JPH01312559A - Copying device - Google Patents

Abstract

PURPOSE:To copy into an OHP sheet to project clear image on a screen at the time of using the OHP by changing copy density characteristics depending on whether the over head projector OHP is a transmission type or a reflection type. CONSTITUTION:A copy device 1 which transfers a toner image corresponding to an electrostatic latent image onto a transfer material 34 is equipped with specifying means which specifies to select a first transmission mode or a second reflection mode whose copy density C.D of the OHP sheet are different at the time of using the OHP sheet and with a signal processing means which changes density information included in an image signal. When the first mode is specified at the time of using the OHP, the maximum copy density level is not changed and copy density C.D is set that gamma value of copy density curve with respect to original density O.D should be gamma =1. When the second mode is specified, copy density C.D is set that either the maximum copy density level or gamma value should be different from the first mode. Thus, the OHP sheet can be copied in two types such as the transmission type and the reflection type for OHP.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a copying apparatus that can use plain paper and OHP sheets as transfer materials.

A conventional OHP (overhead projector) that uses an OHP sheet to project an image formed on the OHP sheet onto a screen is a transmission type OHP100a shown in Fig. 14.
There are two types: and a reflective type 0HP100b shown in FIG.

In FIG. 14, 101 is a housing, 102 is a lamp, 103 is a lens, 104 is a protective glass, and 105 is an O.
HP sheet, 106 is lens, 107 is mirror, 108
is the arm. Light from the light source lamp 102 is focused by a Fresnel lens 103, passes through an OHP sheet 105, and is projected onto a screen via a mirror 107, so that the image formed on the OHP sheet 105 is focused on the screen. FIG. 15 shows a reflective type OHP 100b, in which parts corresponding to the transparent type 0) (Plooa are given the same reference numerals. In this reflective type 0HPIOob, the lamp 102 is placed above the OHP sheet 105,
Further, in place of the Fresnel lens 103 used in the transmission type OHP 100a, a Fresnel mirror 103' whose upper surface is mirror-finished is provided. The light from the lamp 102 first passes through the OHP sheet 105 downward, is reflected by the Fresnel mirror 103', is condensed at the same time, passes upward through the OHP sheet 1'05, and then enters the lens 106.
．． 106', the image is projected onto the screen via the mirror 107, and the image formed on the OHP sheet 105 is formed on the screen. In addition, Fresnel mirror 103' and OH
The OHP sheet 105 is placed in almost intimate contact with the OHP sheet 105, so that the light that passes through the OHP sheet 105 from top to bottom is reflected upward by the Fresnel mirror 103'.
It passes through the same point on the HP sheet 105 and moves upward. In this manner, in the reflective OHP 100b, light passes through the OHP sheet 105 twice and is imaged.

On the other hand, in the transmission type 0HP100a, as mentioned above, the light is
The image is formed by passing through the HP sheet 105 only once.

Therefore, when the transmittance of the OHP sheet 105 is τλ, the transmittance of the transmissive type 0HPPa is τλ, and the transmittance of the reflective type OHP 1
In b, it becomes (τλ)2.

For example, if we consider a blue part of an image formed on an OHP sheet, any point P in this blue part
The spectral ratio of is shown in FIG. Here, the solid line shows the case where the transmission type OHP 100a is used, and the broken line shows the case where the reflection type OHP 100b is used. As is clear from FIG. 16, it is understood that the peak passing rate decreases from 60% to 36% in the reflective type. If peak transmittance τλ-20%, (τλ)2=4
%, and it is understood that the lower the transmittance, the more extreme the fall when squared.

In this way, when creating an OHP sheet with a copying machine, if the OHP sheet image is formed with the same copy density, even if the transmission type 0HP1a can project an optimal image on the screen, the reflective type 0HP1b can project the light amount. This will cause the image to become unclear.

Therefore, when using a copying machine with an OHP mode for a reflective OHP and for a transmissive OHP, each OHP is different.
There has been a desire for a copying machine that can copy HP sheets so that each copy has an optimum density.

In view of the above-mentioned problems, an object of the present invention is to switch copy density characteristics depending on whether the OHP used in the OHP sheet is a transmission type or a reflection type when creating an OHP sheet.
To provide a copying machine capable of copying so that a clear image can be projected on a screen when used in a computer.

The present invention can use plain paper and OHP sheets as transfer materials, and forms an electrostatic latent image on the surface of a photoreceptor by driving a laser light source in response to an image signal. In a copying apparatus in which a developing device to which a developing bias power supply is applied, an amount of toner corresponding to the developing bias is deposited to form a toner image corresponding to an electrostatic latent image, and this toner image is transferred to a transfer material. , a mode specifying means for selectively specifying a first mode and a second mode in which the copy density of the OHP sheet is different when the OHP sheet is used; a signal processing means for changing density information included in the image signal; and the mode specifying means. Based on the mode designation, the voltage value of the developing bias power supply and/or the signal processing means is controlled, and in the first mode, the maximum density level is not changed, and the gamma value γ of the copy density curve with respect to the original density is changed to γ. = 1, and in the second mode, the copying density is controlled so that at least one of the maximum density level and the gamma value γ is different from the first mode.

According to the above configuration, when the first mode is specified when using an OHP, the maximum copy density level is not changed, and the T value of the copy density curve with respect to the original density is set to γ = 1. Set the copy density to , and copy the image onto the OHP sheet. Also, when the second mode is specified when using an OHP sheet, the maximum copy density level and the γ value γ are small (
The copying density is set so that either mode is different from the first mode, and the image is copied onto the OHP sheet.

This makes it possible to automatically switch the copy density and copy onto an OHP sheet. Therefore, for example, by copying in the second mode with the maximum density set to about 1/2 of the normal maximum density and the γ value set to γ#1/2, an OHP film for reflective OHP is created. can do. Also, when copying in the first mode,
A transparent OHP sheet can be created.

[Principle of the present invention] First, the principle of copy density switching according to the present invention will be explained, and then embodiments to which the present invention is applied will be explained.

FIG. 1 is a diagram for explaining the present invention in detail.
FIG. 1 (1) is a graph showing the relationship between original density and copy density, and FIG. 1 (2) is a graph showing the relationship between copy density and image density. Here, original density refers to the image density on the original paper, copy density refers to the density of the copy image created on the OHP sheet, and image density refers to the density of the copy image created on the OHP sheet. Refers to the density of the projected image. Note that the original density is referenced 0. D, and the copy density is C,
D, and the image density is 1. Indicated by D.

By the way, the spectral transmittance of any part of the copy image on the OHP sheet when using a transmission type OHP is τ
λ, the actual spectral transmittance when projecting with a reflective OHP is Tλ, and the spectral reflectance of a portion corresponding to the above-mentioned arbitrary portion on the copy document is rλ, then the first equation holds true.

Tλ=τλ2...■ If we take the logarithm of both sides of this first equation, we get βogTλ=2βogτλ...■. Here, the original density is 0.0=-Ragrλ, and the copy density C, D=-1ogτλ. Therefore, the transmission type O
In the case of an HP, the third equation holds true, and in the case of a reflective OHP, the fourth equation holds.

1, D=C,D...■ 1, D'=20. D...■ Generally, in order not to impair the gradation characteristics of the image,
The relationship between the copy densities C and D and the original density O9D is shown in line 1 in Figure 1 (1)! The copying machine is designed in advance so that the value is around γ#1 as shown by 1. Therefore, when an OHP sheet copied along line β1 is used in a transmission type OHP, its characteristics become line 12 in FIG. 1(2) based on equation 3. On the other hand, when an OHP sheet prepared based on line 11 is used for a reflective OHP, its characteristics become line 13 in FIG. 1(2) based on equation 4.

If an OHP sheet copied in this way with γ=1 is used in a reflective OHP, it will appear to be γ#2, so the density will be too high, resulting in a blackish projected image, especially in the blank part.

Therefore, in order to avoid such problems, reflective type OH
In the case of OHP sheets used for P, Figure 1 (1)
The density characteristic of the copying machine may be switched so that γ#1/2 is obtained as indicated by line 14.

That is, if the OHP sheet copied in the state of this line 14 is projected on a reflective OHP, the original density will be 0. Copy density C for D, γ#1/2 for D, and image density 1 for cohe density C, D. γ#2 in D
Each characteristic of is canceled out, and the image density l for the original density 0.0.

In D, the γ object 1 is obtained, and a projected image with almost the same brightness as when an OHP sheet copied based on line 11 is projected by a transmission type OHP is obtained.

Therefore, when creating an OHP sheet for a transmissive type OHP, copy based on line 11, and when creating an OHP sheet used for a reflective type OHP, set the γ value to about 1/2 and set the maximum density to the normal case. It is understood that copying can be performed by setting the value to about 1/2.

Therefore, in the present invention, a first mode and a second mode are provided as OHP sheet copy modes, and in the first mode, γ=1
In addition to creating an OHP sheet for transmission type OH2, set the maximum density to about 1/2 as the second mode.
and T#1/2 for reflective OHP.
This is an attempt to create a P sheet. As a method for setting T to approximately 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 using the dither method. There is a second method of changing the maximum density and the slope of the copy density curve using only the dither method, and a third method of changing the maximum density by changing only the developing bias voltage.

[First example] Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 2 is a diagram schematically showing the internal structure of a full-color copying machine 1 according to an embodiment of the present invention.

The original placed on the original glass 2 is exposed to the CCD line sensor 5 by the exposure lamp 3 and the lens array 4, and the R
It is read as color signals of three primary colors: (red), G (green), and B (blue).

This RGBO color signal is converted into Y (yellow), M (magenta), C (cyan) or B by the image processing circuit.
The signal is converted into a 3-value or 4-value signal with k (black) added thereto, and is transmitted to the laser optical system 6 as an output signal. The copying machine of this embodiment does not have an image memory for three colors, and the image league unit 7 scans each time when forming an image for each color, and based on this, sequentially Y, M, C or Y, M,
The C and Bk signals are transmitted to the laser optical system 6.

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 image-forming laser light for each color based on the M, C, or Y, M, C, and Bk signals. In this embodiment, the photosensitive drum 11 is rotated in the direction of the arrow.
is negatively charged by the charger 12. Around the photosensitive drum 11, there are a drum cleaner 13, a toner collection roll 14, an eraser lamp 15, and the charger 1.
2 and three types of developing devices are also provided.

The first developer 16 supplies yellow toner, the second developer 17 supplies magenta toner, and the third developer 18 supplies cyan toner. It is negatively charged. Further, toner replenishment is carried out by transporting the toner of each color stocked in the toner hopper 20 to each developing device 16, 17, 18 via a vibrator (not shown) at appropriate times based on a replenishment signal.

Recording materials 34 such as plain paper and OHP sheets are stored in a stacked state in a paper feed cassette 21-2.21b, and are fed into the apparatus one by one by paper feed rollers 22a, 22b, etc. Then, when the leading edge contacts the registration roller 23, the recording material 34 is temporarily stopped to determine the subsequent timing, and skinny correction is performed at the same time. 24 is 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. A suction charger 2 is disposed inside the transfer drum 25 along the circumferential direction.
6, a transfer charger 27, and a separation charger 28 are arranged. Further, a transfer charger 29 is provided within 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, facing 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 roller 23 is
It is gripped by the gripper 31 and attracted onto the transfer drum 25 by the suction charger 26 as the transfer drum 25 rotates, and as the transfer drum 25 rotates, the transfer drum 2
It can be wrapped around 5.

Then, while the recording material 34 passes between the transfer chargers 27 and 29, the toner image on the photosensitive drum 11 is transferred onto the recording medium 34. Note that in the full color mode, the recording material 34 is rotated three times by the transfer drum 25. After the transfer process has been completed, the recording material 34 is neutralized by the separation charger 28 , 30 , separated from the transfer drum 25 by the separation claw 32 , and fed to the heat fixing device 36 by the conveyance belt 33 .

The transferred recording material 34 is heated and pressurized by passing through a thermal fixing device 36 , the image is fixed on the recording material 34 , and then the recording material 34 is discharged to a paper discharge tray 35 .

FIG. 3 is an enlarged view of the vicinity of the developing units 16, 17, and 18.

Each developing device 16, 17, 18 has a developing bias power supply Vl, V
2. The developing bias voltages VBI and VB2 are determined by V3, respectively.
．． VB3 (generally referred to as VB) is applied.

Further, a voltage from a charging power source 0 is applied to the charging charger 12. By the way, changing the maximum copying density is achieved by changing the developing bias voltage. That is, by changing the developing bias voltage VB of the developing bias power source without changing the charging voltage and making the developing bias voltage smaller than normal, electrostatic adsorption is performed to move the toner to the surface of the photosensitive drum 11. The force can be reduced, thereby reducing toner adhesion. That is, the maximum copying density can be reduced.

FIG. 4 is a front view of the operation section 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 changeover switch 44 for OHP sheet and plain paper, and each OHP for transmissive type OF2 and reflective type OHP.
A HP sheet creation mode changeover switch 45 and the like are provided. The numeric keypad 42 is used, for example, to set the number of copies, etc., and the liquid crystal touch panel 43 is used to specify an area of a document to be copied or a color copy area in twin-color copying. The mode changeover switches 44 and 45 are switches whose switching modes alternately and sequentially change between on and off when pressed sequentially. The first mode is the copying mode of the OHP sheet used for
It is possible to set each of the second mode, which is a HP sheet copy mode, and the third mode, which is a plain paper copy mode.

FIG. 5 is a block diagram showing an electrical configuration related to image forming processing of the copying machine 1. As shown in FIG. The image reading signal from the CCD line sensor 5 is sent to an analog/digital conversion circuit 50.
is converted into a digital signal and applied to the buffer memory 51.

A CPU 52 is connected to this buffer memory 51, and the input/output operation of image signals to and from the buffer memory 51 is controlled by this CPU 52. The image signal from the buffer memory 51 is given to a comparison circuit 53.

On the other hand, a memory 54a storing a dark value matrix 60A and a memory 54b storing a dark value matrix 60B are individually connected to the CPU 52. The value matrix data is given to the selector circuit 55, and C
Either one of the dark value matrices selected by the switching signal from the PU 52 is given as a reference signal to the comparison circuit 53. The comparison circuit 53 compares the reference signal from the selector circuit 55 and the image signal from the buffer memory 51, and if it is greater than or equal to each darkness value of the darkness value matrix, a logic "1" is generated.
'', and when the value is less than the dark value, a binary signal with logic rOJ is output, and this binary signal is stored in the image memory 56 as image data. Then, when an image for one page is read by the CCD line sensor 5, image data corresponding to the original image is stored in the image memory 56.

In the copying operation, an image signal is read out from the image memory 56 and applied to the semiconductor laser of the laser optical system 8, and a laser beam is driven to emit light in accordance with the image data.

Note that the operation signal from the operation unit 40 is given to the CPU 52, and based on the operation command signal, the CPU 52 adjusts the developing bias voltages VBI to VB3 of the developing bias power supplies V1 to V3,
The driving of the copying mechanism section 57 including the main motor for driving the photosensitive drum 11 is switched off.

Next, the procedure for creating the dark value matrices 60A and 60B stored in the memories 54a and 54b will be explained. Note that the dark value matrices 60A and 60B stored in the memories 54a and 54b are dither matrices and are used to adjust the γ value. By the way, the dither method is a method of the so-called area gradation method, and uses, for example, a 4×4 darkness value matrix 60 shown in FIG. 6. The dark value matrix 60 is, for example, a Bayer type, and the dark values A1 to A16 are configured in a pattern shown in FIG. Here, as shown in FIG. 7, it is assumed that in a document portion 61 corresponding to the darkness value matrix 60 of the document, a diagonally shaded area 62 is a toner-attached area and the remaining area is a toner-non-adherent area. . The reflectance of this toner-attached area is R (0 to 1.0), and the reflectance of the toner-free area is 1.
0, and when the area ratio of the toner-attached portion in the darkness value matrix 60 is a, the reflection density of the document portion 61 is expressed by the fifth equation.

By the way, if the maximum density of a part of the original copy is 1.6, the original density is 0. In order to finish a copy density C, D (0 to 1.6) from an original with D (0 to 1.6), it is necessary to realize a copy density curve of γ = 1 as shown by line m1 in Fig. 8. . In order to obtain such a song 'aml' using the area modulation method, when copying the original, the area ratio curve m shown in Figure 8 must be used.
2, a copy image having the characteristics of the curve m1 can be obtained. Note that the curve m2 was obtained using the fifth equation. If the area ratio curve 2 is set in advance in this way and copying is performed based on this area ratio m2, the curve m1 can be obtained. Therefore, the levels of each of the darkness values A1 to A16 of the darkness value matrix 60A used in the first mode when copying a transmission type OHP sheet may be set in advance so as to correspond to the area ratio curve m2. Threshold A1
The threshold levels of ~A16 are shown in Table 1, for example.

Dark value matrix 60A having such dark value levels
is stored in advance in the memory 54a.

[Margin below]

Table 1 In such a dark value matrix 60A, for example, if the level of the pixel signal is 521, the dark value matrix 60
The threshold levels of A1 to A7 are larger than the pixel level, so in the dithered image, as shown in FIG. 9, pixels corresponding to dark values Al to A7 are black, and the remaining pixels A8 to A16 are black.
becomes white.

That is, the dithered image shown in FIG. 8 is set to the seventh density level in 16 gradations.

On the other hand, in order to create the darkness value matrix 60B for the reflective OHP stored in the memory 54b, it is necessary to obtain the area ratio curve m3 shown in FIG. This area ratio curve m3 indicates that the maximum density of copying is 1/2 of the normal maximum density, that is, 0.
．． 8 and the darkness value level of the darkness value matrix 60B is changed to obtain the copy density curve m1. This area ratio curve m3 is calculated using the fifth equation to calculate the development density (
The area ratio curve was determined so that the copy density (0 to 0.8) linearly corresponds to the density (0 to 1.6). still,
The calculation is performed assuming that the maximum density corresponding to the black part of the copy is 0.8. Based on such an area ratio curve m3, O
When the HP sheet is copied, r shown with reference mark m4 in FIG.
An ideal copy density curve of -4/2 is obtained.

Therefore, in order to obtain the area ratio curve m3, specifically, the dark value level of the threshold value matrix 60B stored in the memory 54b is set to the area ratio curve m3 different from the dark value level of the aforementioned dark value matrix 60A. This can be achieved by setting the darkness value level so that The values of each of the dark values A1 to A16 of the dark value matrix 60B obtained based on this idea are shown in Table 1, for example. In this way, the darkness value matrix 60B for reflective OHP
is formed and stored in the memory 54b.

FIG. 10 is a flowchart showing the copying process of the copying machine 1. In step s1, it is determined whether or not the OHP mode is set. That is, it is determined by the on/off state of the mode changeover switch 44. When the mode changeover switch 44 is in the OFF state, it is assumed that the mode is not 0HP mode, and the process moves to step s2, where the third mode for recording normal copy paper is set and the copying process is executed.

When the mode selector switch 44 is in the on state in step S1, it is assumed that the OHP mode is set, and the process moves from step s1 to step s3, where it is determined whether or not copying is for a reflective OHP sheet. That is, it is determined by the on/off state of the mode changeover switch 45. When the mode changeover switch 45 is in the OFF state, the copying mode for transparent OHP sheets, that is, the first mode is designated as the copying mode, and the process moves from step s3 to step S4. Then, in step s4, the developing bias voltage is set to be the same as the voltage in the normal state. , therefore, there is no change in the maximum copying density. Also, step S5
Then, select the area ratio curve m1. That is, CPU50
derives the selector 55 switching signal to select the dark value matrix 60A stored in the memory 54a. In this way, the first mode is set, and in step s6, the process of copying onto the P sheet is executed. That is, the CCD line sensor 5 reads the original image, and the analog/digital conversion circuit 50 converts it into a digital signal. ,
The data is stored in the buffer memory 51. In this way,
The buffer memory 51 stores one page of a document. Thereafter, each image density level signal corresponding to the dark value matrix is provided from the buffer memory 51 to the comparison circuit 53. In the comparator circuit 53, each threshold value of the IJ socks 0A is sequentially given as a reference signal via the selector 55, and this threshold value is compared with the density level signal from the buffer memory 51 to determine, for example, the image density level. If the density level is higher than the threshold value, it is stored in the image memory 56 as a logic "1" indicating black, and when the density level is smaller than the darkness value, it is stored in the image memory 56 as a logic "ArO" indicating white. In this way, the image data in the buffer memory 51 is compared with the dark value matrix 60A by the comparison circuit 53,
Image data for one page of the original image is stored in the image memory 56.

The image data in the image memory 56 is stored in the laser optical system 6
The semiconductor laser is thereby driven to emit light with an amount of light corresponding to the image signal. As a result, an electrostatic latent image is formed on the surface of the photoreceptor drum 11 as described above, and then the original image is copied onto the OHP sheet by a normal electrostatic copying process. In this first mode, the copy density of the OHP sheet corresponds to the case where the original image is copied based on the area ratio curve m3 by the dither method using the darkness value matrix 60A, and therefore the copy density curve m1 in FIG. An image along the line is formed on the OHP sheet. Therefore, it can be used for transmission type OHP.

Further, in step s3, the mode changeover switch 4
5 is turned on, it is determined that the second mode is the copying mode for reflective OHP sheets, and the process moves from step s3 to step s7. In step s7, the maximum copying density is set to approximately 1/2. That is, the developing bias voltage is made smaller than that in the first mode, and the toner adhesion amount is made smaller than that in the first mode.
The developing bias voltage is set to be approximately 1/2 that of the mode. Then, in step S8, the area ratio curve m3 is selected. That is, the CPU 52 outputs a switching signal to the selector 55 to select the dark value matrix 60B stored in the memory 54b.

As a result, the second mode is set, and the process of copying onto an OHP sheet is executed in step s6. The copying process in this second mode is basically the same as the copying process in the first mode described above. The only difference is that the darkness value matrix used is 60B, and the developing bias voltage is small and the maximum density is set to about 1/2. Therefore, the copying process is executed based on the area ratio curve m3. As a result, a copy image along the copy density curve m4 where r=1/2 can be created on the OHP sheet. Therefore, when this OHP sheet is used for a reflective OHP, it is possible to project a clear image on a screen that is almost the same as when a transmissive OHP sheet is used for a transmissive OHP.

[Second Embodiment] In the above-mentioned embodiment, when copying a reflective OHP sheet, the density curve m4 was obtained by the developing bias voltage and the dither method using the dark value matrix 60B. In other words, the maximum density is the same as the normal maximum density, and the reflection type O
It is also possible to copy an HP sheet and obtain an image along the density curve m4. In this case, a new dark value matrix 60C (see Table 1) is used in place of the dark value matrix 60B stored in the memory 54b. In creating this threshold matrix Nox 60C, the first
The area ratio curve m5 shown in FIG. 1 may be set. This area ratio curve m5 is an area ratio curve for obtaining the copy density curve m4 where γ=1/2 based on the fifth equation described above. The darkness value matrix 60C based on such an area ratio curve m5 may be set in advance. The threshold level of each threshold value A1 to A16 of this dark value matrix 60C is shown in Table 1, for example. Such a threshold value matrix 60C is used in the memory 54b instead of the dark value matrix 60B.
is stored in advance.

The copy control process in this second embodiment is shown in FIG. The copying process will be explained using FIG. 12. First, in step 1, it is determined whether the OHP mode is selected, and if not, the plain paper copying mode is set, and plain paper copying processing is executed.

When the OHP mode is selected in step 1, the process moves to step 3, where the normal maximum copy density is set. Then, in step 4, it is determined whether or not the OHP sheet is reflective, and if not, the process moves to step 5, where the area ratio curve m2 is selected. That is, a dither method using the dark value matrix 60A is used. Then, in step 6, copying processing to an OHP sheet is executed. Therefore, a copy image along the copy density curve m1 is formed on the OHP sheet.

If the transmission type is selected in step 4, the process moves from step 4 to step 7, and area ratio curve m5 is selected. That is, a dither method using the dark value matrix 60C is performed. Then, in step 6, copying processing to an OHP sheet is executed.

As a result, a copy image along the copy density curve m4 is formed on the OHP sheet.

In this way, it is possible to change both the maximum copying density and the γ value using only the dither method.

[Third Embodiment] As another embodiment, a reflective OHP sheet can be produced by changing the developing bias voltage without using the dither method. That is, this third embodiment is realized by changing the developing bias voltage and setting the maximum copying density to about 1/2.

The processing will be explained with reference to FIG. In step r1, it is determined whether or not the OHP mode is selected, and if not, the plain paper copying mode is set, and step r
In step 2, copying processing onto plain paper is executed.

When the OHP mode is determined in step r1, the process moves to step r3, where it is determined whether the OHP is a reflective type or not.
If it is not a reflective type, the maximum density for copying is set in step r4, which is the same as that for normal copying. That is, the developing bias voltage is set to the same value as in the normal case. Then, in step r5, copying processing onto an OHP sheet is executed. As a result, the OHP sheet has a copy density curve of ml.
A copy image along the lines is formed.

If the OHP is a reflective type in step r3, the process moves to step r6, and the maximum copying density is about 1/1 that of the normal case.
Set to 2. That is, the developing bias voltage is reduced and set to such a value that the maximum density is approximately 1/2 of the normal density. Then, in step r5, copying processing onto an OHP sheet is executed. By this, OHP
An original image with a lower density is formed on the sheet than in the case of a transmission type, and it can be used for a reflection type OHP.

In addition, in this control method of changing only the developing bias voltage, the copy density curve becomes that shown by reference numeral m6 in FIG. 8 mentioned above. As is clear from this figure 8,
A slight deviation occurs from the ideal copy density curve m4 for γ-1/2.

Therefore, in the method of changing only the developing bias voltage,
This method has a drawback that the image characteristics on the screen are slightly degraded compared to the first and second embodiments described above. However, compared to the conventional case of using a transmissive OHP sheet for a reflective OHP, sufficient clarity can be obtained.

Although the copying machine 1 was a color copying machine in the above-mentioned embodiment, it may be a monochrome copying machine, and the present invention can also be applied especially when creating an image with black and white gradation on an OHP sheet. It can be suitably implemented.

In the above explanation, the image data is stored in the buffer memo U51.
Although we have shown an example in which dither processing is performed using a buffer memory, it can also be applied to a method of referencing a table using a dither pattern without a buffer memory.

As described above, according to the present invention, the copy density and γ are automatically adjusted.
It becomes possible to copy copied images with different characteristics onto an OHP sheet. Thus, for example, at the normal maximum concentration and γ
= 1, and copying with the copy density set to about 1/2 of normal and γ set to about 1/2.
Copies can be made in two ways: for P and for reflective OHP.

Therefore, it is possible to solve the problem that in the conventional copying machine, when the produced OHP sheet is used for a reflective OHP, the image on the screen is dark and unclear.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the present invention in detail, FIG. 2 is a diagram showing a simplified internal structure of a full-color copying machine l according to an embodiment of the present invention, and FIG. 3 is a diagram for explaining the developing units 16, 17, . 4 is a front view of the operation section of the color copying machine 1, FIG. 5 is a block diagram showing the electrical configuration related to the image forming process of the copying machine 1, and FIG. 6 is a diagram showing the dark value matrix. A diagram showing the pattern, FIG. 7 is a diagram showing the manuscript portion 61, and FIG. 8 is a diagram showing the first
A graph showing the copy density curve of the embodiment, FIG. 9 is a diagram showing the display mode of the dithered image, FIG. 10 is a flowchart showing the copying process of the first embodiment, and FIG. 11 is a copy density curve of the second embodiment. 12 is a flowchart showing the copying process of the second embodiment, FIG. 13 is a flowchart showing the copying process of the third embodiment, FIG. 14 is a diagram showing the configuration of the transparent OHP, and FIG. 15 16 is a diagram showing the structure of a reflective OHP, and FIG. 16 is a diagram showing blue spectral ratios of a transmission OHP and a reflective OHP. 1... Copying machine, 5... CCD line sensor, 11.
... Photosensitive drum, 16, 17, 18. ...Developer,
34... Recording material, 44.45... Mode changeover switch, 50... Analog/digital conversion circuit, 52...
...CPU, 53-...Comparison circuit, 54a, 54b...
・Memory, 55...Selector, 56...Image memory, 60A, 60B, 60C...Darkness value matrix, V
B...Development bias voltage. Patent applicant: Minolta Camera Co., Ltd. Figure 1 D C,O- Figure 2 Figure 3 O ＼ Figure 4 Figure 5 Figure 6/60 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 0, D-→ Fig. 12 Fig. 13 Fig. 14

Claims (1)

[Claims] (1) Plain paper and OHP sheets can be used as transfer materials, and a laser light source is driven in response to an image signal to form an electrostatic latent image on the surface of a photoconductor, and a development bias power source is applied. A toner image corresponding to the electrostatic latent image is formed by depositing toner in an amount corresponding to the developing bias using a device,
A copying apparatus configured to transfer the toner image onto a transfer material includes a mode specifying means for selectively specifying a first mode and a second mode in which the copying density of the OHP sheet is different when an OHP sheet is used; a signal processing means for changing the density information to be displayed; and a signal processing means for changing the maximum density level in the first mode by controlling the voltage value of the developing bias power supply and/or the signal processing means based on the mode designation of the mode designation means. In addition, the gamma value γ of the copy density curve with respect to the original density is set to γ=1, and in the second mode, the copy density is controlled so that at least one of the maximum density level and the gamma value γ is different from the first mode. A copying apparatus characterized by comprising: a means for doing so;