An image density control device for use in an image forming apparatus

Abstract: An image density control device for use in an image forming apparatus has an exposure lamp (42) for illuminating a document, a photoreceptor (7) for receiving light reflected from the document to produce an electrostatic latent image, and a charger (8) for charging the photoreceptor. The image density control device includes a detector (61) for detecting the density of the document, a controller (32) responsive to the detector for holding the amount of light emitted from the exposure lamp at a predetermined fixed value when the document density lies in a predetermined range, and means (33) responsive to the detector for controlling the charger to change the surface potential of the photoreceptor in accordance with a change in the document density when the document density lies in the predetermined range. The predetermined range may extend between a minimum document density and a reference document density, or between another, higher reference document density and a maximum document density, the arrangement being such that the image density variation is reduced within said ranges.

Description: This invention relates to an image density control device for controlling the density of an image formed on copy paper in a copying machine or like image forming apparatus. Conventionally, an image density control device for use in an image forming apparatus such as a copying machine has been designed to control the density of an image formed on copy paper (hereinafter referred to as image density throughout the Specification) by changing an amount of light emitted from a lamp to be projected onto a document while maintaining the surface potential of a photosensitive drum at a fixed level. For example, in a copying machine provided with an automatic exposing device of the prescanning type, a predetermined amount of light is emitted from a lamp and projected onto a document, and a prescanning operation is executed while the light reflected by the document being detected by a photodetector. The density of the document (document density) is calculated on the basis of the amount of light detected in the prescanning. Based on the calculated document density is determined an amount of light to be emitted from the lamp in a main scanning operation wherein copying of an image is actually executed. There are problems associated with such automatic adjustment of the amount of light emitted and in US 4796060 it has been proposed that upper and lower limits are set to the automatic adjustment of the amount of light. If there are greater variations, no further adjustment is made. As a further example, Fig. 5 is a graph showing a conventional relationship between the document density and the amount of light emission of the lamp, according to which the light emission is determined based on the document density. In Fig. 5, indicated at A, B, C are reference document densities corresponding to reference documents G1, G2, G3. For instance, in the case where the detected or set document density is substantially equal to that of the reference document densities A, B or C, light emission of the lamp is a, b, or c. In the conventional image density control device, the lamp emits less light as the document density becomes lower. Accordingly, in the case where an image of a document whose density is lower than the density B is to be copied, there is a likelihood that the amount of light emitted from the lamp is too small relative to the actual density of the document, whereby causing the fog on the copy to have higher density than necessary. Hereinafter, the density of the fog is referred to as fog density. In other words, a white area in the document is formed into a grey area in the copy. In order to prevent the fog density from increasing to an undesirably high level, it may be considered that the relationship between the document density and the amount of light emission is defined in such a fashion as to make the inclination of the curve in the graph of Fig. 5 smoother below the density B. Thereby, the amount of light emission of the lamp can be adjusted to be larger than the conventional light emission in the case where the document density is relatively low. However, if the density of a document image is low, the resulting image density is low as a whole, whereby forming an unclear image. This is especially prominent if the document image consists of characters, or a diagram. On the other hand, in the case where an image of a document whose density is higher than the density C of the reference document G3 is to be copied, the amount of light emission of the lamp is c, which corresponds to the reference document G3. Similar to the foregoing case, there is a likelihood that the fog density increases to a higher level than is desirable due to deficiency in light emission. In order to avoid this problem, the use of a lamp capable of emitting a larger amount of light may be considered. However, this results in increased costs and an increase in the temperature of a scan/exposure system due to the heat generated from the lamp, thereby necessitating implementation of measures against the heat generated. According to the present invention, there is provided an image density control device for use in an image forming apparatus having an exposure lamp for emitting light to illuminate a document, a photoreceptor for receiving light reflected from the document to produce an electrostatic latent image, and a charger for charging the photoreceptor, the device comprising detector means for detecting the density of the document, means for controlling the amount of light emitted from the exposure lamp, said light control means being arranged to increase the amount of light emitted as a function of increasing detected document density within a limited range, means for controlling the charger to effect said charging of the surface potential of the photoreceptor, said charger control means being arranged to decrease said photoreceptor surface potential as a function of increasing detected document density in at least one further range of the document density beyond said limited density range, the light emission control means being further arranged to control the amount of light emitted in said at least one further range to keep it fixed or to increase it as a function of increasing detected document density at a lower rate than in said limited range. With the image density control device thus constructed, when the document density lies in said at least one further range, in coordination with the amount of light emission, the surface potential can be changed in accordance with a change in the document density. Accordingly, the image density can be controlled at higher precision. Said at least one further range may be a range below said limited range, with its upper limit defined by a minimum document density and a predetermined reference value, and the charger control means controls the charger to increase the surface potential in accordance with a decrease in the document density in said further range. With the image density control device thus constructed, in the case where the detected document density falls below said limited range, the light emission amount of the exposure lamp may be set at the predetermined fixed value while the surface potential of the photoreceptor is increased as the document density becomes lower. Accordingly, even in the case where the base color of the document is white and the density of the document image is low, it is possible to prevent the fog density increasing to such an extent as to make the copied document image unclear. It is also possible, in said further range, to reduce the light emission with increase of surface potential. It is also possible to have a further range as a range extending above said limited range from a maximum document density and a predetermined reference value, and the charger control means controls the charger to decrease the surface potential in accordance with an increase in the document density in said further range. With this arrangement, in the case where the detected document density rises into said further range, the light emission amount of the exposure lamp may be set at the predetermined fixed value while the surface potential of the photoreceptor is reduced as the document density becomes higher. Accordingly, even in the case where a document to be copied has such density as to require more light than the exposure lamp can emit, it is possible to prevent the fog density increasing to a high level due to deficiency in the amount of light emission from the lamp. It is also possible, in said further range, to increase the light emission with reduction of surface potential. It will be clear that it is possible to control the amount of light and of surface potential in the manners indicated through respective further ranges above and below said limited range. That is to say, the surface potential is controlled so that when the document density lies in the lower range the surface potential control means increases the surface potential in accordance with decrease in document density while the light emission control means sets a fixed amount of light emission or reduces the light emission, and when the document density lies in the upper range the charger control means decreases the surface potential in accordance with increase in document density while the light emission control means sets a fixed amount of light emission or increases the light emission. These and other features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings. In the drawings: Fig. 1 is a block diagram showing an image density control device for use in an image forming apparatus embodying the invention; Fig. 2 is a front view in section showing an overall construction of an image forming apparatus provided with the image density control device; Fig. 3 is a graph showing relationship between the document density and amount of light emission of a lamp and relationship between the document density and the surface potential of a photosensitive drum; Fig. 4 is a graph showing relationship between the document density and the image density; and Fig. 5 is a graph showing relationship between the document density and the image density in an conventional image density control device. Fig. 2 is a diagram showing an overall construction of an image forming apparatus provided with an image density control device embodying the invention. A scan/exposure system L comprises a lamp 42 consisting of a halogen lamp for projecting the light onto a document, and an optical system having a reflector 43, mirrors 44, 52 to 54, and a lens 6. The optical system is adapted for introducing a document image to a photosensitive drum 7 disposed therebelow. The lens 6 carries a photodetector 61 for detecting the light reflected by the document. The lamp 42, reflector 43, and mirror 44 are included in a first moving frame body 41 while the mirrors 52, 53 are included in a second moving frame body 51. These first and second moving frame bodies 41, 51 are movable reciprocatingly in a horizontal direction (in directions indicated by arrows A) below a platen glass 2. The scan/exposure system L illuminates the document placed on the platen glass 2 in the form of a slit at a specified intensity while causing the first and second moving frame bodies 41, 51 to move reciprocatingly between a position corresponding to a leading end of the document and a position corresponding to a trailing end thereof. In this way, the document image is introduced to the photosensitive drum 7, whereupon an exposure is executed. Around the photosensitive drum 7 are disposed a main charger 8, a blank lamp 9, a developing device 10, a transfer charger 11, a separation charger 12, a cleaning device 13, etc. The main charger 8 sets a surface potential of the drum 7. The blank lamp 9 removes charges in a region of the surface of the drum 7 which are not to be used in an image forming operation. The developing device 10 develops a latent image formed on the surface of the drum 7 into a toner image. The transfer charger 11 transfers the developed document image to copy paper. The separation charger 12 separates the copy paper having the document image transferred thereto from the surface of the drum 7. The cleaning device 13 removes the toner residual on the surface of the drum 7. The drum 7 and the peripheral devices thereof constitutes an image forming station. To one side surface (right side surface in Fig. 2) of the copying machine 1 is removably mounted copy paper cassettes 14. The copying machine 1 is internally provided with a copy paper feeding station, which is disposed between the cassettes 14 and the photosensitive drum 7. The copy paper feeding station comprises feed rollers 15, pairs of transport rollers 16, 17, and a pair of register rollers 18. Each feed roller 15 dispenses a sheet of copy paper from the cassette 14. The pairs of transport rollers 16, 17 transport the copy paper sheet to the pair of register rollers 18, which in turn feed the copy paper sheet transported thereto to the drum 7 at a specified feed timing so that the feed of the copy paper sheet will synchronize with the scan of the scan/exposure system L. On the other side (left side in Fig. 2) of the copying machine 1 is disposed a discharge tray 22 onto which the copy paper sheet having the document image copied thereon is discharged. A fixing/discharging station is provided between the photosensitive drum 7 and the discharge tray 22 in the copying machine 1. The fixing/discharging station comprises a transport belt 19, a fixing device 20, and a pair of discharge rollers 21. The transport belt transports the copy paper sheet having the document image transferred thereto to the fixing device 20. The fixing device 20 fixes the transferred document image onto the copy paper sheet. The pair of discharge rollers 21 discharges the copy paper sheet onto the discharge tray 22. Fig. 1 is a block diagram showing a construction of the image density control device embodying the invention. In Fig. 1, indicated at 30 is a central processing unit (hereinafter referred to as CPU), at 31 an optical system drive circuit, at 32 an emission control circuit, at 33 a surface potential control circuit, and at 34 a copy key. The CPU 30 centrally controls the image forming operation of the copying machine 1. The drive circuit 31 controls the scan/exposure system L to drive the first and second moving frame bodies 41, 51. The emission control circuit 32 controls the emission of the lamp 42. The surface potential control circuit 33 controls the surface potential of the photosensitive drum 7. The copy key 34 is manipulated to instruct the image forming operation. Upon the copy key being manipulated to instruct the image forming operation, the CPU 30 sends to the drive circuit 31 a control signal for the prescanning operation so as to cause the first and second moving frame bodies 41, 51 to move a predetermined amount. In addition, the CPU 30 sends to the emission control circuit 32 a control signal for the emission so as to cause the lamp 42 to emit a predetermined amount of light. In the prescanning operation, the light reflected by the document is detected by the photodetector 61 which in turn sends to the CPU 30 a sensor signal indicative of the level of detected reflection light. Upon receipt of the sensor signal from the photodetector 61, the CPU 30 determines the document density on the basis of the detected level of reflected light. The amount of light emission of the lamp 42 and the surface potential of the drum 7 for a main scanning operation are determined based on the calculated document density. Since the predetermined amount of light is emitted from the lamp 42 in the prescanning operation, the level of reflected light obtained by the photodetector 61 corresponds to the density of the document prescanned. More specifically, in the case where the document density is low, i.e., the document as a whole is relatively white, the level of reflected light is high. On the other hand, in the case where the document density is high, i.e., the document as a whole is relatively dark, the level of reflected light is low. In this way, the document density is determined on the basis of the corresponding level of reflected light. Further, the amount of light emission of the lamp 42 and the surface potential of the drum 7 in the main scanning operation are calculated based on relationship between the document density and the amount of light emission, and that between the document density and the surface potential respectively. Further description of these relationships will be given later. It should be understood that the document density may be determined, and the amount of light emission of the lamp 42 and the surface potential of the drum 7 may be calculated by way of conversion tables prestored in the CPU 30. In an image forming operation, the surface potential of the drum 7 is set at the calculated surface potential, and the lamp 42 is caused to emit the calculated amount of light, whereupon the document is exposed. Fig. 3 shows graphically the relationship between the document density and the light emission amount, and that between the document density and the surface potential used to obtain the amount of light emission of the lamp 42 and the surface potential of the photosensitive drum 7 in the main scanning operation. In Fig. 3, indicated at A, B, C are document densities of reference documents G1, G2, G3 respectively. The reference document G1 is, for example, a blank document having a specified brightness. The reference document G2 is, for example, a document having a brightness similar to the reference document G1 in which characters are typed at a specified density on the reference document G1. The reference document G3 is, for example, a document of a specified brightness whose base color is gray and in which characters are typed at a specified density. The reference document G3 corresponds to a standard newspaper sheet. In the graph of Fig. 3, a horizontal axis represents the document density. The document density increases tothe right and reduces to the left along the horizontal axis. A curve P represents the relationship between the document density and the light emission of the lamp 42, whereas a curve Q represents the relationship between the document density and the surface potential of the drum 7. As will be seen from Fig. 3, the light emission P of the lamp 42 is set at a fixed value L1 in the case where the document density is not higher than that of the reference document G2 (density B or lower). The limit emission of the lamp 42 is set at a fixed value L2 in the case where the document density is not lower than that of the reference document G3 (density C or higher). Further, in the case where the document density lies in a range between B and C (where B and C are exclusive), the light emission of the lamp 42 is in proportion to the document density. On the other hand, the surface potential Q of the drum 7 is set at a fixed value V1 in the case where the document density lies in the range between B and C (where B and C are exclusive). The surface potential of the drum 7 changes according to the document density in the case the document density is not higher than B or not lower than C. The surface potential of the drum 7 is inversely related to the document density. More specifically, when an image of a document whose density is higher than B and lower than C is to be copied, the image density is controlled by changing the amount of light emission of the lamp 42. On the other hand, when an image of a document whose density is not higher than B or not lower than C is to be copied, the image density is controlled by changing the surface potential of the drum 7. Fig. 4 shows relationship between the document density and the image density wherein the surface potential of the photosensitive drum 7 is used as a parameter. In Fig. 4, curves F1, F2, F3 represents the relationship in states where the surface potential of the drum 7 is set at f1, f2, f3 (f1 < f2 < f3) respectively. As will be seen from Fig. 4, when the surface potential of the drum 7 is set at a high level, the image density changes abruptly relative to a change in the document density. In other words, the image density becomes more sensitive to the document density. Thus, contrast of the image formed on the copy paper can be made higher for the document whose density is low by increasing the surface potential of the drum 7. More specifically, when a document having a density D is to be copied, for example, the image density rises from n to m if the surface potential of the drum 7 is increased from f1 to f2. As a result, the contrast of the formed image increases as much as the overall image density becomes high. Conversely, if the surface potential of the drum 7 is reduced from f2 to f1, the image density falls from m to n, and thereby the overall image density becomes low. Accordingly, in the case where a document whose density is B or lower is to be copied, the amount of light emission of the lamp 42 is set at the fixed value L1 corresponding to the reference document G2 regardless of the low document density, thereby limiting any increase in the fog density by making the overall image density lower. On the contrary, the surface potential of the drum 7 is increased as the document density becomes lower, whereby making the contrast of the formed image higher. Therefore, it is made possible to form clear images from characters typed lightly or with thin lines, which would otherwise be copied as unclear images. In this case, it may be appropriate that the light emission of the lamp 42 be smoothly reduced according to a decrease in the document density as represented by a two-dot-chain line in Fig. 3 instead of being set at the fixed value L1. The reference value for controlling the image density is not limited to B, but can be set at any desired reference value. On the contrary, in the case where a document whose density is C or higher is to be copied, the amount of light projected onto the document is set at the fixed value L2 corresponding to the reference document G3 regardless of the high document density, and therefore the amount of light is deficient relative to the document density. However, if the surface potential of the photosensitive drum 7 is reduced as set forth above, the overall image density becomes low. In view of this, the surface potential of the drum 7 is reduced as the document density becomes higher, enabling the overall image density to be sufficiently low. Thus, it is made possible to form an image at a desired image density without causing the fog density to increase to a level higher than necessary. In this case, it may be appropriate that the light emission of the lamp 42 be smoothly increased according to an increase in the document density as represented by another two-dot-chain line in Fig. 3 instead of being set at the fixed value L2. Although the present invention has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art within the scope of the present invention.

Claims: An image density control device for use in an image forming apparatus having an exposure lamp (42) for emitting light to illuminate a document, a photoreceptor (7) for receiving light reflected from the document to produce an electrostatic latent image, and a charger (8) for charging the photoreceptor, the device comprising: detector means (61) for detecting the density of the document; means (32) for controlling the amount of light emitted from the exposure lamp; said light control means (32) being arranged to increase the amount of light emitted as a function of increasing detected document density within a limited range; means (33) for controlling the charger to effect said charging of the surface potential of the photoreceptor; said charger control means (33) being arranged to decrease said photoreceptor surface potential as a function of increasing detected document density in at least one further range of the document density beyond said limited density range; the light emission control means (32) being further arranged to control the amount of light emitted in said at least one further range to keep it fixed or to increase it as a function of increasing detected document density at a lower rate than in said limited range. An image density control device as defined in claim 1 wherein there are respective further ranges of document density above and below said limited range, and wherein said charger controlling means (33) controls the charger (8) in each said further range outside the limited range to decrease said surface potential as a function of increasing detected document density. An image density control device as defined in claim 1 or claim 2 wherein the light emission control means and said charger control means (33) comprise a memory holding data for correlation of the amount of light emission and the surface potential of the photoreceptor to the document density that has been determined.

Inventors: MIZUDE KAZUHIRO, MIZUDE, KAZUHIRO
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO. LTD.
Application year: 1991
Main classification: G03G15
IPC codes: G03G15