IL42445A - Method and apparatus for recording images on electro-photographic film - Google Patents

Abstract

1438870 Electro-photographic process COULTER INFORMATION SYSTEMS Inc 5 June 1973 [8 June 1972] 26889/73 Heading G2H An electro-photographic process comprises charging a photo-conductive film F at 24 to a peak voltage, immediately after charging imagewise exposing (e.g. using a projector 56) and, immediately after exposure, toning at 84; a photo-cell 32 detecting image light intensity in order to control the peak voltage in inverse relation to the light intensity. The process is particularly intended for making high resolution microimages. Preferably the film comprises a Mylar (TRM) base coated with indium oxide then CdS. As shown, differential amplifier 36 is connected to photo-cell 32 and an electrometer 34 to control a relay 42 connecting voltage source 46 to coronade 24. When amp 36 terminates charging a shutter 58 is opened to expose the film and is closed when the charge on an unexposed area sensed by electrometer 34 has decayed to a predetermined level. As soon as exposure is terminated, liquid toner is pumped to a head 84 and applied under the influence of an electrode 92, whose bias is optionally controlled by electrometer 34. A pump 109 removes excess toner and a heater 126 fixes the image, In Fig.3 (not shown) a rotating drum (109) carries the various processing stations past the film. [GB1438870A]

Description

METHOD AND APPARATUS FOR RECORDING IMAGES ON ELECTRO-PHOTOGRAPHIC FILM This invention relates to a method and apparatus for recording high quality images on electrophotographic film.

There presently exists a large variety of electrostatic image recorders which rely for their operation on the basic steps of charging a previously discharged photo-conductive medium, exposing the medium to a light pattern to form a latent image thereon, applying toner to the medium to form a visible image thereon and then fixing the image either on the medium directly or after transferring the image to another surface such as paper -Most of the prior apparatus are in the form of office copying machines such as the xerographic copiers and the electrofacsimile copiers.

It has also been proposed to use the same basic techniques to form microimages on film. Thus far, however, so far as known, such efforts have not resulted in practical results. One reason for this lies in the fact that the photographic reduction to microfilm size requires subsequent magnification of the recorded image. Not only is the recorded information magnified, however, but also any imperfections in the image are magnified to the same extent, typically on the order of twenty times.

In addition, in order to obtain a suitable full-size picture with reasonably good resolution of the microimage, i.e. five lines per millimeter, the system for making the microimage must have a resolution on the order of 100 lines per millimeter. Systems proposed heretofore are not capable of producing such high resolution images on microfilm. Moreover, these prior systems cannot achieve on the film the total density and purity that is required for high-quality electrophotographic microimages .

Heretofore, the various steps in electrophotography have been treated as a succession of static, unrelated events. First, the photosensitive medium is charged, then it is exposed to an image. Light falls on the portions of the medium corresponding to the light areas of the image, causing the charge on those areas to dissipate while those portions of the medium corresponding to the dark areas of the image retain their charge. In this way, a latent photographic image is formed on the medium. Following this, toner is applied to the medium which tends to adhere to those portions thereof which still retain an electric charge, thereby reducing the latent image to visible form. Finally, the toner is fused to the medium so that the image thereon becomes permanent .

The aforesaid steps are performed in successive time intervals, usually at different locations in the reproduction apparatus.

More significantly, since the prior processes are concerned with impressing a relatively low resolution image on a relatively large image area, they operate at relatively slow copying speeds, i.e. 1 to 10 seconds.

A typical photoconductive medium such as selenium has a characteristic dark decay curve. Once charged to its customary initial voltage, e.g. 500-600 volts, it exhibits a fairly rapid rate of decay, e.g. 50-100 volts/min. during the first minute or so. Then the rate of decay gradually becomes less until the voltage reaches a substantially constant residual background value of about 30^50 volts. Conventional xerography systems, being fairly slow as noted above, process the medium at a time when the rate of decay of the charge on the medium is fairly slow.

It has been found that the development of high-quality images on microfilm requires an entirely different approach. This is because the area to be imaged is very small. Also, the resolution and tonal range requirements of microfilm images are much higher than is the case with larger xerographi prints.

Accordingly, the invention provides a method of producing images on a photoconductive film member which includes the steps of charging the member, exposing the charged member to light and tonin the member after exposure, which comprises charging the member to a peak voltage the magnitude of which is controlled inversely in accordance with the intensity of said light, exposin said charged member immediately following the completion of said chargin and effecting toning immediately subsequent to the completion of the exposure of the member.

The invention further provides apparatus for recording images on a photoconductive medium and including means for charging the medium to a peak voltage, means for exposing the charged medium to a light image immediately following such charging to the peak voltage and means for applying toner to the exposed medium immediately following the completion of exposure, means responsive to the intensity of the light to which the member is exposed and means for controlling the peak voltage to which the medium is charged inversely in accordance with said light intensity, The preferred embodiments of this invention will now be described, by way of example, with reference to the drawings accompanying this specification in which: Figure 1 is a functional block diagram of a system for recording images on electrophotographic film embodying the principles of this invention; Figure 2 is a graph illustrating in greater detail the operation of the Figure 1 system; and Figure 3 is a diagrammatic view of another embodiment of my invention) and Figure 4 is a graph illustrating the operation of the Figure 3 system.

By way of summary prior to a detailed description of the illustrated embodiments, the technique proposed herein involves controlling the, steps of the xerography process as a dynamic series of interrelated steps, some of which are performed concurrently and all of which are performed on a greatly collapsed time scale as compared with conventional xerographic processes. Furthermore, these steps are all carried out at a time when the charge on the medium is decaying most rapidly, i.e. very early on the characteristic dark decay curve of the particular medium. For best results, it is desirable that the photo conductive film being processed be fairly fast and have a fairly rapid dark adaption. Applicant has developed for this purpose a film comprised of a Mylar polyester substrate, an intermediate layer of indium oxide (500 A thick) and a top layer of an n-type cadmium sulfide (3000 A thick). This film has a rapid dark decay rate on the order of 800 volts/sec. initially. It is transparent so that its image may be projected directly as explained hereinafter.

The film (or more particularly its photo conductive layer) is charged, with the charge and light conditions being monitored by an exposure meter so that the charge builds up to an optimum value for that light condition.

Furthermore, contrary to the prevailing practice, the film is subjected to a very high charging voltage (i.e.. a shock) which is often substantially above the saturation level of the particular photo conductive medium. In other words, whereas conventional xerography involves charging the medium to saturation, i.e. to the point where the charge buildup on the medium equals the charge leaking off the medium, the present system often charges the medium very quickly to a voltage far in excess of saturation, exposed to lower the surface voltage, before the film breaks down. This enables the charging step to be performed in a very short time, e.g. 500 microseconds.

As soon as the charge level on the film reaches this desired peak, the film is exposed to the image being reproduced with the automatic exposure meter controlling the exposure in relation to the attainment of the peak charge.

Following this, the toning process is initiated in relation to a selected monitored voltage on the film surface which provides the desired maximum density, i.e. maximum blackness., and the related scale of greys, i.e. half-tones, in the recorded image. This will be discussed later in greater detail.

Next, toner is applied to the film surface very quickly and uniformly. Furthermore, the toner is applied in the presence of a bias voltage in close proximity to the film surface to accelerate the particles toward the film to provide even particle distribution and to minimize lateral particle migration that might tend to cause the well-known edge effect which is characteristic of conventional xerographic reproductions.

Finally, if required, which is not always the case, any excess toner is immediately swept from the film surface and the remaining toner fused to the surface of the film so that the entire process is completed before the surface voltage on the film dissipates to its lowest or background level.

When processed in the foregoing way, the microimage on the film is characterized by a high degree of resolution, good contrast and an exceptionally clean background. Consequently, when the image is projected on a greatly enlarged scale for viewing or copying purposes, the resultant enlarged image still has good resolution and is relatively free of impurities. The benefits of the invention obviously can be applied to the recording of larger images as well.

Referring to Figure 1 of the drawings, the present system advances a strip of film which may comprise microfilm F using any conventional advancement arrangement.

The film F being of the photoconductive type mentioned above includes a transparent plastic substrate 10, a photoconductive layer 12 and an intermediate conductive or ground layer 14. A pair of resilient grounded contacts 16a and 16b slidably engage the opposite edges of the intermediate ground layer 14 to maintain that layer at ground potential.

Prior to processing each frame F' of film F, the frame is advanced past a grounded discharge head 18 in order to remove any electrical charge that may be present on the photoconductive layer 12.

Following this, the frame F is rapidly charged to a peak voltage which is determined by the prevailing light past a charge head 22. Head 22 has a recess 22a in its side wall facing film F which is coextensive with the area of the film frame F ' . A corona discharge wire 24 extends across the recess opposite the film. When a relatively high potential is applied to wire 24 which is negative with respect to ground, a corona discharge develops in the vicinity of the wire 24. The corona causes the photoconductive layer 12 corresponding to frame F' to become negatively charged.

The film is charged very rapidly to a voltage which is dependent upon the prevailing light conditions at the time. This assures that the images impressed on film F will have uniform tonal qualities despite varying conditions. This aspect of the invention will be better understood by reference to Figure 2 which is a graph of the surface voltage on film F versus time.

Each film or, more particularly, its photoconductive layer 12, has a characteristic dark decay curve. In other words, when the film is charged to a selected peak voltage, the charge decays exponentially until it falls to a selected background voltage, e.g. 50-100 volts. If the film is charged to a relatively high potential P as shown by the dotted line curve A in Figure 2, the initial rate of decay following charging is quite rapid. That is, a relatively .small time (and, hence, quantity of light) suffices to cause a relatively large drop in the surface charge to a smaller voltage P* , e.g. 200 volts, the rate of decay is much less as indicated by the dotted line curve B in Figure 2. In other words, a longer time and hence, a greater quantity of light is required to cause the same drop in the surface charge of the film. The voltages shown are exemplary only and will vary with the nature of the photoconductive layer 12.

As illustrated in Figure 1, a photocell 32 is placed adjacent the film frame being exposed so that its output re fleets the average amount of light incident on the film. In the present case, the output is inverted so that it varies inversely with the amount of light. The charge on the film, on the other hand, is monitored by an electrometer 34 which is incorporated into head 22. The electrometer develops a voltage which is proportional to the surface charge on a nonilluminated portion of the frame (i.e. a dark corner beyond the image area) . Accordingly, its output follows the dark decay curve for the film.

The outputs of the photocell and electrometer then are applied to a differential amplifier 36 with high gain so that when its two inputs become equal, its output voltage drops rapidly. A variable resistor 38 which varies the input from the photocell 32 provides an adjustable reference setting.

The output of the amplifier 36 is applied to a current driver 38 which is, in turn, connected to the coil 42a of a relay 42, the other end of which is connected by a switch 44 to ground. Coil 42a controls a switch 42b which connects the corona wire 24 in head 22 to a negative voltage supply indicated by the battery 46.

Switch 44 is normally open, as is the relay switch 42b.

Switch 44 is closed when the film frame F ' is properly positioned in front of head 22 as indicated in Figure 1.

The switch closure may be effected manually or automatically by way of the mechanism which incrementally moves the film. In any event, it is closed for the duration of the charging operation.

The closing of switch 44 energizes the relay coil 42 which, in turn, closes the relay switch 42b. This energizes the corona wire and commences the charging operation. As the surface charge on the film builds up, the value thereof at a nonilluminated portion of the film is sensed by the electrometer 34 which develops an output proportional to that charge. As soon as that output equals the voltage applied to amplifier 36 by the photocell 32, the output of- the driver 38 drops and the relay 42 is de-energized.

This opens switch 42b, thereby completing the charging operation.

If the photocell 32 senses that the light incident of the film frame is quite intense, then a relatively low voltage is applied to the amplifier 36. This means that a relatively small charge on the film frame F' will cause an equal output from the electrometer 34 and thereby terminate the charging operation relatively quickly. In this case, the film will be charged to a relatively low peak voltage as indicated by point P' on the curve B in Figure 2. On the other hand, if the photocell 32 senses that the incident light is not as bright, then a higher voltage is applied from the photocell to the differential amplifier.

Consequently, it "will take a greater charge on the film to develop the output from the electrometer 34 that will terminate the charging operation. In this event, the film is charged to a higher peak voltage as typified by the point P on curve A in Figure 2.

Desirably, the film is charged to the correct peak voltage as quickly as possible. This is accomplished by subjecting the film to a relatively high voltage which may be in excess of the saturation voltage for the film and may even approach the breakdown voltage of the film. This is possible with the present system because immediately after the film is charged, it is exposed to the image being reproduced, as will be described presently. Thus, the charge on the film is reduced before actual breakdown can occur.

As soon as the film frame is charged to the selected peak voltage as aforesaid, it is exposed to the image being reproduced. The portions of frame F' receiving light (corresponding to the light areas of the image) will rapidly discharge. On the other hand, those portions of the frame which receive no incident light (corresponding to the dark portions of the image being reproduced) will retain their charge. In other words, assuming the film is charged to point P in Figure 2, the charge on the areas of the frame F ' receiving light will drop very rapidly as indicated by the curve segment C in Figure 2, while the charge on the nonilluminated areas thereof will continue to decay in accordance with the dotted line curve A.

In accordance with the present technique, the frame F* is exposed until the charge thereon as monitored by the electrometer 34 falls to a selected point on the curve C in Figure 2. From that point on, the charge on the film suffers the usual exponential dark decay to the nominal background level as indicated by the dotted line extension of curve C.

The point is selected so that the charge on the film corresponding to the latent image will retain enough of the toner particles which are applied during the toning step to give the image the proper degree of contrast between the black and white areas of the image. In other words, if the exposure step continues for only a brief period, say, until the dark decay reading is 500 volts in Figure 2, the contrast between the light and dark portions of the image impressed on the frame is less than is the case when exposure continues for a longer period. This is because, as shown by the Figure 2 graph, the rate of dark decay is relatively rapid at the outset.

Accordingly, at the beginning of the exposure period, the charge upon the nonilluminated areas of the frame will drop almost as rapidly as the charge on the illuminated areas. Consequently, if the exposure step is terminated relatively quickly, there will be a minimum charge differential on the illuminated and nonilluminated portions of the film frame.

On the other hand, if exposure proceeds for a longer period of time, the difference between the charge levels on the illuminated and nonilluminated portions of the frame becomes considerably greater. For example, if ex- . posure is continued until the charge on the nonilluminated portions of the frame drop to 480 volts, as seen from the Figure 2 curve, the charge on the illuminated areas will have fallen to approximately 180 volts. Accordingly, in this example, the nonilluminated portions of the frame will retain much more toner than the illuminated portions and therefore, the contrast in the resultant image will be much more pronounced. Also, the illuminated or background portions will be whiter in the resultant picture.

Thus, by selecting the point at which the exposure process is terminated, one can control very precisely not only the degree of blackness of the nonilluminated portions of the latent image, but also the density of the half-tones and the scale of greys in the image. In effect, the sensitivity of the film is automatically chosen on the basis of the ambient light conditions measured.

Turning again to Figure 1, the present system exposes the film by means of a conventional projector 56 which projects the image to be reproduced onto the frame Fl . A normally closed shutter system 58 is positioned between the projector and the film to control the duration of exposure. The shutter is actuated to open as soon as the charging operation is completed by the drop in. the output voltage of the differential amplifier 36. A differentiator 62 detects the negative transition of the amplifier 36 output and applies a signal to the SET input of a flip-flop 64. The output of the flip-flop there- . upon opens shutter 58.

All during this period, the charge on the non-illuminated portions of frame F ' is dropping in accordance with curve A in Figure 2, the instantaneous charge being monitored by the electrometer 34. The output of the electrometer is applied to a high gain differential amplifier 66 which also receives the output from an adjustable reference voltage source 68. The output of the amplifier is, in turn, applied by way of an inverter 72 to the RESET input of flip-flop 64. The adjustable source 68 is set to terminate the exposure step when the charge on the nonilluminated portion of the frame reaches a selected value as determined by the electrometer. When this point is reached, the output of amplifier 66 drops, thereby resetting flip-flop 64 and closing shutter 58.

Immediately upon completion of. the exposing step, toner is applied to the film frame. F' . Furthermore, the toner is applied in the presence of a bias field which propels the toner particles toward the film. Not only does this speed up the toning process, it also distributes the toner particles over the charged portions of the frame so as to minimize the edge effect which characterizes images made by the usual xerographic processes.

In the present system, the commencement of the toning step is initiated by a signal from inverter 72 which is applied to a ONE SHOT multivibrator 76 having a variable time constant. The output of the multivibrator is, in turn, applied to a normally closed solenoid valve 78 which is connected in the line between a liquid toner supply 82 and a toner dispenser 84 situated adjacent the film frame F' . As soon as the ONE SHOT 76 is triggered, valve 78 opens, causing toner to flow to dispenser 84. The dispenser has an aperture 84a which is coextensive with frame F' so that liquid toner bathes the entire surface of the frame.

An electrode 92 extends around the edge of aperture 84a. This electrode is connected by way of a relay switch 94 to one terminal of a voltage source, illustrated by a battery 96 whose other terminal is connected to ground. The output of ONE SHOT 76 also is applied to the relay coil 94a controlling switch 94 so that when the ONE SHOT is triggered, the relay is closed. This applies a strong, positive potential to the electrode which helps to propel the toner particles toward the film to obtain more uniform tonal qualities. as noted above.

Toner particles will adhere to those portions of the frame which were not illuminated during the exposing step and, to a lesser extent, to those portions which received a minor amount of illumination. If it is de sired, also, the bias voltage applied during the toning step can be varied inversely with the output of electrometer 34 so that a higher bias voltage is applied when one operates further down on the Figure 2 curve. This assures that even though the charge on the nonilluminated portions of the frame is fairly low, enough toner will be applied to create a sufficiently black image.

The toning operation ceases upon the resetting of the ONE SHOT 76, the time interval depending upon its time constant setting. As soon as the ONE SHOT rests, its output voltage drops. This transition is detected by a differentiator 102 and applied to a second variable time constant ONE SHOT 104.

The output of ONE SHOT 104 is applied to a solenoid valve 108 connected in the line between a vacuum pump 109 and a hood 112 which is open to the film frame F' . The triggering of ONE SHOT 104 opens valve 108 so that a vacuum is drawn in the area adjacent film F. This immediately sucks excess toner away from the illuminated portions of the frame which retain little or no charge and evaporates the toner solvent so that the latent image on the frame is now reduced to visible form.

Immediately following the toner removal, the toner remaining on the film is fused to the film to create a permanent visible image. More particularly, the return of the ONE SHOT 104 to its initial state is detected by a differentiator 120. The output of the differentiator is used to trigger a third ONE SHOT 124 whose output turns on a heater, illustrated as an infrared lamp 126 backed by a reflector 128 which directs heat toward the film. After a given time interval determined by the setting of the ONE SHOT 124, the heater is turned off, thereby completing the developing process.

The electrophotographic film can be processed in accordance with the foregoing technique on a straight line basis with the several stations depicted in Figure 1 being located along a line perpendicular to the path of film travel. As a film frame incrementally moves into position, the successive stations can be advanced sequentially past that frame.

Figure 3 shows an embodiment of the present system wherein most of the processing components depicted in Figure 1 are mounted on the periphery of a cylindrical rotary drum 110. The drum is mounted on a hollow shaft 111 driven by a servomotor 112. The film F is trained around the periphery of drum 110 and is moved from right to left frame-by-frame through a ready station shown generally at 113. The components of the Figure 3 apparatus which are similar to those depicted in Figure 1 bear similar On the way to station 113, each frame passes the discharge head 18 which removes any residual charge on the film. After the frame reaches station 113, the head is rotated one revolution counterclockwise , o advance the various processing components past the film frame. First the corona wire 24 is swept past the film to charge the frame, the charge being monitored by electrometer 34 as described above. Next the frame is exposed. In this case, the shutter 58 of Figure 1 is replaced by a tunnel 114 extending diametrically through drum 110. When the drum is positioned as shown in Figure 3, the tunnel 114 is aligned parallel to the optical axis 0 of a projector 56 which is illuminating the frame. In this case, the projector has a curved field lens to compensate for the curvature of the image plane containing the film frame.

In the Figure 3 system, the duration of the exposure is dependent upon the angular velocity of drum 110. This can be controlled by controlling speed of servomotor 112 using the same inputs that controlled shutter 58 as the mouth of the tunnel sweeps by the frame.

Next, toner is applied to the film frame when an aperture 84a moves past the frame. Manifold 116 inside drum 110 communicates with the aperture 84a and toner is supplied to this manifold by way of a pipe 118 which extends down through shaft 111 and communicates via a rotary coupling 115 with a toner supply. Also, an electrode 92 is positioned around slit 84a to propel the Immediately following the toning step, excess toner is removed from the film when the aperture 112a rotates past the frame at station 113. The aperture 112a communicates with a manifold 122 inside drum 110 which is connected by a suitable pipe 124 via coupling 115 to a vacuum pump.

The advance of aperture 112a past the film marks one complete revolution of drum 110. A heater in the form of an infrared lamp 126 positioned directly behind the film frame at station 113 is then energized as described above to fuse the toner remaining on the film frame, thereby completing the processing operation. Following this, the film F is incremented so that the next frame is brought into station 113 for processing.

Actually, the durations of the charging, exposing and toning steps can all be controlled by varying the speed of drum 110 over its cycle.

Figure 4 is a graph which indicates how the angular velocity of the drum 110 can be varied during different portions of each operating cycle to vary the time when each processing station is operative on the film frame. The control signals required to do this are developed in much the same way as described above in connection with the processing steps and apparatus described in connection with Figure 1.

In a somewhat similar approach, the various processing rotary disk. The film to be processed is advanced past the disk near its periphery and the disk is rotated to bring the various stations into position opposite the ready frame. The operation of that system would be similar to that of the apparatus shown in Figure 3. 137, 319 42445/2 Case 1 August 5-

Claims (1)

1. A method of producing images on a tive film member which includes the steps of charging the exposing the charged member to light and toning the member after exposure V charging the member to a peak voltage the magnitude of which is controlled inversely in ance with the intensity of said exposing said charged member immediately following the completion of said charging and effecting toning immediately sequent to the completion of the exposure of the membe The method according to claim 1 and the step of additionally fusing the toner which is applied to and adheres to the The method according to claims 1 or 2 and the step of additionally applying a bias field adjacent to the member during toning whereby to the toner toward the The method according to any one of claims 2 or 3 and the step of additionally discharging the member prior to charging it to remove any residual charge 319 The method according to claim 1 and the steps of additionally fusing the toner which adheres to member and additionally applying a bias field adjacent to the member during toning so as to propel the toner toward the The method according to claim 1 the steps of additionally fusing the toner which adheres to the additionally applying a bias field adjacent to the member during toning so as to propel the toner ward the member and additionally discharging the member prior to charging it to remove any residual charge The method according to any one of claims 2 to 6 and the step of immediately removing excess toner from the member prior to fusing the The method according to any one of claims 1 to 7 in which the exposing step is continued until the charge on the member reaches a selected The method according to any one of claims 1 to 8 and the steps of additionally discharging the member prior to charging it to remove any residual charge additionally fusing the toner adheres to the additionally applying a bias field adjacent to the member during toning so as to propel the toner toward the member and additionally discharging the member prior to charging it to move any residual charge The method according to any one of claims 2 3 or 4 and the steps of immediately removing excess toner from the member prior to fusing the toner and in which the exposing step is continued until the charge on the member reaches a selected The method according to any one of claims 1 to 10 in which the voltage to which the member is charged is determined by comparing signals ing the instantaneous charge on the member and the intensity of the light incident on the Apparatus for recording images on a conductive medium and including means for charging the medium to a peak means for exposing the charged medium to a light image immediately ing such charging to the peak voltage and means for applying toner to the exposed medium immediately lowing the completion of means responsive to the intensity of the light to which the member is exposed and means for controlling the peak voltage The apparatus according to claim 12 in which there are means for electrically propelling the toner to the medium to achieve even distribution thereof on said The apparatus according to claims 12 or 13 in which there are means for fusing the toner to the medium after application The apparatus according to claims 13 or 14 in which there is a rotary support member and said charging exposing toner applying means and fusing means are all distributed around eral portions of a rotary support member and means are provided for moving the medium as a belt in engagement with said peripheral The apparatus according to any one of claims 13 or 14 in which there are means for sensing said intensity of light and ing a first signal representative sensing means for sensing the instantaneous charge on said medium and deriving a second signal ative and comparing means for comparing said first and second signals and deriving a control nal representative of said comparison and applying The apparatus according to claim 16 in which there are means for removing excess toner after applying The apparatus according to any one of claims 13 or 14 in which there are means for sensing said intensity of light and deriving a first representative means for sensing the instantaneous charge on said medium and deriving a second signal representative and comparing means for comparing said first and second signals and deriving a control signal representative of said comparison and applying said control signal to said charging means and controlling means for controlling the toner applying means after exposure for a length of time determined by the decrease of charge to a predetermined level from its The apparatus according to any one of claims 12 to 18 inclusive and projector means for projecting an image toward the medium and a lable shutter mechanism for exposing the medium to the projected image for a selected period of time m insufficientOCRQuality