JP2002248808A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an imaging apparatus having an LED print head in which a toner image can be formed with good gradation proportional to that of image data. SOLUTION: Period of a lighting reference clock for varying the lighting time of an LED depending on the gradation of image data can be altered depending on the gradation of the image data. Since linearity can be ensured between the image data and the gradation of an image formed by toner, image quality can be enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, and a printer.
The present invention relates to an image forming apparatus having an LED print head for exposing a photoconductor surface.

[0002]

2. Description of the Related Art There is known an image forming apparatus such as a copying machine or a printer which forms an image by an electrophotographic method, in which an electrostatic latent image is formed by an LED print head. In such an apparatus, the LED print head faces the surface of the photosensitive drum and has a shape elongated in the axial direction of the photosensitive drum. LED printheads, for example,
It has a configuration in which about 7,000 LEDs are arranged in a straight line.

[0003] Each LED of the LED print head
The lighting time is controlled by the lighting clock, and the lighting time is changed according to the gradation of the image data. As a result, if the lighting time is long, the surface of the photosensitive drum is strongly exposed, the amount of toner adhering to the exposed area increases, and a dark image can be formed. On the other hand, when the lighting time is short, the amount of exposure on the surface of the photosensitive drum is small, the amount of toner on the exposed area is small, and a thin image is formed.

[0004]

By the way, the conventional LE
The lighting clock for controlling the lighting time of each LED of the D print head has a constant cycle, and is one of the lighting clocks.
The lighting time of the LED could be controlled only by an integral multiple of the cycle. However, the relationship between the lighting time of the LED, that is, the exposure time of the surface of the photosensitive drum, and the amount of toner on the exposed area is not a linear relationship but a non-linear relationship. That is, even if the exposure time is linearly increased in proportion to the gradation of the image data, the relationship between the gradation of the image data and the density of the formed toner image is linear because the development characteristics are non-linear. There is a problem that the gradation of the image data and the density of the formed toner image are slightly different from each other.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and an object of the present invention is to provide an image forming apparatus having an LED print head, which can form an image with good gradation. And

[0006]

Means for Solving the Problems and Effects of the Invention According to the first aspect of the present invention, there are provided a plurality of L for exposing the surface of a photoreceptor.
In an electrophotographic image forming apparatus having an LED print head in which EDs are arranged, a lighting time setting unit in which a relationship between an image density and a lighting time is set in advance, and lighting according to a gradation of input image data. An LED driving means for reading a time from the lighting time setting means and lighting each LED of the LED print head with the read lighting time.

According to a second aspect of the present invention, the lighting time setting means sets the relationship between the image density and the lighting time by a lighting reference clock having a different cycle according to the gradation of the image. A gradation data counter that outputs a count number corresponding to the gradation of the input image data, and a frequency division counter that divides the lighting reference clock read from the lighting time setting means until the count becomes the same as the count number. 2. The image forming apparatus according to claim 1, wherein an output pulse of the frequency dividing counter is given as a lighting time.

According to a third aspect of the present invention, the lighting time setting means is a CPU of a CPU which is a control center of the image forming apparatus.
3. The image forming apparatus according to claim 2, further comprising a look-up table storing a relationship between the number of clocks (reference clocks) and gradations, and generating a lighting reference clock based on the look-up table. Device.
The invention according to claim 4, wherein the relationship between the number of clocks of the reference clock and the gradation is determined based on each gradation confirmation patch printed by the image forming apparatus. Image forming apparatus.

The invention according to claim 5 is the image forming apparatus according to claim 3, wherein a plurality of the look-up tables are prepared, and any one of the look-up tables can be selected. The invention according to claim 6 is the image forming apparatus according to claim 3, wherein the look-up table is rewritable. The invention according to claim 7, wherein the lighting time setting means is provided in a CPU, which is a control center of the image forming apparatus, and the CPU controls the LED from the LED.
3. The image forming apparatus according to claim 2, wherein a lighting reference clock is supplied to the driving unit.

In a preferred embodiment of the present invention, the lighting time setting means is provided in an LED driving means, and the reference clock is supplied to the LED driving means from a CPU which is a control center of the image forming apparatus. The image forming apparatus according to claim 2, wherein: According to a ninth aspect of the present invention, there is provided an electrophotographic image forming apparatus having an LED print head in which a plurality of LEDs for exposing the surface of a photoreceptor are arranged.
LED driving means for lighting D, and a lighting time signal for generating a lighting reference clock to be given to the LED driving means for changing the lighting time of each LED of the LED print head according to the gradation of image data And a lighting time signal generating means capable of changing a cycle of a lighting reference clock according to a gradation of image data.

According to a tenth aspect of the present invention, there is provided an LED print head in which a plurality of LEDs for exposing the surface of a photoreceptor are arranged, wherein a relationship between an image density and a lighting time is set in advance. And an LED drive unit for reading a lighting time according to a gradation of input image data from the lighting time setting unit, and lighting each LED with the read lighting time. It is. According to the present invention, the lighting time for lighting each LED of the LED print head can be set non-linearly, so that the LED can be lighted with the optimum lighting time according to the gradation of the input image data. As a result, it is possible to flexibly cope with the non-linear characteristics of the gradation and the exposure time of the image data, to associate the two well, and to give the formed image good gradation.

That is, the linearity of the gradation of the image data and the image formed by the toner can be secured, and the image quality can be improved. Further, in the image forming apparatus, the relationship between the LED lighting time and the print image density differs depending on the inherent characteristics, for example, the characteristics of the photosensitive drum, and the setting conditions, for example, the settings of the development potential and the transfer potential. For this reason, the relationship between the number of reference clocks and the gradation is prepared in a plurality of look-up tables so that any one of the look-up tables can be selected so as to support different relationships. preferable.

Preferably, the look-up table is rewritable. According to the present invention, an image forming apparatus having good image forming performance, particularly excellent in gradation expression, can be provided by the above configuration.

[0014]

Embodiments of the present invention will be specifically described below with reference to the drawings. FIG. 1A,
FIG. 1B is an illustrative view showing an arrangement relationship between the photosensitive drum 12 and the LED print head 5 provided in the image forming apparatus according to the embodiment of the present invention. As shown in FIGS. 1A and 1B, the photoconductor drum 12 is a cylindrical body that is long in the axial direction, and has a photoconductor layer on its surface. The photosensitive layer on the surface of the photosensitive drum 12 is charged to a predetermined potential by a discharger (not shown).

The LED print head 5 extends in the axial direction of the photosensitive drum 12 and faces the surface of the photosensitive drum 12. The LED print head 5 includes a large number of LEDs arranged in the length direction. For example,
This image forming apparatus can print 6 sheets of paper
When an image with a resolution of 00 dpi can be formed, the LED print head 5 is provided with 7000 LEDs arranged in series. Each LED is provided with a driver for controlling the lighting of the LED, and the lighting / extinguishing and the lighting time are controlled for each LED.

If the LED lighting time is long, the surface of the photosensitive drum 12 is strongly exposed, and the amount of toner on the exposed area increases. Conversely, when the LED lighting time is short, the exposure intensity on the surface of the photosensitive drum 12 is weak, and the amount of toner on the exposure area decreases. The gradation of the formed toner image changes depending on the amount of toner on the surface of the photosensitive drum 12. FIG. 2 is a configuration block diagram of a printer having the LED print head 5. In FIG.
Includes a CPU 2, a ROM 3, a RAM 4, an LED print head 5, an LED driving unit 6, an LED head 7, a network interface 8, and an internal bus 9. The image data is sent, for example, from a personal computer 11 connected via a network cable 10.

FIG. 3 shows the structure of the LED print head 5. The LED print head is of a gradation type, in which m (m is an integer) chips are arranged in an LED array 71 on which n (n is an integer) dots of LEDs (7211 to 721n) are integrated. LED driving circuits 61 to be driven are arranged in m chips. The following LED array 71
Will be described. For example,
In the case of a 4-bit gradation type LED print head, each of the LED drive circuits (611 to 61m) has 4 bits ×
Latch circuit for n dots (631-63m), 4bit
× shift register for n dots (621 to 62m), n
It has AND circuits (6511 to 65mn) for dots. The grayscale data is supplied from the data lines D1 to D4 to the first LED drive circuit 611 in synchronization with the shift clock of the shift clock signal line SCLK. The grayscale data moves through the shift register 621, and when the (n + 1) th data is given, the grayscale data input first is shifted by the shift register 622 of the second LED drive circuit 612.
Enter. As described above, 4-bit gradation data sequentially moves through the shift registers (621 to 62m), and nx
When m sets of gradation data are input, the gradation data is input to all the shift registers (621 to 62m). Next, a latch signal is supplied from the latch signal line LA, and the grayscale data is latched by the latch circuits (631 to 63m) at the timing of the latch signal. Next, this 4 bit
The lighting time signal generator 64 outputs a lighting time signal corresponding to each gradation from the gradation data and the lighting reference clock CLK1. The AND circuit (6511 to 65mn) performs an AND operation on the lighting time signal and the strobe signal from the strobe signal line ST, and the AND circuit (6511 to 65mn)
Output from the LED (7211 to 72m
n) is turned on. G is a ground line.

The lighting time signal generator 641 has, for example, a configuration as shown in FIG. That is, it has a 4-bit counter 6411 and a frequency division counter 6412. When gradation data is input from the data lines D1 to D4, the 4-bit counter 6411 outputs a value of 0 to 15 (count number) according to this value. The count number and the lighting reference clock from CLK1 are input to the frequency division counter 6412, the number of falling edges of the lighting reference clock is counted, and a lighting time signal is output until the count becomes equal to the input count. .

For example, as shown in the time chart of FIG. 5, the falling time of the strobe signal from the strobe signal line ST is used as a trigger to generate a lighting time signal. One such lighting time generator 64 is provided for each LED. However, in the above-described control, the gradation data and the lighting time of the LED can only be set in a proportional relationship. This is because the lighting reference clock uses a rectangular wave having a constant frequency, and is divided by the count based on the gradation data, so that the count and the lighting time are in a proportional relationship.

On the other hand, in an electrophotographic image forming apparatus, the exposure time and exposure intensity of the photoconductor and the printed image density (ID ) Is not proportional. For example, when 4-bit image data is used as gradation data without performing gamma conversion and printed, as shown in FIG. 6, the lighting time (gradation data having a linear proportional relationship with the lighting time) is proportional to the print image density. Rather, it is known that the relationship has a γ characteristic.
For this reason, in a region where the gradation data is small, the increase of the print image ID for one gradation data is small, and in a region where the gradation data is large, the increase of the print image ID for one gradation data is large. Would. Therefore, when input image data from a personal computer or a scanner is converted into gradation data used for the printer unit, by performing γ conversion using a γ conversion lookup table corresponding to the γ characteristic of the printer engine unit, It is necessary to perform processing so that the value of the input image data and the ID of the image to be printed have a proportional relationship.

[0021]

[Table 1]

Table 1 shows γ for 4-bit image data.
The conversion is performed to generate 4-bit gradation data, and the image density when printing with the 4-bit gradation data is measured. In this case, the relationship between the original image data and the printed image density becomes substantially proportional as shown in FIG. However, the LED print head does not have a strict proportional relationship because there is an image density that cannot be output even in an effective image density area. Originally, 4-bit grayscale data must be able to express 16 grayscale image densities, but here, only 11 grayscales can be expressed. As a result, there is a new problem that a region having a large ID of a printed image cannot be expressed in detail. In particular, if the image density cannot be expressed finely in a halftone area, there is a problem that a false contour is likely to occur. If possible, the relationship between the gradation data input to the print head and the ID of the print image is preferably a proportional relationship or one that is intentionally easy to operate.

Therefore, in this embodiment, the lighting reference clock is determined as follows. The procedure for determining the lighting reference clock (signal used for CLK1) will be described with reference to FIGS. First, using this printer, a patch is printed with arbitrary gradation data, and this is read by a scanner to generate image data, and the relationship between the lighting time and the print image density is obtained from the measured data. When the obtained relation is relatively close to linear, the γ curve (γ
In the case of a relatively high γ, a γ curve (a γ pattern B printer) as shown in FIG. 9 is obtained. Such γ characteristics are determined by the characteristics of the photosensitive drum, the development potential, and the setting of the transfer potential.

Next, the print image density is divided at equal intervals, and the lighting time at each density is determined. Here, assuming that the effective maximum image density of the printer is 1.5 and 4-bit gradation data (16 gradations) is used, an image density of 0.1
The lighting time corresponding to each image density is obtained in steps.

[0025]

[Table 2]

In Table 2, the lighting time of each density is represented by the number of pulses of a reference clock (CPU clock), and this data is represented as a lighting reference clock lookup table in FIG.
Is stored in the RAM 4. Next, the procedure for determining the waveform of the lighting reference clock from the lighting reference clock lookup table will be described again with reference to FIGS. As described above, the lighting time signal generator 6 shown in FIG.
Reference numeral 41 counts the input gradation data, divides the lighting reference clock in accordance with the count, and outputs a lighting time signal.

Therefore, in the case of the printer of the γ pattern A, the count becomes 0 in response to the input of the gradation data 0000.
There is no output from the lighting time signal generator 641 and the count becomes 1 for the input of the gradation data 0001, and the lighting reference clock is 1 with 170 reference clock pulses.
It suffices if there is a second fall, the gradation data 0010 has a count of 2, the reference clock is 340 pulses, and the lighting reference clock needs to have the second fall.
··· The gradation data 1111 has a count of 15, and it is sufficient that the lighting reference clock has the 15th falling edge at 1500 pulses of the reference clock. The waveform is determined by determining the fall time of the lighting reference clock for each count in this manner.

Similarly, in the case of a printer of γ pattern B,
The count is 0 for the input of the gradation data 0000,
There is no output from the lighting time signal generator 641, the count becomes 1 for the input of the gradation data 0001, and the lighting reference clock only needs to have the first falling at 500 pulses of the reference clock. The data 0010 has a count of 2, the reference clock has 625 pulses, and the lighting reference clock needs to have the second falling edge... The gradation data 1111 has a count of 15, and the reference clock has 1500 pulses and the lighting reference clock has Needs only to have the 15th fall. Thus, the waveform of the lighting reference clock is determined.

The lighting reference clocks obtained by these procedures are shown in the lower part of FIGS. 8 and 9, and are the rectangular waveforms of the lighting reference clock for γ pattern A correction and the lighting reference clock for γ pattern B correction, respectively. Becomes When each LED of the printer is driven by the lighting reference clock for correcting each of the γ patterns, as shown in FIGS.
A lighting time signal is generated for each of 4-bit grayscale data of 0 to 1111. When the printer is operated with this lighting time signal, the image densities in the 4-bit gradation data of 0000 to 1111 are in a proportional relationship as shown in FIGS. 11A and 11B, and printing is performed at a required image density. Can be.

The above-mentioned lighting reference clock is used for .gamma. Correction. However, when it is desired to control the print image density with respect to the input gradation, the lighting reference clock shown in FIGS. It is good to prepare a pattern. FIG. 7A shows the lighting reference clock for γ correction described above. (B) ~
If the lighting reference clock of (D) is used, the output characteristic of the relationship between the gradation data and the image density as shown on the right side of FIG. A plurality of look-up tables for generating such a plurality of lighting reference clocks may be stored in the RAM 4 and selected and used according to a user's instruction. A new look-up table may be created and a new lighting reference clock pattern may be added, or an existing lookup table may be changed to change the lighting reference clock pattern.

FIG. 13 shows a block diagram of a printer according to another embodiment of the present invention. Since the main configuration is the same as that of the embodiment shown in FIG. 2, the same components are denoted by the same reference numerals. In this embodiment, the CPU 2 does not input the lighting reference clock CLK1 to the LED print head 5,
A reference clock (CPU clock) is input from a reference clock signal line CLK, and a signal for controlling the LED print head 5 is input from a signal line S.

The LE of the printer according to another embodiment
FIG. 14 shows details of the D print head 5. The main configuration is the same as that of the embodiment shown in FIG. 3, and the same components are denoted by the same reference numerals. In this embodiment, a lighting reference clock generator 66 and an LUT 67 are added as new components. The LUT 67 stores the same look-up table that stores the image density and the lighting time by the number of pulses of the reference clock, which is stored in the RAM 4 in the previous embodiment. Since the reference clock is inputted to the lighting reference clock generator 66, the lighting reference clock as shown in FIG. The lighting reference signal generated here is input to each lighting time signal generator 64 via CLK1, and is used in the same manner as in the previous embodiment.

As described above, in the above embodiment, the CPU 2 and the R
The load on the CPU can be reduced by providing the LED print head 5 with the lighting reference clock generated using the look-up table of AM4.
The selection, change, and new creation of the LUT 67 can also be performed from the signal line S. FIG. 15 is a block diagram for simplifying and explaining the contents of the present invention. Ie LED
One LED unit 2 provided in the print head 5
FIG. 2 is a block diagram showing a configuration of one (one LED unit 21 is provided with one LED) lighting control circuit. The LED unit 21 receives a lighting request signal (ST
ROBE). Also, the lighting clock generator 6
4 supplies a lighting reference clock. In the LED unit 21, the light emission time of the LED is controlled based on the lighting request signal and the lighting reference clock.

The lighting reference clock is a lighting time signal generator 6
Given from 4. The lighting time signal generator 64 includes a frequency dividing circuit 31 for dividing a frequency of a CPU clock of, for example, 2 GHz.
Is provided. 2 GHz CP by frequency dividing circuit 31
The U clock is divided into a plurality of types of clocks. Then, the divided clock is supplied to the selector 32. The selector 32 can be switched by image data, and the clock selected by the selector 32 is output as a lighting reference clock. As a result, the lighting time signal generator 6
The cycle of the lighting reference clock output from 4 can be changed.

FIG. 16 shows the conventional LED lighting control timing for reference, and FIG. 17 shows the LED lighting control timing according to this embodiment performed by the circuit of FIG. As is clear from the comparison between FIG. 16 and FIG. 17, the lighting time of the conventional LED is controlled by the falling timing of the lighting reference clock whose period does not change. Therefore, L corresponds to the gradation of the image data.
It can be seen that the lighting time of the ED is controlled to a length that is an integral multiple of the cycle of the lighting reference clock.

On the other hand, in this embodiment, as shown in FIG. 17, the cycle of the lighting reference clock, in other words, the fall timing of the lighting reference clock can be changed. For this reason, it can be seen that the falling timing of the lighting reference clock can be changed according to the gradation of the image data, and the lighting time of the LED can be adjusted to an arbitrary length that is not limited to an integral multiple of the cycle of the lighting reference clock. The lighting time can of course be adjusted based on the rising timing of the lighting reference clock.

Next, the relationship among the gradation of the image data, the LED lighting time, and the density of the toner image to be formed will be described. FIG. 18 is a graph showing the relationship between the theoretical gradation of image data, the lighting time, and the density of the toner image to be formed. As shown in FIG. 18, if the gradation of the image data, the lighting time, and the density ID of the toner image to be formed are all in a linear relationship, it is not necessary to change the cycle of the lighting clock. A toner image having a corresponding density can be formed.

However, actually, as shown in FIG. 19, the gradation of the image data and the density I of the toner image to be formed are determined.
D is not in a linear relationship but in a non-linear relationship. Therefore, when the lighting time is controlled by a lighting reference clock having a constant cycle according to the gradation of the image data,
There is a disadvantage that the gradation of the image data is not reflected on the density of the toner image formed as it is, and for example, the gradation reproducibility of a thin portion of the toner image is deteriorated.

Therefore, in this embodiment, as shown in FIG. 20, by changing the period of the lighting reference clock (in other words, the falling timing of the lighting reference clock), the LEDs are turned on in accordance with the gradation of the image data. The lighting time can be controlled not by an integer multiple of the lighting reference clock but by non-linear characteristics. As a result, the amount of toner adhering to the exposed area by turning on the LED can be adjusted, and the density of the formed toner image can be linearly associated with the gradation of the image data.

As described above, the present invention relates to an LED
By using a lighting reference clock for controlling the lighting time of each LED included in the print head as a clock whose cycle can be changed, an apparatus capable of forming an image with good tone reproducibility can be provided. The present invention is not limited to what has been described in the embodiments, and various changes can be made within the scope of the claims.

[Brief description of the drawings]

FIGS. 1A and 1B are illustrative views showing an arrangement relationship between a photosensitive drum and an LED print head provided in an image forming apparatus according to an embodiment of the present invention.

FIG. 2 shows a configuration block diagram of a printer having an LED print head.

FIG. 3 is a block diagram illustrating a configuration of an LED print head.

FIG. 4 is a block diagram showing a configuration of a lighting time signal generator.

FIG. 5 is a timing chart for explaining the operation of the lighting time signal generator according to the related art.

FIG. 6 is a diagram for explaining that the lighting time and the print image density are not proportional and have a γ characteristic.

FIG. 7 is a diagram for explaining the relationship between image data and image density after γ conversion.

FIG. 8 is a diagram for explaining a procedure for determining a lighting time from image density data.

FIG. 9 is a diagram for explaining a procedure for determining a lighting time from image density data.

FIGS. 10A and 10B are diagrams for explaining a relationship between 4-bit gradation data and a lighting time signal.

FIGS. 11A and 11B are diagrams for explaining the relationship between image data and image density. FIGS.

12A, 12B, 12C and 12D are diagrams for explaining patterns of various lighting reference clocks.

FIG. 13 is a configuration block diagram of a printer according to another embodiment of the present invention.

FIG. 14 is a perspective view of an L of a printer according to another embodiment of the present invention.
FIG. 3 is a block diagram illustrating details of an ED print head.

FIG. 15 is a block diagram for simply describing the contents of the present invention.

FIG. 16 is a reference diagram showing a conventional LED lighting control timing.

FIG. 17 is a diagram showing lighting control timings of LEDs performed by the lighting control circuit according to the embodiment of the present invention.

FIG. 18 shows the theoretical gradation of image data, lighting time,
4 is a graph showing a relationship between the density of a toner image to be formed and the density of the toner image.

FIG. 19 is a graph showing a relationship between a gradation of image data, a lighting time, and a density of a toner image to be formed according to a conventional lighting control timing.

FIG. 20 is a graph showing a relationship between a gradation of image data, a lighting time, and a density of a toner image to be formed in the case of lighting control timing according to an embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Printer main body 2 CPU 3 ROM 4 RAM 5 LED print head 6 LED drive means 7 LED head

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshifumi Ishii 1-2-28 Tamazo, Chuo-ku, Osaka-shi, Osaka Inside Kyocera Mita Co., Ltd. (72) Hideo Umezawa 1-2-2 Tamazo, Chuo-ku, Osaka-shi, Osaka No. 28 Kyocera Mita Corporation F term (reference) 2C162 AE14 AE28 AE47 AF20 AF44 AF72 AF75 AF89 FA04 FA17

Claims (10)

[Claims] 1. A large number of LEDs for exposing a photoreceptor surface
In an electrophotographic image forming apparatus having an LED print head in which are arranged, a lighting time setting means in which a relationship between an image density and a lighting time is preset, and a lighting time according to a gradation of input image data. Is read from the lighting time setting means, and each LED of the LED print head is read by the read lighting time.
An LED driving unit for turning on the LED. 2. The lighting time setting means sets a relationship between an image density and a lighting time by a lighting reference clock having a cycle different according to a gradation of an image. A gradation data counter that outputs a count number corresponding to a gradation, and a frequency division counter that divides a lighting reference clock read from the lighting time setting means until the count becomes equal to the count number. 2. The image forming apparatus according to claim 1, wherein an output pulse of the counter is given as a lighting time. 3. The lighting time setting means has a look-up table which stores a relationship between the number of CPU clocks (reference clocks) of a CPU which is a control center of the image forming apparatus and gradations. 3. The image forming apparatus according to claim 2, wherein a lighting reference clock is generated based on a table. 4. The image forming apparatus according to claim 3, wherein the relationship between the number of clocks of said reference clock and gradation is determined based on each gradation confirmation patch printed by said image forming apparatus. apparatus. 5. A plurality of lookup tables are prepared, and any one of the lookup tables can be selected.
The image forming apparatus according to claim 3. 6. The image forming apparatus according to claim 3, wherein said look-up table is rewritable. 7. The lighting time setting means is provided in a CPU which is a control center of the image forming apparatus.
3. The image forming apparatus according to claim 2, wherein a lighting reference clock is supplied from U to the LED driving unit. 8. The LED driving means, wherein the lighting time setting means is provided in the LED driving means, and the reference clock is supplied to the LED driving means from a CPU which is a control center of the image forming apparatus. Item 3. The image forming apparatus according to Item 2. 9. A large number of LEDs for exposing a photoreceptor surface
An electrophotographic image forming apparatus having an LED print head having a plurality of LED print heads, wherein: LED driving means for lighting each LED of the LED print head; and each LED of the LED print head according to the gradation of image data Lighting time signal generating means for generating a lighting reference clock to be given to the LED driving means in order to change the lighting time of the LED driving means, wherein the lighting time signal generating means comprises a lighting reference clock according to a gradation of image data. An image forming apparatus characterized in that the period of the image can be changed. 10. A plurality of LEs for exposing the surface of a photoreceptor.
D is an LED print head in which D is arranged, wherein a lighting time setting means in which a relationship between an image density and a lighting time is set in advance, and a lighting time according to a gradation of input image data, An LED print head comprising: LED driving means for reading from the means and lighting each LED at the read lighting time.