV) curing system. Specifically, embodiments are described that relate to UV curing of inks and coatings.  
      2. Description of the Related Art  
      UV lamps having reflectors that direct UV light towards a surface may be used to cure inks and coatings. These reflectors are typically specular reflectors with shapes that are elliptical and focusing or parabolic and defocusing. The elliptical reflectors have a high intensity UV source at one focus of an ellipse with the emitted UV rays focused at the second focus on the ellipse.  
      These reflectors are also generally made of an anodized aluminum material or a dichroic multiple thin film material, with each material having its own reflective properties. The anodized aluminum reflectors are specular reflectors with a reflectivity of about 70% at 250 nm and about 20% at 200 nm. These reflectors do not transmit the UV spectrum of a lamp to an exposed surface without changing the spectrum of UV radiation. As a result, the entire spectrum of UV radiation is not transmitted and applied to the surface of exposure.  
      In comparison, the dichroic reflectors are specular reflectors with a reflectivity of about 92 to 94% in a specific wavelength, for example 220 to 260 nm, but have a comparatively poor reflectivity outside those bands. These reflectors are composed of multiple layers of oxides deposited on metal or glass surfaces.  
      An example of an elliptical specular reflector is made by Fusion UV Systems, Inc. for use in microwave lamps. The Fusion UV reflector has a reflectivity that ranges approximately from 20% at 200 nm to 70% at 240-270 nm and 86% at visible wavelengths. It also has an elliptical shape with a bulb at the first focus of the ellipse. The second focus of the ellipse is a few inches outside of the lamp housing, though in some applications it is intentionally de-focused to create a more uniform flux outside the housing.  
      The Fusion UV elliptical specular reflector is made of Alzak, an anodized aluminum material. The reflector forms a portion of a microwave cavity that couples microwave energy into a high intensity UV bulb lamp, which is linear and electrodeless. The light exits the cavity through a metallic screen, usually made of fine tungsten wire so it contains the microwaves and allows light to pass through with about 5 to 10% absorption due to the wires. The reflector incorporates slots for coupling the microwave energy from the magnetron into the lamp cavity formed by the reflector and metallic screen. Other holes are placed in the reflector to allow cooling air to flow through the reflector, across the bulb and out of the cavity.  
      In the case of the Fusion UV F300S lamp, about 690 Watts is radiated as light and 181 watts is radiated in UV between 200 and 300 nm. About 30% exits the reflector directly, about 5% is lost through the cooling holes and slots in the reflector, and about 65% is incident on the Alzak reflector surface which reflects at 70%. Thus, about 54 watts exit the lamp directly and about 82 watts are focused by the reflector for a total available UV power of 136 watts. The need therefore exists for an improved reflector for UV curing systems that is more efficient.  
     SUMMARY OF THE INVENTION  
      The inventions described herein provide for UV curing with an increase of the total flux of UV delivered to a surface. For example, a high intensity UV lamp-reflector combination is described wherein the total flux of light is increased over present methods while maintaining a reflectivity greater than 95% over the entire UV wavelength region between 200 nm and 400 nm.  
      In one embodiment of the improved reflector for UV curing, a diffuse material is used to line the inside of a an elliptical specular reflector, thereby increasing the total UV flux from the reflector towards the surface to be irradiated and transmitting the UV spectrum with high fidelity. Similarly, in a second embodiment a reflector for a high intensity arc lamp is lined with a diffuse reflecting material and in a third embodiment a circular shaped reflector is lined with a diffuse reflecting material. A cooling system may also be provided where necessary so the temperature of the diffuse reflecting material does not exceed a softening point or a maximum operating temperature.  
      These and other objects and features of the present invention will become more fully apparent from the following description and appended claims taken in conjunction with the following drawings, where like reference numbers indicate identical or functionally similar elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram of an elliptical reflector.  
       FIG. 2  is an illustration of light reflecting from a diffuse reflector.  
       FIG. 3A  is a diagram of a high intensity arc lamp used for UV curing.  
       FIG. 3B  is a diagram of a diffuse reflector and fan.  
       FIG. 4  is a diagram of a diffuse reflector and fan illustrating peak flux.  
       FIG. 5  is a diagram of a UV curing system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      Embodiments of the invention will now be described with reference to the accompanying Figures, wherein like numerals refer to like elements throughout. The terminology used in the description presented herein is not intended to be interpreted in any limited or restrictive manner, simply because it is being utilized in conjunction with a detailed description of certain specific embodiments of the invention. Furthermore, embodiments of the invention may include several novel features, no single one of which is solely responsible for its desirable attributes or which is essential to practicing the invention herein described.  
      The first embodiment is to line a shaped surface of metal, glass, or other structural material with a diffuse reflector material, such as Teflon or any other suitable diffuse reflective material. As illustrated in  FIG. 1 , a diffuse reflector comprises an elliptical surface  100  that is lined with a diffuse reflecting material  101 . Such a reflector is also referred to as a Lambertian reflector.  
      A Lambertian or diffuse reflector is a reflecting surface which reflects incident light in all directions regardless of the angle it is incident on the diffuse reflector. A Lambertian surface is defined as a surface from which the energy emitted in any direction is substantially proportional to the cosine of the angle which that direction makes with the normal to the surface. As illustrated in  FIG. 2 , an incident ray of light  201  incident on a diffuse reflecting surface  200  emits rays  202  with a substantially cosine distribution with respect to the reflecting surface  200  in a substantially cosine dependence on theta, where theta is the angle between the perpendicular to the surface and the direction of emission. For example, if a diffuse reflector comprises a portion of a panel in a reflecting chamber, incident light will be scattered from the panel in all directions regardless of the shape of the diffuse reflector and the relationship of other panels in the reflecting chamber. With the diffuse reflector, the fluence within a reflective chamber may be substantially uniform regardless of the chamber geometry, UV source geometry, and UV source location within the chamber. Thus, a substantially uniform illumination inside a reflective chamber is possible regardless of the geometric shape of the chamber and the location of the emitter within the chamber.  
      If the elliptical reflector  100  of  FIG. 1  were a specular reflector, such as the Fusion UV elliptical specular reflector, then high intensity UV source located at one focus  102  of the ellipse and would have its emitted rays focused at the second focus  103  of the ellipse. In contrast, if the Fusion reflector was lined with a diffuse material, thereby creating a diffuse Lambertian reflector, the same ellipse would not focus the rays on the second focus of the ellipse, but would increase the total UV flux from the reflector towards the surface to be irradiated and would transmit the UV spectrum with high fidelity, i.e. the spectra striking the surface to be irradiated would be close to the same as that emitted by the high intensity UV source.  
      Accordingly, one embodiment of the invention is to line the existing reflector of a lamp (such as the F300S Fusion lamp) with a diffuse material. Different types of diffuse material may be used to create a Lambertian or diffuse reflector. For example, diffuse reflecting Teflon ePTFE material (trade name DRP) from the W. L. Gore company has a reflectivity above 95% from 200 nm to 400 nm, a diffuse reflecting Teflon PTFE material (trade name Spectralon) from Labsphere, Inc. has a reflectivity of greater than 92% from 200 nm to 400 nm, and a Teflon material called Zenith from Sphere Optics has a reflectivity greater than 94%. Teflon materials are not the only suitable diffuse reflective material. Any material providing the desired reflective properties may be used.  
      In the case of lining a surface that forms a surface for coupling microwaves into a microwave cavity containing an electrodeless lamps, wherein the surface has slots used for coupling microwaves from the microwave source to the lamp, the lining of diffuse material should be sufficiently recessed from the slots to be outside the high electric field region created by the microwaves passing through the slots. If the lining is not recessed, arcing may occur in the slot due to the high electric fields that exist in the slot. This arcing may spoil the coupling between the magnetron and the lamp. About ¼″ clearance is sufficient in the case of the F300S Fusion lamp. Other appropriate clearance values may be obtained depending on the configuration of the reflecting system. This clearance should be kept as small as possible because it exposes the lower reflectivity backing material.  
      The lining of diffuse material should also have an adequate cooling system to ensure that the temperature of the lining material does not exceed its softening point or maximum operating temperature. For example, holes within the lining material may allow cooling air to enter the microwave cavity. Cooling air for the electrodeless lamp may then flow over and around the lining, which is adhered to the reflector, thereby helping to maintain the temperature of the lining below its softening point or maximum operating temperature.  
      As an example of this embodiment, a F300S Fusion lamp reflector was lined with DRP from the W. L. Gore Company and the output measured and compared to the output with an unlined reflector. Calorimeter measurements were taken on axis at 9.25″ and 21.75″ away from the mesh. A 295 nm cutoff filter was used to measure the far UV content. The temperature of the DRP was also measured to be 73 C., well below its limit of about 300 C.  
      The data from the above described example is shown in Table 1. The lamp was operated for about 16 hours and no degradation in total optical output power or far UV output power was observed.  
                                   TABLE 1                       Dis-           Output with   Output   % of       tance       Total optical   295 cutoff   &lt;295   output       from       output flux   filter   nm   &lt;295 nm       mesh   Reflector   (W/cm 2 )   (W/cm 2 )   (W/cm 2 )   (W/cm 2 )                                                         9.25″   Standard   0.392   0.225   0.167   42.6%           DRP lined   0.489   0.281   0.208   42.6%                             Increase   25%           due to           lining                                     21.75″   Standard   0.066   0.039   0.027   40.9%           DRP lined   0.100   0.061   0.039   39.0%                             Increase   52%           due to           lining                      
 
      A second embodiment comprises a high intensity arc lamp with a reflector lined with a diffuse material, as depicted in  FIG. 3A . High intensity arc lamps are commonly used for UV curing. A typical high intensity arc lamp reflector  300  is elliptical with the lamp  304  mounted at the focus of the ellipse. The typical reflector also has a longitudinal slot  302  along its apex for cooling of the lamp by convection through the slot. These reflectors are also commonly lined with a specular reflector made of Alzak.  
      Instead of using the specular reflector material, the second embodiment lines the reflector with a diffuse reflecting material  301  without blocking the cooling opening. In this embodiment, the airflow over the surfaces  300  or the  301  should be increased to prevent overheating of the lining material. For example, a Teflon liner may be subject to overheating because the typical lamp has power levels of 200-600 watts per linear inch. In such cases, as illustrated in  FIG. 3B , a fan  303  may be located over the slot  302  to increase the rate of airflow away from the lamp. This fan may blow or suck air from one end of the reflector. The air flow should preferably be sufficient to maintain the temperature of the diffuse reflecting material below a maximum operating temperature.  
      In a third embodiment, a diffuse reflecting reflector with a circular shape with radius of curvature R will create a peak flux on the center of a circle  403 . This is because the highest emission direction is perpendicular to a surface. Because specular reflection has been used in conventional reflectors, which necessitates elliptical or parabolic reflector shapes, circular lamp reflector configurations have not previously been considered. Such a reflecting surface  401  is shown in  FIG. 4 . The support surface of the reflector  400  is lined with a diffuse reflecting material  401 . The peak flux would be located at the center of the circle of revolution  403 . The lamp may be located anywhere proximate to the surface  401  and a cooling fan  404  may also be used to move air by the lined surface to ensure its temperature is maintained below the lining&#39;s softening point. It will be appreciated that either spherical or cylindrical reflectors may be utilized as each forms a surface defining a substantially circular arc.  
      In a fourth embodiment, as depicted in  FIG. 5 , a UV light source  501  is advantageously incorporated into a curing system  500 . In this system, the UV light source  501  comprises a shaped reflector with at least of portion of the reflector lined with a diffuse reflecting material. In this configuration, at least a portion of UV light emitted from the UV light source reflects off of the diffuse reflecting material thereby exposing an item  502  to UV. The curing chamber also comprises an input area  503  for uncured items to enter the system and an output area  504  for cured items to exit. This curing system may also incorporate a cooling system, such as a cooling fan, to move air by the diffuse lining material to ensure its temperature is maintained below its softening point.  
      Specific parts, shapes, materials, functions and modules have been set forth, herein. However, a skilled technologist will realize that there are many ways to fabricate the system of the present invention, and that there are many parts, components, modules or functions that may be substituted for those listed above. While the above detailed description has shown, described, and pointed out the fundamental novel features of the invention as applied to various embodiments, it will be understood that various omissions and substitutions and changes in the form and details of the components illustrated may be made by those skilled in the art, without departing from the spirit or essential characteristics of the invention.