Apparatus for forming image according to image formation condition

An image forming apparatus adjusts a gamma-LUT of a gamma correction circuit in accordance with density data on a measurement image formed on a photosensitive drum. A CPU selects a conversion table in association with the image formation condition such as laser power of a semiconductor laser, fixing temperature of a fixing device, or a charge in a developer. A luminance/density converting portion converts luminance data on the measurement image into density data using the conversion table selected by the CPU. The CPU adjusts a contrast potential and a gamma-LUT using this density data.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus such as a copy machine or a laser beam printer, for example.

2. Description of the Related Art

Image formation conditions (for example, a gamma correction table, laser power, fixing temperature, electric charge of toner) used in an image forming apparatus to form a high-quality image need to be changed as appropriate depending on an installation environment (for example, temperature and humidity) of the image forming apparatus, or operating time thereof.

Japanese Patent Laid-Open No. 2002-072574 proposes a technique for performing color adjustment by forming a measurement image such as a patch or a pattern image on a transfer member, detecting the amount of applied toner of the measurement image using a regular reflection sensor, and feeding back the detected amount applied toner to a lookup table. Thus color stability is maintained without bothering a user.

Meanwhile, Japanese Patent Laid-Open No. 2003-215981 proposes a technique for controlling the amount of used recording material at an optimum value by operating outputs of an irregular reflection sensor and a regular reflection sensor.

Meanwhile, the abovementioned conventional techniques still have the following problem, and there is a room for improvement.

Generally, a measurement image detecting portion detects light reflected by the measurement image, and outputs a signal corresponding to a received reflected light amount (reflection output) to a density conversion circuit. Because the reflection output is a kind of luminance signal, the density conversion circuit converts the reflection output into a density signal. Usually, a relationship exists where as an attached toner amount becomes larger and an image density becomes higher, a reflection output becomes smaller. The density conversion circuit converts the reflection output having such a characteristic into an image density at the time of being formed on a recording material.

Incidentally, the image formation conditions are adjusted in every use of an image forming apparatus, and therefore change from time to time. For example, a target density and a target potential are maintained at appropriate values by density adjustment, potential control, or the like. However, it has been found that when the image formation conditions change, a correspondence between the reflection output and the image density held by the density conversion circuit becomes different from the actual relationship. If the correspondence between the reflection output and the image density becomes different from its initial state, accurate density control or potential control cannot be performed. If the conversion between the reflection output and the image density cannot be performed, a high-quality image will not be able to be formed.

SUMMARY OF THE INVENTION

Therefore, the present invention provides an image forming apparatus that forms a high-quality image even if an image formation condition changes by switching, upon the change in the image formation condition, a conversion table for converting a luminance of a measurement image into a density to one associated with that image formation condition.

The present invention in its first aspect provides an image forming apparatus comprising the following elements. A detection unit is configured to detect a measurement image formed on an image carrier and output luminance data on the measurement image. A storage unit is configured to store a plurality of conversion tables for converting the luminance data output by the detection unit into density data, the conversion tables being prepared in advance in association with different image formation conditions. A selection unit is configured to select a conversion table associated with an image formation condition used by the image forming apparatus from among the plurality of conversion tables stored in the storage unit. A tone correction unit is configured to convert the luminance data on the measurement image output by the detection unit into density data using the conversion table selected by the selection unit, and perform tone correction in accordance with the converted density data.

The present invention in its second aspect provides an image forming apparatus comprising the following elements. An adjustment unit is configured to adjust an image formation condition in accordance with a physical parameter measured without using a measurement image. A detection unit is configured to detect a measurement image formed on an image carrier and output luminance data on the measurement image. A storage unit is configured to store a plurality of conversion tables for converting the luminance data output by the detection unit into density data, the conversion tables being prepared in advance in association with different image formation conditions. A selection unit is configured to select, every time the image formation condition is adjusted by the adjustment unit, a conversion table associated with the adjusted image formation condition from among the plurality of conversion tables stored in the storage unit. A tone correction unit is configured to convert the luminance data on the measurement image output by the detection unit into density data using the conversion table selected by the selection unit, and perform tone correction in accordance with the converted density data.

The present invention in its third aspect provides an image forming apparatus comprising the following elements. A detection means detects a measurement image formed on an image carrier and output luminance data on the measurement image. A storage means stores a plurality of conversion tables for converting the luminance data output by the detection means into density data, the conversion tables being prepared in advance in association with different image formation conditions. A selection means selects a conversion table associated with an image formation condition used by the image forming apparatus from among the plurality of conversion tables stored in the storage means. A tone correction means converts the luminance data on the measurement image output by the detection means into density data using the conversion table selected by the selection means, and performs tone correction in accordance with the converted density data.

DESCRIPTION OF THE EMBODIMENTS

As an example of an image forming apparatus according to the present embodiment, a digital monochrome copy machine will be used in the following description. The feature of the present embodiment lies in that a density conversion table is changed in accordance with laser power of a light source among several types of image forming conditions. Note that it is well known that a measurement image such as a patch or a pattern image is formed on an image carrier or a recording material and an LUT for a gamma correction circuit is corrected based on density of the measurement image, and therefore, the description thereof will be omitted. The “LUT” is an abbreviation of a look-up table. The recording material is also called recording medium, sheet, or sheet material in some cases.

InFIG. 1, an image forming apparatus100performs tone correction in accordance with density data on a measurement image formed on an image carrier. A control portion200is a control unit for comprehensively controlling the overall image forming apparatus100. The control portion200executes tone correction using the measurement image, and adjusts an image formation condition in accordance with physical parameters measured without using the measurement image.

For example, the control portion200adjusts an image formation condition such as a developing bias potential Vdc applied to a developing device4, a driving current for a semiconductor laser32, a grid potential Vg applied to a primary charging device2, or the like.

A light source12irradiates a document D with illumination light. An optical system13forms a document image on a CCD21. The “CCD” is an abbreviation of a charge-coupled device. The light source12, the optical system13, and the CCD21are provided on a reader unit. The document D is scanned by the reader unit moving along the arrow. A luminance signal of the document image is digitalized by an A/D conversion circuit22and output as image data to the control portion200.

The control portion200performs image processing on the image data as necessary, and drives the semiconductor laser32, which is a light source, based on the image data. The semiconductor laser32is a light source that is adjusted to have a prescribed reference light amount by auto light power control (APC). After the semiconductor laser32is continuously used, its laser power gradually lowers even if a current at a constant level is kept flowing therethrough. Therefore, while laser light3is scanned over a non-image forming zone, the driving current is adjusted by APC so as to obtain a constant reference light amount. In the case where local sensitivity reduction is occurring on the surface of a photosensitive drum1, the laser power is possibly increased when that portion is irradiated with the laser light3. Generally, an area of a beam spot per dot is changed by performing pulse width modulation on the driving current for the semiconductor laser32. When the beam spot area is changed, an amount of applied toner is also changed, and as a result, an image tone is expressed. Note that the non-image forming zone is provided with an optical sensor for receiving the laser light3and outputting a signal indicating a light-receiving level (light amount). The level of reception of the laser light3is an example of the physical parameters measured without using a measurement image.

The control portion200controls the primary charging device2to uniformly charge the surface of the photosensitive drum1. The laser light3emitted by the semiconductor laser32is deflected by a polygon mirror33, and the photosensitive drum1, which is an image carrier, is irradiated therewith. Thus, an electrostatic latent image corresponding to the input image data is formed on the photosensitive drum1.

The developing device4, whose developing bias potential Vdc is controlled by the control portion200, develops the electrostatic latent image on the photosensitive drum1using a developer containing toner to form a toner image. The recording material P is transported in synchronization with an image leading edge of the toner image. A transfer charging device6is a transfer unit for transferring the toner image from the photosensitive drum1onto the recording material P. The fixing device10heats and presses the toner image transferred onto the recording material P to fix the toner image onto the recording material P.

On the upstream side of the developing device4in the rotation direction R1of the photosensitive drum1, a potential sensor S0is provided. The potential sensor S0measures surface potential of the photosensitive drum1. As is known well, the control portion200controls the grid potential Vg of the primary charging device2and the developing bias potential Vdc of the developing device4.

Further, on the downstream side of the developing device4in the rotation direction R1of the photosensitive drum1, a detecting portion50having a LED52and a photosensor51is arranged. The detecting portion50functions a detection unit for detecting the measurement image formed on the photosensitive drum1and outputting luminance data on this measurement image. The LED52functions as a light-emitting unit for emitting light towards the measurement image on the photosensitive drum1. The photosensor51functions as a light-receiving unit for receiving light reflected by the measurement image and outputting an analog luminance signal corresponding to the amount of the received reflected light.

Next, the flow of an image signal from the CCD21to the laser light3according to the present embodiment will be described with reference toFIG. 2. A CPU28, such as a microprocessor, executes various kinds of control in accordance with control programs stored in a ROM210or various data. The RAM212is a storage device used as a work area for the CPU28. The CPU28is connected to the abovementioned LED52and photosensor51. The photosensor51may have a built-in A/D converter53for converting the analog luminance signal into digital luminance data, or the A/D converter53may alternatively be provided outside the photosensor51.

The CPU28is connected to a temperature sensor54for measuring a temperature of the fixing device10. Note that a sensor for detecting an absolute water content in the atmosphere environment may be connected to the CPU28. The absolute water content is used by the CPU28to determine an initial value of a contrast potential Vcont. Also, the absolute water content is an example of the physical parameters measured without using a measurement image.

A luminance signal corresponding to the document image obtained by the CCD21in a reader portion is digitalized by the A/D conversion circuit22. A shading circuit23adjusts an amplification gain for each sensor cell in a sensor cell group in the CCD21to reduce influence of uneven sensitivity of individual sensor cells arrayed in a row. A LOG conversion circuit24converts an output signal from the shading circuit23from a luminance scale into a density scale. Thus the luminance signal is converted into a density signal.

A gamma-LUT25is a conversion table that can be rewritten by the CPU28. The gamma-LUT25converts and outputs a tone of an input density signal. The CPU28adjusts the gamma-LUT25, which serves as a gamma correction circuit, in accordance with density data on the measurement image formed on the photosensitive drum1.

A pulse width modulation circuit26functions as a modulation unit for changing the driving current for driving the semiconductor laser32depending on image data, and thereby forming a latent image corresponding to the image data on the photosensitive drum1. The pulse width modulation circuit26converts the density signal into a signal corresponding to light-emission duration time of the laser light3, and delivers the signal to a laser driver31. The light-emission duration time of the laser light3is associated with the density (tone) of an image to be formed. The semiconductor laser32repeats turning on and off in accordance with this signal.

Incidentally, a pattern generator29is mounted on the control portion200. The pattern generator29holds a tone pattern (measurement image) shown inFIG. 3. The pattern generator29directly delivers a signal to the pulse width modulation circuit26in accordance with an instruction given by the CPU28. In other words, when the measurement image is formed, the gamma-LUT25does not affect the image signal.

A luminance/density converting portion42has a plurality of conversion tables42ato42dfor converting luminance data into density data. The conversion tables42ato42dare prepared in advance in association with mutually different image formation conditions. The CPU28selects, from among the conversion tables42ato42d, a conversion table associated with the image formation condition currently used in the image forming apparatus100. The CPU28, upon detecting the change in the image formation condition, selects a conversion table associated with the changed image formation condition. Exemplary types of image formation conditions include laser power (reference light amount) of the semiconductor laser32, fixing temperature of the fixing device10, and toner charge in the developer. Because adjustment of those image formation conditions is executed under the control of the CPU28, the CPU28can detect the change in the image formation conditions.

The luminance/density converting portion42converts the luminance data on the measurement image detected by the detecting portion50into density data using the conversion table selected by the CPU28.

Next, the role of the gamma-LUT25will be described.FIG. 4is a four-quadrant chart showing characteristics with which density of a document image is reproduced. Quadrant I shows a characteristic of the reader portion in the image forming apparatus100that converts document density into a density signal. Quadrant II shows a characteristic of the gamma-LUT25that converts the density signal into a laser output signal. Quadrant III shows a characteristic of a printer portion in the image forming apparatus100that converts the laser output signal into output density (density of a toner image on the recording material P). Quadrant IV shows a relationship between the output density and the document density. Thus, those characteristics represent overall tone characteristics in the image forming apparatus100. Note that in the case where processing is performed with an 8-bit digital signal, the number of tones is 256.

If the document image is copied to form a duplicate, it is expected that density of the document image agree with density of the duplicate. Therefore, the image forming apparatus100corrects a curved section in a recording characteristic of the printer portion shown in Quadrant III based on the gamma-LUT25shown in Quadrant II, thereby keeping a tone characteristic shown in Quadrant IV to be linear. The gamma-LUT25can be easily created by inverting an input-output relationship in the characteristic in Quadrant III. That is, a laser output signal at the time when a measurement image is formed needs only be replaced with a density signal obtained from the measurement image. Thus the gamma-LUT25converts the density signal of the document image into the laser output signal.

The detecting portion50detects light reflected by the measurement image and outputs a reflection output at the time when the measurement image comes to a position opposite the detecting portion50. The reflection output is a kind of a luminance signal. The luminance/density converting portion42converts the reflection output for the measurement image into a density signal using the conversion table selected by the CPU28. In the present embodiment, elements employed for the LED52and the photosensor51have a light emission peak and a light reception sensitivity peak of 960 nm, respectively.

As described above, the luminance/density converting portion42has the conversion tables42ato42d. The conversion tables42ato42dmay be stored in the RAM212, or in a memory built in the luminance/density converting portion42. In either case, the CPU28creates the conversion tables42ato42dand stores them in that storage device. Then, the CPU28selects a conversion table associated with the latest image formation condition currently in use. The luminance/density converting portion42converts a luminance signal into a density signal using the conversion table selected by the CPU28. Although four conversion tables42ato42dare used as an example here, the number of the conversion tables may be any number of 2 or larger. The method for creating the conversion tables42ato42dis already known, and the detail description thereof will be omitted.

As shown inFIG. 5A, the conversion tables42ato42dare prepared in advance in accordance with the light amount (laser power) of the light source, which is an example of the types of the image formation conditions. Note that the plurality of conversion tables may be substantially achieved by the CPU28performing prescribed calculation (coefficient multiplication or the like) in accordance with the laser power with respect to one conversion table used as a reference. According toFIG. 5A, the laser power is expressed by 8 bits.

The light reflected by the toner that enters the photosensor51is near infrared light. InFIG. 2, the photosensor51converts the reflected light into an electric signal. This electric signal is a kind of luminance signal that varies in the range of 0 V to 5 V. The A/D converter53converts this electric signal into a digital luminance signal of a level ranging from 0 to 255 in proportion to a voltage level of the electric signal. In other words, the A/D converter53functions as an AD conversion unit for converting the analog luminance signal output by the photosensor51and outputting digital luminance data. The digital luminance signal is delivered, via the CPU28, to the luminance/density converting portion42. The CPU28refers to the correspondence table shown inFIG. 5A, selects the conversion table associated with the laser power set currently for the semiconductor laser32, and sets the selected conversion table for the luminance/density converting portion42. The luminance/density converting portion42converts the digital luminance signal into a density signal using the conversion table selected by the CPU28. The density signal is input to the CPU28.

In the present embodiment, one-component magnetic toner is employed as a black developer. The one-component magnetic toner has a good performance in running cost reduction for monochrome copy.FIG. 6Ashows a spectral characteristic of the black toner. As shown inFIG. 6A, the reflectivity of near infrared light (960 nm) from the black toner is about 10%. Note that two-component toner may alternatively be employed as the black developer. Also, the photosensitive drum1in the present embodiment is an OPC (organic photo conductor) drum, and its reflectivity with respect to near infrared light (960 nm) is about 40%. The photosensitive drum1may be an amorphous silicon drum.

FIG. 6Bshows an example of the conversion tables42ato42dheld by the luminance/density converting portion42. The vertical axis indicates image density, and the horizontal axis indicates reflection output. As a coverage factor (image density) of a black toner area that covers the photosensitive drum1becomes larger, the output of the photosensor51gradually becomes smaller. The black toner containing carbon black absorbs light of 960 nm. Accordingly, as the amount of attached toner becomes larger, the image density becomes higher, while the reflection output becomes smaller. By selectively using the conversion tables42ato42dshown inFIG. 6Bin accordance with the image formation condition, the CPU28can obtain the density signal with high accuracy. The photosensor51is adjusted so that the reflection output is 4 V when detecting the light reflected by the surface (base material) of the photosensitive drum1.

Incidentally, the inventors formed measurement images of the same density using different levels of laser power, and measured the reflection outputs thereof. As a result, it was found that the reflection output tends to be higher as the laser power is lower. This is because scattering of the toner increased due to the lower laser power, and the reflection output lowered. However, the density of the toner image after being subjected to fixing processing is in proportion to the total toner amount on the recording material P. Therefore, even if the reflection output lowers, the image density does not change. Accordingly, if the conversion tables associated with different levels of laser power are separately prepared in advance, density conversion that is hardly affected by the change in the laser power can be achieved.

The reason why the maximum densities in the conversion tables42bto42dare lower than that in the conversion table42ais because, as shown inFIG. 5A, the laser powers associated with the conversion tables42bto42dare originally lower than the laser power associated with the conversion table42a. The absolute maximum image density is in proportion to the laser power. Note that in the case where the density of the image detected by the detecting portion50is significantly smaller than the set level, the CPU28may determine that some malfunction has occurred in the image forming apparatus100, and display an error message on a display device (not shown) in an operation panel.

A procedure of tone characteristic control executed immediately after the image forming apparatus100is activated, that is, setting of the gamma-LUT25by the CPU28will be described with reference to the flowchart inFIG. 7. This flowchart is executed by the CPU28. Note that the method for adjusting the gamma-LUT25is already know well, and therefore will be described simply.

In step S1, the CPU28measures a fixing temperature using the temperature sensor54for measuring the temperature of the fixing device10. The temperature sensor54functions as a temperature measurement unit for measuring the fixing temperature of the fixing device10.

In step S2, the CPU28determines whether or not the measured fixing temperature is equal to or lower than a prescribed temperature threshold (for example, 150° C.). The temperature threshold is a fixing temperature to be an index for determination of whether or not the tone characteristic control is necessary. If the fixing temperature is not equal to or lower than the temperature threshold, the CPU28skips the tone characteristic control and ends the processing according to the present flowchart. Meanwhile, if the fixing temperature is equal to or lower than the temperature threshold, the CPU28determines that the tone characteristic control needs to be performed, and proceeds to step S3.

In step S3, the CPU28waits until the components of the image forming apparatus100enter into a standby state. For example, upon the laser temperature of the semiconductor laser32reaching a prescribed temperature, the CPU28determines that the semiconductor laser32has shifted from a warm-up state to a stand-by state. Here, the CPU28performs potential control, which is part of image stabilization control. The CPU28measures the surface potential Vd of the photosensitive drum1using the potential sensor S0provided to the photosensitive drum1. The CPU28adjusts the grid potential Vg of the primary charging device2and the developing bias potential Vdc of the developing device4based on the measured value of the surface potential Vd, and corrects change in discharge magnitude of the primary charging device2and sensitivity deterioration of the photosensitive drum1.

In step S4, the CPU28controls the pattern generator29to cause it to output image data on the measurement image of the maximum density (for example, level255), and thereby forms the measurement image on the recording material P. Note that the contrast potential Vcont used at this time is derived from the absolute water content in the atmosphere environment. For example, the ROM210stores a table indicating a relationship between the absolute water content and the contrast potential Vcont, and the CPU28obtains, from the table, the contrast potential associated with the absolute water content measured using a sensor. Also, the CPU28stores the laser power value used at this time in the RAM212. The laser power value is determined in advance by auto light power control (APC).

In step S5, the CPU28reads the measurement image formed on the recording material P with the CCD21in the reader portion.

In step S6, the CPU28reads the laser power value from the RAM212, selects the conversion table associated with the value from the correspondence table shown inFIG. 5A, and sets the selected conversion table for the luminance/density converting portion42.

In step S7, the CPU28converts the luminance data on the measurement image of the maximum density into the density data using the luminance/density converting portion42. Thus, the maximum density DA at the time when the relative drum surface potential is A is obtained.

In step S8, the CPU28computes the contrast potential Vcont corresponding to a target largest density. Here, the specific method for determining the contrast potential Vcont will be described.

FIG. 8Ashows a relationship between the relative drum surface potential and the image density. In the abovementioned step S4, the CPU28uniformly charges the photosensitive drum1with the primary charging device2, and causes the semiconductor laser32to output the laser light3for forming the measurement image of the maximum density to form a latent image on the surface of the photosensitive drum1. Further, the CPU28measures the surface potential Vd in a latent image area with the potential sensor S0. The CPU28obtains a difference (relative drum surface potential A) between measured values of the developing bias potential Vdc and the surface potential Vd of the photosensitive drum1that are currently in use. According toFIG. 8A, the maximum image density obtained with respect to the relative drum surface potential A is the abovementioned DA. The maximum image density DA is obtained in steps S5and S6, as described above. If the conversion tables42ato42bare selected in accordance with the laser power, in most cases the image density corresponding to the relative drum surface potential is linear, as shown by a solid line L. Meanwhile, in a two-component development system, if the toner density in the developing device4decreases, in some cases the image density has a nonlinear characteristic in the vicinity of the maximum density, as shown by a broken line N. The target image density is 1.6 according toFIG. 8A, while the target image density may be set to be 1.7 in consideration of a margin of 0.1. Here, it is assumed that the target image density is 1.7. The relative drum surface potential (contrast potential B) at the time when the target image density is set to be 1.7 can be obtained using the relative drum surface potential A, the maximum density DA, and the following expression:
B=A×1.7/DA

The method for deriving the grid potential Vg and the developing bias potential from the contrast potential based on the relationship between the grid potential Vg of the primary charging device2and the surface potential of the photosensitive drum1will be simply described.FIG. 8Bshows the relationship between the grid potential Vg and the surface potential Vd of the photosensitive drum1. The surface potential Vd at the time when the photosensitive drum1is scanned after the grid potential Vg is set to be −200 V and the level of the laser light3is set to be the minimum value in its configurable range is VL. The surface potential Vd at the time when the level of the laser light3to be the maximum value in the configurable range is VH. The CPU28measures surface potentials VL and VH using the potential sensor S0. Similarly, the CPU28measures the surface potentials VL and VH at the time when the grid potential Vg is set to be −400 V. The CPU28obtains the relationship between the grid potential Vg and the surface potential Vd shown inFIG. 8Bby interpolating and extrapolating between the −200 V data and the −400 V data.

The CPU28sets the developing bias potential Vdc by subtracting, from VL, Vbg (for example, 100 V) that is set so that fogging toner is not attached onto the image. As shown inFIG. 8B, the contrast potential Vcont is a difference voltage between the developing bias potential Vdc and VH. Further, as shown inFIG. 8A, as the relative drum surface potential (contrast potential Vcont) is larger, the maximum density becomes higher.

The CPU28calculates the grid potential Vg and the developing bias potential Vdc based on the contrast potential Vcont=B computed in step S8and the relationship shown inFIG. 8B. Note that the contrast potential Vcont=B is set to be higher by 0.1 than the usual target maximum density, namely 1.6.

In step S9, the CPU28causes the pattern generator29to output the tone pattern shown inFIG. 3. As is clear from the configuration inFIG. 2, the gamma-LUT25is not configured to affect the image data on the tone pattern output by the pattern generator29. The image data on the tone pattern is output via the pulse width modulation circuit26to the laser driver31, and drives the semiconductor laser32. Thus the tone pattern serving as the toner image shown inFIG. 3is formed on the surface of the photosensitive drum1.

In step S10, the CPU28selects the conversion table associated with the laser power of the laser light3for forming the tone pattern. If the laser power is 128, the conversion table42cis selected from the correspondence table inFIG. 5A.

In step S11, the CPU28detects the tone pattern on the photosensitive drum1with the detecting portion50, and converts the reflection output of the photosensor51in the detecting portion50into the density data with the luminance/density converting portion42.

In step S12, the CPU28specifies, from the formation position of a tone pattern at every tone level, the laser power used when the tone pattern is formed, creates the gamma-LUT25in association with the density data on that tone pattern, and stores the created gamma-LUT25in the RAM212. The number of sets of the density data is not more than the number of the tone patterns, and therefore, in some cases the gamma-LUT25cannot be calculated only with the existing density data. In this case, the CPU28may generate the laser power in association with the density data at all levels ranging from 0 to 255 by interpolation.

After the above-described control ends, the CPU28displays a message “Copier: Ready” on the display device in the operation panel, and enters into a copy stand-by state. During the copy operation, the contrast potential Vdc and the gamma-LUT25calculated by the abovementioned method are used, and so toner images having a linear tone characteristic are formed. By regularly performing the above control, images having excellent tonality can be formed for a long time.

In the first embodiment the present invention is applied to a single-color image forming apparatus, while in the second embodiment the invention is applied to a multi-color image forming apparatus. In the multi-color image forming apparatus, the above-described conversion tables42ato42dare prepared as many as the colors. That is, the characteristic of the second embodiment lies in that the conversion tables associated with image formation conditions are prepared for each color.

FIG. 9shows a block diagram of the control portion200in the present embodiment. ComparingFIGS. 2 and 9, those two figures are different at least in the point thatFIG. 9additionally has a Bk generation circuit91, and the conversion tables42ato42dare prepared for each color. Thus, in the second embodiment, a plurality of conversion tables are provided for each of the plurality of developers of different colors. The CPU28selects a conversion table for each color associated with the image formation condition set for the color from among the plurality of conversion tables.

FIG. 10shows the image forming apparatus100for forming a multi-color image. Note that the components already mentioned above are assigned the same reference numerals to simplify and the description thereof. The developing device4in the image forming apparatus100shown inFIG. 10is a rotary-type developing device. The developing device4generates toner images of the respective colors in order by switching a developing sleeve that abuts the photosensitive drum1. InFIG. 10, a yellow developing sleeve abuts the photosensitive drum1. The developing device4may be a so-called tandem-type developing device. This is because the present invention does not depend on the type of the developing device.

The recording material P contained in a paper feed cassette81is supplied, via a paper feed roller82, a transport roller83, and a registration roller84, to a transfer drum5. The recording material P is wound around the transfer drum5. Every time the transfer drum5rotates once, toner images of Y (yellow), M (magenta), C (cyan), and Bk (black) are transferred onto the recording material P in this order. That is, with four times of rotation of the transfer drum5, the toner images of four colors are transferred onto the recording material P so as to be superimposed on each other. After the transfer ends, the recording material P is separated from the transfer drum5, and the toner images are fixed by the fixing device10.

Meanwhile, the CCD21in the reader portion obtains an RGB signal of a document image via a color separation filter for three colors, namely R (red), G (green), and B (blue). The A/D conversion circuit22converts an analog RGB luminance signal into digital luminance data, and outputs the digital luminance data to the shading circuit23. The shading circuit23executes the abovementioned shading correction. The LOG conversion circuit24converts the RGB luminance data into CMY density data. The Bk generation circuit91generates black density data from the CMY density data, and creates density data on four colors, namely MCYBk. The gamma-LUT25executes the tone control on the density data. The density data drives the semiconductor laser32via the pulse width modulation circuit26and the laser driver31.

Note that the developer for multi-color image formation used in the image forming apparatus100is toners of four colors, namely yellow, magenta, cyan, and black. The YMC toners are formed using styrene copolymer resin as a binder by dispersing color materials of yellow, magenta, and cyan, respectively. The black toner is made by mixing three colors of YMC.

The spectral characteristics of the yellow, magenta, cyan, and black toners are shown inFIGS. 11A,11B,11C, and11D, respectively. With all toners, an 80% or higher reflectivity was obtained with respect to 960 nm. Further, the image forming apparatus100employs, in image formation using those color toners, the two-component development system, which has advantages in terms of color purity and transparency. The photosensitive drum1is an OPC drum, and has a reflectivity of about 40% with respect to 960 nm. As shown inFIG. 12A, the reflection output becomes larger as more toner is applied to the photosensitive drum1.

In the present embodiment, because of the difference in the reflectivity of the color materials of the respective colors, an independent density conversion circuit for each color that are associated with the laser power is necessary. In the present embodiment, the detecting portion50is set so that a drum reflection output in a state where the toner is not attached is 1 V.

FIG. 12Bshows an example of a relationship between the reflection output of the photosensor51for a yellow image and an actual image density. Also,FIG. 5Bshows a table indicating a correspondence between the laser power and the conversion tables. In the present embodiment, as the laser power of the semiconductor laser32becomes lower, the reflection output of the photosensor51tends to becomes larger. Therefore, the CPU28selects the conversion table of each color associated with the laser power using the correspondence table shown inFIG. 5B.

The flowchart for the second embodiment is basically the same as the flowchart shown inFIG. 7. The CPU28executes the steps S4to S12for the four colors of YMCBk individually. That is, in step S4a measurement image of the maximum density is formed with respect to each of YMCBk, and is read in step S5. In step S6, the CPU28selects the conversion table for each color in accordance with the laser power. In step S7, the luminance data is converted into the density data with respect to each color. In step S8, the contrast potential and the like are determined with respect to each of YMCBk. In step S9, a tone pattern of each of YMCBk is formed and read. In step S10, the CPU28selects the conversion table for each color in accordance with the laser power. In step S11, the luminance data on the tone pattern of each color is converted into the density data using the selected conversion table. In step S12, the gamma-LUT25is created for each color.

According to the present embodiment, the same advantage as in the first embodiment is obtained by applying the present invention to the image forming apparatus100that forms multi-color images. That is, the image forming apparatus100is capable of providing high-quality multi-color images with excellent tonality and good gray balance for a long time.

In the above-described example, the photosensitive drum1is used as an image carrier of the image forming apparatus100. However, the image carrier is not limited to the photosensitive drum1, and may alternatively be a photosensitive sheet in a sheet shape or a photosensitive belt in a belt shape with a photosensitive layer on its surface, for example.

In the image forming apparatus100that forms multi-color tone images, if the CPU28changes the laser power of the semiconductor laser32during job execution, the CPU28selects the conversion table for each color in association with the changed laser power value.

Incidentally, although in the first and second embodiments the conversion table is changed in accordance with the change in the laser power, the CPU28may change the conversion table in accordance with the change in another type of the image formation conditions, such as the fixing temperature or the toner charge in the developer. Also in this case, a correspondence table indicating the relationship between the image formation conditions and the conversion tables are stored in the ROM210, and is referred to by the CPU28.

For example, as the fixing temperature becomes lower, the actual image density tends to become lower. Therefore, the CPU28selects or creates a conversion table with which the reflected light amount is reduced every time fixing temperature decreases. Accordingly, the CPU28functions as a selection unit for selecting, from among a plurality of conversion tables, a conversion table associated with the fixing temperature measured by a temperature measurement unit.

Also, the CPU28selects or creates a conversion table with which the reflected light amount is increased every time the toner charge decreases. The CPU28may predict the toner charge from the number of formed images, or estimate the toner charge from data obtained using the potential sensor S0or the detecting portion50. Those function as a charge measurement unit for measuring the toner charge in the developer. Thus the CPU28functions as a selection unit for selecting, from among a plurality of conversion tables, a conversion table associated with the toner charge in the developer measured by the charge measurement unit. The toner charge in the developer is an example of the physical parameters measured without using a measurement image.

According to the present invention, upon an image formation condition changing due to use of an image forming apparatus100, a conversion table associated with the changed image formation condition is selected, luminance data on a measurement image is converted into density data based on the selected conversion table, and the gamma correction circuit is adjusted according to the density data. With this configuration, the gamma correction circuit can be adjusted without bothering a user when the image formation condition changes, and therefore, the image forming apparatus is capable of continuously forming high-quality images. For example, it is possible to keep a good output density range in the image forming apparatus100, and further maintain a stable tone characteristic ranging from highlight to shadow.

This application claims the benefit of Japanese Patent Application No. 2011-200972, filed Sep. 14, 2011, which is hereby incorporated by reference herein in its entirety.