Abstract:
An apparatus for laser marking indicia on a moving photosensitive web substantially reduces fog spots on the web caused by impinging laser energy. A laser beam attenuating member arranged in the laser beam tube enables control of peak laser beam power thereby significantly reducing the incident of fog spots on the film surface.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of laser marking systems. More particularly, the invention concerns an apparatus that uses laser energy for marking indicia on photosensitive web with a dramatic reduction in the occurrence of fog on the photosensitive web. 
     BACKGROUND OF THE INVENTION 
     Conventional edge marking in photographic film manufacturing involves printing some sort of identification indicia along the edge of film rolls during the finishing operation. Edge marked film has direct verification of roll identity, sheet identity and waste identity during all stages of the manufacturing process. Importantly, edge marked film provides accurate footage identification that enables operators to quickly identify, trace and remove film imperfections, thereby minimizing the amount of product waste. More generally, edge marked film increases process understanding by allowing process interactions to be more closely identified with their corresponding effect on the product. Traditional embossing marking techniques are being replaced by laser edge marking. Current mechanical embossing techniques (embossing wheels) are not programmable, generate poor quality marks and require excessive maintenance. Laser edge marking, on the other hand, is particularly advantageous in the industry because it provides a permanent record and can be read before and after film processing. 
     Advances in laser technology enabled the use of a dot matrix CO 2  laser marking system to be used to replace existing embossing technology. Off the shelf laser marking equipment will mark the film at required throughput rate, however, an unacceptable level of fog spots occurred. 
     Thus, a particular shortcoming of these advanced high powered laser systems used for edge marking photosensitive film is that they produce a by-product that impinges on the film surface. Laser energy by-products in the form of a plume of energized smoke and irradiated debris on the film surface is known to cause the localized fogging on the film. Experience has shown that localized fogging is not easily eliminated even when the film is immersed in a 99.8% nitrogen atmosphere. 
     More recent developments in laser technology enabled the development of high speed marking systems using short pulse lasers. Short pulse laser exposure on photosensitive film shows some promise in reducing the occurrences of fog spots. Our experience also indicates that an air jet directed at the laser impingement point on the film surface further reduce the occurrence of fog. Statistical methods have been employed to gain information on fog incidence reduction when laser marking photosensitive film. It has been experimentally proven that laser pulse width does not have a significant effect on fog. Importantly, however, our experience does suggest that laser peak power has a dramatic effect on the reduction of occurrences of fog spots by a factor of about 30. In addition, significant statistical benefits can be derived from an air jet that we believe can further reduce the incidences of fog spots by a factor of about 10. 
     Hence, laser marking without controlling peak power will result in 14% to 50% of the laser-generated dots of dot matrix characters to have fog spots around the dots. There are no present attempts known to the inventors to control peak power in laser edge marking devices because embossing techniques still remain prevalent in the industry and, more importantly, the fog spots remain a significant quality issue during the finishing process. 
     Therefore, a need persists for variable information to be permanently marked on the edge of each sheet of photosensitive web, such as photographic film, without significant incidences of fog spots on the surface of the film. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the invention to provide an apparatus for laser marking indicia on a moving photosensitive web while substantially reducing the occurrence of deleterious fog spots on the photosensitive web. 
     It is another object of the invention to provide an apparatus for exposing a moving photosensitive web to laser energy while controlling the peak power of the laser energy. 
     Yet another object of the invention is to provide an apparatus for laser printing indicia on a photosensitive web by further directing a jet of air onto the laser energy impinged surface of the photosensitive web. 
     It is a feature of the invention that the apparatus for laser marking indicia on a moving photosensitive web has a means for controlling the output peak laser power to each of a plurality of lasers directed at the moving photosensitive web. 
     To accomplish these and other objects and features and advantages of the invention, there is provided, in one aspect of the invention, an apparatus for marking indicia on a moving photosensitive web, comprises: 
     a source of laser energy for producing a range of laser power; 
     laser printer means operably connected to said source of laser energy, said laser printer means being provided with a laser beam tube having an active end positioned proximate to said moving photosensitive web, a plurality of lasers disposed in said laser beam tube for generating a plurality of laser beams, a lens arranged in said laser beam tube for focusing each one of said plurality of laser beams along a predetermined optical path and into impinging contact with said moving photosensitive web thereby producing indicia thereon; and, 
     means for controlling peak laser power to each one of said plurality of lasers. 
     It is, therefore, an advantageous effect of the present invention that laser edge markings on photosensitive web can be accomplished with an apparatus and method that is easy to operate, simple and cost effective to produce and that substantially reduces the occurrence of fog spots on the photosensitive web. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical features that are common to the figures, and wherein: 
     FIG. 1 is a schematic diagram of a laser edge marking system of the invention; 
     FIG. 2 a  is a front elevational view of the laser head showing an attenuating member therein; 
     FIGS. 2 b  and  2   b  are top elevational views of the mesh screen; 
     FIG. 2 c  is an isometric view of the beam splitter; 
     FIG. 3 is a graph of the relationship between focus position effect (inversely proportional to peak power) on fog spots formed on the photosensitive film; and, 
     FIGS. 4 and 5 show the effects of an attenuating screen of the invention on incidents of fog spots. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, and in particular to FIG. 1, the apparatus  10  according to the principles of the invention for printing indicia  2  on a moving laser impingeable surface, such as a moving photosensitive web  1 , is illustrated. According to FIG. 1, apparatus  10  has a source  12  of laser energy for producing a range of laser power. A laser printer means  14  is operably connected to the source  12  of laser energy. 
     Referring to FIG. 1, laser printer means  14 , preferably a Domino DDC2 Digital Laser Coder, manufactured by Domino Lasers, Inc. of Gurnee, Ill., is provided with laser head  16  and a laser beam tube  18  structurally associated with the laser head  16 . Laser beam tube  18  has an active end  20  positioned proximate to the moving photosensitive web  1  and a plurality of lasers  22  disposed in the laser beam tube  18  for generating a plurality of laser beams. 
     According to FIG. 2, in the preferred apparatus  10 , seven lasers  22  are employed each being a medium power CO 2  laser that operates at about 30 watts maximum power. Each laser  22  corresponds to a row of dots in a dot matrix character. This type of laser  22  has enough power to mark small characters or indicia into photosensitive materials, for example emulsion coated film. At least one lens  24  is arranged in the laser beam tube  18  for focusing each one of the plurality of laser beams along a predetermined optical path  23  (FIG. 2 a ) and into impinging contact with the laser impingeable material, such as photosensitive web  1  thereby producing indicia thereon. 
     Referring to FIGS. 2 a-   2   c,  means for controlling peak power, preferably a laser beam attenuating member  26  (FIG. 2 a ), is disposed in the optical path  23  for attenuating the laser beams passing through the laser beam tube  18 . In the preferred embodiment, laser beam attenuating member  26  is a metallic mesh screen  30  (FIG. 2 b ) arranged in the laser beam tube  18 . Preferably, metallic mesh screen  30  is made of materials selected from the group consisting of brass, steel, copper, and metal alloys. We consider copper to be most preferred because it has more suitable thermal conductivity and reflective characteristics of the wavelengths contemplated by the invention. Moreover, the mesh screen  30  has a plurality of openings  32 . Openings  32  each have a wire diameter in the range of from about 0.00025 inches (0.000635 cm) to about 0.025 inches (0.0635 cm) and a clear opening having a dimension in the range from about 0.001 inches (0.00254 cm) to about 0.100 inches (0.254 cm). In the preferred embodiment, mesh screen  30  has clear opening dimension of 0.055 inches (0.140 cm), and a wire diameter of 0.016 inches (0.041 cm). 
     As shown in FIG. 2 c,  alternatively, laser beam attenuating member  26  may include at least one beam splitter  27  arranged along the optical path  23  in the laser beam tube  18 . Moreover, laser beam attenuating member  26  may include a neutral density filter (not shown). 
     Referring to FIG. 3, peak power of each of the plurality of lasers  22  was determined to be a primary factor controlling the incidents of fog spots occurring on the photosensitive web  1  after impingement by laser energy. According to FIG. 3, we observed that the incidents of fog spots decreased as the focus position of the lens moved further out of focus. This corresponded to an effective reduction in peak power that enabled the inventors to select controlling peak power for minimizing the incidents of fog spots. 
     Referring to FIGS. 4 and 5, performance of mesh screens  30  used as laser beam attenuating member  26  in the apparatus  10  of the invention are illustrated. According to both FIGS. 4 and 5, the incidents of fog spots are well below expected levels generally experienced in the industry. 
     In another embodiment of the invention, a method of controlling peak power of a laser marking apparatus  10  adapted for marking predetermined indicia  2  on a moving photosensitive web  1  comprises the steps of providing a source  12  of laser energy. A laser printing means  14  (described above) is structurally connected to the source  12  of laser energy which has a laser head  16 , a laser beam tube  18  connected to the laser head  16 . As indicated above, the laser beam tube  18  has an active end  20  positioned proximate to the moving photosensitive web  1 . A plurality of lasers  22  is disposed in the laser head  16  for generating a plurality of laser beams. A lens  24  is arranged in the laser beam tube  18 , preferably near the active end  20 , for focusing each one of the plurality of laser beams along a predetermined optical path  23  and into impinging contact with the moving photosensitive web  1  thereby producing indicia  2  thereon. 
     Further, the source  12  of laser energy is activated so as to energize each one of the plurality of lasers  22  for impinging laser beams forming predetermined indicia  2  on the moving photosensitive web  1 . Importantly, the peak power to each one of the plurality of lasers  22  is controlled, as described above, for minimizing fog spots on the web  1 . 
     The invention has been described with reference to a preferred embodiment. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST: 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 photosensitive web 
               
               
                 2 
                 indicia 
               
               
                 10 
                 apparatus of the invention 
               
               
                 12 
                 source of laser energy 
               
               
                 14 
                 laser printer means 
               
               
                 16 
                 laser head 
               
               
                 18 
                 laser beam tube 
               
               
                 20 
                 active end of laser beam tube 18 
               
               
                 22 
                 lasers 
               
               
                 23 
                 optical path 
               
               
                 24 
                 lens 
               
               
                 26 
                 means for controlling peak power or laser beam attenuating 
               
               
                   
                 member 
               
               
                 27 
                 beam splitter 
               
               
                 30 
                 mesh screen 
               
               
                 32 
                 opening in mesh screen 30