JP2759967B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus that can use plain paper and OHP sheets as transfer materials.

2. Description of the Related Art There are two types of OHPs (overhead projectors) for projecting an image formed on an OHP sheet on a screen using an OHP sheet, a transmissive OHP 100a shown in FIG. 19 and a reflective OHP 100b shown in FIG. There is.

In FIG. 19, 101 is a housing, 101 is a lamp, 10
3 is a Fresnel lens, 104 is a protective glass, 105 is an OHP sheet, 106 is a lens, 107 is a mirror, and 108 is an arm.
Light from the light source lamp 102 is collected by the Fresnel lens 103, passes through the OHP sheet 105, is projected on the screen via the mirror 107, and an image formed on the OHP sheet 105 is formed on the screen. . Figure 20 shows the reflective OHP100b
And the portions corresponding to the transmission type OHP 100a are denoted by the same reference numerals. In this reflection type OHP 100b, the lamp 102 is disposed above the OHP sheet 105, and a full-face mirror 103 'whose lower surface is mirror-finished is provided in place of the full-nel lens 103 used in the transmission-type OHP 100a. I have. The light from the lamp 102 first passes through the OHP sheet 105 downward,
At the same time as being reflected by the Fresnel mirror 103 ', the light is condensed and passed upward through the OHP sheet 105, and the lenses 106, 10
6 ′, projected on the screen via mirror 107, OHP
The image formed on the sheet 105 is formed on the screen. It should be noted that the Fresnel mirror 103 'and the OHP sheet 105 are almost in close contact with each other.
The light that has passed through 05 from above to below is reflected upward by the Fresnel mirror 103 ', and then travels upward almost through the same point on the OHP sheet 105. In this way, the reflective OHP100
In b, light passes through the OHP sheet 105 twice to form an image.
On the other hand, in the pass-through type OHP100a, the light is
The image is passed only once through 105.

Therefore, when the transmittance of the OHP sheet 105 is τλ, the transmission type
The transmittance is τλ in OHP1a, and (τλ) ^{2 in} reflective OHP1b.
Becomes

This means that, for example, considering a blue portion of an image formed on an OHP sheet, an arbitrary point P
21 are shown in FIG. Here, the solid line is a case where the transmission type OHP100a is used, and the broken line is a case where the reflection type OHP100b is used.
Is used. As is apparent from FIG. 21, it is understood that the peak transmittance decreases from 60% to 36% in the case of the reflection type. If the peak transmittance τλ =
If it is 20%, (τλ) ² = 4%, and it is understood that the smaller the transmittance is, the more extreme the drop when squared becomes.

When an OHP sheet image is formed at the same copy density when creating an OHP sheet with a copying machine in this manner, a transmission type OHP sheet is used.
In 1a, the optimal image can be projected on the screen, but in the reflective OHP 1b, the amount of light is insufficient and the image becomes unclear.

Therefore, in a copying machine capable of forming an image on an OHP sheet, an image formed on an OHP sheet is different between when used for a reflective OHP and when used for a transmissive OHP. It is desirable to vary the concentration.

An object of the present invention is to provide an image capable of forming an image at an optimum density for each of a transmissive OHP sheet and a reflective OHP sheet by switching density characteristics as necessary in view of the above problems. It is an object to provide a forming device.

Means for Solving the Problems The present invention forms an electrostatic latent image on a photoconductor based on an image signal, develops the electrostatic latent image with a developing device, and transfers the developed toner image to an OHP sheet. In an image forming apparatus capable of forming an image on an OHP sheet at a first density,
A mode, a deciding unit for deciding which of the second modes for forming an image on the OHP sheet at a second density different from the first density, and density information included in the image signal Signal processing means for changing the image density formed on the OHP sheet by controlling at least one of the maximum density level and the gamma value γ based on the determination of the determining means. And control means for changing.

Further, according to the present invention, a laser light source is driven based on an image signal to form an electrostatic latent image on the surface of the photoreceptor, and the electrostatic latent image is developed by a developing device to which a developing bias voltage is applied. In an image forming apparatus capable of transferring a toner image to an OHP sheet, a first mode in which an image is formed on an OHP sheet at a first density, and an image is formed on an OHP sheet at a second density different from the first density Determining means for determining which mode of the second mode to perform image formation; signal processing means for changing density information included in the image signal; and the signal processing means based on the determination of the determining means. Controlling means for changing at least one of the maximum density level and the gamma value γ to change the image density formed on the OHP sheet.

Further, according to the present invention, the OHP sheet is provided with a mark for designating the first mode or the second mode in advance, and the determining unit detects the mark when forming an image on the OHP sheet. In this manner, a mode to be executed for the OHP sheet is determined.

Still further, in the present invention, the control means preferably includes the second control means.
The control is performed such that the maximum density level and the gamma value γ in the mode are approximately 1/2 of the maximum density level and the gamma value γ in the first mode, respectively.

According to the above-described configuration, the determining means sets the first concentration at the first concentration.
The first mode for forming an image on an OHP sheet and the second mode for forming an image on an OHP sheet at a second density different from the first density are determined, and the determination is made. Based on the determination of the means, the signal processing means is controlled to change at least one of the maximum density level and the gamma value γ to change the image density formed on the OHP sheet. According to the distinction between the sheet for transmission and the sheet for transmission type OHP, it is possible to form an image with an optimum density. In addition, since the density is changed by changing at least one of the maximum density level and the gamma value γ, images with different densities can be formed without significantly impairing the gradation.

In addition, the OHP sheet is previously stored in the first mode or the second mode.
A mark for designating the mode is attached, and the determination means reliably determines the mode to be executed on the OHP sheet by detecting the mark when forming an image on the OHP sheet.

Further, the control means controls the maximum density level and the gamma value γ in the second mode to be approximately ほ ぼ of the maximum density level and the gamma value γ in the first mode, respectively. If the second mode is executed when an image is formed on the reflective OHP sheet, the reflective OHP can be projected on the screen as clearly as when the transmissive OHP is used even if the reflective OHP is used. .

Embodiment [Principle of the Present Invention] First, the principle of copy density switching according to the present invention will be described, and then an embodiment to which the present invention is applied will be described.

FIG. 1 is a diagram for explaining the principle of the present invention. FIG. 1 (1) is a graph showing the relationship between original density and copy density, and FIG. 1 (2) is copy density and image density. 6 is a graph showing a relationship with the graph. Here, the original density refers to the image density on the original paper, the copy density refers to the density of the copy image created on the OHP sheet, and the image density refers to the projected image when projected on the screen by the OHP sheet Refers to the concentration of The original density is indicated by reference numeral OD, the copy density is indicated by CD,
The image density is indicated by ID.

By the way, when a transmission type OHP is used for an arbitrary portion on a copy image on an OHP sheet, the spectral transmittance is τλ.
If the actual spectral transmittance at the time of projection by the reflection type OHP is Tλ, and the spectral reflectance at a portion corresponding to the above-mentioned arbitrary portion on the copy original is rλ, the first expression is established.

Tλ = τλ ² ... LogTλ = 2logτλ... Where the original density OD = −logrλ,
The copy density CD = -logτλ. Therefore, transmission type
In the case of OHP, the third equation holds, and in the case of a reflective OHP, the fourth equation holds.

ID = CD ... ID '= 2C.D. In general, in order not to impair the gradation characteristics of an image, the relationship between the copy density CD and the original density OD is the first.
The copying machine is designed in advance so that γ ≒ 1 or so near as shown by the line 11 in FIG. So, this line l1
When the OHP sheet copied in step (1) is used for a transmissive OHP, its characteristic is represented by the line l2 in FIG. On the other hand, when the OHP sheet created based on the line l1 is used for the reflection type OHP, its characteristic becomes the line l3 in FIG. When the OHP sheet copied with γ = 1 in this way is used in a reflection type OHP, γ ≒ 2 is apparently obtained, so that the density becomes too large, and a blackish projected image is formed particularly in a color portion.

Therefore, in order to avoid such a problem, a reflective OH
In the case of an OHP sheet used for P, the density characteristics of the copying machine may be switched so that γ ≒ 1/2 indicated by the line 14 in FIG. That is, if the OHP sheet copied in the state of the line 14 is projected by the reflection type OHP, γ ≒ 1/2 in the copy density CD corresponding to the original density OD and γ ≒ 2 in the image density ID corresponding to the copy density CD And the image density ID with respect to the original density OD becomes γ ≒ 1, and a projected image having almost the same brightness as that obtained when the OHP sheet copied based on the line l1 is projected by the transmissive OHP is obtained.

Therefore, when creating an OHP sheet for transmission type OHP, line l1
It is only necessary to set the γ value to about 1/2 when the OHP sheet used for the reflection type OHP is prepared, and to set the maximum density to about 1/2 compared to the normal case. It is understood that.

Therefore, in the present invention, a first mode and a second mode are provided as the copy mode of the OHP sheet, and in the first mode, γ = 1
The OHP sheet for the transmissive OHP is set as the second mode, and the maximum density is set to about 1/2 as the second mode, and the OHP sheet for the reflective OHP is set to γ ≒ 1/2. Is what you want to create. In addition, when setting the mode, add a mode designation mark on the OHP sheet in advance.
This mark is identified by the identification means when the HP sheet is created, and the first mode or the second mode is automatically determined based on the identification result.
The mode is set. And γ is almost 1/2
The first method is to change the maximum density by changing the developing bias voltage, and to change the slope (corresponding to the γ value) of the copy density curve by using the dither method, and to use only the dither method. A second method for changing the gradient of the maximum density and the copy density curve and a third method for changing only the developing bias voltage to change the maximum density can be considered.

First Embodiment Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Second
FIG. 1 is a diagram schematically showing the internal structure of a full-color copying machine 1 according to one embodiment of the present invention.

The original placed on the original glass 2 is exposed to a CCD line sensor 5 by an exposure lamp 3 and a lens array 4,
(Red), G (green), and B (blue) are read as color signals of three primary colors. The RGB color signals are converted into Y (yellow), M (magenta), C
The signal is converted to a ternary or quaternary signal obtained by adding (cyan) or Bk (black) thereto, and transmitted to the laser optical system 6 as an output signal. The copying machine of this embodiment has image memories for three colors, and sequentially stores Y and Y based on the memories.
The signal of M, C or Y, M, C, Bk is transmitted to the laser optical system 6. In the case of a copying machine having no image memory, the image reader unit 7 needs to scan each time an image is formed for each color.

The laser optical system 6 includes a scanning polygon mirror 8, an Fθ lens 9, a reflection mirror 10, and the like.
Exposure is performed by irradiating the photosensitive drum 11 with an image forming laser beam for each color based on the Y, M, C, and Bk signals. The photosensitive drum 11 is driven to rotate in the arrow direction. In this embodiment, the photosensitive drum 11 is negatively charged by the charger 12. Around the photosensitive drum 11, a drum cleaner 13, a toner collecting roll 14, an eraser lamp 15, and the charging charger 12 are provided, and three types of developing devices are provided. The first developing unit 16 supplies a yellow toner, the second developing unit 17 supplies a magenta toner, and the third developing unit 18 supplies a cyan toner. It is negatively charged. Further, the toner is supplied by transporting the toner of each color stored in the toner hopper 20 to each of the developing devices 16, 17, and 18 via a pipe (not shown) as appropriate based on a supply signal.

The plain paper 80 is stored in a stacked state in the paper feed cassette 21b, and is fed one by one toward the inside of the apparatus by the paper feed roller 22b. The OHP sheets 81 are stored in a stacked state in the sheet feeding cassette 21a, and are fed one by one into the apparatus by the sheet feeding roller 22a. OHP sheet 81 is the third
The transmissive OHP sheet 81a shown in the figure and the reflective OHP sheet shown in FIG.
There are two types of P sheets 81b. As shown in FIG. 3, the OHP sheet 81a has an opaque member 82 attached to the leading end in the sheet feeding direction. A mark M1 for designating a mode for designating the first mode is provided at one end in the longitudinal direction (the right end in FIG. 3) of the opaque member 82. On the other hand, OHP sheet 81b
As in the case of the OHP sheet 81a, as shown in FIG. 4, an opaque member 82 is attached to the leading end of the sheet in the sheet feeding direction, and one end of the opaque member 82 in the longitudinal direction (the right end in FIG. 4) And a mode designation mark M2 for designating the second mode. The mark M1 and the mark M2 are surface-treated so that the reflected light amounts are different from each other so that the optical sensor can identify the mark.
The first mode is a transmission type OHP sheet copy density setting mode, and the second mode is a reflection type OHP sheet copy density setting mode.

An optical sensor 83 as identification means for identifying the marks M1 and M2 is provided near the downstream side of the sheet feeding roller 22a in the sheet feeding direction of the sheet feeding cassette 21a as shown in FIG. This optical sensor 83 is a light emitting element as shown in FIG.
This is a reflection type optical sensor having a light receiving element 85 and a light receiving element 85. still,
The optical sensor 83 may be a transmission type optical sensor as shown in FIG. Further, these OHP sheets 81a and 81b may be fed by manual feeding instead of feeding from the inside of the sheet feeding cassette.
Paper is fed from the a side. Recording material 34 such as OHP sheets 81a, 81b or plain paper 80 fed by paper feed rollers 22a, 22b
The recording material 34 is temporarily stopped at the point where the leading end of the recording material 34 contacts the registration roller 23, thereby taking the following timing, and simultaneously performing the skew correction. Reference numeral 24 denotes a paper sensor used for this purpose.

Reference numeral 25 denotes a transfer drum, which is driven to rotate in the direction of the arrow. Inside the transfer drum 25, a suction charger 26 is provided along the circumferential direction.
A transfer charger 27 and a separation charger 28 are provided.
Further, a transfer charger 29 is provided in the photosensitive drum 11 corresponding to the transfer charger 27, and a separation charger 30 is provided on the outer side of the transfer drum 25 so as to face the separation charger 28. .

The electrostatic latent image formed on the photosensitive drum 11 by exposure by the laser optical system 6 is developed as a toner image by one of the developing units 16 to 18. On the other hand, the recording material 34 fed from the registration rollers 23 is gripped by the gripper 31, is attracted onto the transfer drum 25 by the suction charger 26 as the transfer drum 25 rotates, and is transferred onto the transfer drum 25 as the transfer drum 25 rotates. Wrapped around. Then, while the recording material 34 passes between the transfer chargers 27 and 29, the toner image on the photosensitive drum 11 is transferred to the recording material 34. In the full color mode, the recording material 34 is rotated three times by the transfer drum 25. After the transfer process, the recording material 34 is
28, 30 and the transfer drum 25 is separated by the separation claw 32.
It is separated from above and fed to the thermal fixing device 36 by the conveyor belt 33. The transferred recording material 34 is heated and pressurized by passing through a heat fixing device 36, an image is fixed on the recording material 34, and is discharged to a paper discharge tray 35.

FIG. 8 is an enlarged view of the vicinity of the developing units 16, 17, and 18. Developing bias voltages VB1, VB2, and VB3 (denoted by VB when collectively referred to) are applied to developing units 16, 17, and 18 by developing bias power supplies V1, V2, and V3, respectively.

Further, a voltage from a charging power supply V0 is applied to the charging charger 12. By the way, the maximum copy density is changed by changing the developing bias voltage. That is, by changing the developing bias voltage VB of the developing bias power supply without changing the charging voltage,
By making the developing bias voltage smaller than usual, the electrostatic attraction force for the toner to move to the surface of the photosensitive drum 11 can be reduced, thereby reducing the amount of toner adhesion. That is, the maximum copy density can be reduced.

FIG. 9 is a front view of the operation unit of the color copying machine 1. The operation unit 40 includes a copy button 41, a numeric keypad 42, a liquid crystal touch panel 43, a mode switch 44 for switching between the OHP sheet 81 and the plain paper 80, and the like. The numeric keypad 42 is used to set, for example, the number of copies, and the like.
Is used to designate an area of a document to be copied or a color copying area in twin-color copying. The mode changeover switch 44 is a switch whose switching state alternately and sequentially changes between ON and OFF by being sequentially pressed. When the switch key 44 is pressed and the OHP sheet is used, the mark is identified by the optical sensor 83. Is performed, the first mode, which is a copy mode of an OHP sheet for a transmission type OHP, the second mode, which is a copy mode of an OHP sheet used for a reflection type OHP, and the third mode, which is a copy mode of plain paper, are used. Each mode setting of the mode can be performed.

FIG. 10 is a block diagram showing an electrical configuration related to the image forming process of the copying machine 1. The detection signal from the optical sensor 83 is given to the CPU 52. The image reading signal from the CCD line sensor 5 is supplied to an analog / digital conversion circuit 50.
Is converted to a digital signal and supplied to the buffer memory 51. A CPU 52 is connected to the buffer memory 51, and the CPU 52
The input / output operation of the image signal to / from is controlled. The image signal from the buffer memory 51 is provided to the comparison circuit 53.
On the other hand, a memory 54a storing a threshold matrix 60A and a memory 54b storing a threshold matrix 60B are individually connected to the CPU 52.
The threshold matrix data read from 4a, 54b is supplied to a selector circuit 55, and one of the threshold matrices selected by a switching signal from the CPU 52 is output to a comparison circuit 53.
Is given as a reference signal. The comparison circuit 53 compares the reference signal from the selector circuit 55 with the image signal from the buffer memory 51, and when it is equal to or more than each threshold of the threshold matrix, sets it to logic "1", and when it is less than the threshold, sets the logic "1" The binarized signal is output as "0", and the binarized signal is stored in the image memory 56 as image data. And
When an image of one page is read by the CCD line sensor 5, the image data corresponding to the original image is stored in the image memory 56. In the copying operation, an image signal is read from the image memory 56,
The laser light is supplied to the semiconductor laser of the laser optical system 8 and the laser light is driven to emit light in accordance with the image data.

The operation signal from the operation unit 40 is given to the CPU 52, and based on the operation command signal and the detection signal from the optical sensor 83, the CPU 52 executes the development bias voltages VB1 to VB3 of the development bias power supplies V1 to V3, the photosensitive drum 11 The driving of the copying mechanism 57 including the main motor for driving is controlled.

Next, a procedure for creating the threshold matrices 60A and 60B stored in the memories 54a and 54b will be described. Note that the threshold matrix 60 stored in the memories 54a and 54b
A and 60B are dither matrices used to adjust the γ value. Incidentally, the dither method is one of the so-called area gradation methods, and for example, a 4 × 4 threshold matrix 60 shown in FIG. 11 is used. The threshold matrix 60 is, for example, a buyer type, and the thresholds A1 to A16 are configured in a pattern shown in FIG. Here, it is assumed that, as shown in FIG. 12, in a document portion 61 corresponding to the threshold matrix 60 of the document, a hatched area 62 is a toner-attached portion, and a remaining portion is a toner non-adhered portion. When the reflectance of the toner-attached portion is R (0 to 1.0), the reflectance of the non-toner-attached portion is 1.0, and the area ratio of the toner-attached portion in the threshold matrix 60 is a, the original portion 61
Is represented by the following equation (5).

By the way, if the maximum density of the original copy section is 1.6, the original density OD (0 to 1.6)
To finish the CD (0 to 1.6), the line m1 in FIG.
It is necessary to realize a copy density curve of γ = 1 as shown in FIG. In order to obtain such a curve m1 by the area gradation method, if a document is copied based on the area ratio curve m2 in FIG. 13, a copy image having the characteristic of the curve m1 is obtained. In addition, curve m2
Is obtained by using the fifth formula. In this way, the area ratio curve 2 is set in advance, and if the copy is performed based on the area ratio m2, the curve m1 can be obtained. Therefore, transmission type
The levels of the thresholds A1 to A16 of the threshold matrix 60A used in the first mode at the time of copying the OHP sheet 81a,
What is necessary is just to set in advance so as to obtain the area ratio curve m2. The threshold levels of the thresholds A1 to A16 are shown in, for example, Table 1. A threshold matrix 60A having such threshold levels is stored in the memory 54a in advance.

In such a threshold matrix 60A, for example, when the level of the pixel signal is 521, the threshold levels of the thresholds A1 to A7 of the threshold matrix 60A are larger than the pixel levels,
Accordingly, in the dither image, as shown in FIG. 9, the pixels corresponding to the thresholds A1 to A7 are black, and the remaining A8 to A16 are white. That is,
The dither image shown in FIG.
The third density level is set.

On the other hand, in order to create the threshold matrix 60B for the reflection type OHP stored in the memory 54b, it is necessary to obtain the area ratio curve m3 shown in FIG. This area ratio curve m3 is an area ratio curve for obtaining a copy density curve m1 by setting the maximum copy density to 1/2 of the normal maximum density, that is, 0.8, and changing the threshold level of the threshold matrix 60B. This area ratio curve m3
Is an area ratio curve obtained by using the formula (5) so that the copy density (0 to 0.8) linearly corresponds to the development density (0 to 1.6). The calculation is performed with the maximum density corresponding to the black portion of the copy portion being 0.8. When the OHP sheet 81b is copied based on such an area ratio curve m3, an ideal copy density curve of γ = 1/2 indicated by reference numeral m4 in FIG. 13 is obtained.

Therefore, in order to obtain the area ratio curve m3, specifically, the threshold level of the threshold matrix 60B stored in the memory 54b is different from the threshold level of the above-described threshold matrix 60A so that the area ratio curve m3 is obtained. This can be realized by setting the threshold level. The values of the thresholds A1 to A16 of the threshold matrix 60b obtained based on such a concept are shown in, for example, Table 1. In this way, the threshold matrix 60B for the reflection type OHP is formed, and the memory 54b
Stored in

FIG. 15 is a flowchart showing the copying process of the copying machine 1. First, in step s1, the recording material 34 is fed,
At s2, it is determined whether the mode is the OHP mode. That is, the determination is made based on the on / off state of the mode changeover switch 44. When the mode changeover switch 44 is in the off state, OHP
If the mode is not the mode, the process proceeds to step s3, and the mode is set to the third mode for recording ordinary copy paper, and the steps s4, s5
The processing shifts to s6 and s7, where normal development, transfer, fixing, and paper discharge processing are executed.

When the mode changeover switch 44 is in the ON state in the step s2, the process proceeds from the step s2 to the step s8 assuming that the mode is the OHP mode, the output of the optical sensor 83 is read, and in the step s9, the copy for the transmission type OHP sheet is performed. Is determined. In the copy mode, when the detection result of the transmission optical sensor 83 is the first mode, the process proceeds from step s9 to step s10. Then, in step s10, the developing bias voltage is set to be the same as the voltage in the normal state. Therefore, the maximum copy density does not change at all. Step s1
In step 1, the area ratio curve m1 is selected. That is, the CPU 50 derives the selector 55 switching signal and selects the threshold matrix 60A stored in the memory 54a. Thus, the first mode is set, and development and transfer processing to the OHP sheet 81a is executed in steps s4 and s5. That is, the original image is read by the CCD line sensor 5, converted into a digital signal by the analog / digital conversion circuit 50, and stored in the buffer memory 51. In this manner, one page of the document is stored in the buffer memory 51. After that, each image density level signal corresponding to the threshold value matrix is supplied from the buffer memory 51 to the comparison circuit 53. Comparison circuit 53
In, each threshold of the threshold matrix 60A is sequentially given as a reference signal via the selector 55, and this threshold is compared with the density level signal from the buffer memory 51. For example, the image density level is higher than the threshold. In this case, it is stored in the image memory 56 as logic "1" as black, and as logic A "0" indicating white when the density level is smaller than the threshold value. In this manner, the image data in the buffer memory 51 is compared with the threshold matrix 60A by the comparison circuit 53.
The image data for one page of the document image is stored in the image memory 56.

The image data in the image memory 56 is stored in the laser optical system 6.
And the semiconductor laser is driven to emit light with a light emission amount corresponding to the image signal. As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 11 as described above, and thereafter, the transfer image in which the original image is transferred to the OHP sheet by a normal electrostatic copying process is moved to steps s6 and s7. A fixing / discharge process is executed. This first
In the mode, the copy density of the OHP sheet 81a is equivalent to the case where the original image is copied based on the area ratio curve m3 by the dither method using the threshold matrix 60A, and therefore, the image along the copy density curve m1 in FIG. Is formed on the OHP sheet 81a. Therefore, it can be used for transmission type OHP.

If the detection result of the optical sensor 3 is the second mode which is the copy mode of the reflection type OHP sheet in step s9, the process proceeds from step s9 to step s12.
In step s12, the maximum copy density is set to about 1/2. That is, the developing bias voltage is set to be smaller than that in the first mode, and the developing bias voltage is set so that the amount of applied toner is about half that in the first mode. Then, in step s13, the area ratio curve m3 is selected. That is, the CPU 52 derives the switching signal to the selector 55, and selects the threshold matrix 60B stored in the memory 54b. As a result, the second mode is set, and the OHP sheet is set in steps s4, s5, s6, and s7.
Copy processing to 81b is executed. The copying process in the second mode is basically the same as the copying process in the first mode. The only difference is that the threshold matrix used is 60B and that the developing bias voltage is reduced and the maximum density is set to about 1/2. Therefore, the copying process has been executed based on the area ratio curve m3.
As a result, a copy density along the copy density curve m4 where γ = 1/2 can be created on the OHP sheet 81b. Therefore, when this OHP sheet 81b is used for a reflection type OHP, it becomes possible to project a clear image on the screen almost similar to the case where the transmission type OHP sheet 81b is used for a transparent type OHP.

[Second Embodiment] In the above embodiment, the density curve m4 is obtained by the developing bias voltage and the dither method using the threshold matrix 60B when copying the reflection type OHP sheet 81b, but the developing bias voltage is changed. In other words, the maximum density is the same as the normal maximum density, and the reflection type OH is obtained only by the dither method.
An image along the copy density curve m4 can be obtained from the P sheet 81b. In this case, a new threshold matrix 60C (see Table 1) is used instead of the threshold matrix 60B stored in the memory 54b. In creating the threshold matrix 60C, an area ratio curve m5 shown in FIG. 16 may be set. This area ratio curve m5 is an area ratio curve for obtaining a copy density curve m4 in which γ = 1/2 based on the above-described formula (5). Such area ratio curve
A threshold matrix 60C based on m5 may be set in advance. The threshold levels of the thresholds A1 to A16 of the threshold matrix 60C are shown in, for example, Table 1. Such a threshold matrix 60C is stored in advance in the memory 54b instead of the threshold matrix 60B.

The copy control processing in the second embodiment is shown in FIG. The copying process will be described with reference to FIG. First, at step k1, the recording material 34 is fed,
At k2, it is determined whether the recording material 34 is the OHP sheet 81,
Otherwise, plain paper copy mode is set, and plain paper copy processing is executed.

If it is the OHP sheet 81 in step k2, the process proceeds to step k8, and the normal copy maximum density is set.
Then, in step k9, the output of the optical sensor 83 is read, and in step s10, it is determined whether or not the OHP sheet 81 is the transmission type OHP sheet 81a.If so, the process proceeds to step k11, where the area ratio curve m2 is selected. You. That is, a dither method using the threshold matrix 60A is used. And step k
The process proceeds to 4, k5, k6, and k7, where copying processing to the OHP sheet 81a is performed. Therefore, a copy image along the copy density curve m1 is formed on the OHP sheet 81a.

When the reflection type is set in step k10, the process proceeds from step k10 to step k12, and the area ratio curve m5 is selected. That is, the dither method using the threshold matrix 60C is performed. Then, in steps k4, k5, k6, and k7, a copy process to the OHP sheet 81b is executed. With this, the OHP sheet 81b
A copy image is formed along the copy density curve m4.

In this way, it is possible to change two of the maximum copy density and the γ value only by the dither method.

[Third Embodiment] As still another embodiment, the reflection type OHP sheet 81b can be formed even if the developing bias voltage is changed without using the dither method. That is, the third embodiment is realized by changing the developing bias voltage to set the maximum copy density to about 1/2.

The processing will be described with reference to FIG. In step r1, the recording material 34 is fed, and in step r2, the recording material 34
It is determined whether or not the sheet is the OHP sheet 81, and if not, the plain paper copying mode is set in step r3, and the
At r4, r5, r6, r7, a copy process to plain paper is executed.

When the sheet is the OHP sheet 81 in step r2, step r
Move to 8, the output of the optical sensor 83 is read, and step r9
It is determined whether or not the OHP sheet 81 is of the transmission type. If the transmission type is not of the transmission type, the copying maximum density is set to the same value as in normal copying in step r10. That is, the developing bias voltage is set to the same value as in the normal case. Then, in steps r4, r5, r6, r7, a copying process to the OHP sheet 81a is executed. This allows the OHP sheet 81a to have a copy density curve
A copy image along m1 is formed.

When the OHP sheet 81 is of the reflection type in step r9, the process proceeds to step r11, and the maximum copy density is set to about 1/2 of that in the normal case. That is, the developing bias voltage is reduced, and the developing bias voltage is set such that the maximum density becomes about half of the normal density. And steps r4, r5, r6, r7
Then, the copying process to the OHP sheet 81b is executed. As a result, a document image having a lower density is formed on the OHP sheet 81b than in the case of the transmission type, and can be used for the reflection type OHP.

In the control method in which only the developing bias voltage is changed, the copy density curve is indicated by the reference numeral m6 in FIG. As is clear from FIG. 13,
A slight deviation occurs from the ideal copy density curve m4 for γ = 1/2. Therefore, the method in which only the developing bias voltage is changed has a disadvantage that the image characteristics on the screen are slightly reduced as compared with the above-described first and second embodiments. However, reflection-type OHP is replaced by transmission-type OHP.
Sufficient sharpness can be obtained as compared with the conventional case using a sheet.

Although the copying machine 1 is a color copying machine in the above-described embodiment, it may be a monochrome copying machine. In particular, the present invention is also applicable to a case where an image having black and white gradation is formed on an OHP sheet. It can be suitably implemented.

In the above description, an example in which the image data is stored in the buffer memory 51 and the dither processing is performed has been described.

In the above-described embodiment, the marks M1 and M2 are optically identified. However, the mark M1 and M2 may be magnetically processed and identified by a magnetic sensor.

Effects of the Invention As described above, according to the configuration of the present invention, the first mode in which the image forming is performed on the OHP sheet at the first density by the determining unit, and the OHP sheet at the second density different from the first density It decides in which mode of the second mode in which image formation is to be performed, and controls the signal processing means based on the determination of the determination means to determine at least one of the maximum density level and the gamma value γ. Since the image density formed on the OHP sheet is changed by changing, for example, according to the distinction between the reflection type OHP sheet and the transmission type OHP sheet,
It is possible to form images with optimum densities. Moreover, since the density change is performed by changing at least one of the maximum density level and the gamma value γ, a high-quality image can be formed without significantly impairing the gradation even when the density is changed. .

Further, a mark designating the first mode or the second mode is attached to the OHP sheet in advance, and the determining unit detects the mark when forming an image on the OHP sheet, thereby detecting the OHP sheet. Mode to be executed for the first time can be reliably determined.

Further, the control means may determine that the maximum density level and the gamma value γ in the second mode are the first density level and the gamma value γ, respectively.
In the mode, the maximum density level and the gamma value γ are controlled so as to be approximately 1/2, so that the second mode is executed when an image is formed on the reflective OHP sheet.
Even when a reflective OHP is used, it can be projected on a screen as clearly as when a transmissive OHP is used.

[Brief description of the drawings]

FIG. 1 is a view for explaining the principle of the present invention, FIG. 2 is a view showing a simplified internal structure of a full-color copying machine 1 according to one embodiment of the present invention, and FIG. 3 is a perspective view of an OHP sheet 81a. 4 is a perspective view of the OHP sheet 81b, FIG. 5 is a perspective view of the vicinity of the paper feed roller 21a, FIG. 6 is a cross-sectional view of the optical sensor 83, and FIG. FIG. 8 shows developing units 16, 17, and 18.
FIG. 9 is a front view of an operation unit of the color copying machine 1, FIG. 10 is a block diagram showing an electric structure related to image forming processing of the copying machine 1, and FIG. FIG. 12, FIG. 12 is a diagram showing a document portion 61, FIG. 13 is a graph showing a copy density curve of the first embodiment, FIG. 14 is a diagram showing a display mode of a dither image, and FIG. FIG. 16 is a flowchart showing a copy density curve of the second embodiment, FIG. 17 is a flowchart showing a copy process of the second embodiment, and FIG. 18 is a copy process of the third embodiment. FIG. 19 is a diagram showing a configuration of a transmission type OHP, FIG. 20 is a diagram showing a configuration of a reflection type OHP, and FIG. 21 is a diagram showing a blue spectral ratio of a transmission type OHP and a reflection type OHP. is there. 1 ... copier, 5 ... CCD line sensor, 11 ... photosensitive drum, 16,17,18, ... developer, 34 ... recording material, 44 ...
Mode switch, 50: Analog / digital conversion circuit, 52: CPU, 53: Comparison circuit, 54a, 54b: Memory, 55: Selector, 56: Image memory, 60A, 60B, 60C
…… Threshold matrix, 81a …… Transmissive OHP sheet, 81
b: Reflective OHP sheet, 83: Optical sensor, VB: Development bias voltage, M1, M2: Mark for mode designation.

Continuation of the front page (51) Int.Cl. ⁶ identification code FI G03G 21/00384 (56) References JP-A-62-43660 (JP, A) JP-A-57-210355 (JP, A) JP-A Sho 61-28961 (JP, A) JP-A-1-193885 (JP, A) JP-A-62-157063 (JP, A) JP-A-1-276154 (JP, A) JP-A-63-274549 (JP, A) A) JP-A-1-124873 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G03G 15/00 303 G03G 15/04 120 G03G 21/00 370 G03G 21/00 376- 384

Claims (4)

(57) [Claims] An image forming apparatus capable of forming an electrostatic latent image on a photosensitive member based on an image signal, developing the electrostatic latent image by a developing device, and transferring the developed toner image to an OHP sheet. Which of the first mode for forming an image on the OHP sheet at the first density and the second mode for forming an image on the OHP sheet at a second density different from the first density Determining means for determining the density information included in the image signal; and controlling the signal processing means based on the determination by the determining means to determine at least one of a maximum density level and a gamma value γ. Control means for changing one of them to change the image density formed on the OHP sheet. 2. An electrostatic latent image is formed on a photoreceptor surface by driving a laser light source based on an image signal, and the electrostatic latent image is developed by a developing device to which a developing bias voltage is applied. In an image forming apparatus capable of transferring an image to an OHP sheet, a first mode for forming an image on an OHP sheet at a first density and a second mode for forming an image on an OHP sheet at a second density different from the first density are provided. Determining means for determining which of the two modes image formation is to be performed; signal processing means for changing density information included in the image signal; controlling the signal processing means based on the determination of the determining means And control means for changing at least one of the maximum density level and the gamma value γ to change the image density formed on the OHP sheet. 3. The OHP sheet is preliminarily provided with a mark for designating the first mode or the second mode, and the determining means detects the mark when forming an image on the OHP sheet. 3. The image forming apparatus according to claim 1, wherein a mode to be executed for the OHP sheet is determined based on the determined mode. 4. The control means controls the maximum density level and the gamma value γ in the second mode to be approximately の of the maximum density level and the gamma value γ in the first mode, respectively. The image forming apparatus according to claim 1, wherein: