ELECTROSTATIC COPYING MACHINE

Abstract: An electrostatic copying machine having an optical system for exposing to light an original (10) placed on a contact glass plate (1), a sensor (27) for detecting the density of the original (10) and a control system for control­ ling image density in accordance with a detection signal from the sensor (27), the glass plate (1) being provided with a light blocking white portion (51) at a reference position thereon where the front end of the original is placed. A light blocking colored area (52) reflecting a smaller quantity of light than the white portion (51) is provided in the white portion (51) at least at the part thereof corresponding to the position of detection by the sensor (27). Although the white portion (51) provided for preventing the front end of copy paper from fitting around a fixing roller would result in a reduced amount of exposure for the front end portion of the original (10) to produce an increased image density at the paper end portion and all over the paper in the case of the prescanning system, the presence of the colored area (52) assures proper automatic density control free of such objection.

Description: FIELD OF THE INVENTION AND RELATED ART STATEMENT The present invention relates to improvements in electrostatic copying machines having the function of automatically controlling the image density in accordance with the density of originals. Examined Japanese Patent Publication SHO 54-­ 36502, for example, discloses a contact glass plate provided with a light blocking white portion at a reference position thereon to form a nonimage area having a width of about 0.5 to about 20 mm at the front end of copy paper and thereby prevent the front end of the copy paper from fitting around a heat fixing roller when the copy paper is brought into pressing contact with the roller for fixing so that the copy paper can be separated off free of trouble by a separating pawl. Electrostatic copying machines are known which have the function of automatically controlling the image density by detecting the density of an original from the light reflected therefrom and varying the quantity of light to be emitted by the optical system or the bias voltage of the developing unit. When the above-mentioned light blocking white portion is provided in such a machine, there arises the problem that the front end of copy paper becomes black for the following reason. When the light blocking white portion is exposed to light, an increased quantity of light is reflected therefrom, causing the automatic density control system to greatly decrease the amount of exposure during a period A-B corresponding to the light blocking portion as shown in Fig. 5. Although the original is present subsequent to the light blocking portion after the optical system has passed this portion at time B, a delay involved in the operation of the control system results in an increased image density and produces a dark image over an area corresponding to a period B-C required for the amount of exposure to return to a value corresponding to the density of the original. The drawback may be obviated by setting a timer so as to hold the control system out of operation during the exposure of the light blocking white portion, but this renders the control system complex in construction. Furthermore, copying machines equipped with a variable magnification device have the problem that it is difficult to assure accurate control since the scanning speed of the optical system changes. With systems wherein the density of the original is measured by prescanning to obtain an average density and determine the amount of exposure based on the average value, the presence of the light blocking white portion produces a lower average exposure value, entailing the problem that the overall image to be formed on copy paper becomes dark. OBJECT AND SUMMARY OF THE INVENTION The object of the present invention which has been accomplished to overcome the foregoing problems is to provide an electrostatic copying machine which has an automatic density control function and which is adapted to properly preclude the likelihood that the image density at the front end of copy paper will increase to a level higher than is needed and to obviate a reduction in the overall density of copy images, using a simple arrangement. The present invention provides an electrostatic copying machine having an optical system for exposing to light an original placed on a contact glass plate, sensor means for detecting the density of the original and control means for controlling image density in accordance with a detection signal from the sensor means, the glass plate being provided with a light blocking white portion at a reference position thereon where the front end of the original is placed. A light blocking colored area reflect­ ing a smaller quantity of light than the light blocking white portion is provided in the white portion at least at the part thereof corresponding to the position of detection by the sensor means. With the copying machine described above, the quantity of light emitted by the optical system and reflected by the less reflective light blocking colored area is detected by the sensor means, and the image density is controlled based on the detected value. Conse­ quently, despite the provision of the light blocking white portion, the presence of the colored area precludes the likelihood that an image to be formed at the front end portion of copy paper and an overall image in the case of the prescanning system become dark. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram showing an embodiment of electrostatic copying machine according to the present invention; Fig. 2 is a block diagram showing an embodiment of control means; Fig. 3 is a bottom view showing an embodiment of light blocking portion; Fig. 4 is a diagram showing the exposure charac­ teristics afforded by the present invention; and Fig. 5 is a diagram showing the exposure charac­ teristics of a conventional system. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 schematically shows the interior construc­ tion of an electrostatic copying machine. The drawing shows a contact glass plate 1 on the top of the machine frame. Provided within the machine frame are an optical system 2, a photosensitive drum 3 and devices arranged around the drum 3. The optical system 2 comprises an exposure lamp 21, a reflector 22, first to third mirrors 23 to 25, a condenser lens 26 and a fourth mirror 27. The exposure lamp 21, etc. reciprocatingly travel along the lower surface of the glass plate 1. An original 10 placed on the glass plate 1 is exposed to light during the forward travel of the movable members. The drum 3 is drivingly rotatable in the direc­ tion of arrow X by unillustrated drive means. Arranged around the drum 3 are a main charger 32 having a charge wire 31, a developing unit 34 having a developing roller 33, a transfer charger 35, a separating charger 36 and a cleaner 37. With the rotation of the photosensitive drum 3, the surface of the drum 3 is sensitized by the charge wire 31 of the main charger 32 to which a high voltage is applied. The image of the original is then projected on the drum 3 by the optical system 2 to form an electrostatic latent image, which is developed by the unit 34. The developed image is transferred by the transfer charger 35 to copy paper dispensed from a paper cassette 11 and fed by a transport assembly 12. The copy paper is then separated from the drum 3 by the charger 36, thereafter sent to a fixing unit 14 by a discharge belt 13 and delivered onto a copy tray 15. The optical system 2 is provided with sensor means 27 comprising a photodiode or the like for detecting the density of the original 10 from a portion of the light emitted by the exposure lamp 21 against the glass plate 1 and reflected from the original 10. Stated more specif­ ically with reference to Fig. 2, light is emitted by the exposure lamp 21 in response to an instruction from a print key 46, and the reflected light is detected by the sensor means 27. The detection signal is fed to a light quantity detecting circuit 41 of control means 4 and converted to a voltage signal. The voltage signal is delivered from the circuit 41 to a comparison circuit 42, in which the signal is compared with a reference voltage from a reference voltage circuit 43. For example, when a small quantity of light is reflected from the original because the background density thereof is high, the voltage signal from the light quantity detecting circuit 41 will be lower than the reference voltage. In this case, an instruction signal for causing the exposure lamp 21 to produce an increased quantity of light is fed to a lamp voltage generating circuit 45 via a limiting circuit 44. The circuit 45 applies a voltage in accordance with the instruction signal to the exposure lamp 21 to scan the original with a controlled quantity of light. The limit­ ing circuit 44 is provided to prevent application of a voltage higher than a limit to the exposure lamp 21 and thereby prevent burnout of the lamp, for example, when the original has a very high density. As shown in Fig. 3, a light blocking portion 5 is provided on the lower surface of the contact glass plate 1 at a reference position where the front end of the original 10 is placed, i.e., in the vicinity of the start position of the optical system 2. The light blocking portion 5 comprises a light blocking white area 51 formed, for example, by adhering a white tape with a width a to the glass plate 1 almost over the entire width thereof, and a light block­ ing gray area 52 formed on the surface of the white area 51 approximately at the widthwise midportion of the glass plate 1 where the original density sensor means 27 is disposed. The gray area 52 is provided by a tape or the like having a width b and a length c and is disposed outward of the reference position Y where the front end of the original is to be located. The light blocking gray area 52 has a lightness corresponding to the average density of originals (about N3 to 7 according to the lightness scale of JIS Z8721). The color of the gray area 52 is not always limited to gray insofar as the area 52 reflects a smaller quantity of light than the white area 51. For example, any color is usable which has a wavelength outside the region of sensitivity wavelengths of the sensor of the original density sensor means 27. The following descrip­ tion is given with reference to the area 52 which is gray. The operation of the electrostatic copying machine having the above construction will be described below. An original 10 is placed on the upper surface of the glass plate 1 with its front end at the reference position Y. Consequently, the surface of the front end portion of the original 10 is covered with the rear end of the light blocking white area 51 over a length d in the scan direction. When the covered portion is exposed to light by the optical system 2, a nonimage area corre­ spong to the length d is formed at the front end of copy paper. With the start of exposure of the original 10 by starting the optical system 2, the quantity of light reflected from the original 10 is detected by the sensor means 27, and the quantity of light of the exposure lamp 21 is controlled by the control means 4 according to the detected value along with development and transfer opera­ tion. With the start of the exposure operation, the light reflected from the light blocking white area 51 impinges on the sensor means 27, with the result that the comparison circuit 42 recognizes an excessive quantity of light. Accordingly, during a period A-D corresponding to the white area 51 shown in Fig. 4, the quantity of light emanating from the exposure lamp 21 is so controlled that the quantity gradually decreases and then levels off. Subsequently, with the start of exposure of the light blocking gray area 52, the quantity of reflected light incident on the sensor means 27 markedly decreases, so that the control means 4 gives an instruction to increase the voltage to be applied to the exposure lamp 21. Consequently, during a period D-B' corresponding to the gray area 52, the quantity of light from the exposure lamp 21 gradually increases. The exposure of the original 10 is started at time B when an approximately standard quantity of light is reached, almost free of the influence of a period B'-B corresponding to the portion of the white area 51 having a very small width d as will be described below. When the exposure operation is conducted with automatic control of the image density by the control means 4, the presence of the gray area 52 in the midportion of the light blocking white area 51 obviates the objec­ tionable result that the amount of exposure for the front end portion of the original decreases under the influence of the white area 51 to darken the image at the front end of copy paper and the overall image in the case of the prescanning system. Further the white area 51 overlapping the foremost end of the original 10 over the length d forms a nonimage area on the copy paper for preventing the paper from fitting around the fixing roller. Although the quantity of reflected light temporarily increases at the portion of the white area 51 haivng the length d , the length d , which can be as small as about 5 mm for forming the nonimage area and need not be very large, will not result in a marked reduction in the amount of exposure. Moreover, the present invention eliminates the need to use a complex arrangement such as one including a timer for interrupting the density control operation during the exposure of the white area and further has the advantage of assuring proper control at all times for machines equipped with a variable magnification device. According to the embodiment described above, the image density is controlled by varying the quantity of light of the exposure lamp 21, whereas the present invention is similarly applicable to apparatus wherein the image density is controllable by varying the bias voltage of the developing unit 34. The light blocking white area 51, as well as the gray area 52, may be provided by applying a coating composition to the lower or upper surface of the glass plate 1. Furthermore, the position, size, etc. of the light blocking gray area 52 can be altered variously as required, provided that the gray area 52 is located at the position of detection by the density sensor means 27. The present invention is applicable not only to copying machines of the type described wherein the optical system is movable but also to those having a movable document carriage.

Claims: 1. An electrostatic copying machine having an optical system for exposing to light an original placed on a contact glass plate, sensor means for detecting the density of the original and control means for controlling image density in accordance with a detection signal from the sensor means, the glass plate being provided with a light blocking white portion at a reference position there­ on where the front end of the original is placed, the copying machine being characterized in that a light blocking colored area reflecting a smaller quantity of light than the light blocking white portion is provided in the white portion at least at the part thereof corresponding to the position of detection by the sensor means. 2. A copying machine as defined in claim 1 wherein the light blocking colored area is a gray area having a lightness corresponding to the average density of originals. 3. A copying machine as defined in claim 1 wherein the light blocking colored area has a color with a wave­ length outside the region of sensitivity wavelengths of the sensor means. 4. A copying machine as defined in claim 1 wherein the light blocking gray area is disposed outward of a reference position for the front end of the original. 5. A copying machine as defined in claim 1 wherein the sensor means for detecting the density of the original is adapted to detect the quantity of reflected light resulting from the exposure of the original by the optical system. 6. A copying machine as defined in claim 1 wherein the control means is adapted to control the quantity of light of an exposure lamp to control image density. 7. A copying machine as defined in claim 6 wherein the control means comprises a light quantity detecting circuit for receiving the detection signal from the sensor means and converting the signal to a voltage signal, a reference voltage circuit, a comparison circuit for comparing the voltage signal delivered from the detecting circuit with a reference voltage given by the reference voltage circuit, a limiting circuit for preventing application of a voltage above a limit to the exposure lamp, a lamp voltage generating circuit for receiving an output from the comparison circuit and determining the quantity of light of the exposure lamp. 8. A copying machine as defined in claim 1 wherein the control means is adapted to control the bias voltage of a developing unit to control image density.

Inventors: OKAJO, YUKIO
Applicants: MITA INDUSTRIAL CO. LTD.
Application year: 1986
Main classification: G03G15
IPC codes: G03G15