Abstract:
A method of electronic processing of a latent image from a photographic element, the method employing pulsed radiation and radio frequency photoconductivity apparatus having a sample capacitor with a gap, including the steps of: placing the element in an electromagnetic field adjacent the sample capacitor; providing an advance mechanism for advancing the photographic element past the capacitor; scanning the element through the gap in the sample capacitor with a pulsed, focused beam of radiation; directly measuring the photoelectron response of the element and recording the resulting signals from the radio frequency photoconductivity apparatus; and advancing the element and repeating the exposing and measuring steps to provide a two dimensional readout of the latent image on the photographic element at ambient temperature or below.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electronic processing of exposed photographic material. In particular, this invention relates to the use of a radio frequency photoconductivity measurement to scan a photographic element to detect a latent image in the exposed photographic material. 
     BACKGROUND OF THE INVENTION 
     The latent image in silver halide crystals is formed through the excitation of free charge carriers by absorbed photons and their subsequent trapping and reaction with interstitial silver ions within the silver halide grain structure to form latent image centers (i.e. electron trapping centers). The use of electromagnetic radiation to detect latent image formation in exposed silver halide grains has been recognized in the photographic art. For example, the January/February 1986 issue of Journal of Imaging Science, Vol. 30, No. 1, pp. 13-15, in an article entitled “Detection of Latent Image by Microwave Photoconductivity”, describes experiments designed to detect latent image formation in silver halide using microwave photoconductivity. The technique, which is operated at room temperature, is recognized as potentially useful in detection of latent images without the need for conventional chemical development solution processing. 
     Carriers which are thought to play an important role in the formation of latent image centers in silver halide grains are believed to be electrons, holes, and interstitial silver ions. The mobility of electrons is far greater than that of holes or interstitial silver ions so that conductivity attributed to photoelectrons is expected to be detectable by measurement of photoconductivity of silver halide grains through use of microwave radiation. Such a measurement has been reported using low temperatures, L. M. Kellogg et al., Photogr. Sci. Eng. 16, 115 (1972). 
     U.S. Pat. No. 4,788,131, issued Nov. 29, 1988 to Kellogg et al., entitled “Method of Electronic Processing of Exposed Photographic Material” discloses a method for electronically processing exposed photographic materials for detection and measurement of latent images contained therein. The method includes the steps of placing the element in an electromagnetic field and cooling the element to a temperature between about 4 to about 270K to prevent further image formation; subjecting the element to a uniform exposure of relatively short wavelength radiation; exposing the element to pulsed, high intensity, relatively longer wavelength radiation to excite electrons out of image centers; and measuring any resulting signal with radio frequency photoconductivity apparatus. 
     The shortcomings of this approach are that it needs to be performed at low temperatures, and there is no easy technique disclosed for making a two dimensional scan of the element. 
     Accordingly, there is a need for an improved technique for detection and measurement of latent images in silver halide photographic materials. 
     SUMMARY OF THE INVENTION 
     The need is met according to the present invention by providing a method of electronic processing of a latent image from a photographic element, the method employing pulsed radiation and radio frequency photoconductivity apparatus having a sample capacitor with a gap, that includes the steps of: placing the element in an electromagnetic field adjacent the sample capacitor; providing an advance mechanism for advancing the photographic element past the capacitor; scanning the element through the gap in the sample capacitor with a pulsed, focused beam of radiation; directly measuring the photoelectron response of the element and recording the resulting signals from the radio frequency photoconductivity apparatus; and advancing the element and repeating the exposing and measuring steps to provide a two dimensional readout of the latent image on the photographic element at ambient temperature or below. 
     In a preferred embodiment, the photographic element has a Ruthenium hexacyano doped tabular grain emulsion with a grain size greater than 2 μm, and the measurement of the photoelectron response is conducted at ambient temperature. 
     The present invention has the advantage of eliminating the need for chemical processing of photographic film for development. A simpler film format can be employed with the present invention that does away with the need for dispersions or interlayers, thereby simplifying and reducing the cost of the film manufacturing process. Only one emulsion per color is required since the resulting signal from individual silver halide grains is proportional to the exposure level of the grain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a radiofrequency photoconductivity measurement apparatus for use in the electronic process of the present invention; 
     FIG. 2 is a detailed view of the tuned LC circuit of FIG. 1; 
     FIG. 3 is a detailed view of the capacitor electrode configuration of the present invention; 
     FIG. 4 is a flow diagram showing the individual steps in the electronic process of the present invention; 
     FIG. 5 is a detailed view of an alternative embodiment of the electrode configuration; 
     FIG. 6 is a schematic view of a further alternative embodiment of the electrode configuration, wherein the electrodes are segmented; 
     FIG. 7 is a schematic view of a still further alternative embodiment of the electrode configuration wherein segmented electrodes are provided with LED arrays for scanning the photographic element; 
     FIG. 8 is a schematic diagram of a film element particularly useful with the present invention; 
     FIG. 9 is a schematic diagram useful in describing the orientation of tabular film grains with respect to the electric field produced by sample capacitor according to a preferred mode of practicing the present invention; and 
     FIG. 10 illustrates a silver density vs. exposure curve obtained by chemical processing and a signal in mV vs. exposure curve obtained by electronic processing of identically exposed photographic samples. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The light sensitive elements in photographic systems are silver halide emulsion grains. These grains are photoconductors, i.e. when they are exposed, either in the intrinsic absorption region or in the dye absorption region, electrons are excited into the conduction band and these electrons are free to move through the grain. If these grains are placed in an electromagnetic field and then exposed, this photoconductivity can be detected by measuring the change in the field. 
     In the examples presented here the photoconductivity is measured in the following way. A photographic film is placed in a measurement capacitor in a tuned radio frequency circuit. The change in the capacitance of this tuned circuit is then measured when the silver halide grains in the film are exposed and the free electrons are excited into the conduction band. 
     This technique can be used to detect the level of exposure the silver halide grains have received because new electron traps are formed in the grains as a result of the exposure. These traps, which decrease the photoconductivity, are formed when mobile interstitial silver ions in the silver halide grain react with the photoelectrons generated during exposure and form Ag n   0  centers which associate with interstitial ions and act as electron traps. The photoconductivity, then, decreases as the exposure level the grain has received, increases. 
     In order to use this technique to scan an image, it is necessary to provide a measurement capacitor that is sensitive enough to detect a small spot size for good image resolution, and would allow the film to be scanned in two dimensions. The following characteristics are necessary to achieve these goals: 
     1. The sample should be placed in the capacitor so the long dimension of the tabular grain is parallel to the (RF) field. 
     2. The capacitor gap should be very small, i.e. on the order of the image resolution required. 
     3. The photographic element should pass through or over the electrodes to allow 2 dimensional imaging. 
     Referring to FIG. 1, the method of the present-invention is carried out on radio frequency photoconductivity measurement apparatus  10  that includes a radiofrequency signal generator  12  and a radiofrequency bridge  14 . In association with bridge  14  is a 50 ohm terminator  16  and a tuned LC circuit  18 . A preamplifier  20  is provided as is detector  22 . FIG. 2 illustrates, in greater detail, the tuned LC circuit  18  of FIG. 1 wherein is shown inductor  24  along with sample capacitor  26  and variable capacitor  28 . FIG. 3 shows in detail the sample capacitor  26 , which includes two plates  26   a  and  26   b  arranged coplanar with each other and adjacent a photographic film element  29 . A pulsed focused scanning light beam  30  is directed onto the photographic film element  29  through a gap  32  formed by the capacitor plates  26   a  and  26   b . Preferably the gap is small, having a size on the order of the diameter of the scanning beam  30  (e.g. 20-100 μm). A drive mechanism includes drive wheel  34  and idle wheel  36  and a motor  38  connected to drive wheel  34 . After the light beam  30  scans the element  28 , the advance mechanism advances the element  28  by one scan line, and the scan is repeated. 
     Referring to FIG. 4, the method of the present invention includes the steps of providing ( 48 ) an exposed photographic element; placing ( 50 ) the element  29  adjacent to the sample capacitor  26 ; and scanning ( 52 ) the element  29  with the pulsed beam of light  30 . The photoelectron response is directly measured and recorded ( 54 ) by the radio frequency photoconductivity apparatus  10  and the element  29  is advanced ( 56 ) by one scan line. A check ( 58 ) is made to determine if the element has been completely scanned. If not, the next line is scanned ( 52 ) and the process is repeated until the element  29  has been completely scanned. After the element  29  has been scanned to read out the latent image, the image signal can be displayed ( 60 ) or stored ( 62 ) for later viewing. 
     FIG. 5 shows in detail an alternative configuration for sample capacitor  26  which includes two plates  26   a  and  26   b  with slots  27   a  and  27   b  through the center of each plate. These plates are arranged coplanar with each other. A photographic element  29  passes through slots  27   a  and  27   b  into the (RF) field established between the two plates. A pulsed focused scanning light beam  30  is directed onto element  29  through gap  32  formed by the capacitor plates  27   a  and  27   b.    
     FIG. 6 shows in detail a possible capacitor array  26  which includes multiple (e.g.  5 ) plates  26   a  arranged coplanar with corresponding plates  26   b . All of these plates are adjacent to a photographic element  29 . These plates are separated by insulating regions  40   a  and  40   b . A pulsed focused scanning light beam  30  is directed onto element  29  through the gap  32  between the plates. This arrangement increases the sensitivity of the apparatus by employing smaller capacitors. The drawback to this arrangement is that it has gaps between the capacitors where the film cannot be scanned. In order to scan the entire width of the film element  29 , a second capacitor array and scanning beam shifted with respect to the first array can be provided, such that the locations of the capacitor plates in the second array occur in the gaps of the insulators in the first array. It will be understood that although each capacitor plate in FIG. 6 is shown with 5 elements, more or fewer than 5 may be used in the practice of the present invention. 
     FIG. 7 shows an alternative embodiment of the present invention having a capacitor array including capacitor  41  with coplanar plates  41   a  and  41   b  and capacitor  42  with coplanar plates  42   a  and  42   b . Associated with these capacitors are LED arrays  44  and  46  respectively for scanning the photographic element through the gaps between the capacitor plates. Each capacitor and associated LED array scans a separate portion of the film, and are shown staggered in the direction of film travel so that they can be easily arranged to scan the entire width of the film. Although two such arrays are shown it should be understood that any number of such arrays can be employed across the width of the film. 
     FIG. 8 shows a schematic diagram for a color photographic film element useful with the present invention. This color film element consists of a film base  78  coated with a gel pad and antihalation layer  80 . An emulsion layer  82  is coated over the gel pad. Preferably this emulsion layer includes tabular light sensitive silver halide grains. This emulsion layer contains both the green and the red sensitized emulsions. On top of the red and green sensitized emulsion layer is a yellow filter layer  84  to prevent blue radiation from reaching the red and green emulsion layer  82 . A blue sensitized emulsion layer  86  (preferably also a tabular grain emulsion) is coated on top of the filter layer  84  and a gelatin overcoat  88  is coated over the blue emulsion layer  86  for protection. The color information is recovered from an exposed film element of this type by scanning the element separately with red, green and blue beams of light. 
     FIG. 9 illustrates the orientation of the tabular silver halide grains  90  and the film base  92  with respect to the electric field  94  in the preferred embodiment of the film element. For other emulsion types other field orientations may be useful. 
     EXAMPLE 1 
     A 4.0 μm×0.11 μm Ag(Br,I) (4% I) T-Grain emulsion doped with Ru(CN) 6   −3  at 25 ppm and dyed with 0.5 mmol/Ag mol of a blue sensitizing dye was coated at a silver coverage of 2.6 g Ag/m 2  and 4.3 g gel/m 2  over a film support previously coated with an antihalation (AHU) layer. 
     Five 35 mm×300 mm samples were prepared for measurement in a radiofrequency (RF) photoconductivity measurement apparatus according to FIG.  1 . One sample was unexposed and the remaining samples were exposed to the 10 −2  s exposure of an EG&amp;G sensitometer with a different neutral density filter in the exposure beam for each strip. 
     One strip at a time was placed next to the sample capacitor in the apparatus of FIG.  1 . The system was tuned and the room temperature photoconductivity signal was measured several times by moving the sample up to an unexposed position after each measurement. The measurement exposure was a strobe exposure that was filtered with a Wratten 47b (blue) filter and focused into a 100 μm optical fiber. The other end of the optical fiber was placed in a holder in close proximity to the gap in the sample capacitor. Only a portion of the entire sample was exposed during the measurement. The same strips that were measured were then processed in Kodak Rapid X-ray (KRX) developer (3 minutes@20° C.). Table 1 below records the exposure, the photoconductivity signal observed, and the corresponding developed density of the comparison coating: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 10 −2  EG&amp;G 
                 Photoconductivity 
                 Comparative 
               
               
                   
                 Exposure 
                 Signal (mV) 
                 Developed Density 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 No Exposure 
                 83 ± 3 
                 .54 
               
               
                   
                   
                   
                 (AHU) 
               
               
                   
                 +2.1 ND 
                 70 ± 1 
                 .76 
               
               
                   
                 +1.5 ND 
                 64 ± 1 
                 1.88 
               
               
                   
                 +1.0 ND 
                 55 ± 1 
                 2.38 
               
               
                   
                 No ND 
                 39 ± 1 
                 2.98 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 10 is a plot of this data and shows a comparison for the blue sensitized emulsion of the density versus log relative exposure curve  96  obtained by chemical processing and the signal in mV versus log relative exposure  98  obtained by measuring the photoconductivity response of the film while scanning with a light beam. The background density on the chemically processed curve  100  represents the background density due to the antihalation layer (AHU). The response of the red and green sensitized layers is similar. 
     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 spirit and scope of the invention. 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                  10 
                 radio frequency photoconductivity measurement apparatus 
               
               
                  12 
                 radio frequency signal generator 
               
               
                  14 
                 radio frequency bridge 
               
               
                  16 
                 50 ohm terminator 
               
               
                  18 
                 tuned LC circuit 
               
               
                  20 
                 preamplifler 
               
               
                  22 
                 detector 
               
               
                  24 
                 inductor 
               
               
                  26 
                 sample capacitor 
               
               
                  26a,b 
                 capacitor plate 
               
               
                  27a,b 
                 slots in capacitor plates 
               
               
                  28 
                 variable capacitor 
               
               
                  29 
                 photographic film element 
               
               
                  30 
                 scanning light beam 
               
               
                  32 
                 gap 
               
               
                  34 
                 drive wheel 
               
               
                  36 
                 idle wheel 
               
               
                  38 
                 motor 
               
               
                  40a,b 
                 insulating regions 
               
               
                  41 
                 capacitor 
               
               
                  41a,b 
                 coplanar plates 
               
               
                  42 
                 capacitor 
               
               
                  42a,b 
                 coplanar plates 
               
               
                  44 
                 LED array 
               
               
                  46 
                 LED array 
               
               
                 (48) 
                 exposed photographic element step 
               
               
                 (50) 
                 place element adjacent sample capacitor step 
               
               
                 (52) 
                 scanning element step 
               
               
                 (54) 
                 measure and record step 
               
               
                 (56) 
                 advance step 
               
               
                 (58) 
                 check step 
               
               
                 (60) 
                 display step 
               
               
                 (62) 
                 storage step 
               
               
                  78 
                 film base 
               
               
                  80 
                 antihalation layer 
               
               
                  82 
                 red and green emulsion layer 
               
               
                  84 
                 yellow filter layer 
               
               
                  86 
                 blue sensitized emulsion layer 
               
               
                  88 
                 gelatin overcoat 
               
               
                  90 
                 silver halide grains 
               
               
                  92 
                 film base 
               
               
                  94 
                 electric field 
               
               
                  96 
                 density vs. relative exposure curve 
               
               
                  98 
                 signal vs. relative exposure curve 
               
               
                 100 
                 background density curve