Electrophotographic Apparatus

The invention relates to an electrophotographic apparatus in which the image of an original is projected through an exposure controller onto a light-sensitive member which is previously uniformly charged, to form an electrostatic latent image thereon, and more particularly to such apparatus in which a positive copy image is obtained for either a negative or positive original by using a toner of a fixed polarity as a developer and by switching the polarity with which the light-sensitive member is charged.

In a copying apparatus of the type described, when it is desired to adjust the optical density of the copy, the dial on the exposure controller is turned to change the amount of light emitted by the exposure lamp. However, the direction of such adjustment is reversely related depending on whether the original is a negative or a positive image. Specifically, for a negative original, the amount of light from the lamp is increased in order to obtain an increased optical density while the light quantity is decreased for a reduced density. The reverse is true with a positive original. Thus, in the case of a positive original, the light quantity must be increased to provide a reduced optical density while an increased optical density is achieved with a reduced light quantity. As a result, the dial on the exposure controller must be turned in the opposite direction for a positive image from the direction in which it is turned for a negative image. This is inconvenient in practical operations, and often causes malfunctioning.

It is an object of the invention to eliminate the foregoing disadvantages of copying apparatus of the type described above.

In accordance with the invention, the above object is accomplished by providing a change-over switch in the exposure controller, which switch is changed depending on whether the original is a positive or a negative image. A particularly advantageous electrophotographic apparatus can be obtained by the provision of a photoelectric detector sensitive to a variation in the optical density in the marginal edge of the original for discriminating between a negative and a positive image, and switching means adapted for interlocked operation with the change-over switch in the exposure controller and responsive to a detection signal from the detector to change the polarity of the charger.

Other features and advantages of the invention will become apparent from the following description of an embodiment of the invention shown in the attached drawings.

FIG. 1 is a schematic illustration of the exposure controller circuit used in the invention;

FIG. 2 is a plan view of a film carrying a negative original;

FIG. 3 is a plan view of a film carrying a positive original;

FIG. 4 is a schematic view of a microreader-printer in which the invention is incorporated; and

FIG. 5 is a circuit diagram of the electrical circuit used in the microreader-printer shown in FIG. 4.

Referring to FIG. 1, there are shown a pair of terminals 1, 1 across which a power supply (not shown) is connected. A pair of branch circuits are connected across the supply terminals, one including a print start switch 2 in series with a print drive circuit 3 and the other being an exposure controller circuit 4. The circuit 4 includes a pair of contacts 5 of an exposure timer, a change-over switch 6 having its movable arm connected with one of the contacts 5 and having a pair of change-over contacts 6A, 6B across which an exposure controller 9 formed by a potentiometer 10 is connected, and a transformer 8 having its primary connected with a movable tap on the potentiometer, all of these elements being connected in series across the supply terminals 1, 1. The circuit 4 further includes an exposure lamp 7 which is connected across the secondary of the transformer 8. The potentiometer 10 serves to adjust the primary voltage on the transformer 8 and thereby to control the amount of light emitted by the lamp 7.

The change-over switch 6 is thrown to either contact 6A or contact 6B depending on whether the original is a negative or a positive image. In the example shown, it is assumed that the change-over switch 6 is thrown to the contact 6A as shown in solid line when the original is a negative image. In this instance, the primary voltage of the transformer 8, and consequently the amount of light emitted by the exposure lamp 7 is at its maximum when the movable tap 9A of the controller 9 is located on the end 10A of the potentiometer 10, and is at its minimum when the tap 9A is located on the end 10B of the potentiometer. Thus the optical density on the copy can be decreased by moving the movable tap 9A from the end 10A toward the end 10B of the potentiometer 10.

When the original is a positive image, the single pole, double throw changeover switch 6 is thrown to the contact 6B, as indicated in broken lines in FIG. 1. The amount of light from the exposure lamp 7 is now at its maximum when the movable tap 9A is located on the end 10B of the potentiometer 10 and is at its minimum when the tap 9A is located on the end 10A, thus reversing the effect of a given direction of adjustment of the potentiometer 10 of the controller 9. As mentioned previously, when it is desired to obtain a positive copy from a positive original, the optical density of the copy increases as the amount of light from the lamp is reduced. Consequently, in order to reduce the optical density of the copy, the movable tap 9A may be moved from the end 10A toward the end 10B of the potentiometer 10. It will be noted that this direction of movement of the movable tap is the same as that employed (to decrease the optical density of the copy) when the original is a negative image. In other words, the direction in which the dial of the controller (connected with the movable tap 9A) is moved to produce a desired optical density remains the same for both the positive and negative image, thus eliminating the possibility of an inadvertent operation. The direction of controller adjustment remains the same for both positive and negative images because the effect of a given direction of controller adjustment is selectably reversed by change-over switch 6, depending on whether the original is a positive or negative original.

An embodiment of the invention as applied to an apparatus for automatically switching the polarity of charging in a microreader-printer will be described below with reference to FIGS. 2 to 5. FIG. 2 shows a film 11 carrying a negative original while FIG. 3 shows a film 12 carrying a positive original. The optical density in the marginal regions 11A, 12A of the respective films is established such that the marginal region 11A has a reduced density while the marginal region 12A has an increased density. In the apparatus of the invention, a photoelectric element determines the transmissive or reflective light quantity of such regions by comparison, thereby enabling switching of the polarity of charging to be performed automatically. The photoelectric element may be located at any desired position.

FIG. 4 shows a microreader-printer in which light emitted by the exposure lamp 7 associated with a back reflecting mirror 13 passes through a condenser 15 and a microfilm 16 and is focused by a projection lens 17 onto a screen 19 through a reflecting mirror 18 interposed therebetween. The microfilm 16 may comprise either film 11 or film 12. The arrangement is such that the effective image area of the microfilm 16 is projected onto the screen 19 while the light which is transmitted through the remainder or the marginal region of the film is projected outside the screen 19 with a portion of such light impinging on a photoelectric element 20 disposed at the lower end of the screen 19.

When a copy is to be obtained, the reflecting mirror 18 is moved to the position indicated by phantom lines 18A so as to be disposed out of the path of light, thus allowing the light projected by the lens 17 to impinge on a light-sensitive paper 21 disposed directly below it. The light-sensitive paper 21 is reeled off a roll 22 and fed by pairs of feed rollers 23 and 24, the paper being charged by a charger 25 before it is fed into the exposure station shown. Subsequent to the exposure, the paper 21 is fed by a pair of feed rollers 26 into a developing unit 27, and the developed paper 21 is discharged externally by a pair of feed rollers 28. It will be appreciated that the light sensitive paper 21 is cut to size at a suitable stage in the process.

The charger 25 is connected through a high tension switch 29 with a high voltage generator 30. The polarity of the voltage supplied is switched as will be described hereinafter. The amount of light supplied by the exposure lamp 7 is controlled by the exposure controller 4 which is in turn operated by the exposure dial 14.

The operation of the above apparatus will be described with reference to FIG. 5 which shows a circuit diagram. In FIG. 5, assuming an initial condition that a main switch 31 is closed alone, the reflecting mirror 18 remains in the position shown in solid line in FIG. 4, and the copying apparatus is in its inoperative condition, whereby the projection of the image of the microfilm 16 takes place onto the screen 19. When it is desired to obtain a copy, a print button 32 is depressed, whereby the reflecting mirror 18 is moved out of the path of projection light and the copying apparatus commences its operation. Specifically, a relay coil RA is energized and maintains its self-holding circuit by way of its contact RA2. Simultaneously, another contact RA1 associated with the relay coil RA activates the print drive circuit 3, and a further associated contact RA3 supplies power to the charging polarity switching circuit. In the charging polarity switching circuit, resistors R1, R2, R3, R4, Zener diode D1 for supplying a reduced voltage, photoelectric element 20, reference Zener diode D2, transistor Tr, output silicon controlled rectifier SCR and output relay RB form a detector for detecting the amount of light. The photoelectric element 20 has an incidence of light from the marginal region of the microfilm 16 thereon, and the amount of such light is reduced for a microfilm 16 of a positive type and is increased for a microfilm of a negative type, as mentioned previously. Assuming that the microfilm 16 is of a positive type, the amount of light incident on the light receiving element 20 is reduced, whereby it exhibits a resistance which is substantially greater than that of the reference resistor R3. Conversely, for a negative microfilm, the amount of incident light will be increased to make the resistance of the element 20 substantially less than that of the reference resistor R3.

As a consequence, for a positive microfilm 16, the voltage developed across the reference resistor R3 will be less than the voltage across the reference Zener diode D2, with a result that the transistor Tr is not operated; therefore, the relay coil RB is not energized through the SCR. A solenoid 34 which is connected across the power supply through the contact RB2 associated with the relay coil RB is not energized, so that the high tension switch 29 remains thrown to the negative side. Thus, under such condition, the charger 25 charges the light-sensitive paper to a negative polarity to permit developing with a toner having a positive polarity, thus producing a positive copy image. Such a relationship between the negative or positive nature of the image in the original film and the polarity of charging as well as toner favors a wet developing process. However, it should be understood that such relationship can be modified depending on the charging characteristic of the light-sensitive paper used.

When the microfilm 16 is of a negative type, the voltage developed across the reference resistor R3 will exceed the voltage across the reference Zener diode D2, with the consequence that the transistor Tr is rendered conductive to permit energization of the relay coil RB through SCR. Then the contact RB2 associated with the realy coil RB is closed to energize the solenoid 34, whereby the high tension switch 29 is thrown to the positive side, changing the polarity of the charger 25 to the positive polarity. In this instance, the charger 25 charges the light-sensitive paper to the positive polarity, and a toner of positive polarity is used to effect a so-called reversal development, thereby producing a positive copy image. It should be understood that the relationship in this instance again can be pre-established in any desired manner. Such a copying process is continued until a stop switch 35 is opened to break the self-holding circuit for the relay coil RA, thereby disconnecting the print drive circuit. While in the embodiment described above, the switching by way of detection of the film takes place at the time the print operation is initiated, the switching can be effected at any desired time, for example, when the original is set into the machine.

As shown in FIG. 5, the exposure controller circuit 4 according to the invention is associated with the electrical circuit of the microreader-printer described above. The circuit 4 includes a switch 35A which is adapted for interlocked operation with the stop switch 35 in the charging polarity switching circuit, and operates to connect the exposure controller circuit when the switch 35 is thrown to the charging side. In addition, the change-over switch 6 constitutes one of the relay contacts associated with the relay coil RB. When the relay coil RB is energized for a negative microfilm 16, the change-over switch 6 is thrown to the contact 6A as shown, but is connected to the contact 6B when the relay coil RB is not energized. In other respects, the exposure controller circuit is similar to that described previously in connection with FIG. 1.

From the foregoing, it will be appreciated that a determination whether the original carries a negative or a positive image is accomplished automatically, and the exposure controller circuit is also automatically changed based on such determination, thereby allowing the direction in which the dial of the controller should be operated to be maintained uniform without requiring a manual intervention of an operator.

It is to be understood that the invention is not limited to the features and advantages hereinabove specifically set forth but may be carried out in other ways without departure from its spirit.