Patent Application
20200233357
The present invention relates to an image forming apparatus which includes a plurality of image forming modes.
Color gamut is an image quality index used with an image forming apparatus. The color gamut of an image forming apparatus is a color reproduction range which the image forming apparatus can output, and as the color gamut widens, the color reproduction range widens, which means that the image forming apparatus has advanced features. A possible method of expanding the color gamut is adding thick developers of four-colors (YMCK) to the regular developers of YMCK, or increasing the amount of developer on the recording material. Japanese Patent Application Publication No. 2013-137577 discloses an image forming apparatus for performing quality printing in various print modes.
A configuration having other image forming modes to reduce process speed, besides the standard image forming mode, has been proposed. “Other image forming modes” include a thick paper mode. For such a configuration having a plurality of image forming modes, it is proposed to calculate the density in other image forming modes by arithmetic processing from the measured density information in the standard image forming mode. Thereby tinge can be adjusted in the other image forming modes without any additional downtime.
However, the above mentioned configuration having a plurality of image forming modes has the following problems. Specifically input image data, which is measured as density 0 in the standard image forming mode, may be input image data which is measured as density 0 as well in another image forming mode, or may be input image data which is detected as a density that is not 0. Therefore, in the case of the configuration having a plurality of image forming modes, as mentioned above, the density of the low density portion is calculated by extrapolation based on the result of calculating the density of the high density portion.
Herein an image data I1, in which the measured density becomes a value close to a boundary 700, which is now assumed to a boundary of a certain density range, will be considered. It is assumed that when the image data value is I1, an actually measured value of the density of an image formed in a normal print mode is D1. This is plotted as the measurement result 701a. Then based on this measurement result 701a, a calculation point 701b, which is the result of calculating the density in the wide color gamut print mode, is calculated.
In the same manner, it is assumed that when the image data value is I2, an actually measured value of the density of the formed image is D2, and the actual measurement result 702a is plotted. Then based on this actual measurement result 702a, a calculation point 702b is calculated.
Then based on the calculation points 701b and 702b, an approximation line 703a is calculated. Using this approximation line 703a and the value of the image data corresponding to a low density portion LD, the calculation points 704, 705, 706 and 707 are calculated.
However, when the calculation points 701b and 702b are determined from the measurement results 701a and 702a, an error in a range indicated by the upward and downward arrows from each measurement result is included. Because of this error, the approximation line can change in the 703b to 703c range. As a result of this change of the approximation line, each of the calculation points 704, 705, 706 and 707 may include an error in the range indicated by the upward and downward arrows from each calculation result. This error is larger compared with an error that is generated when a density is generated in a wide color gamut print mode based on the density measurement result of the low density portion LD in the normal print mode. This error further increases as the image data becomes smaller, and the image data departs more from the calculation point 701b of calculating the approximation line.
An image forming apparatus that operates in a first mode in which an image is formed in a first color gamut, and a second mode in which an image is formed in a second color gamut which is different from the first color gamut, comprising:
a photosensitive drum;
an exposure unit that forms an electrostatic latent image by exposing the photosensitive drum;
a developing roller that forms a toner image by developing the electrostatic latent image which is formed using a toner on the photosensitive drum by the exposure unit;
an intermediate transfer member to which the toner image formed on the photosensitive drum by the developing roller is transferred;
a density detection unit that detects the density of the toner image transferred to the intermediate transfer member; and
a controller that adjusts the density of the toner image on the basis of a value of input image data which is inputted, wherein a dithering process performed when the controller controls the exposure unit is different depending on whether the image forming apparatus is operating in the first mode or the second mode, and in at least a part of the input image data in which the density of an image to be formed is on a low density region side, the density of the toner image which is formed by the dithering process in the first mode is higher than the density of the toner image which is formed by the dithering process in the second mode.
Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings).
Preferred embodiments of the present invention will now be described with reference to the drawings. Dimensions, materials, shapes, relative positions and the like of the components described below may be appropriately changed depending on the configuration and various conditions of the apparatus to which the invention is applied. Therefore, the following description is not intended to limit the scope of the present invention.
General Configuration of Image Forming Apparatus
The image forming apparatus 200 includes image forming stations SY, SM, SC and SK corresponding to each color. For example,
The process cartridge 204 includes a photosensitive drum 301 (image bearing member). The photosensitive drum 301 is rotary-driven by a driving unit (not illustrated) in the arrow B direction indicated in
A toner replenishing roller 306 rotates in the arrow D direction indicated in
By the intermediate transfer belt 205 rotating in the arrow A direction indicated in
While the recording material 203 is held by and transported between the secondary transfer roller 211 and the secondary transfer counter roller 212, voltage having reverse polarity to toner is applied to the secondary transfer roller 211 from a power supply device (not illustrated). Since the secondary transfer counter roller 212 is grounded, a transfer electric field is formed between the secondary transfer roller 211 and the secondary transfer counter roller 212. By this transfer electric field, the toner image is transferred from the intermediate transfer belt 205 to the recording material 203. After passing through the nip between the secondary transfer roller 211 and the secondary transfer counter roller 212, the recording material 203 is heated and pressed by a fixing apparatus 213. Thereby the toner image on the recording material 203 is fixed to the recording material 203. Then the recording material 203 is transported to a paper delivery tray 215 via a paper outlet 214, and the image forming process completes. The toner remaining on the intermediate transfer belt 205, which was not transferred by the secondary transfer unit, is removed from the intermediate transfer belt 205 by a cleaning member 216, whereby the intermediate transfer belt 205 is refreshed to a state where image forming can be performed again.
Control Block Diagram
Configuration of Photosensitive Drum Layer
In this configuration, in addition to the normal print mode as the first mode, a wide color gamut print mode is included as the second print mode. The wide color gamut print mode is a print mode to widen the color gamut of the normal print mode. This is implemented by increasing the toner amount on the photosensitive drum 301 compared with that in the normal print mode. In order to increase the toner amount on the photosensitive drum 301, the peripheral velocity ratio of the developing roller 303, with respect to the photosensitive drum 301, and potential setting are optimized in this embodiment.
Difference of Peripheral Velocity of Developing Roller and Toner Supply Amount
The relationship between the peripheral velocity ratio and the toner amount on the photosensitive drum 301 will be described with reference to
Here it is assumed that the peripheral velocity of the developing roller 303 is Van, the peripheral velocity of the photosensitive drum 301 is Vbn, the length of the surface of the developing roller 303 developed per unit time is Lan, and the length of the surface of the photosensitive drum 301 developed per unit time is Lbn. These parameters have a relationship given by Expression (1).
Va
n
/Vb
n
=La
n
/Lb
n (1)
In the wide color gamut print mode as well, just like the normal print mode, it is assumed that the peripheral velocity of the developing roller 303 is Vaw, the peripheral velocity of the photosensitive drum 301 is Vbw, the length of the surface of the developing roller 303 developed per unit time is Law, and the length of the surface of the photosensitive drum 301 developed per unit time is Lbw, as illustrated in
Va
w
/Vb
w
=La
w
/Lb
w (2)
Van/Vbn and Vaw/Vbw are called “peripheral velocity ratios”. In this embodiment, it is assumed that the peripheral velocity ratio in the normal print mode is Van/Vbn=1.4, and the peripheral velocity ratio in the wide color gamut print mode is Vaw/Vbw=2.2. In the case of Lbn=Lbw, Law/Lan=2.2/1.4 is established. This means that if the development efficiency from the developing roller 303 to the photosensitive drum 301 is 100%, the peripheral velocity ratio indicates the ratio of the toner amount on the surface of the photosensitive drum 301. Setting the peripheral velocity of the developing roller 303 to Van or Vaw, the peripheral velocity of the photosensitive drum 301 to Vbn or Vbw, as described above, can be implemented by the CPU 2021 instructing operation to the drive unit 2026.
Surface Potential of Photosensitive Drum
To make the development efficiency 100% in both the normal print mode and the wide color gamut print mode, the potential is set as indicated in
Therefore, in the normal print mode according to the configuration of this embodiment, Vdn=−500V, Vdcn=−350V and V1n=−100V are used. Further, in the wide color gamut print mode, Vdw=−850V, Vdcw=−600V and V1w=−120V are used. Here the charging bias Vd, the developing potential Vdc and the exposure potential V1 are denoted as Vdn, Vdcn and V1n in the normal print mode, and are denoted as Vdw, Vdc and V1w in the wide color gamut print mode. Each potential in each print mode is set to a value that is sufficient to develop the toner coated on the surface of the developing roller 303.
The above mentioned Vdn=−500V, Vdcn=−350V, Vdw=−850V and Vdcw=−600V are implemented by the CPU 2021 controlling and instructing the high voltage power supply (not illustrated) connected to the charging roller 302 and the developing roller 303. Here the high voltage power supply 2025 described above is assumed to be a generic term of the high voltage power supply connected to each member. The high voltage power supply to each member may not be an independent power supply, but may be a common high voltage power supply which outputs various high voltages by resistive voltage division.
Density Detection
In the electrophotographic type image forming apparatus, the tinge of the printed matter changes depending on various conditions, such as the durability of the cartridge and the operating environment. Therefore, it is necessary to measure the density at an appropriate timing and to feedback the measurement results to the control mechanism of the main unit.
Controller Processing Flow
Now how tinge information (value determined by converting the density value into the chromaticity difference) acquired by the density detection sensor 218 is used for correction will be described.
γ Characteristic Based on Dithering of Comparative Example
The third quadrant indicates a state of converting the input image data to the γ correction unit 225 into actual input image data using a look-up table (LUT). The “actual input image data” refers to the input image data after the conversion using the look-up table, which is data to be inputted to the function block (half toning unit 226) that comes after the γ correction unit 225.
The input image data before the conversion increases in the left direction of the abscissa, and has an 8-bit (256 gradation) resolution in this embodiment. Actual input image data after the conversion, on the other hand, increases in the downward direction of the ordinate. A table that indicates the relationship of this input data is called a “look-up table”, and the γ correction unit 225 performs the γ correction by changing this look-up table.
In the look-up table 501 which is not γ-corrected, the value of the input image data and the value of the actual input image data change in the same manner, that is, in a linear relationship. In terms of the accuracy of the γ correction, it is preferable that the actual input image data has a higher resolution compared with the input image data, and in the case of the configuration of this embodiment, the actual input image data has a 10-bit (1024 gradation) resolution. The look-up table 511 after the γ correction (γ-corrected look-up table 511) is the look-up table that is finally acquired in the comparative example.
The fourth quadrant indicates the relationship between an exposure condition (i.e. the laser irradiation rate) converted as the result of performing the dithering with respect to the actual input image data when exposure is performed. This relationship indicated in the fourth quadrant is called “dithering” in this embodiment. The laser irradiation rate indicates an area ratio (ratio) of an area irradiated by laser with respect to the unit area, which increases in the right direction of the abscissa. For example, when the laser irradiation rate is 50%, half of the unit area is exposed by the laser. In concrete terms, when the laser is irradiated, the light quantity is not changed, but the irradiation area is changed by the PWM modulation. In
The first quadrant indicates the relationship between the laser irradiation rate and ΔE, and this relationship is called the “engine γ characteristic” in this embodiment.
A value in the upward direction of the ordinate indicates a chromaticity difference (ΔE) between a portion on which toner is exists and a portion on which toner does not exist, and increases in the upward direction of the ordinate. In this embodiment, ΔE is the correction target of the γ correction unit 225. The target, however, is not limited to the chromaticity difference (ΔE), and may be the density or the like instead of ΔE. For example, the chromaticity difference may be the difference between the detected and converted chromaticity and the chromaticity of the white portion of a specific type of paper. The chromaticity of the white portion may be changed when necessary.
The engine γ characteristic, which indicates the correspondence of the laser irradiation rate (exposure condition) and the density indicated in the first quadrant changes depending on the image forming mode, the time dependent conditions (e.g. use state of cartridge, use state of main unit), and the environmental conditions (e.g. use amount of toner, installation environment of main unit). Therefore, while continuously operating the image forming apparatus, it is necessary to measure ΔE and perform γ correction using the γ correction unit 225 when necessary. In this case, the engine stops print operation, enters the calibration mode, and performs calibration sequence operation.
In the calibration sequence, an image is formed using the look-up table 501 without γ correction. In the normal print mode, the density is detected by the density detection sensor 218, and calculates the result ΔE. Furthermore, using this ΔE, the density detection sensor 218 also calculates ΔE in the wide color gamut print mode. Therefore, an error, which is generated when the ΔE in the wide color gamut print mode from the ΔE in the normal print mode, is expressed as an error of the engine γ characteristic. Using the acquired γ characteristics, the γ correction unit 225 corrects the look-up table. Thereby the γ correction completes.
The relationship between the input image data and ΔE acquired above is called the “input/output γ characteristic”, and is expressed in the second quadrant.
The flow of γ correction will be described using a concrete example. It is assumed that an image based on the input data image of which value is 40h is formed. This input image data is written in the graph as a number “1” that is enclosed within a circle. Hereafter, the number “1” that is enclosed within a circle in the graph is expressed as “sign (1)” in this description. This is the same for subsequent circled numbers. According to the look-up table 501 before γ correction, the actual input image data is 255 (sign (2)). The input image data 255 is converted into the laser irradiation rate by the dithering 502, and the result is 25% (sign (3)).
Further, it is assumed that the measurement result of the density detection sensor 218 is ΔE=5 (sign (4)). The intersection between the sign (3) and the sign (4) indicates the engine γ characteristic when the value of the input image data is 40h (sign (5)). For other input image data as well, the conversion into the laser irradiation rate and the measurement of ΔE are performed, then the engine γ characteristic 503 in the normal print mode is acquired.
Based on the measurement result ΔE=5 when the input image data is 40h, the point 504 is acquired (sign (6)). By performing the plotting in the same manner for the relationship between the other input image data and ΔE, the input/output γ characteristic 505 in the normal print mode is acquired.
Here it is assumed that the relationship in which ΔE changes linearly in accordance with the value of the input image data is an ideal input/output γ characteristic 506 in the normal print mode. Then the ideal input/output γ characteristic 506 in the normal print mode and the input/output γ characteristic 505 in the (actual) normal print mode have different profiles, which means that γ correction is required. The ideal input/output γ characteristic 506 in the normal print mode here is the case of the normal print mode, and in the case of the wide color gamut print mode, an image having a larger ΔE than the case of the normal print mode is formed based on the same input image data as the normal print mode, hence the ideal input/output γ characteristic 514 is the target in the wide color gamut print mode.
In the ideal input/output γ characteristic 506 in the normal print mode, the input image data that implements ΔE=5 is 10h (point 507). In order to establish this relationship, the actual input image data should be 255 when the input image data is 10h, since the laser irradiation rate when ΔE=5 is 25% according to the engine γ characteristic 503 in the normal print mode, and the actual input image data to implement the laser irradiation rate 25% is 255. As a result, the point 508 is derived. By performing the plotting in the same manner for the other input image data, the γ-corrected look-up table 511 is derived.
The γ-corrected look-up table 511 can also be derived as follows. According to the point 509 of the ideal input/output γ characteristic 506 in the normal mode, ΔE should be ΔE=21 if the input image is 40h. This means that the laser irradiation rate must be 41% based on the engine γ characteristic 503. By plotting the relationship between the actual input image data and this input image data based on the dithering 502, the point 510 is derived.
However if the γ-corrected look-up table 511 acquired like this is used for the engine γ characteristic 512 in the wide color gamut print mode and an image is formed, the input/output γ characteristic becomes the actual wide color gamut γ characteristic (input/output γ characteristic) 513 instead of the ideal wide color gamut γ characteristic (input/output γ characteristic) 514.
For example, it is assumed that the value of the input image data is 40h (sign (1)). First a point 510 is determined by the γ-corrected look-up table 511. Then this data is converted into the laser irradiation data using the dithering 502 (sign (7)). Then ΔE is determined based on the engine γ characteristic 512 in the wide color gamut print mode (sign (8)). Then ΔE (sign (8)) in the wide color gamut mode and the value 40h of the input image data are plotted (sign (9)). By performing this plotting for the other input image data, the input/output γ characteristic 513 in the actual wide color gamut print mode is acquired.
Here as indicated in the first quadrant, the normal engine γ characteristic 503 and the engine γ characteristic 512 in the wide color gamut print mode are different. This difference is generated due to the difference in the latent image formation and the number of toner layers, for example. In other words, in the case of the wide color gamut print mode, a number of toner layers is larger and the light quantity by the scanner unit 207 is higher, which makes the latent image slightly larger, compared with the normal print mode, hence ΔE becomes larger than the case of the normal print mode when compared with the same laser irradiation rate.
As a consequence, a γ-corrected look-up table for the wide color gamut is required separately from the γ-corrected look-up table 511 for the normal print mode. For this, it is necessary to acquire the engine γ characteristic 512 in the wide color gamut when necessary. However, if the image formation and density detection are performed, and the engine γ characteristic is acquired in the same manner as in the normal print mode to create the look-up table in the wide color gamut print mode, the downtime of the image forming apparatus is prolonged. Therefore, in this embodiment, ΔE in the wide color gamut print mode is calculated from ΔE in the normal print mode in order to decrease the downtime.
Chromaticity Calculation Table in Wide Color Gamut Print Mode
In the case where the drum lifetime and the developing device lifetime are not included in
The sub-table 521 and the sub-table 522 to interpolate the drum lifetime 90% are selected. Further, the small tables 521a and 521b (used when the drum lifetime is 100%) and the small tables 522a and 522b (used when the drum life is 80%) to interpolate the developing device lifetime 90% are selected.
The small table 521c and 522c for the developing device lifetime 90% are derived by performing linear interpolation based on the developing device lifetime.
The sub-table 523 for the drum lifetime 90% and the developing device lifetime 90% is derived by performing linear interpolation based on the drum lifetime.
The values indicated in the sub-table 523 are ΔE (LGT)-ΔE (Normal). Therefore, by adding ΔE (Normal) to a value indicated in the table, ΔE (LGT) is calculated/converted. Thereby ΔE (LGT) is calculated, but the table may be sub-divided so that the factors that change the tinge (e.g. installation environment of main unit) are included. If the required value of ΔE (Normal) is not in the sub-table 523, the liner interpolation may be further performed.
Here the state of each composing element of the image forming apparatus is determined as a component lifetime. This component lifetime can be regarded as a degree of component use. The degree of component use can be acquired by the controller 201 measuring the operation time of each component or a number of rotations (in the case of a drum and roller), and comparing the result with an assumed operation time or an assumed number of rotations, for example. The table in accordance with the operation time or the number of rotations, instead of the component lifetime, may be created. Further, to determine ΔE (LGT), a mathematical expression that indicates the relationship between ΔE (LGT) and ΔE (Normal) may be created and used, instead of the above mentioned predetermined table.
To create the table in
γ Characteristic Based on Dithering of Embodiment 1
First it is assumed that an image is formed when the input image data is 40h in the normal print mode (sign (1)). According to the look-up table 501 without γ correction, the actual input image data is 255 (sign (2)). Then the actual input image data is converted into the laser irradiation rate by the dithering 525 in the normal print mode (sign (3)). Then based on ΔE measured by the density detection sensor, the engine γ characteristic 503 in the normal print mode is acquired (sign (4)). Thereby ΔE, when the input image data is 40h in the normal print mode, can be plotted in the second quadrant (sign (5)). By performing this plotting for the other input image data values as well, the input/output γ characteristic 526 in the normal print mode is acquired. The engine γ characteristic 503 in the normal print mode and the engine γ characteristic 512 in the wide color gamut print mode are the same in
In the wide color gamut print mode, ΔE is calculated using the table in
The engine γ characteristic depends on the state of use, but the dithering 525 in the normal print mode is determined so that the liner input/output γ characteristic, with respect to the input image data, can be acquired to an extent even if this change occurs. As a result, the input/output γ characteristic 526 in the normal print mode has high linearity, which is relatively close to the ideal input/output γ characteristic 506 in the normal print mode indicated in
In a region in which the input image data is small, the dithering 527 in the wide color gamut print mode must be set so that ΔE (LGT)<ΔE (Normal) is established. In this embodiment, for example, the dithering 527 is set so that ΔE (LGT)<ΔE (Normal) is always established when the value of the input image data is 40h or less. In concrete terms, in the case of 40h, ΔE (Normal) 771, which is determined using the dithering 525 and the engine γ characteristic 503 in the normal print mode, is larger than ΔE (LGT) 772, which was determined using the dithering 527 and the engine γ characteristic 512 in the wide color gamut print mode.
533 is a look-up table which was corrected so that the ideal input/output γ characteristic 514 in the wide color gamut print mode is implemented. The broken line 534 indicates each value in the case where the corrected look-up table 533 is used when the input image data is 40h. In other words, in the case of the wide color gamut print mode, the actual input image data is determined using the corrected look-up table 533 (sign A), the laser irradiation rate is determined using the dithering 527 in the wide color gamut print mode (sign B), ΔE is determined based on the engine γ characteristic 512 in the wide color gamut print mode (sign C), and the ideal input/output γ characteristic 514 in the wide color gamut print mode is determined based on the input image data 40h and the plot of ΔE (sign D).
To create the corrected look-up table 533, ΔE of the image formed in the wide color gamut print mode may actually be measured, but ΔE in the wide color gamut print mode may be calculated from the measurement result in the normal print mode using the method in
The input/output γ characteristic 514 in the wide color gamut print mode, the look-up table 501 without γ correction, the dither pattern for the dithering 527 in the wide color gamut print mode, and the dither pattern for the dithering 525 in the normal print mode are assumed to be stored in the memory 2022 in advance. As the dither pattern, a well-known pattern may be used as appropriate, hence detailed description here is omitted. The other characteristic curves change depending on the detection values of the density detection sensor 218 at each detection, and the changed characteristic curves are stored in the memory 2022 until the next density measurement.
Optimum Dithering in Accordance with Engine γ Characteristic
A reason why it is preferable to adjust the dithering in accordance with the engine γ characteristic will be described with reference to
If the actual image data next to RI1 is RI2, the graduation between ΔE1 and ΔE3 cannot be expressed using the second dithering 530. With the first dithering 529, on the other hand, ΔE2, which is an image between ΔE1 and ΔE3, can be formed. In other words, compared with the first dithering 529, the change in ΔE with respect to the actual input image data is large and gradation of the image is inferior if the second dithering 530 is used.
Now it is assumed that an image is formed using a specific dithering in accordance with a second engine γ characteristic 532, which is different from the first engine γ characteristic 531. The second engine γ characteristic 532 will be considered in the same manner as the above-mentioned first engine γ characteristic 531. If the first dithering 529 is used, the chromaticity difference becomes ΔE1 when the actual input image data is RI1, and becomes ΔE3 when the actual input image data is RI2. If the second dithering 530 is used, the chromaticity difference becomes ΔE1 when the actual input image data is RI1, and becomes ΔE4 when the actual input image data is RI2.
Summarizing the above description on the dithering, the degree of change of the laser irradiation rate with respect to the input image data in the case of performing the first dithering 529 is smaller than the degree of change of the laser irradiation rate with respect to the input image data in the case of performing the second dithering 530. In other words, when the ordinate and the abscissa are set as
Summarizing the above description on the engine γ characteristic, the gradation of the image is better when the first engine γ characteristic 531 is used, compared with using the second engine γ characteristic 532 if the same pair of input image data is inputted. This is because the degree of change of ΔE, with respect to the laser irradiation rate when the first engine γ characteristic 531 is used, is smaller than the degree of change of ΔE, with respect to the laser irradiation rate when the second engine γ characteristic 532 is used. In other words, when the ordinate and the abscissa are set as
In order to compensate for the deterioration of the engine γ gradation because of the sharpness of the inclination of the engine γ characteristic (degree of change of ΔE with respect to the laser irradiation rate is large), the gradation is improved by making the inclination of the dithering sharper (decreasing the degree of change of laser irradiation rate with respect to the input image data). On the other hand, in a region where the inclination of the engine γ characteristic is moderate and the engine γ gradation of the image is relatively good, gradation of density can be maintained in general, even if the gradation deteriorates by making the inclination of the dithering moderate.
As a consequence, it is preferable that the inclination of the dithering is sharp in the image data region, which indicates the engine γ characteristic with which gradation of the image deteriorates, and the inclination of the dithering is moderate in the image data region which indicates the engine γ characteristic with which gradation of the image is good. Thereby the gradation of the image can be maintained with good balance with respect to all the image data.
The above is the reason why adjusting the dithering in accordance with the engine γ characteristic is desirable. In this embodiment, the dithering 525 for the normal print mode is used in the normal print mode, and the dithering 527 for the wide color gamut print mode is performed in the wide color gamut print mode. The engine γ characteristic, which changes depending on the state, should be designed considering overall balance.
γ Characteristic in Low Density Region in Accordance with Difference of Image Forming Mode
In this embodiment, the low density region is defined as a region in the 00h to 20h range. In some cases the low density region, where output is not stable, does not strictly depend on the input image data. In other words, ΔE (Normal)=0 may continue for a while even if the input image data is increased, or may change to ΔE (Normal)≠0 relatively quickly. In the case of the configuration of this embodiment, ΔE (Normal)≠0 occurred stably if the density region is at least 20h, hence the low density region is defined as s region in the 00h to 20h range. The value 20h is a predetermined upper limit value of the low density region, but this upper limit value changes depending on the dithering or the like, and is not always uniquely determined, that is, the upper limit value must be changed in accordance with the engine γ characteristic, dithering and the like. When the input image data is divided into a side of the low density region and a side of the high density region, the “input image data corresponding to the low density region” refers to the input image data on the side where a minimum value is included, or to the input image data on the side including the density of the image to be formed that is small, to be detected by the density detection sensor 218.
An example of the method of determining the input image data corresponding to the low density region will be described. First the input image data is set to a minimum value (00h in this example), then while gradually increasing the value, density detection is repeated using the density detection sensor 218. Thereby an appropriate “predetermined upper limit value” is determined.
Influence of Chromaticity Error in Comparative Example
The effect of this embodiment will be described next with reference to
The normal print mode of the comparative example will be described first. The input image data I3 and I4 are converted into I3′ and I4′ using the look-up table 501 without γ correction, and are converted into the laser irradiation rates R3 and R4 by the dithering 525. ΔE3′ and ΔE4 are acquired by forming an image in the state of the engine γ characteristic 503 in the normal print mode, and sensing the density by the density detection sensor 218. The result is plotted in the second quadrant, and the input/output γ characteristics P3′ and P4′ in the normal print mode are acquired. Further, other input image data are plotted, and the input/output γ characteristic 526 in the normal print mode are acquired.
A correction method from the normal print mode to the wide color gamut print mode according to the comparative example will be described next. As described above, from the measured chromaticity differences ΔE3′ and ΔE4′ in the normal print mode, ΔE3 and ΔE4 in the color gamut print mode are calculated. Here a calculation error is generated. For example, in the case of the input image data I3 or I4 of which values are relatively large, the calculation error is relatively small, as indicated in P3 and P4. However, if a value in the low density region (region near I1 and I2, where the value of the input image data is relatively small) is determined by extrapolation, the influence of this calculation error increases, and a large calculation error, such as P1 or P2, is generated. Because of this calculation error, the inclination of the extrapolated line can change in the range between the extrapolated line 535 and the extrapolated line 536. The errors in the input image data I1 and I2 are determined by the extrapolated line 535 and the extrapolated line 536, and become E1 and E2 expressed by the arrow length in
In other words, the upper limit value and the lower limit value of each error range of P1 to P4 are compared with the ideal wide color gamut input/output γ characteristic (input/output γ characteristic) 514. For example, it is assumed that the upper limit value of ΔE4, when the input image data is I4, is ΔE4 (max), and the lower limit value thereof is ΔE4 (min). This corresponds to the values of the upward and downward arrows of P4 in the second quadrant in
In the case where the error is the lower limit value, if the look-up table 501 without γ correction is used, the chromaticity difference becomes ΔE4 (min) when the input image data is I4. Therefore, in order to make the chromaticity difference when the input image data is I4 become a point on the ideal input/output γ characteristic 514 in the wide color gamut print mode (sign (1)), the conversion into actual input image data is performed using the point on the look-up table 538 (sign (3)).
The region between the look-up table 537 and the look-up table 538 determined like this is an error of the look-up table. For example, in the case of the input image data I1, the range of the input image data is ΔI1 indicated by the arrow in
Influence of Chromaticity Error in Embodiment 1
An error of ΔE (LGT) after γ correction in the case of using the dithering and the calculation method according to this embodiment will be described next with reference to
At this time, for the dithering, the dithering 527 for the wide color gamut print mode, which is determined in accordance with the engine γ characteristic 512 in the wide color gamut print mode, is used. As a result, an error at each point is smaller than the comparative example, and is approximately constant, which is between the first input/output γ characteristic 540 and the second input/output γ characteristic 541. Hereafter an error of the look-up table and the output error generated thereby are calculated in the same manner as the case of
Flow Chart of γ Correction by Image Forming Apparatus
The processing related to the γ correction by the image forming apparatus 200 will be described with reference to the flow chart in
In they correction for the normal print mode which is started from S1901, the image forming apparatus 200 uses the look-up table 501 and forms patches. The γ correction is not performed to the look-up table 501. Alternatively, the image forming apparatus 200 may use the corrected look-up table when the image forming apparatus 200 forms toner patches on the intermediate transfer belt 205 for the γ correction for the normal print mode.
Then in S1902, the density detection sensor 218 detects the density of each patch formed on the intermediate transfer belt 205. As described with reference to
In S1903, the measured value of the reflected light is acquired by the CPU 2021. The density value acquired by the CPU 2021 may be a value determined by subtracting a diffused reflection detection output 402 from a normal reflection detection output 401, or a value determined by further converting this value into a density value. A density value determined by eliminating the influence of the base of the intermediate transfer belt 205 on which the patches are formed may be used.
Then in S1904, the CPU 2021 inputs the density value of each gradation, computed in S1903, to a first conversion table which is stored in the memory 2022 in advance, and acquires the converted value (ΔE (Normal)) of the density value of each gradation. The conversion table is provided for each color, and the output value from the first conversion table is ΔE (Normal) for each color.
In S1905, the CPU 2021 inputs ΔE (Normal) for each color and for each gradation acquired in S1904, to a second conversion table, which is also stored in the memory 2022 for each color in advance, and acquires the output value ΔE (wide color gamut) from the second conversion table described in
Also, the CPU 2021 obtains the actual image data 255 (sign (2)) by using the look-up table 501 to which they correction is not performed when the input image data 40h (sign (1)) is input in the wide color gamut print mode. Next, the CPU 2021 performs the dithering process 527 for the wide color gamut print mode which exchange the actual input data to the laser irradiation rate (sign (6)). The ΔE is determined by the γ characteristic 512 in the wide color gamut print mode (sign (7)). Thereby ΔE, when the input image data is 40h in the wide color gamut print mode, can be plotted in the second quadrant (sign (8)). Also, the CPU 2021 calculates each ΔE (sign (5)) for the each gradation value such as 20h for the normal print mode and each ΔE (sign (8)) for the each gradation value for the wide color gamut print mode. Then, the CPU 2021 generates the second conversion table based on the relationship between (i) the ΔE for the normal print mode and (ii) the ΔE for the wide color gamut print mode.
The ΔE indicated by (sign (8)) calculated for the wide color gamut print mode correspond to the ΔE (wide color gamut) converted in S1905. Here, in at least a part of the input image data in which the density of an image to be formed is on a low density region side, the ΔE indicated by (sign (8)) is smaller than the ΔE indicated by (sign (5)).
Finally in S1906, the CPU 2021 corrects the look-up table 533 based on ΔE (wide color gamut) for each color and for each gradation acquired in S1905, stores the corrected look-up table 533 in the memory 2022, and uses the corrected look-up table 553 for the subsequent execution in the wide color gamut print mode. The computing of the look-up table 533 by the CPU 2021 is as described above, mainly with reference to
Also, as long as the ΔE in the wide color gamut print mode calculated for the input data on a low density region side is smaller than the ΔE in the normal print mode calculated for the same input data (the same value), any combinations of (i) the dither pattern for the wide color gamut mode and (ii) the look-up table to which the γ correction is performed may be applied for the wide color gamut mode.
As described above, according to the image forming apparatus of this embodiment, when the tinge in the image forming mode, to implement another color gamut, is calculated from a tinge in the standard image forming mode, errors do not increase even in the calculation of the tinge in the low density region. In the configuration of this embodiment, the control target is the chromaticity difference from the non-image forming portion, but the control target is not limited to the chromaticity difference, and may be density, for example. Further, in the configuration of this embodiment, the peripheral velocity ratio of the developing roller 303 is used to implement the wide color gamut print mode, but this is not limited to the peripheral velocity ratio, and may be another parameter to control the toner supply amount.
A difference of Embodiment 2 from Embodiment 1 is in the modes in which the image forming apparatus operates. Embodiment will be described using an example of having a normal print mode (first mode) and a toner saving print mode (second mode) to save toner consumption will be described. In other words, in Embodiment 2, a standard image forming mode is the normal mode, and a density-variable image forming mode is the toner saving mode. The configuration of the image forming apparatus, however, is the same as Embodiment 1, including having the first conversion table to convert the detection value (density value), detected by the density detection sensor 218, into ΔE (Normal), hence the description thereof is omitted.
Surface Potential of Photosensitive Drum
The surface potential of the photosensitive drum 301 in the normal print mode and the toner saving print mode will be described with reference to
Therefore, in the normal print mode according to the configuration of Embodiment 2, the peripheral velocity ratio 1.4, Vdn=−500V, Vdcn=−350V and V1n=−100V are used. In the toner saving print mode, the peripheral velocity ratio 1.1, Vds=−380V, Vdcs=−250V and V1ns=−50V are used. Here the charging bias Vd, development potential Vdc and exposure potential V1 are denoted by Vds, Vdcs and V1s respectively.
γ Characteristic Based on Dithering of Embodiment 2
When the input/output γ characteristic 606 in the normal print mode and the input/output γ characteristic 607 in the toner saving print mode are compared in the low density region, ΔE in the toner saving print mode is smaller than ΔE in the normal print mode. Therefore, it is not necessary to calculate the engine γ characteristic 605 in the high density region to determine the engine γ characteristic 605 in the high density region. As a result, the variation of the engine γ characteristic 605 in the low density region can be minimized.
As described above, according to the image forming apparatus of Embodiment 2, which has a configuration to calculate the tinge in the toner saving image forming mode based on the tinge in the standard image forming mode, errors do not increase even in the calculation of the tinge in the low density region.
Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-Ray Disc (BD)™), a flash memory device, a memory card, and the like.
As described above, according to the above disclosure, the error in the tinge of an image can be decreased without increasing the downtime, in a configuration where an image can be formed in the image forming mode, in which the color gamut is different from the standard image forming mode.
While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2019-9779, filed on Jan. 23, 2019, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2019-009779 | Jan 2019 | JP | national |
