Abstract:
A method for discrimination between a first ( 308 ) and a second surface ( 304 ) type based on reflectivity has a light source ( 416 ) that illuminates on a media surface. A photosensor ( 420 ) receives and measures the reflection value from the surface. A first gain element adjusts a voltage from the photosensor and a second gain element adjusts a current measurement supplied to the light source. A subtractor ( 530 ) for subtracting the first adjusted voltage ( 534 ) and the adjusted measurement ( 538 ) are subtracted to provide an output value close to zero with respect to the second surface and near a maximum with respect to the first surface. The adjusted reflection value and a threshold reference value ( 428 ) are compared ( 124 ) and indicates whether the first surface or the second surface is present.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 12/874,249 (now U.S. Pat. No. 8,427,648), filed Sep. 2, 2010, entitled APPARATUS FOR DISCRIMINATING BETWEEN OBJECTS, by Burkatovsky; the disclosure of which is incorporated herein. 
     FIELD OF THE INVENTION 
     The present invention relates in general to a method for discriminating between two objects based on surface reflectivity differences, and more specifically to discriminating between printing plates covered by a polymer emulsion and interleaf paper between the plates. 
     BACKGROUND OF THE INVENTION 
     A computer-to-plate (CTP) device  204 , shown in  FIG. 2 , is used for direct imaging on printing plates. The plates are loaded in a magazine or cassette and delivered one by one to be exposed by the imaging device. Alternatively the plates can be provided by an automatic plate loader (APL)  104 , wherein a plates stack  108  is inserted into the APL as is shown in  FIG. 1 . The plates  304  provided in the cassette or in a plate stack  104  are usually separated by interleaf paper  308 , shown in  FIG. 3 , interposed between the plates to prevent the emulsion-covered surfaces of the plates from being damaged. 
     In the course of imaging plates, the plate placed at the top of the stack is picked and transferred to the exposure area for imaging. When an interleaf paper (slip-sheet) is at the top of the stack, the paper is picked and disposed of, before picking the plate. There is a need for discriminating between plate and the interleaf paper are used, to correctly identify the topmost object on the stack. 
     U.S. Pat. No. 6,825,484 (Burkatovsky) describes a discriminating device based on measurements of the light reflections from the surfaces with different roughness. For example, discrimination between paper and non-covered by emulsion printing plate will be reliable due to substantially different roughness of paper as opposed to a smooth and glossy plate metal surface. But discrimination between paper and emulsion covered plate will often be inaccurate due to the small difference between their roughness properties. 
     Another method for discriminating the slip sheets and emulsion covered printing plate described in U.S. Pat. No. 7,157,725 (Kawamura). This method is based on the difference between absorbance (reflectance) of a slip sheet and a plate, irradiated by light of 570-740 nm wavelengths. 
     Reflectance of emulsions and papers produced by different manufacturers may vary substantially. An example of reflections from papers and emulsions of different manufacturers is shown in  FIG. 6 . X-axis depicts lighting source current or irradiating intensity I and Y-axis depicts the reflection intensity represented by photosensor output voltage Vr. The reflections from paper slip sheet and emulsion covered plate manufactured by the first manufacturer and measured at predetermined height and light source current is shown by lines  604  and  608  respectively. Reflections from paper slip sheet and emulsion manufactured by the second manufacturer and measured at the same height and light source current is shown by lines  612  and  616 . The threshold Vth 1  for discrimination between paper and emulsion produced by the first manufacturer according to Kawamura et al., should have value greater than emulsion reflection and less than paper reflection values. While irradiation caused by the same light source current the reflections from paper and emulsion produced by the second manufacturer are smaller. This might happen due to different processes applied for emulsion covered printing plates and slip sheet by different manufacturers. In this case the chosen threshold Vth 1  will be greater than paper and emulsion reflections and media discriminating will be impossible. 
     It should be noted that not only manufacturer media variations and differences between the batches of media lead to reflection deviations. Changing parameters such as distance to media, light source, ambient light are also impact on reflections thus making difficult to practical implementation of the method suggested by Kawamura et al. 
     The purpose of this invention is to improve the paper slip sheet and emulsion covered plate discrimination capability. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention a method for discrimination between a first and a second surface type based on reflectivity has a light source that illuminates on a media surface. A photosensor receives and measures the reflection value from the surface. A first gain element adjusts a voltage from the photosensor and a second gain element adjusts a current measurement supplied to the light source. A subtractor for subtracting the first adjusted voltage and the adjusted measurement are subtracted to provide an output value close to zero with respect to the second surface and near a maximum with respect to the first surface. The adjusted reflection value and a threshold reference value are compared and indicates whether the first surface or the second surface is present. 
     The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an automatic plate loader (APL) loaded with stack of plates (prior art); 
         FIG. 2  shows an imaging device connected to an APL (prior art); 
         FIG. 3  is a schematic illustration of a plates stack, showing plates separated by slip sheets (prior art); 
         FIG. 4  is a schematic illustration of paper/plate discrimination device known in the art; 
         FIG. 5  is a schematic illustration of the discrimination device proposed by the current disclosure; 
         FIG. 6  is shows a behavior function of reflection from various media objects; 
         FIG. 7  is shows a graph of amplification factors selection; and 
         FIG. 8  is shows the discrimination behavior of the suggested device in response to the distance from the measured target. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure. 
     While the present invention is described in connection with one of the embodiments, it will be understood that it is not intended to limit the invention to this embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents as covered by the appended claims. 
     The schematic illustration of a discriminating device known the art is shown in  FIG. 4 .  FIG. 5  shows components of the proposed invention, where some of the components are also being used in the Kawamura et al., as are shown in  FIG. 4 . The illumination control unit  404  intended for setting the level of media  408  illumination connected through the light source driver  412  to the light source  416 , the light source can be a LED (light emitting diode). The reflections from media  408  are measured by photosensor  420 . 
     An adjustment element  542  is connected by its inputs to the outputs of the photosensor  420  and the illumination control unit  404 . The output of the adjustment element  542  is connected to the first input of comparator  124  while the second input of comparator  124  is connected to the threshold reference  428 . 
     The irradiation of the tested media provided by the light source  416  controlled by the illumination set point signal Vi produced by illumination control unit  404  through the light source driver  412 . The photosensor  420  generates signal V R  proportional to the reflection from tested media. This signal is amplified by first input amplification factor  534  (G R ) of adjustment element  542 , simultaneously the illumination set point signal Vi is gained (divided) by second input amplification factor  538  (Gi) and than subtracted by subtractor  530  from the gained V R  signal. The result of the subtraction is represented by subtractor output value signal V S  complying with following equation:
 
 V   S   =V   R   *G   R   −V   i   *G   i   (1)
 
 FIG. 7  explains the selection of G R  and Gi which is substantial for suggested discriminating device. Assuming G R0 =1 and Gi 0 =0.
 
     In this case according to Equation (1) VS will equal to VR.
 
 V   S   =V   R   (2)
 
In the case when VS equals VR, according to Equation (1), the behavior of the proposed discrimination device shown in  FIG. 5  will behave as the device described by Kawamura et al, which is shown in  FIG. 4 .
 
     Line  704  in  FIG. 7  shows a response function representing the reflection from paper slip sheet and line  708  represents reflection from emulsion covered plate surface respectively. These lines are identical to lines  604  and  608  of  FIG. 6 . Respectively Vsp=Vpl and Vse=Vel. The relation between the Vsp and Vse determines the capability of the device to perform a reliable discrimination between paper slip sheets and emulsion covered plates. The relation between amplified paper reflection and amplified (divided) emulsion reflection is defined as discrimination factor DF.
 
 DF=Vsp/Vse   (3)
 
     As much as the Vsp value is bigger than Vse value, the discrimination will be more reliable, due to covering of a larger reflection range and thus decreasing the sensitivity of reflection deviations. 
     In other words in order to improve the discrimination capability of a discrimination device the value of DF needs to be increased. This can be achieved by adjusting the amplification factors  534  (G R ) and  538  (Gi). The Gi adjustment should be provided while emulsion covered printing plate is examined. Adjustment may start with mentioned above values of Gi 0  and G R0 . (Gi O =0, G RO =1). According to equations (2) and (3) discrimination factor for these values will be defined as
 
 DF   0   =Vsp   0   /Vse   0 .  (4)
 
     Now by increasing  538  (Gi) up to the moment when Vse will be close to zero we obtain the situation when Vse is practically not dependent upon the light source  416  current and remains low within the light source current possible range (line  716 ). Respectively after adjusting  538  (Gi) while examining the paper slip sheet, the Vsp line  704  will change its slope. The Vsp dependence on light source  416  current after Gi adjustment is presented by line  712 . 
     Increasing the light source  416  current to Im by means of illumination control unit  404  we obtain Vsph value while examining paper and Vsel value while examining emulsion covered plate. As Vsph is bigger than Vsp and Vsel is lower than Vse thus according to equation (3) the value of representing discrimination factor
 
 DF 1 =Vsph/Vsel   (5)
 
will be much bigger than DF 0  (4), thus yielding a substantially improved discrimination capabilities. The maximum value of Vsph is restricted by power supply voltage. In other words the threshold margin is enough to support the discrimination of plates and emulsions from various manufacturers.
 
     It should be noted that the DF1 value may be achieved also without light source  416  current changing (from In to Im). This can be obtained by increasing the amplification factor  534  (G R ) while maintaining line  716  close to the X-Axis (as is shown in  FIG. 7 ), by adjusting the amplification factor  538  (Gi). 
       FIG. 8  depicts the subtractor output value V S  signal used in  FIG. 7  as a function of media to sensor distance shown as axis H ( 804 ). Comparing to  FIG. 7  point Hn of axis H determines the media to sensor distance while illuminating current value is In. Reflections from paper slip sheet and emulsion covered plate at this point are the same as shown on  FIG. 6  (Vpl and Vel). Respectively the subtractor output values while Gi O =0 and G RO =1 are Vsp and Vse, the same as shown on  FIG. 7 . Now while maintaining the constant value of LED current. In the amplification factors  534  (G RO ) and  538  (Gi O ) should be adjusted such as subtractor output value V S  is close to zero Vsel in response to lower reflectance surface type (emulsion covered plate). As a result of the subtractor output value V S  is maximal Vsph in response to higher reflectance surface type (slip slit paper). The subtractor output value behavior represents the emulsion covered plate reflection after gains adjustment is depicted by line  812 . Respectively the subtractor output value behavior for the slip sheet paper reflection after gains adjustment is represented by behavior function  808  (sensor to media distance is bigger than Hn) and line Vsph—maximum voltage value restricted by power supply voltage (sensor to media distance is smaller than Hn). 
     As it can be seen from  FIG. 8  the range from smaller media to sensor distance Hm to higher media to sensor distance Hn shows practically same subtractor output values such as Vsel close to zero in the case of emulsion testing and Vsph close to power supply voltage in case of slip sheet paper testing. Within this (Hm−Hn) range the discrimination factor has maximum allowable value DF=(Vsph/Vsel) according to Equation (3) and DF=constant as well. Referring to the prior art performance according to  FIG. 8 , in the Hn sensor to media distance point the prior art device will have a worse DF than the suggested device as is shown by Equations (4) and (5). In addition the Hm sensor to media distance point in prior art device will not work at all, due to very high incoming reflection. Specific gains adjustment allows discrimination performance practically independent of sensor to media distance and extending of sensor to media distance range where discriminating is possible. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 104 
                 automatic plate loader 
               
               
                 108 
                 plate stack 
               
               
                 124 
                 comparator 
               
               
                 204 
                 computer-to-plate (CTP) device 
               
               
                 304 
                 plates (with emulsion surface up) 
               
               
                 308 
                 interleaf paper (slip sheets) 
               
               
                 404 
                 illumination control unit 
               
               
                 408 
                 media 
               
               
                 412 
                 light source driver 
               
               
                 416 
                 light source 
               
               
                 420 
                 photosensor 
               
               
                 428 
                 threshold reference 
               
               
                 530 
                 subtractor 
               
               
                 534 
                 first input amplification factor 
               
               
                 538 
                 second input amplification factor 
               
               
                 542 
                 adjustment element 
               
               
                 604 
                 sheet slip reflection graph a first plate manufacturer 
               
               
                 608 
                 emulsion surface reflection graph a first plate manufacturer 
               
               
                 612 
                 sheet slip reflection graph from a second plate manufacturer 
               
               
                 616 
                 emulsion surface reflection graph a second plate manufacturer 
               
               
                 704 
                 slip sheet reflection graph a first plate manufacturer 
               
               
                 708 
                 emulsion surface reflection graph a first plate manufacturer 
               
               
                 712 
                 adjusted slip sheet reflection graph 
               
               
                 716 
                 adjusted emulsion surface reflection graph 
               
               
                 804 
                 height axis 
               
               
                 808 
                 slip sheet reflection behavior function after gains 
               
               
                 812 
                 emulsion surface reflection behavior function after gains