JPH0872297A - Image forming apparatus and method - Google Patents

Abstract

PURPOSE: To prevent printing density unevenness depending upon light emitting means and to form excellent image by stable optical output by providing an altering means for altering the value of light emitting data to be applied to a plurality of light emitting means in response to the output level detected by a detecting means. CONSTITUTION: In the red image formation, a photosensitive drum 110 is rotated in a direction of an arrow, charged to a desired potential by a primary charger 112, then emitted with a level light 128 from an exposure controller 120 to form a red electrostatic latent image. The image formed on the drum 110 is developed by a red developing unit 122 as visualized as red toner image. Then, in the black image formation, an angle photosensitive drum 110 is recharged to a desired potential by a recharger 113, then emitted with a laser light 129 from the controller 120 to form a black electrostatic latent image. The image formed on the drum 110 is developed by a black developing unit 121 this time and a black toner image is visualized.

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 and method for forming an image using a plurality of light sources.

[0002]

2. Description of the Related Art Conventionally, an image forming apparatus has been proposed which forms an image using a plurality of light sources. When a plurality of light sources of such an image forming apparatus are caused to emit light, conventionally, light modulation data is given to the respective light sources.

[0003]

However, even when the same light modulation data is given to a plurality of light sources, these light sources do not emit light at the same output level. Therefore, conventionally, when an image is formed by using a plurality of light sources, there is a drawback that the image output from the image forming apparatus has uneven density.

[0004]

In order to achieve the above object, the invention according to claim 1 is an image forming apparatus for forming an image by irradiating a photoreceptor with light emitted from a plurality of light emitting means. In the above, the light emission control means for giving light emission data to each of the plurality of light emitting means to emit light, the detecting means for detecting the output level of each light emitted from the plurality of light emitting means, and the detecting means for detecting And a changing unit that changes the value of the light emission data given to each of the plurality of light emitting units according to the output level.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the changing means sets the output level of each light emitted from the plurality of light emitting means to be the same. It is characterized in that it has a level adjusting means for changing the value of the light emission data.

According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the changing unit sets a predetermined output level of each light emitted from the plurality of light emitting units. Further comprising a target value control means for changing the value of the light emission data so as to approach the target value, the measuring means for measuring the temperature of the plurality of light emitting means, and the detection based on the temperature measured by the measuring means. Temperature compensation means for compensating the data indicating the output level output from the means relative to the target value.

According to a fourth aspect of the present invention, in an image forming method for forming an image by irradiating a photoconductor with light emitted from a plurality of light emitting means, light emission data is given to each of the plurality of light emitting means. Each of the plurality of light emitting means in accordance with the light emission control step of causing the light to be emitted, a detection step of detecting the output level of each light emitted from the plurality of light emitting means, and the output level detected by the detection step. And a change step of changing the value of the light emission data given to the.

According to a fifth aspect of the present invention, in the image forming method according to the fourth aspect, the changing step is performed so that the output levels of the lights emitted from the plurality of light emitting means are the same. It is characterized by including a level adjusting step for changing the value of the light emission data.

According to a sixth aspect of the invention, in the image forming method according to the fourth or fifth aspect, in the changing step, the output level of each light emitted from the plurality of light emitting means is predetermined. Further comprising a target value control step of changing the value of the light emission data so as to approach the target value,
A measuring step of measuring the temperature of the plurality of light emitting means,
A temperature compensating step for compensating the data indicating the output level output in the detecting step relative to the target value based on the temperature measured in the measuring step.

[0010]

According to the present invention, in the case where an image is formed by irradiating a photoconductor with light emitted from a plurality of light emitting means such as a laser diode or a light emitting diode, light modulation data (light emission of the scope of claims) (Corresponding to the data) is applied to the plurality of light sources, the light modulation data is corrected so that the output levels of the lasers emitted from the plurality of light sources are kept the same.

In particular, according to the invention described in claim 1 or 4, light emission data is given to each of the plurality of light emitting means to emit light, the level of each light is detected, and a plurality of light emitting means are detected in accordance with the detected levels. The value of the light emission data given to each of the light emitting means is changed.

According to the invention of claim 2 or 5,
The value of the light emission data is changed so that the level of each light emitted from the plurality of light emitting means is the same.

According to the invention of claim 3 or 6,
The value of the light emission data is changed so that the output level of each light emitted from the plurality of light emitting means approaches a predetermined target value. Further, the temperature of the light emitting means is measured, and the data indicating the output level of the light emitted from the light emitting means is relatively compensated for the target value based on the measured temperature.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) FIG. 1 is a cross-sectional structural view for explaining an example of an image forming apparatus of the present invention, in which 100 is a copying apparatus main body and 180 is a circulation type automatic document feeder for automatically feeding originals ( RDF), 190 is a sorter or sorter. The RDF 180 and the sorter 190 can be used by freely combining with the main body.

Reference numeral 101 is a document table glass as a document placing table. Reference numeral 102 denotes a document illumination lamp 103 and scanning mirror 1.
In the scanner configured by 04 or the like, the scanner is reciprocally scanned in a predetermined direction by a motor (not shown), and the reflected light of the original is imaged on the CCD sensor 109 through the lens 108 through the scanning mirrors 104 to 106. CCD sensor 109
The original image formed on the
(Red), G (Green), B (Blue) are converted into analog electric signals. Image processing unit 203 described later
By using the image information, red and black image data, which are toner colors of the printer unit, are created.

Reference numeral 120 denotes an exposure control section composed of a monolithic multi-laser, a polygon scanner and the like. The monolithic multi-laser modulated based on each of the red and black image signals separated by the image processing unit 203 becomes the light modulation 128 for red and the light modulation 129 for black and irradiates the photoconductor drum 110. Around the photosensitive drum 110, a primary charger 112, a red developing device 122, and a recharging device 1
13, black developing device 121, transfer charging device 118, separation charging device 119, cleaning device 116, pre-exposure lamp 142
Is provided. With one photosensitive drum 110,
The red image and the black image are formed at once.

In the red image type, the photosensitive drum 110 is rotated in the direction of the arrow shown in the figure by a motor (not shown), and is charged to a desired potential by the primary charger 112, and then the exposure controller 120 The laser light 128 of is irradiated to form an electrostatic latent image for red. Photoconductor drum 110
The electrostatic latent image formed above is developed by the red developing device 122 and visualized as a red toner image. Next, in the black image formation, the photoconductor drum 1 is re-charged by the recharger 113.
After recharging 10 to a desired potential, laser light 129 from the exposure controller 120 is irradiated to form a black electrostatic latent image. The electrostatic latent image formed on the photosensitive drum 110 is developed by the black developing device 121 this time and visualized as a black toner image.

On the other hand, the transfer paper fed from the upper cassette 131 or the lower cassette 132 by the pickup rollers 133 and 134 is sent to the main body by the paper feed rollers 135 and 136, and is visualized by the registration roller 137 together with the toner image and the paper. To the transfer position and the transfer charger 118 transfers the red paper on the photosensitive drum to the transfer paper.
The black toner image is transferred, and the transfer paper is separated from the photosensitive drum by the separation charger 119. On the exposed body drum after the transfer, the residual toner is cleaned by the cleaner device 116, and the residual charge is erased by the pre-exposure lamp 114.

The transfer sheet after separation is sent to the fixing device 141 by the transport belt 130, fixed by pressure and heating, and discharged to the outside of the main body 100 by the discharge roller 142. 1
Reference numerals 39 and 140 denote pre-fixing chargers that supplement the toner suction force of the transfer paper after separation and prevent image disturbance. A paper sensor 148 detects the leading end of the transfer member fed onto the registration guide, and is used as a synchronization signal in the paper feeding direction (sub-scanning direction).

The main body 100 is equipped with a deck 150 capable of accommodating, for example, 4000 sheets of transfer paper. The lifter 151 of the deck 150 rises according to the amount of transfer paper so that the transfer paper always contacts the paper feed roller 152. Also, a multi-manual feed 153 capable of accommodating 100 sheets of transfer paper
Is provided. A paper discharge flapper 154 switches between the double-side recording side or the multiple recording side and the discharge side (sorter). The transfer paper sent from the discharge roller 142 is switched to the double-sided recording side or the discharge side by the paper discharge flapper 154. Reference numeral 158 is a lower conveyance path. At the time of double-sided copying, the transfer paper sent from the discharge roller 142 is turned upside down via the reversing path 155 to re-feed the tray 1.
Lead to 56. Reference numeral 157 denotes a multiple flapper that switches between double-sided recording and multiple recording paths. By tilting this to the left, the transfer paper is directly guided to the lower transport path 158 without passing through the reverse path 155.

Reference numeral 159 denotes a paper feed roller for feeding the transfer paper to the photosensitive drum 110 through the path 160. A discharge roller 161 is disposed near the discharge flapper 154 and discharges the transfer sheet switched to the discharge side by the discharge flapper 154 to the outside of the machine. During double-sided recording (double-sided copying) or multiplex recording (multiplexing copying), the paper discharge flapper 154 is raised to transfer the copied transfer paper to the transport paths 155, 15
It is stored in the sheet re-feed tray 156 in the state of being turned upside down via 8. At this time, the double flapper 157 is tilted to the right during double-sided recording, and the double flapper 157 is moved during multiple recording.
Tilt 7 to the left. At the time of back side recording or multiplex recording to be performed next, the transfer sheets stored in the re-feed tray 156 are routed one by one from the bottom by the sheet feeding roller 159.
Through the registration roller 137 of the main body.

When the transfer paper is reversed and ejected from the main body, the paper ejection flapper 154 is moved upward and the flapper 157 is ejected.
To the right, the copied transfer paper is conveyed to the conveyance path 155 side, and after the trailing end of the transfer paper has passed the first feed roller 162, it is conveyed to the second feed roller side by the reversing roller 163. By the discharge roller 161, the copy is turned over and discharged to the outside of the machine.

(Outline of Processing) FIG. 2 shows a hardware block diagram of the image forming apparatus of the present invention. The image forming apparatus includes an image reading unit 201, an image processing unit 203, a printer unit 204, and a CPU unit 205.

The image reading unit 201 includes a lens 108,
CCD sensor 109 and analog signal processing unit 202
Have. The original image formed on the CCD sensor 109 via the lens 108 is R
(Red), G (Green), B (Blue) are converted into analog electric signals. The converted image information is input to the analog signal processing unit 202, sampled and held for each color of R, G, and B, and after dark level correction or the like is performed, analog / digital conversion (A / D conversion) To be done.

The digitized full-color signal is input to the image processing section 203. The image processing unit 203 performs correction processing necessary for the reading system such as shading correction, color correction, and γ correction, smoothing processing, edge enhancement, other processing, and processing, and outputs an image signal to the printer unit 204.

The printer unit 204 has the exposure control unit 120 including a laser, the image forming unit 110, the transfer paper conveyance control unit, and the like, which have been described with reference to the cross-sectional configuration diagram of FIG.
An image is recorded on the transfer paper according to the input image signal.
The CPU circuit unit 205 includes a CPU 206, a ROM 207,
It also has a RAM 208, controls the image reading unit 201, the image processing unit 203, the printer unit 204, and the like, and controls the sequence of this apparatus in a centralized manner.

FIG. 3 is a detailed hardware block diagram of the image processing unit 203. The analog signal processing unit 20 of FIG.
2 from the digital image signal is the shading correction unit 30.
Input to 1. The shading correction unit 301 corrects variations in sensors that read a document and the light distribution characteristics of a document illumination lamp. The corrected image signal
The gradation correction unit 30 converts the brightness signal into the density signal.
Input 2 and create density image data. The image signal converted into the density signal is input to the two-color separation circuit unit 303 of the present invention, and from the density signals C (cyan), M (magenta), and Y (yellow), the toner color of the printer unit is red. , And black image data is created. The red image signal 305 output from the two-color separation circuit 303 is output to the printer unit. Further, the black image signal 306 is delayed in the buffer memory 304 for a predetermined time. this is,
This is to correct the physical displacement on the photoconductor that exposes the red image and the black image. The black image data is output to the printer unit 204 after being delayed for a predetermined time in the buffer memory 304.

FIG. 4 shows the structure of a monolithic multi-laser diode. In FIG. 4, 401 is a submount and 402 is a light emitting layer made of GaAlAs.
Reference numeral 403 is a wire on the anode side of the first laser diode, 404 is a wire on the anode side of the second laser diode, and 405 is a wire on the cathode side of the first and second laser diodes.

FIG. 5 shows a transmission circuit of a monolithic multi-laser diode. Reference numeral 501 is a first laser diode (referred to as LD501), and 502 is a second laser diode (referred to as LD502). 503 is shown in FIG.
Is a pin photodiode (referred to as PD) for detecting the light emission amount of a laser (not shown).

FIG. 6 shows the mechanical structure of the exposure controller 120 (FIG. 1) and the printer 204 (FIG. 2). In FIG. 6, reference numeral 601 is a monolithic multi-laser element. The two beams emitted from the monolithic multi-laser 601 are converted into almost two parallel rays by a collimator lens 602 and a cylindrical lens 603 and are incident on a polygon (rotating polygon mirror) 604 with a predetermined beam diameter. The polygon rotates at a constant angular velocity in the direction of the arrow. With this rotation, the incident beam is reflected as a deflected beam whose angle is continuously changed. The two laser beams that have become the deflected beams are condensed by the f-θ lens 605. f-
Since the θ lens simultaneously corrects the distortion aberration that guarantees the temporal linearity of the scanning, the beam is imaged at a constant speed on the photoconductor (not shown) as the image carrier in the direction 606 of the arrow. To be scanned.

Reference numeral 606 is a sensor for converting a light output into an electric signal, which constitutes one element of a synchronizing mechanism for detecting the timing of writing image information on the photosensitive member. Normally, the sensor 606 is installed at a position outside the scanning region so that the beam emission timing is set for each scanning. A writing signal is sent after a predetermined t seconds from the time when the scanning beam is detected by the sensor 606. The output from the sensor 606 is converted into an electric signal by 607 and is input to the image processing unit of 203 (referred to as a BD signal). A laser driver 610 drives the 601 monolithic multi-laser. The laser driver 610 is also connected to the image processing unit 203.

FIG. 7 shows a hardware block diagram of the laser driver 610. In FIG. 7, reference numeral 701 is a control signal for switching the selectors 702 and 703, which is given from the CPU 206. The selector 702 includes red image data 305 and CPU bus 20 respectively.
6 is connected. A CPU bus is connected to the selector 703 as with the black image data 306. The outputs of the selectors 702 and 703 are conversion tables 704.
And 705.

Conversion data can be set in these conversion tables by the CPU. Therefore, after the conversion data is set in the CPU, the red and black image data are input to the conversion tables 704 and 705, converted into desired data and output as 716 and 717. Image data 71
6 and 717 are input to pulse width modulation 706 and 707 (referred to as PWM) to generate pulses corresponding to image data. Reference numerals 708 and 709 are high-speed switches connected to the LD 501 and LD 502. These high-speed switches switch in synchronization with the image data 718 and 719.

A constant current circuit 71 is provided downstream of 708 and 709.
0 and 711 are connected. This circuit keeps the current value flowing through the laser constant and stabilizes the optical output. Further, the laser device 60 is controlled by a temperature control block (not shown).
1 is controlled so as to have a constant temperature. The output of the PD 503 is passed through the resistor 712 and the amplifier unit 713 and is output to the A / D 71.
4 is input. The optical output 715 from the laser converted into digital data is input to the CPU 206. The image data of red and black are PWM modulated and LD501,5
02 added. The LDs 501 and 502 emit laser light to form image data on the photoconductor.

FIG. 8 shows a state of optical output with respect to PWM data of the twin laser. In FIG. 8, 718 is LD5
This is a PWM signal added to 01. A PWM signal 719 is applied to the LD 502. The pulse widths of 718 and 719 are the same. 801 and 802 are LD50 respectively
1 is a light output waveform from the LD 502. As can be seen from FIG. 8, even when the same PWM data is input, the optical waveform output by the laser is different.

In FIG. 9, the horizontal axis represents the data width of the PWM data,
LD501 and L with the maximum optical output level plotted on the vertical axis
The graph which showed the relationship of D502 is shown. 901 is LD5
The light emission level of 01 and the light emission level of 902 are shown by 502. Comparing 901 and 902, it can be seen that 901 has a larger light output for the same PWM data.

A method of correcting the data width of the PWM data will be described with reference to FIG. PWM data 1001 is given to the LD 501 and is 20 (H). 1002 is LD
The PWM data given to 502 is also 20 (H). However, as described in FIG. 8, 1004 and 1005
The optical output waveforms of are different. Then P as shown in 1003
The WM data is added to the LD 502 and the PWM width is corrected by the width indicated by 1007. Then, the optical output shown by 1006 is obtained, and the same optical waveform as 1004 is obtained. 10
07 corresponds to 8 of PWM data, and the data given by 1003 is 28 (H).

The correction width is determined as follows. First,
The CPU 206 sets the LD50 according to the data of 20 (H).
1 is made to emit light, and the optical output is read by PD503 and RA
Store in M208. Next, set LD502 to data 20
The light is emitted at (H), and the output is compared with the data stored in the RAM 208. If the output from LD502 is smaller, increase the data given to LD502,
02 is made to emit light and compared with the data stored in the RAM 208. On the contrary, if the output from the LD 502 is larger, L
The data given to D502 is reduced. In this embodiment, L
When the data given to D502 is 28 (H), the outputs match. This conversion data is converted into a conversion table 7 shown in FIG.
Set to 05.

A data conversion method in the conversion table 705 will be described with reference to FIG. Reference numeral 306 is black image data shown in FIG. 717 is a conversion table 705
Shows the output from. In this embodiment, all input data is output as 8. For example, 0 is converted to 8 and 20 is converted to 28.

LD50 when the above correction is added to FIG.
1 shows the relationship between PWM and light output of LD 502. LD5
02 output level 901 and LD 502 output level 90
It can be seen that 2 matches.

(Embodiment 2) FIG. 13 shows the wavelength characteristics of the PD. The vertical axis represents sensitivity and the horizontal axis represents wavelength. As shown in FIG. 13, the PD sensitivity varies depending on the wavelength. When the laser emits light, heat is generated and the temperature rises. Further, the emission wavelength of the laser changes depending on the temperature. This is called mode hopping. In this embodiment, the wavelength of 0.3 nm / ° C. changes. Therefore, even if the light emission level is the same, the level detected by the PD changes due to the heat generation of the LD.

FIG. 14 shows a block diagram of the laser driver of the second embodiment. In FIG. 14, the same functional elements as those in FIG. 7 are denoted by the same reference numerals as those in FIG. 1401 is a thermistor, LD50
The temperature of the twin laser is detected near 1,502. The output 1402 of the thermistor 1401 passes through the A / D 1403 and is input to the CPU 206. This makes LD5
The CPU recognizes the temperatures of 01 and 502. In this example, the center value of the emission wavelength of the laser is 670 nm.

The temperature guarantee procedure will be described with reference to the flowchart of FIG. Step (abbreviated as S) 1501
It is checked whether or not the temperature has changed, and if it has not changed, nothing is done and the process loops in S1501. If the temperature changes, the sensitivity of the PD is corrected in S1502. For example, if it changes 10 degrees,

[0045]

From 0.3 * 10 = 3 nm, the wavelength shifts by 3 nm. Therefore, the wavelength of the emitted laser is

[0046]

## EQU2 ## 670 + 3 = 673 nm. The PD sensitivity at this time is, as shown in FIG.
This is 97% of the case where the wavelength is 670 nm. Therefore,
In S1502, the input laser light level is 100/9.
Multiply by 7. Next, in step S1503, the value calculated in step S1502 is input to the PD 503 and set in the working memory of the CPU. Then, the process returns to S1501.

(Others) As an example of the "light emitting means" described in the claims, a laser diode was used in the examples. However, the light emitting means is not limited to the laser diode, and may be a light emitting element of a light bulb, a fluorescent lamp, or a diode array.

Further, the "light emission data" described in the claims is PWM-modulated and given to the light-emitting means in the embodiment. However, by AM modulation, the voltage or current value is changed and given to the light-emitting means. You can also

As an example of the "measuring means" described in the claims, a thermistor is used in the embodiment, but the wavelength of the light emitted from the light emitting means is detected, or a predetermined color filter is passed. The temperature of the light emitting means may be measured by measuring the light output level.

As an example of the "target value control means" described in the claims, the target value of the emission level is changed in the embodiment, but the measured value of the emission level may be corrected instead.

It is apparent from the scope of the claims that all of these alternatives are included in the scope of the technical matters described in the claims.

[0052]

As described above, the first and second aspects of the present invention are as follows.
According to the invention described in 4 or 5, by correcting the light modulation data before outputting the light modulation data to the laser diode, the output levels of the plurality of light sources can be equally compensated. Therefore, even when the same light modulation data is given to a plurality of light emitting means, for example, a laser diode or a light emitting diode, the light emitting means can emit light of the same level. Therefore, according to the present invention, it is possible to prevent uneven printing density depending on the light emitting means.

Further, according to the invention of claim 3 or 6, the influence of mode hopping is eliminated, so that a more stable light output can be obtained and a good image can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view of an image forming apparatus of the present invention.

FIG. 2 is a block diagram of an image forming apparatus.

FIG. 3 is a hardware block diagram of an image processing unit 203.

FIG. 4 is a structural diagram of a monolithic multi-laser.

FIG. 5 is an equivalent circuit diagram of a monolithic multi-laser.

6 is a block diagram of an exposure control unit 120. FIG.

FIG. 7 is a block diagram of a laser driver 610.

FIG. 8 is an explanatory diagram showing a waveform of optical output.

FIG. 9 is an explanatory diagram showing PWM waveforms and optical output waveforms of LD 501 and LD 502.

FIG. 10 is an explanatory diagram showing PWM correction data.

FIG. 11 is an explanatory diagram showing a data structure of a conversion table 705.

FIG. 12 is an explanatory diagram showing a relationship between PWM of LD 501 and LD 502 and corrected light output.

FIG. 13 is a characteristic diagram showing frequency characteristics of a PD.

FIG. 14 is a hardware block diagram of a laser driver according to a second embodiment.

FIG. 15 is a flowchart illustrating an operation of the CPU 206 according to the second embodiment.

[Explanation of symbols]

 100 Copier main body 104-106 Scanning mirror 108 Lens 109 CCD sensor 110 Photosensitive drum 112 Primary charger 113 Recharger 120 Exposure controller 128 Laser light 180 Circular automatic document feeder (RDF) 203 Image processor 206 CPU 401 Submount 402 Luminous Layer 501 Laser Diode (LD) 502 Laser Diode (LD) 503 Pin Photodiode (PD) 601 Laser Element 610 Laser Driver 704, 705 Conversion Table 706, 707 Pulse Width Modulator 708, 709 Switch 710, 711 Constant current circuit 712 Resistor 713 Amplifier section 714 A / D converter 716,717 Image data

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Claims (6)

[Claims] 1. An image forming apparatus for forming an image by irradiating a photosensitive member with light emitted from a plurality of light emitting means, and a light emission control means for giving light emission data to each of the plurality of light emitting means to emit light. Detecting means for detecting the output level of each light emitted from the plurality of light emitting means, and according to the output level detected by the detecting means,
An image forming apparatus comprising: a changing unit that changes a value of the light emission data given to each of the plurality of light emitting units. 2. The image forming apparatus according to claim 1, wherein the changing unit changes a value of the light emission data so that an output level of each light emitted from the plurality of light emitting units becomes the same. An image forming apparatus having a level adjusting means. 3. The image forming apparatus according to claim 1, wherein the changing unit is configured to bring the output level of each light emitted from the plurality of light emitting units close to a predetermined target value. A target value control means for changing the value of the light emission data is further provided, and a measuring means for measuring the temperature of the plurality of light emitting means, and the output output from the detecting means based on the temperature measured by the measuring means. An image forming apparatus comprising: a temperature compensating means for compensating data indicating a level relative to the target value. 4. An image forming method for forming an image by irradiating a photoconductor with light emitted from a plurality of light emitting means, wherein a light emission control step of giving light emission data to each of the plurality of light emitting means to emit light. A detection step of detecting an output level of each light emitted from the plurality of light emitting means, and a value of the light emission data given to each of the plurality of light emitting means according to the output level detected by the detection step. An image forming method, comprising: 5. The image forming method according to claim 4, wherein in the changing step, the value of the light emission data is changed so that the output levels of the lights emitted from the plurality of light emitting units are the same. An image forming method comprising a level adjusting step. 6. The image forming method according to claim 4, wherein in the changing step, the output level of each of the lights emitted from the plurality of light emitting units is brought close to a predetermined target value. The method further comprises a target value control step of changing the value of the light emission data, a measurement step of measuring the temperatures of the plurality of light emitting means, and the output level output in the detection step based on the temperature measured by the measurement step. And a temperature compensating step for compensating the data indicating the above relative to the target value.