# Patent

## Title
Image forming apparatus and image forming method

## Abstract

An image forming apparatus including (a) a γ conversion unit having a γ conversion table γαi of a first one Vcontα of a designation development contrast in a case that a development contrast correction amount ΔVcont1 is a first value, and a γ conversion table γβi, different from the γ conversion table γαi, of a second one Vcontβ, different from the first one Vcontα, of the designation development contrast in a case that the development contrast correction amount ΔVcont1 is a second value different from the first value, and (b) an arithmetic operation unit for calculating a γ conversion table γNowi of a third one VcontNow of the designation development contrast in a case that the development contrast correction amount ΔVcont1 is a third value, based on a formula: γNowi=γαi+(γβi−γαi)×(Vcontα−VcontNow/(Vcontα−Vcontβ), wherein the subscript i is a value of an output density signal in the γ conversion table. The γ conversion unit conducts the γ conversion of the third one VcontNow of the designation development contrast, based on the γ conversion table γNowI calculated by the arithmetic operation unit.

## Background
This is a divisional of U.S. patent application Ser. No. 11/749,884, filed May 17, 2007, which is a divisional of U.S. patent application Ser. No. 11/080,490, filed Mar. 16, 2005, now U.S. Pat. No. 7,259,885, which is a divisional of U.S. patent application Ser. No. 09/797,991, filed on Mar. 5, 2001, now U.S. Pat. No. 6,987,576.
This application claims the right of priority under 35 U.S.C. § 119 based on Japanese Patent Application No. 2000-060284, filed on Mar. 6, 2000, and Japanese Patent Application No. 2000-223726, filed Jul. 25, 2000, which are hereby incorporated by reference herein in their entirety as if fully set forth herein.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying apparatus or a printer, and, more particularly, to gamma (γ) conversion for use therein.
2. Related Background Art
For adjusting image density in an image forming apparatus, there are known digital image processing method and analog adjustment methods. The digital image processing methods include an adjustment with luminance-density conversion means for converting a received original luminance signal into an original density signal, an adjustment with output density conversion means for converting an original density signal into an output density signal, and an adjustment constituted by an inverse function of printer characteristics, representing an output density signal corresponding to an output signal. In general, the adjustment by the gamma conversion means is easiest for adjusting the image density, and is more direct and accurate in determining the printer characteristics.
For analog adjustment, it is common to adjust the potential of the latent image or the development potential. There are other adjustable parameters, such as toner amount of the development means, development frequency, various adjusting gaps in the development means and a gap between the development means and the electrophotographic photosensitive member, but the latent image potential and the development potential have conventionally been employed in consideration of each of the adjustments. Particularly, the most common parameter is the development contrast, which is the difference between the latent image potential inducing the toner adhesion on the electrophotographic photosensitive member and the development potential of the development means.
The digital image adjustment represented by the adjustment of the content of conversion of the gamma conversion means is capable of relatively adjusting the density in the halftone area according to the desired amount, but cannot adjust the density of an area in which the toner is deposited with the maximum amount (hereinafter represented as Dmax) or a fog level in an area where the toner is not deposited.
On the other hand, the analog image adjustment represented by the adjustment of the development contrast can adjust, not only the density of the halftone area, but also, the maximum amount Dmax in mutual linkage. The fog level in the area without the toner adhesion scarcely changes unless the back contrast, relating to the difference between the development potential and the potential in the area without the toner adhesion, is altered.
Also, the digital image adjustment and the analog image adjustment, as described in the foregoing, may conventionally be designated individually, but there has not existed an algorithm or a sequence capable of automatically linking the two in the image forming apparatus.
It is to be noted that there may result a situation where the development contrast has to be altered, not for the purpose of image adjustment, but, for example, for reducing the toner consumption amount or for avoiding leakage of the charging means, for providing the photosensitive member with a potential. As explained in the foregoing, a change in the development contrast alone results in a change in the density of the halftone area and Dmax in linkage. The resulting image, therefore, provides an impression of significantly deteriorated image quality.
SUMMARY OF THE INVENTION
In consideration of the foregoing, the object of the present invention is to provide an image forming apparatus and an image forming method capable of maintaining a halftone density by gamma conversion, thereby outputting an image with little deterioration in the image quality, even when the image contrast has to be altered, not for the purpose of image adjustment.
The present inventors have investigated means for avoiding the impression of deteriorated image quality in the image, even in a case of altering the development contrast for the purpose of reducing the toner consumption amount or for avoiding leakage of the charging means. As the basis of the present invention, the present inventors have found the following two experimental rules.
A first rule is that the impression of deterioration of an image is principally caused by the density of the halftone area, and is, therefore, not much dependent on Dmax, and that an impression satisfactory to most users can be given, even if Dmax is somewhat lowered, as long as the density can be maintained in the halftone area. For example, in the ordinary visual observation of the image, the image quality does not appear to be deteriorated if a reflective density at least equal to 1.1 is obtained.
A second rule is that the toner consumption amount is reduced by a lower Dmax, even if the density of the halftone area is made somewhat higher. This phenomenon is based on a fact that the deposited amount of toner is larger in the Dmax area than in the halftone area.
In consideration of these two experimental rules, an optimum image can be provided if the halftone density can be maintained by an adjustment of the content of conversion in the gamma conversion means when the development contrast is altered, since such a change in the development contrast lowers the Dmax to reduce the toner consumption amount and to prevent the leakage in the charging means, while the image does not give a deteriorated impression, because of the maintained halftone density. More specifically, if the development contrast becomes deficient, the content of the gamma conversion means is adjusted so as to provide a halftone density higher than that in a normal case.
In the present invention, therefore, there is adopted a configuration of reading the development contrast in the printer portion, and the gamma conversion means provides a result of conversion corresponding to such a development contrast.
Also, except for the aforementioned adjustment of the development contrast not intended for image adjustment, such as for reducing the toner consumption amount or for avoiding a leak in the charging means, the convent of conversion of the gamma conversion means is not adjusted in a case of the adjustment of the development contrast, for the purpose of image adjustment. This is because, in a case of reducing the halftone density by reducing the development contrast, for example, in the image density adjustment for the purpose of general image adjustment or coping with the environment fluctuation of the development means, if the content of conversion of the gamma conversion means is adjusted, such image processing results in an increase in the halftone density, thus attenuating the effect of the present invention.
The aforementioned object of the present invention can be attained, according to the present invention, by an image forming apparatus provided with:
luminance-density conversion means for converting an original luminance signal into an original density signal;
An image forming apparatus, which visualizes an image, according to a latent image potential, on an image bearing body, which includes a charging unit and an exposing unit, in which a development potential is applied to a developing unit, and a development contrast is a difference between the latent image potential and the development potential, wherein a designation development contrast is defined as a sum of a basic development contrast Vcont, said image forming apparatus comprising:
(a) a γ conversion unit having a γ conversion table γαi of a first one Vcont α of the designation development contrast in a case that the development contrast correction amount ΔVcont1 is a first value, and a γ conversion table γβi, different from the γ conversion table γαi, of a second one Vcontβ, different from the first one Vcontα, of the designation development contrast in a case that the development contrast correction amount ΔVcont1 is a second value different from the first value; and
(b) an arithmetic operation unit for calculating a γ conversion table γNowi of a third one Vcont Now of the designation development contrast in a case that the development contrast correction amount ΔVcont1 is a third value, based on a formula:

γNowi=γαi+(γβi−γαi)×(Vcontα−VcontNow/(Vcontα−Vcontβ)

wherein the subscript i is a value of an output density signal in the γ conversion table, and wherein

the γ conversion unit conducts the γ conversion of the third one VcontNow of the designation development contrast, based on the γ conversion table γNowI calculated by the arithmetic operation unit. an]] output density conversion means for converting the obtained original density signal into an output density signal;
gamma conversion means for converting a signal into an output signal corresponding to the output density signal;
charging means for providing an electrophotographic photosensitive member with a uniform charge;
digital exposure means for exposing the electrophotographic photosensitive member after the charging to the output signal after the gamma conversion, thereby forming a latent image; and
development means for developing the latent image with toner to obtain a visible image,
in which the image is rendered visible by forming a latent image potential, with the surface potential of the electrophotographic photosensitive member formed by the charging means and the exposure means, and a development potential applied to the development means, the apparatus comprising:
environment detection means for detecting at least one of the temperature and the humidity of the location where the image forming apparatus is positioned,
wherein the content of conversion of the gamma conversion means is modulated according to the development contrast, which is the difference between the latent image potential causing the adhesion of the toner flying from the development means to the electophotographic photosensitive member and the development potential, depending on the change amount of the gamma conversion means corresponding to the change amount of the development amount in the result of detection by the environment detection means, and the weight of the gamma conversion means corresponding to the change amount of the contrast, in other cases.
According to the present invention, there is also provided an image forming apparatus provided with luminance-density conversion means for converting an original luminance signal into an original density signal, output density conversion means for converting the original density signal obtained by the luminance-density conversion means into an output density signal, gamma conversion means for executing gamma conversion on the output density signal obtained in the output density conversion means into an output signal, charging means for providing an electrophotographic photosensitive member with a uniform charge, exposure means for exposing the electrophotographic photosensitive member after the charging by the charging means to an exposure by the output signal, thereby forming a latent image, and development means for developing the latent image formed by the exposure means for developing the latent image formed by the exposure means with toner to obtain a visible image, wherein the image is rendered visible by forming a latent image potential, which is the surface potential of the electrophotographic photosensitive member formed by the charging means and the exposure means and a development potential applied to the development means,
wherein the content of conversion of the gamma conversion means is modulated according to the development contrast, which is the difference between the latent image potential causing the adhesion of the toner flying from the development means to the electrophotographic photosensitive member and the development potential.
Other objects of the present invention, and the features thereof, will become fully apparent from the following detailed description, which is to be taken in conjunction with the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing the configuration of the first embodiment;
FIG. 2 is a view showing the electrical configuration of a control board and peripheral components thereof;
FIG. 3 is a view showing the configuration of an image processing unit;
FIG. 4 is a chart showing a conversion table of a luminance-density conversion unit;
FIG. 5 is a chart showing a conversion table of an output density conversion unit;
FIG. 6 is a chart showing a conversion table of a gamma conversion unit;
FIG. 7 is a chart showing a conversion table indicating density characteristics for an output signal;
FIG. 8 is a view showing a latent image potential and a development potential in a background exposure system;
FIG. 9 is a chart showing adjustment items for determining the development contrast;
FIG. 10 is a chart showing a gamma conversion table prepared in advance;
FIG. 11 is a view showing the configuration of a gamma conversion unit;
FIG. 12 is a chart showing an example of γNowi;
FIG. 13 is a chart showing a latent image potential and a development potential in an image exposure system in the second embodiment;
FIG. 14 is a view showing the configuration of an image processing unit in the third embodiment;
FIG. 15 is a view showing a gamma conversion table for a dither screen process;
FIG. 16 is a view showing a gamma conversion table for pulse width modulation (PWM) processing;
FIG. 17 is a chart showing adjustment items for determining the development contrast;
FIG. 18 is a chart showing a change in the toner adhesion amount in a solid image area and in the halftone density;
FIG. 19 is a chart showing a change in the halftone density when the toner adhesion amount in the solid image area is made constant by a change in the development contrast under an environmental fluctuation;
FIG. 20 is a chart showing two gamma conversion tables for an error diffusion process for two development contrasts, provided in advance, in an image forming apparatus;
FIG. 21 is a view showing the configuration of a gamma conversion unit in an image processing unit in the fourth embodiment;
FIG. 22 is a chart showing an example of γNowi obtained from two development contrasts and a current development contrast;
FIG. 23 is a chart showing the latent image potential and the development potential in the image exposure system in the fifth embodiment;
FIG. 24 is a view schematically showing the configuration of an image processing portion in an image forming apparatus with plural image processing methods in the sixth embodiment;
FIG. 25 is a chart showing two gamma conversion tables for dither screen processing for two development contrasts, provided in advance, in the image forming apparatus of the sixth embodiment; and
FIG. 26 is a chart showing two gamma conversion tables for pulse width modulation (PWM) processing for two development contrasts, provided in advance, in the image forming apparatus of the sixth embodiment.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following, the present invention will be clarified in detail by the embodiments thereof, taking a laser printer as an example. However, the present invention may be realized not only by a laser printer, but also, by any image forming apparatus of a digital type.
Also, the present invention may be realized, not only in the form of an apparatus, but also in the form of an image forming method based on the description of the embodiments.
First Embodiment
FIG. 1 schematically shows a laser printer constituting the first embodiment. The printer is provided, at the approximate center of a main body 1, with a cylindrical photosensitive drum 2 constituting an electrophotographic photosensitive member. The photosensitive drum 2 is supported in the main body 1 and is rotated in a direction R1, and, along the periphery of the photosensitive drum 2 and in an order of the rotating direction thereof, a charge eliminating unit 3 for eliminating the potential on the photosensitive drum 2, a primary charger 4 constituting charging means for uniformly charging the surface of the photosensitive drum 2, exposure means 5 for exposing the surface of the photosensitive drum 2, thereby forming an electrostatic latent image, a potential sensor 6 for measuring the potential on the photosensitive drum 2 after exposure, a development unit 7 constituting development means for adhering toner to the electrostatic latent image, thereby forming a toner image, a transfer charger 8 for transferring the toner image onto a transfer material P, a separating charger 9 for separating the transfer material P from the photosensitive drum 2, and a cleaner 10 for removing the toner remaining on the photosensitive drum 2.
The transfer material P, to which the toner image is to be transferred, is fed from a paper feeding deck 11. Below the photosensitive drum 2, namely, in the lower part of the interior of the main body 1 of the apparatus, there is provided the paper feeding deck 11 containing the transfer material P. The transfer material P in the deck 11 is fed by a feeding roller 12, and is supplied through conveying rollers 13 and registration rollers 15 to a space between the photosensitive drum 1 and the transfer charger 8. In this position, the transfer material P receives transfer of the toner image from the photosensitive drum 2, and is conveyed by a conveyor belt 16 to a fixing unit 17. After the fixation of the toner image by heat and pressure provided by the fixing unit 17, the transfer material P is discharged as the final output image by discharge rollers 19 onto a discharge tray 20.
In the exposure means 5 of the aforementioned printer, a laser beam emitted from a semiconductor laser 30 according to an image signal is put into a scanning motion by a polygon mirror 31, and is guided through an imaging lens 32 and a mirror 33 to the photosensitive drum 2.
FIG. 2 is a view schematically showing the electrical configuration of a control board for measuring the potential on the photosensitive drum 2 and peripheral components. Referring to FIG. 2, a ROM storing a control program and a RAM constituting a temporary memory element for the data required in the execution of the control program are connected to a CPU, which is the core of the process execution. Also, an interface (I/O), and A/D and D/A converters for data conversion, are connected with external peripheral devices, whereby the information is inputted into or outputted from the control board. As a peripheral device, a potential sensor is provided for measuring the potential on the photosensitive drum after charging and after exposure. The voltage applied to the primary charger and the output value applied to the semiconductor laser can be controlled in order to form a desired dark voltage and light voltage on the photosensitive drum 2. Also, a development potential can be formed on the development unit 7, thereby causing the toner to fly to the photosensitive drum 2. The potential applied to the development unit 7 is, in practice, composed of superimposed DC and AC components, but, in the present embodiment, the development potential is represented by a DC component.
FIG. 3 is a schematic view showing the configuration of a digital image processing unit. An original image is entered into an image processing unit as an 8-bit signal, either through a reader scanner network constituting a reading apparatus or from a memory apparatus, such as a hard disk. The original image is subjected to a filter process in a filter process unit, in order to enhance the edge portion of the image. Then, a luminance image constituting the original image is subjected to a luminance-density conversion in a luminance-density conversion unit for conversion into a density signal. The density signal constituting the original image is subjected to an output density conversion process in an output density conversion unit, for conversion into the output density corresponding to an output density pattern selected by the user. The output density pattern is selected by the user on an operation panel, not shown, in FIG. 1, but consisting a user interface, and can be a character mode in which emphasis is given to the contrast of the output image for outputting principally a character original, or a photograph mode in which emphasis is given to the gradation of the output image for outputting principally an original photograph, or a character-photograph mode for outputting an intermediate image between the character mode and the photograph mode. A gamma conversion unit executes a gamma conversion in consideration of the output characteristics of the printer, on the thus obtained output density signal. An output signal for obtaining the output density is determined by such gamma conversion. A table in the gamma conversion unit is constituted by determining an inverse function for the output signal to be outputted to the exposure system of a predetermined printer and for a correlation table of the density signal. Such an output signal is converted into a binary level in an error diffusion process unit for conversion from an 8-bit signal to a binary signal. Such a binary output signal is supplied to a semiconductor laser 30, constituting an exposure system, whereby a latent image, converted from the original image, is recorded on the photosensitive drum 2. The error diffusion process unit provides such a binary image that is microscopically a binary image, but can macroscopically be observed as a multi-value image.
FIG. 4 shows the characteristics of a conversion table in the luminance-density conversion unit, which, being an application of logarithmic conversion, serves to compress the signal change in a dark area, which is not clear to the human eyes, and to expand the signal change in a light area where the change is more easily observed by the human eyes.
FIG. 5 shows the characteristics of a conversion table in the output density conversion unit. In the present embodiment, three tables are provided, in which, in the character mode, giving emphasis to the contrast of the output image in order to principally output a character original, the lighter area is more or less skipped, while, in the photograph mode, giving emphasis to the gradation of the output image in order to principally output an original photograph, the density signal of the original and the output density signal are in a relatively smooth linear relationship, thereby maintaining the gradation. On the other hand, the character-photograph mode has an interim character between the character mode and the photograph mode.
FIG. 6 shows the characteristics of a conversion table in the gamma conversion unit. In the gamma conversion unit, the relationship of the output signal for obtaining the desired density is retained, and the gamma conversion table is given by investigating in advance the density characteristics for an arbitrary output signal and determining an inverse function of a table F obtained by approximating or interpolating such density characteristics with curved or straight lines, as shown in FIG. 7.
FIG. 8 is a chart showing a latent image potential and a development potential in the present embodiment. Since the present embodiment employs a background exposure method in which the toner flies principally to an area not exposed to the light of the semiconductor laser, the toner adheres to a dark potential area of the photosensitive drum 2. A light potential area constitutes a so-called white area, though a certain fog is generated. A development potential indicates a DC component of a bias voltage applied to the development unit. Depending upon the magnitude of the development contrast, which is the difference between the dark potential and the light potential, there are varied the adhesion amount of the toner and the halftone density even in the digital apparatus. In the present embodiment, there are adopted basic conditions of a dark potential of 300V, a light potential of 50V, a development potential of 200V and a development contrast of 190V. In the present embodiment, the development contrast is adjusted by the adjustment of the dark potential by a change in the applied voltage to the primary charger. The back contrast, which is the difference between the development potential and the light potential, is not changed in the present embodiment.
FIG. 9 is a view showing adjustment items for determining the development contrast in the present invention. The basic development contrast (Vcont) is 190V, as explained in the foregoing with reference to FIG. 8. For correcting the basic development contrast in the present embodiment, there are added or subtracted a first development contrast correction amount ΔVcont1 and a second development contrast correction amount ΔVcont2. Therefore, the total amount of the development contrast at the actual output operation by the printer is represented by:

total development contrast=Vcont+ΔVcont1+ΔVcont2  (3)

The first development contrast correction amount ΔVcont1 is further divided into the following two applications: namely, for reducing the toner consumption amount and for avoiding leakage in the charger. For the purpose of reducing the toner consumption amount, the ΔVcont1 can be switched to 0V (no change), −20V, −40V or −60V. A reduction of the development contrast provides the ability to reduce the adhesion amount of the toner to the dark potential, thereby lowering the toner consumption amount. To avoid leakage of the charger, it can be switched to 0V (no change), −30V or −60V. The prevention of leakage means the following. When the voltage applied to the primary charger is elevated, the primary charger in the printer located in a low air pressure situation generates spark discharge, and the dark potential has to be lowered in order to avoid the drawbacks resulting from such spark discharge.
The second development contrast correction value ΔVcont2 is also divided into two applications, namely, for image adjustment and for environmental adjustment. For the purpose of image adjustment, it can be finely set within a range from −50V to +50V. It is used by the service person for the printer or the user of the printer for the purpose of adjusting the entire image density to the darker or lighter side under the observation of the outputted image. For the purpose of environmental adjustment, the result of detection is read from a temperature-humidity sensor provided inside the printer for calculating the moisture in the environment where the printer is installed, and the development contrast is automatically and continuously adjusted from 0V to −75V, according to the moisture. This adjustment is executed in order to avoid a phenomenon that the development unit in the printer shows a higher development ability in a lower moisture environment than in a higher moisture environment, whereby the toner adhesion amount becomes higher, even if the surface potential of the photosensitive drum is maintained constant. Naturally, such a tendency is affected by the development characteristics of the development unit, and, in the present embodiment, the same reduction of ΔVcont2 had to be made larger as the moisture becomes lower.
It is to be noted that the second development contrast correction amount ΔVcont2 is for image adjustment upon observation of the output image or for environment adjustment corresponding to the state of the development unit, and is, therefore, to be adjusted manually or automatically according to the image. Consequently, in contrast to the first development contrast correction value ΔVcont1, the ΔVcont2 value varies the development contrast so as to optimize the image quality. Stated differently, the second development contrast correction value ΔVcont2 is used in order to decrease Dmax and the halftone density when they are excessively high, or to increase these when they are excessively low, and the density adjustment beyond these purposes should not be executed in a digital manner.
On the other hand, the first development contrast correction amount ΔVcont1 is used for altering the development contrast regardless of the image quality. In case there is employed only one gamma conversion table, as shown in FIG. 6, the lowering of Dmax indicating the density of the toner adhering to the dark potential also results in a lowering of the macroscopic halftone density, thus giving a strong impression of image quality deterioration to the user.
Therefore, the present embodiment provides the ability to prevent the image deterioration without lowering of the halftone density, by preparing a gamma conversion table according to the first development contrast correction amount ΔVcont1 as the digital density adjustment.
FIG. 10 shows two gamma conversion tables prepared in advance in the printer of the present embodiment. The two gamma conversion tables consist of a gamma conversion table (γα1) for a case in which the first development contrast correction amount ΔVcont1 is 0V, namely, for a situation where Vcont+ΔVcont1 is 190V (Vcontα), including the basic development contrast of 190V, regardless of the second development contrast correction amount ΔVcont2, and a gamma conversion table (γβ1) for a case in which the first development contrast correction amount ΔVcont1 is −50V, namely, for a situation where Vcont1+ΔVcont1 is 140V (Vcontβ), including the basic development contrast of 190V, regardless of the second development contrast correction amount ΔVcont2. The suffix i of the gamma conversion table indicates the eight-bit value of the output density signal of the table and assumes a value within a range of zero to two hundred fifty-five.
The present embodiment functions with the aforementioned two gamma conversion tables, namely, replacing the gamma conversion unit shown in FIG. 3 with that shown in FIG. 11. The gamma conversion unit shown in FIG. 11 is provided with two gamma conversion tables. Also, as shown in FIG. 9, VcontNow is determined as the sum of the basic development contrast Vcont and the first development contrast correction value ΔVcont1. The VcontNow varies within a range of 190V to 70V, based on the set values shown in FIG. 9. As the reduction of the toner consumption amount and the prevention of leakage are not used often in combination, in practice, it is generally used in the range of 190V to 130V. An interpolation calculation unit shown in FIG. 11 executes calculations based on the VcontNow, the relationship between Vcontα and Vcontβ, the conversion table at Vcontα and the conversion table at Vcontβ, according to an interpolation equation:

γNowi=γαi+(γβi−γαi)×(Vcontα−VcontNow)/(Vcontα−Vcontβ)  (3)

wherein the suffix i of the gamma conversion table is an eight-bit value of the output density signal of the table, assuming a value within a range of zero to two hundred fifty-five. The foregoing equation naturally assumes a value γNowi=γαi when VcontNow=Vcontα, or γNowi=γβi when VcontNow=Vcontβ.

FIG. 12 shows an example of the result of calculation by equation (3). In FIG. 12, γNowi is obtained by interpolation from the two γαi, γβi, the VcontNow, which is the sum of the basic development contrast Vcont, and the first development contrast correction value ΔVcont1.
The γNowi, determined by interpolation, is passed by an averaging process provided in the gamma conversion unit shown in FIG. 11, in order to reduce the quantization error resulting from the deficiency in the dynamic range of the eight-bit signal.
The present embodiment employs the calculation according to equation (3), but there may also be employed an interpolating equation (4) with a suffix j:

ΓNowj=Γαj+(Γβj−Γαj)×(Vcontα−VcontNow)/(Vcontα−Vcontβ)  (4)

wherein Γ indicates a table of the output density signal corresponding to the output signal j and is an inverse function to γ.

The above-described embodiment employing the gamma conversion table according to the development contrast has the features shown in the following Table 1, in which a comparison is made with a conventional example 1, in which the analog image adjustment alone is executed, for example, by the change of the development contrast only, and with a conventional example 2, in which the digital image adjustment alone is executed, for example, by an arbitrary change of the gamma conversion table only. The analog image adjustment, as in the conventional example 1, results in a lowering of Dmax, and at the same time, in a lowering of the halftone density, thereby significantly deteriorating the apparent image quality. The lowered density itself results in a phenomena such as a weaker impression of the image, deteriorated gradation and thinning of the lines. On the other hand, the digital image adjustment, as in the conventional example 2, shows a weaker effect of toner reduction, in comparison with the analog image adjustment, thereby resulting in an extremely large loss of the halftone density, in order to achieve the desired toner reducing effect, and has no effect in preventing the leakage of the charger.
In contrast, in the present embodiment, the gamma conversion table is automatically modulated (change of the gamma conversion characteristics) simultaneously with the lowering of the development contrast, thereby maintaining the halftone density, though Dmax is lowered, thus avoiding poor impression of the image. This is due to a fact that the poor impression is not caused because of the weaker response of the human eyes to the density in the darker area, as long as Dmax in the reflective density is at least equal to 1.1. Also, the present embodiment providing the ability to achieve reduction of the toner amount and prevention of the leakage at the same time, thereby providing the ability to attain the image quality and other requirements at the same time, unlike the conventional examples 1 and 2 (see Table 1 below).











TABLE 1







Conventional
Conventional
Present



example 1
example 2
Embodiment



















Analog
used
—
used


(development


contrast lowered)


Digital (gamma
—
used
Automatically


conversion table


modulated


modulated)


according to





development





contrast


Dmax
lowered
maintained
lowered


Halftone density
low
significantly
maintained




lower


Image in total
apparently
poor upon
little influenced



poor
toner amount
by lower Dmax




reduction


Drawback
defects in
leakage cannot
none



image
be prevented









As explained in the foregoing, the present embodiment provides the ability to maintain the image quality as long as possible, even in a situation where a lowering of the development contrast is unavoidable, such as in reducing the toner consumption amount or avoiding the leakage of the charger, by preparing gamma conversion tables corresponding to two development contrasts, and calculating a suitable gamma conversion table according to the current development contrast.
Second Embodiment
The foregoing first embodiment has been explained by a printer of the background exposure type, in which the toner principally flies to the dark potential area, but the present invention is likewise applicable to a printer of the image exposure type, in which the toner principally flies to the light potential area, as shown in FIG. 13, and such an example will be explained as the second embodiment.
In the image exposure system, the difference of the development potential and the light potential constitutes the development contrast, and the difference between the dark potential and the development potential constitutes the back potential. In the present embodiment, there are assumed basic conditions of a dark potential of 400V, a light potential of 50V, a basic development potential of 250V, and a development contrast of 200V.
In the first embodiment, the development contrast is modulated by the modulation of the dark potential, but, in the present embodiment, the development contrast is modulated simultaneously with the dark potential. This is because the dark potential is not directly related to the development contrast in the image exposure system, in contrast to the first embodiment, in which the dark potential has to be lowered for preventing the leakage, as shown in FIG. 9. Therefore, the back contrast remains unchanged by varying the development potential corresponding to the lowering of the dark potential, whereby the development contrast alone can be lowered without a variation, such as an increase or a decrease of the fog level. The back contrast relating to the fog level, if made smaller, results in unnecessary adhesion of the toner called background fog, but, if made excessively large, results in a reversal fog induced by the toner, which is charged in an opposite polarity or an image stain called shadowing. It is, therefore, important to maintain the back contrast within a certain range, and the back contrast can be advantageously maintained constant by reducing the development potential in linkage with the dark potential. The printer of the present embodiment utilizing the lowered development potential provides the ability, as in the first embodiment, to prevent the leakage, to reduce the toner consumption amount and to provide the high image quality, at the same time.
In the first embodiment, the present invention is realized by a printer of the background exposure type, but, the present invention, considered from the standpoint of the development contrast, can also be realized, as in the present embodiment, in the printer of an image exposure system. More specifically, by reducing the development potential simultaneously with the lowering of the dark potential, it is possible to prevent leakage of the charger and reduction of the toner consumption amount resulting from the lowered development contrast, and also, to suppress the loss in the image quality by the optimum gamma conversion.
Third Embodiment
In the first embodiment, shown in FIG. 3, the conversion to the binary level is executed in the error diffusion unit after the gamma conversion. In the present embodiment, there will be explained a printer utilizing plural image processing methods, including the error diffusion process. FIG. 14 is a schematic view showing the configuration of a digital image processing unit of the present embodiment, in which an original luminance signal read from a reader, a scanner, a network or a memory device, such as a hard disk, is subjected to an edge enhancement in a filter unit, then converted into an original density signal in a luminance-density conversion unit, and further converted into a desired output density signal in an output density conversion unit. The present embodiment is provided, for preparing signal data for driving the semiconductor laser, with an error diffusion process explained in the first embodiment, a dither screen process, and a pulse width modulation (PWM) process.
The present embodiment is featured by preparing two gamma conversion tables for each of the aforementioned three image processing methods, or six gamma conversion tables in total, and preparing a gamma conversion table corresponding to the current development contrast from the two gamma conversion tables corresponding to the image processing method. In the dither screen process, the error diffusion process tends to generate more scattered toner dots on the photosensitive drum, thereby producing finer dots. For this reason, the gamma conversion table becomes different, even for a same development contrast. Similarly, the PWM process, having a known tendency of generating dots more scattered in the main scanning direction and more connected in the sub scanning direction, requires gamma conversion tables different from those for the error diffusion process or for the dither screen process. Consequently, gamma conversion tables are necessary, corresponding to each of the three image processing methods. The error diffusion process is used, for example, in the character mode, because of its features that the stability of gradation is inferior to that in other processes, though the resolution is extremely high. On the other hand, the dither screen process is used, for example, in the photograph mode, because of the excellent stability of the gradation, though the resolution is inferior. The PWM process is positioned in properties between these two methods and is, therefore, used, for example, in the character photograph mode.
FIG. 15 shows gamma conversion tables for two development contrasts for use in the dither screen process, and FIG. 16 shows gamma conversion tables for two development contrasts for use in the PWM process. The gamma conversion unit is provided in advance with two gamma conversion tables for each of the three image processing methods, including those shown in FIG. 10, and a gamma conversion table corresponding to the current development contrast is prepared through an interpolation unit and an averaging unit. The present embodiment, in which an optimum gamma conversion table is automatically selected corresponding to the image processing method and the current development contrast, is capable of providing constantly stable images, even in a situation where the development contrast has to be lowered.
As explained in the foregoing, the present embodiment provides, in a printer provided with plural image processing methods, an optimum image according to the image processing method and the current development contrast, by preparing two gamma conversion tables for each of the image processing methods.
The foregoing embodiment provides an image with little deterioration, even when an adjustment of the image contrast is unavoidable, not for the purpose of image adjustment.
Fourth Embodiment
In the foregoing first to third embodiments, the gamma (γ) development contrast VcontGamma (described as VcontNow in the first to third embodiments) for gamma conversion is practically defined as follows:

VcontGamma=Vcont+A·Δcont1+B·ΔVcont2

wherein A=1, B=0, therefore

VcontGamma=Vcont+ΔVcont1  (5)

Therefore, it is defined solely by the first development contrast correction amount ΔVcont1 for correcting the development contrast and the basic development contract Vcont. The first development contrast correction amount is determined by the reduction of the toner consumption amount in the development means or by the prevention of the leakage in the charging means, and the deterioration of the image can be suppressed, even when the development contrast has to be adjusted, by changing the content of conversion of the gamma conversion means according to equation (5). On the other hand, the second development contrast correction amount is determined by the adjustment of the image density or the adjustment for the environmental fluctuation, and is not made to contribute to the content of conversion of the gamma conversion means.
However, there may be encountered a following drawback if the reduction of the toner consumption amount or the prevention of the leakage of the charging means alone is correlated with the content of conversion of the gamma conversion means, as indicated by equation (5).
An environmental fluctuation in the development characteristics causes a change not only in the halftone density, but, also, in the toner adhesion amount, even under a same development contrast. A change in the halftone density may be recognized as a deterioration in the image quality. Also, a change in the toner adhesion amount may affect the toner consumption amount and may induce a drawback, in a case of excessive adhesion, for example, scattering of the image or image deterioration in a portion where the toner surface is subjected to friction, such as an enhanced trace of a separating finger of the fixing roller. In connection with such a phenomenon, the experiments of the present inventors have clarified that the changes in the halftone density and in the toner adhesion amount, resulting from the environmental fluctuation do not take place at a same level. In the image forming apparatus employing digital area gradation, the halftone density is generally determined by the toner deposition rate per area of the sheet, while the toner adhesion amount determining the image deterioration caused by the toner scattering or the trace of the separating finger can be regarded as the toner amount in the height direction with respect to the sheet. It has been found that the toner deposition under the environmental fluctuation does not change with a constant rate in the area and height directions of the sheet, but changes individually in these directions. Therefore, if the total development contrast VcontBody of the main body is determined so as to optimize the toner adhesion amount for the environment fluctuation and, if the content of the conversion of the gamma conversion means is determined without taking the environmental fluctuation into consideration, the halftone density cannot reach the target density, so that the desired toner adhesion amount and the desired halftone density cannot be attained at the same time. If the priority is given to the halftone density, naturally, the total development contrast VcontBody of the main body does not become appropriate, so that the inadequate toner adhesion amount results in an unsatisfactory toner consumption amount or image deterioration, such as toner scattering or a trace of the separating finger.
In the following embodiment, therefore, there are conceived the following means for attaining the toner adhesion amount and the halftone density at the same time. More specifically, the development contrast in the main body is adjusted so as to optimize the toner adhesion amount, in consideration of the influence of the environmental fluctuation. At the same time, as the halftone density also shows a change, though not in direct correlation, the development contrast based on the environmental fluctuation is corrected with a weight. More specifically, the gamma development contrast VcontGamma for gamma conversion is defined in the following manner:

VcontGamma=Vcont+A·ΔVcont1+B·ΔVcont2+C·ΔVcont3

wherein A=1, B=0, and C is a non-zero constant, therefore:

VcontGamma=Vcont+ΔVcont1+C·ΔVcont3  (6)

The first development contrast correction amount ΔVcont1 is used for reducing the toner consumption amount in the development means or for preventing the leakage in the charging means, while the second development contrast correction amount ΔVcont2 is used for adjusting the image density, and the third development contrast correction amount ΔVcont3 is used for an adjustment for the environmental fluctuation. Since the constant C is larger than zero, but smaller than one, the change in the halftone density resulting from the environment fluctuation can be corrected in an optimum manner.
FIG. 17 shows the adjustment items for determining the development contrast. The basic development contrast (Vcont) is 190V, as explained in FIG. 8. In the present embodiment, such a basic development contrast is corrected by adding or subtracting three correction amounts, namely, the first development contrast correction amount ΔVcont1, the second development correction amount ΔVcont2 and the third development contrast correction amount ΔVcont3 to or from the basic development contrast. Consequently, the total development contrast VcontBody at the actual output operation of the image forming apparatus is represented by:

VcontBody=Vcont+ΔVcont1+ΔVcont2+ΔVcont3  (7)

Also, in the present embodiment, the gamma development contrast for gamma conversion is defined according to equation (6).
Also, the development contrast, not including the second development contrast correction amount ΔVcont2, is defined as VcontGamma according to equation (6), as will be explained later.
The first development contrast correction amount ΔVcont1 is further divided into the following two applications: namely, for reducing the toner consumption amount and for avoiding leakage in the charger. For the purpose of reducing the toner consumption amount, the value of ΔVcont1 can be switched to 0V (no change), −20V, −40V or −60V. A reduction of the development contrast reduces the adhesion amount of the toner to the dark potential, thereby lowering the toner consumption amount. For the purpose of avoiding leakage of the charger, it can be switched to 0V (no change), −30V or −60V. The leakage prevention means the following. When the voltage applied to the primary charger is elevated, the primary charger in the printer located in a low air pressure situation generates a spark discharge, and the dark potential has to be lowered in order to avoid the drawbacks resulting from such a spark discharge.
The second development contrast correction value ΔVcont2 is used for image adjustment, and can be finely set within a range from −50V to +50V. It is used by the service person for the image forming apparatus or the user thereof for the purpose of adjusting the entire image density to the darker or lighter side under the observation of the outputted image.
The third development contrast correction value ΔVcont3 is used for environmental adjustment. For the environmental adjustment, the result of the detection is red from a temperature-humidity sensor provided inside the image forming apparatus for calculating the moisture in the environment where the printer is installed, and the development contrast is automatically and continuously adjusted from 0V to −50V, according to the moisture. This adjustment is executed in order to avoid a phenomenon that the development unit in the image forming apparatus shows a higher development ability in a lower moisture environment than in a higher moisture environment, whereby the toner adhesion amount becomes higher, even if the surface potential of the photosensitive drum is maintained constant. Naturally, such a tendency is affected by the development characteristics of the development unit, and, in the present embodiment, the amount of reduction of ΔVcont3 had to be made smaller as the moisture level becomes lower.
The first development contrast correction amount ΔVcont1 is to change the development regardless of the image quality. In case only one gamma conversion table, shown in FIG. 6, is employed, there will result a lowering of the macroscopic halftone image, as well as a lowering in Dmax indicating the density of the toner adhering to the dark potential, thus inducing an impression of significant image deterioration to the user. For this reason, equation (7) determines the development contrast of the image forming apparatus and equation (6) determines the gamma development contrast for gamma conversion, including ΔVcont1 with a same contribution.
The second development contrast correction amount ΔVcont2 is used for image adjustment upon confirmation of the output image and is adjusted manually according to the image. Therefore, a change in the development contrast serves to optimize the image quality. Stated differently, the second development contrast correction value ΔVcont2 is used in order to decrease Dmax and the halftone density when they are excessively high, or to increase these when they are excessively low, and the density adjustment beyond these purposes should not be executed in a digital manner. Therefore, though equation (7) includes ΔVcont2, it is unnecessary in equation (6) for determining the gamma development contrast gamma conversion.
FIG. 18 shows the influence of the environmental fluctuation on the development characteristics, by the toner adhesion amount in a solid image area and by the density in a halftone area. It shows the variation rates of the solid area toner adhesion amount and the halftone density, taking the values thereof as 100 at a high moisture (22 g/kg), to a lower moisture situation (5 g/kg). For a lower moisture, the development unit shows a higher development ability, with increases both in the solid area toner adhesion amount and the halftone density, but the change rate is not the same for the two. FIG. 18 shows that the change rate is higher for the solid area toner adhesion amount than for the halftone density.
Therefore, if the development contrast of the main body of the image forming apparatus is changed, according to the moisture, so as to stabilize the toner adhesion amount in the solid image area, the halftone density does not become constant, as shown in FIG. 19. It will also be understood that, if the development contrast of the main body of the image forming apparatus is changed, according to the moisture, so as to stabilize the halftone density, the toner adhesion amount in the solid image area does not become constant. Thus, if priority is given to the halftone density as in the conventional examples, there cannot be avoided the fluctuation in the toner adhesion amount, eventually leading to an increased toner consumption amount, a scattering of the image or a trace of the separating finger remaining on the sheet.
According to an experiment of the present inventors, the ratio of the change rates of the solid area toner adhesion amount and the halftone density, as shown in FIG. 18, is 7:2 in the image forming apparatus employed for the experiment. This value may naturally be different in another image forming apparatus of another image forming system. The above-mentioned value means that, in a case of determining the potential conditions for providing the optimum image characteristics under a high humidity environment, for example, of 22 g/kg and for obtaining the same image characteristics under a low humidity environment, for example, of 5 g/kg, the development contrast has to be reduced by 50V for the solid area toner adhesion amount and by about 15V for the halftone density. Stated differently, if the development contrast is reduced by 50V, in order to stabilize the solid area toner adhesion amount in a case of a shift from a high humidity environment to a low humidity environment, the halftone density is recovered partially corresponding to 15V, but is still deficient corresponding to the remaining 35V (=50V−15V).
In the present invention, therefore, the half tone density is assured by a feedback of a portion corresponding to such 35V, not to the contrast potential, but to the content of conversion of the digital gamma conversion means. This is represented by a term C·ΔVcont3 in equation (2). In the present embodiment, an optimum result can be obtained in a case C=0.714 (=35/50V=5/7V), so that equation (6) becomes:

VcontGama=Vcont+ΔVcont1+0.714·ΔVcont3  (6′)

In the present invention, it is possible to suppress the image deterioration without losing the halftone density, by preparing gamma conversion tables according to VcontGamma in equation (6′).
FIG. 20 shows two gamma conversion tables prepared in advance in the image forming apparatus of the present invention and consisting of a gamma conversion table (yai) for a case where the contrast correction amounts ΔVcont1, ΔVcont2 and ΔVcont3 are all 0V, namely, a case of VcontGamma=Vcont=190V (Vcontα), and a gamma conversion table (γβi) for a case where Vcont is 140V (Vcontβ), and a suffix i of the gamma conversion table indicates an eight-bit value of the output density signal of the table, assuming a value within a range of zero to two hundred fifty-five.
The present embodiment functions with the aforementioned two gamma conversion tables, namely, replacing the gamma conversion unit shown in FIG. 3 with that shown in FIG. 21. The gamma conversion unit shown in FIG. 21 is provided with two gamma conversion tables. Also, as shown in the equation (6′), VcontGamma is determined as the sum of the basic development contract Vcont and 0.714 times of the third development contrast correction value ΔVcont3. The VcontGamma can vary by about 155V from the set value shown in FIG. 17, namely, within a range of 190V to 35V. As the reduction of the toner consumption amount and the prevention of leakage are not used often in combination, in practice, it is generally used in the range of 190V to 100V. An interpolation calculation unit shown in FIG. 21 executes calculation based on the VcontGamma, the relationship between Vcontα and Vcontβ, the conversion table at Vcontα and the conversion table at Vcontβ, according to the interpolation equation:

γNowi=γαi+(γβi−γαi)×(Vcontα−VcontGamma)/(Vcontα−Vcontβ)  (8)

wherein the suffix i of the gamma conversion table is an eight-bit value of the output density signal of the table, assuming a value within a range of zero to two hundred fifty-five. The foregoing equation naturally assumes a value γNowi=γαi in case VcontGamma=Vcontα, or γNowi=γβi in case VcontGamma=Vcontβ.

FIG. 21 shows an example of the result of calculation by equation (8). In FIG. 12, γNowi is obtained by interpolation from the two gamma conversion tables γαi, γβi, and the VcontGamma, which is the current sum of the basic development contrast Vcont and the first development contrast correction value ΔVcont1.
The γNowi, determined by interpolation, is passed by an averaging process provided in the gamma conversion unit shown in FIG. 21 in order to reduce the quantization error resulting from the deficiency in the dynamic range of the eight-bit signal.
The present embodiment employs the calculation according to equation (8), but there may also be employed an interpolating equation (9) with a suffix j:

ΓNowj=Γαj+(Γβj−Γαj)×(Vcontα−VcontGamma)/(Vcontα−Vcontβ)  (9)

wherein Γ indicates a table of the output density signal corresponding to the output signal j and is an inverse function to γ.

In the exploitation of the present invention, the gamma conversion tables prepared in advance and the current ones derived from equation (8) or equation (9) are subjected to sufficient smoothing at the preparation of the tables prepared in advance, or at the derivation of the current tables, in order not to cause a jumping in the image gradation by the quantization error.
As explained in the foregoing, the present invention is capable of individually optimizing the development characteristics under the influence of the environmental fluctuation and provides the features shown in the following Table 2, in comparison with the conventional examples. In the conventional examples, the change in the development characteristics induced by the environmental fluctuation is not taken into consideration sufficiently, and the development contrast is merely changed so as to stabilize the halftone density, but the toner adhesion amount becomes excessively high in the solid image area, because of the difference in the change rate, from the halftone density, under the environmental fluctuation. Such excessive toner adhesion leads to drawbacks, such as an increased toner consumption amount, scattering of the image in the development, transferring or fixing unit, and an enhanced trace of the fixing finger in the solid image area. On the other hand, in the present invention, the development contrast is changed according to the toner adhesion amount in the solid image area, and the gamma conversion tables are prepared so as to compensate for the deficiency in the halftone image. Consequently, there can be simultaneously attained the toner adhesion amount in the solid image area and the halftone density, and there can be provided an image forming apparatus resistant to the environmental fluctuations (see Table 2 below):










TABLE 2







Conventional
Present



example
Embodiment




















Consideration on
Change in
Change in



development
development
development



characteristics by
contrast only
contrast and



environmental

modulation of gamma



fluctuation

conversion table





with weighting



Solid area toner
Stabilized by
Stabilized by



adhesion amount
development
development contrast




contrast



Halftone density
lower
Stabilized by gamma





conversion table



Drawback
Poor image by
none




lowered halftone




density










In the present embodiment, equation (6′) is defined for an image forming apparatus showing higher values of the solid area toner adhesion amount and the halftone density at a lower humidity environment, but the constant C may assume a negative value depending on the image forming system of the image forming apparatus. Also, there has been explained a case where the change in the solid area toner adhesion amount is larger than that in the halftone density, but it will be apparent that the present invention is applicable also in an opposite case.
Also, in the present embodiment, an environment sensor is provided as the environment detection means and the moisture in the environment where the image forming apparatus is installed is measured from the temperature and humidity from such a sensor, but a sensor for the temperature only or for the humidity only may be employed for the purpose of cost reduction.
Fifth Embodiment
The foregoing fourth embodiment has been explained by an image forming apparatus of the background exposure type in which, as shown in FIG. 8, the toner principally flies to the dark potential area, but the present invention is likewise applicable to an image forming apparatus of the image exposure type in which the toner principally flies to the light potential area, as shown in FIG. 23.
In the image exposure system, the difference of the development potential and the light potential constitutes the development contrast, and the difference between the dark potential and the development potential constitutes the back potential. In the present embodiment, there are assumed basic conditions of a dark potential of 400V, a light potential of 50V, a basic development potential of 250V, and a development contrast of 200V.
In the fourth embodiment, the development contrast is modulated by the modulation of the dark potential, but, in the present embodiment, the development contrast is modulated simultaneously with the dark potential. This is because the dark potential is not directly related to the development contrast in the image exposure system, in contrast to the fourth embodiment, in which the dark potential has to be lowered for preventing the leakage, as shown in FIG. 17. Therefore, the back contrast remains unchanged by varying the development potential corresponding to the lowering of the dark potential, whereby the development contrast alone can be lowered with the variation, such as an increase or a decrease of the fog level. The back contrast relating to the fog level, if made smaller, results in unnecessary adhesion of the toner, called background fog, but, if made exclusively large, results in a reversal of fog induced by the toner, which is charged in an opposite polarity or in an image stain called shadowing. It is, therefore, important to maintain the back contrast within a certain range, and the back contrast can be advantageously maintained constant by reducing the development potential in linkage with the dark potential. The image forming apparatus of the present embodiment, utilizing the lowered development potential, provides the ability, as in the fourth embodiment, to attain the prevention of the leakage, the reduction of the toner consumption amount and the high image quality at the same time, and, even under an environmental fluctuation at the same time, the image is not deteriorated, because the development contrast is modulated for stabilizing the solid area toner adhesion amount, and the gamma conversion table is modulated for stabilizing the halftone density.
Therefore, the present invention employing the gamma conversion corresponding to the development contrast is applicable also to the image forming apparatus of the image exposure type.
Sixth Embodiment
In the fourth embodiment, as shown in FIG. 3, the conversion to the binary level is executed in the error diffusion unit after the gamma conversion. In the present embodiment, there will be explained an image forming apparatus utilizing plural image processing methods including the error diffusion process. FIG. 24 is a schematic view showing the configuration of a digital image processing unit of the present embodiment, in which an original luminance signal read from a reader, a scanner, a network or a memory device, such as a hard disk, is subjected to an edge enhancement in a filter unit, then converted into an original density signal in a luminance-density conversion unit, and further converted into a desired output density signal in an output density conversion unit. The present embodiment is provided, for preparing signal data for driving the semiconductor laser, with an error diffusion process explained in the fourth embodiment, a dither screen process, and a pulse width modulation (PWM) process.
The present embodiment is featured by preparing two gamma conversion tables for each of the aforementioned three image processing methods, or six gamma conversion tables in total, and preparing a gamma conversion table corresponding to the current development contrast from two gamma conversion tables corresponding to the image processing method. In the dither screen process, the error diffusion process tends to generate more scattered toner dots on the photosensitive drum, thereby producing finer dots. For this reason, the gamma conversion table becomes different, even for the same development contrast. Similarly, the PWM process, having an already known tendency of generating dots more scattered in the main scanning direction and more connected in the sub scanning direction, requires gamma conversion tables different from those for the error diffusion process or for the dither screen process. Consequently, there are required two gamma conversion tables corresponding to each of the three image processing methods. The error diffusion process is used, for example, in the character mode, because of its features that the stability of gradation is inferior to that in other processes, though the resolution is extremely high. On the other hand, the dither screen process is used, for example, in the photograph mode, because of the excellent stability of the gradation, though the resolution is inferior. The PWM process is positioned in properties between these two methods and is, therefore, used, for example, in the character photograph mode.
FIG. 25 shows gamma conversion tables for two development contrasts for use in the dither screen process, and FIG. 26 shows gamma conversion tables for two development contrasts for use in the PWM process. The gamma conversion unit is provided in advance with two gamma conversion tables for each of the three image processing methods, including those shown in FIG. 6, and a gamma conversion table corresponding to the current development contrast is prepared through an interpolation unit and an averaging unit. The present embodiment, in which an optimum gamma conversion table is automatically selected corresponding to the image processing method and the current development contrast, is capable of providing constantly stable images, even in a situation where the development contrast has to be lowered.
In the fourth to sixth embodiments, there are prepared in advance the gamma conversion tables corresponding to the two development contrasts, and an optimum gamma conversion table is calculated according to the current development contrast, whereby the image quality represented by the solid area toner adhesion amount and the halftone density can be stabilized not only in a situation where the development contrast has to be lowered, such as in the reduction of the toner consumption amount or in the prevention of the leak of the charger, but also, under any environment (as described in the first embodiment).
In the fourth embodiment, there has been explained an image forming apparatus of the background exposure system, but the present invention is also applicable to an image forming apparatus of the image exposure system, taking the development contrast as a parameter. Thus, by lowering the development potential simultaneously with the lowering of the dark potential, the image quality represented by the solid area toner adhesion amount and the halftone density can be stabilized, not only in a case of reducing the toner consumption amount or in a case of preventing leakage by the charger, but also, under any environment (as described in the fifth embodiment).
Also, in the image forming apparatus provided with plural different image processing methods, an optimum image can be provided according to the image processing method and the development contrast, by preparing two gamma conversion tables for each of the image processing methods (as described in the sixth embodiment).
The present invention has been explained by certain preferred embodiments thereof, but the present invention is not limited to such embodiments, and is subject to various modification and applications within the scope and spirit of the appended claims.

## Claims

1. An image forming apparatus, which visualizes an image, according to a latent image potential, on an image bearing body, which includes a charging unit and an exposing unit, in which a development potential is applied to a developing unit, and a development contrast is a difference between the latent image potential and the development potential, wherein a designated development contrast is defined as a sum of a basic development contrast Vcont, and a development correction amount ΔVcont1, for correcting the basic development contrast Vcont, said image forming apparatus comprising:
(a) a γ conversion unit having a γ conversion table γαi of a first value Vcontα of the designated development contrast in a case that the development contrast correction amount ΔVcont1 is a first value, and a γ conversion table γβi, different from the γ conversion table γαi, of a second value Vcontβ, different from the first value Vcontα, of the designated development contrast in a case that the development contrast correction amount ΔVcont1 is a second value different from the first value of ΔVcont1; and
(b) an arithmetic operation unit for calculating a γ conversion table γNowi of a third value VcontNow of the designated development contrast in a case that the development contrast correction amount ΔVcont1 is a third value, based on a formula:


γNowi=γαi+(γβi−γαi)×(Vcontα−VcontNow)/(Vcontα−Vcontβ),


wherein the subscript i is a value of an output density signal in the γ conversion table, and
wherein the γ conversion unit conducts γ conversion of the third value VcontNow of the designated development contrast, based on the γ conversion table γNowi calculated by the arithmetic operation unit.

2. The image forming apparatus according to claim 1, wherein the first development contrast correction amount ΔVcont1 is used to reduce a toner consumption amount in the developing unit or to deal with leakage of the charging unit, and the first development contrast correction amount may be one of a plurality of levels.

3. The image forming apparatus according to claim 1, wherein the second development contrast correction amount ΔVcont2 is used to adjust image density.

4. An image forming apparatus according to claim 1, further comprising:
an environment detection unit adapted to detect at least one of a temperature and a humidity; and
an adjustment unit adapted to execute image adjustment by changing a target development contrast and a gamma development contrast for gamma conversion, and executing the gamma conversion based on the changed gamma development contrast, according to a detected result of the environment detection unit,
wherein the adjustment unit obtains the target development contrast VcontBody by a sum total of a basic development contrast Vcont, a first development contrast correction amount ΔVcont1 for correcting the basic development contrast, a second development contrast correction amount ΔVcont2, and a third development contrast correction amount ΔVcont3, such that VcontBody=Vcont+ΔVcont1+ΔVcont2+ΔVcont3,
the adjustment unit obtains the gamma development contrast VcontGamma for gamma conversion by VcontGamma=Vcont+ΔVcont1+C·ΔVcont3, where C is a constant other than 0, and
setting of C is defined within a range 0<C<1 according to the detected result of the environment direction unit.

5. The image forming apparatus according to claim 4, wherein the third development contrast correction amount ΔVcont3 is used to adjust a variation due to environment.