JPH0723005B2 - Driving method for optical writing device - Google Patents

Description

The present invention relates to a method for driving an optical writing device that irradiates a photoreceptor with light to expose it.

“Conventional Technology” With the remarkable progress of office automation in recent years, demand for high-quality, high-speed, low-cost non-impact printers has increased. Also in the medical field, the advent of high-quality video printers and photo printers used in medical equipment is desired.

In these fields, laser printers are most widely used to meet such requirements, but the requirements of small size, high speed, high image quality, high reliability, and low price are satisfied at a high level, and both of them are satisfied. Has not reached.

Now, as a driving method for such an optical writing device, an LED is used.
(Light emitting diode) array printers, laser diode array printers, EL (electroluminescence) array printers, liquid crystal optical shutter printers, fluorescent display tube printers, etc., so-called light modulator array in which light emitting parts are arranged in rows. Electrophotographic printers and photo printers that use a computer have attracted attention because of their high reliability and high-speed suitability.

Similar to laser printers, these printers irradiate light onto a photosensitive member to form an electrostatic latent image by selectively emitting light from a light emitting element, develop it, and record it by a so-called electrophotographic method. I'm getting an image. Alternatively, the light modulation element array may be used to irradiate a film of silver salt with light, and the film may be exposed to obtain a negative film similar to a normal photograph for printing.

However, these have not been able to sufficiently satisfy the demand for high image quality. In particular, in an electrophotographic printer using an LED array, variations in manufacturing intensity cause variations in emission intensity among the light emitting elements, which leads to deterioration in image quality, making it impossible to take advantage of its outstanding high-speed suitability. Heretofore, various proposals have been made in order to correct the variation in the emission intensity of each light emitting element of such an optical writing device (Japanese Patent Laid-Open No. 58-7847).
6, Same 60-5387, Same 60-1471, Same 60-6472, Same 60-6473, Same 6
1-47556, 60-64870).

That is, an LED array used in a general optical writing device is constructed by incorporating a control circuit as shown in FIG.

This circuit has a structure in which a large number of LEDs 1 are arranged in a line, and the blinking data stored in the shift register 2 is supplied to these via the latch circuit 3, the AND gate 4, and the driver 5. There is.

In this circuit, when one line of blinking data 2a is input to the shift register 2 at the same timing as the transfer clock 2b, the latch signal 3a is input to the latch 3 circuit and the blinking data is held. Next, when the lighting signal 4a is supplied to the AND gate 4 for a certain period of time, if the blinking data held in the latch circuit 3 is such that the corresponding LED1 is lighted, the time when the LED1 is supplied with the lighting signal Only lights up.

By the way, this way, all the LEDs will be lit for the same fixed lighting time. Therefore, each light emitting element is turned on a plurality of times within the light writing time for one main scan of the LED array, and the total lighting time is differentiated to perform correction.

Specifically, as shown in FIG. 11, the time Tr for one main scan
Is divided into, for example, three time intervals, and a lighting time t having a constant width is selected. On the other hand, for each light emitting element, an appropriate total lighting time is determined in advance according to the light emission intensity. In this example, the total lighting time of each light emitting element is
It is possible to select three types of lighting time of t, 2t, and 3t. Of course, if no exposure is required, the lighting time is zero. Here, the total lighting time of a light-emitting element whose emission intensity is close to the average value is 2t, the total lighting time is 3t when the emission intensity is close to 50% of the average value, and the emission intensity is close to 150% of the average value. The total lighting time t is set. As a result, the photoconductor is irradiated with substantially uniform light energy from the light emitting element to be turned on during the time Tr.

According to the circuit shown in FIG. 10, first, the first blink data 2a is input to the shift register 2 in synchronization with the transfer clock 2b. After that, the latch signal 3a is supplied to the latch circuit 3 to hold the blinking data in the latch circuit 3. Then, when the blinking data and the lighting signal 4a are supplied to the AND gate 4, only the light emitting element having the blinking data "1" is turned on. This operation is repeated three times, and the blinking data in the shift register 2 is replaced each time. If all the blinking data for three times is "1", the total lighting time of the light emitting element is 3t. If two times are "1" and the remaining one is "0", the total lighting time is 2t.

As described above, the variation in the emission intensity of each light emitting element is corrected.

“Problems to be Solved by the Invention” In the above-described apparatus, since the exposure time for one main scan is divided into a plurality of time intervals to correct the light emission time, the correction is performed. The number of divisions must be increased to improve the accuracy of. For example, the accuracy of correction
If you try to double the size of the example in the figure, as shown in Figure 12,
It is necessary to repeat blinking data transfer and lighting twice, that is, six times, within the time for performing exposure for one same main scan.
As a result, in the case of FIG. 12, six types of total lighting time can be selected.

Here, in FIG. 11, the time for exposing the blinking data for one main scanning is Tr (hereinafter referred to as one main scanning writing time).
And the frequency of the transfer clock 2b of the blinking data 2a is f _CLK, and the total number of blinking data stored in the shift register 2 is Nd.
And the number of time intervals is Ni, the relation of the following equation must be established between them.

Ni ≦ Tr {(1 / f _CLK ) × Nd} (1) Here, {(1 / f _CLK ) × Nd} corresponds to the transfer time of the blinking signal for one line.

As described above, since the division number Ni has a certain upper limit, the correction accuracy cannot be improved to some extent.

The correction error Ec can be expressed by the following equation.

Ec = 1 / Ni ≧ Nd / (f _CLK ) × Tr) (2) Further, one main-scanning writing time Tr is determined by the dot density ds in the sub-scanning direction required for the recorded image and the process speed Up.

Tr = 1 / (Up × ds) (3) Substituting equation (1) into equation (2) yields the following equation.

Ec = Nd × ds × Up / f _{CLK In other} words, high speed (Up → Large) high quality (ds →
To achieve (large and Ec → small), the number of serial signal lines for transferring blinking data to the shift register is increased and parallel transfer is performed, and the serial data transfer number Nd
Or the shift register 1 or latch 2 capable of high-speed data transfer must be used to increase the data transfer frequency f _CLK . However, increasing the number of serial signal lines is not preferable because it increases the cost of the device and lowers the reliability. In addition, the use of high-speed elements increases the cost by itself, and high-frequency noise countermeasures are expensive, which is also undesirable.

The present invention has been made to solve the above points, and an object of the present invention is to provide a driving method of an optical writing device capable of realizing high speed and high image quality at low cost.

"Means for Solving Problems" In the method of the present invention, a light modulation element array in which a plurality of light modulation elements are arranged in a row is made to face a photoconductor, and the light modulation elements are selectively caused to emit light so that one main The operation of performing exposure for scanning, moving the photoconductor and the light modulation element relatively in the sub-scanning direction, and performing exposure for one main scan again is repeated, and the photoconductor is exposed corresponding to the recorded image. To do. Here, the exposure time for one main scan is divided into a plurality of time intervals, and at least two or more intervals are selected with different predetermined lighting times to generate a lighting signal. At this time, at least a lighting signal other than the lighting signal having the longest lighting time is composed of a plurality of pulse-shaped lighting signals obtained by evenly dividing the lighting signal, and the pulse-shaped lighting signal is divided into a predetermined period within a time section. Disperse evenly over the width region. Then, for each light modulation element, sections are selected to be turned on so as to obtain an appropriate total lighting time according to the emission intensity, and the light modulation element is turned on only in the selected section to perform one main scanning. Do a minute exposure.

If the width of the region where the pulsed lighting signal is dispersed and the width of the lighting signal with the longest lighting time are made equal, and the width of the region and the width of the lighting signal with the longest lighting time are expanded / contracted at the same ratio. The total lighting time can be corrected without changing the ratio of the respective total lighting times corrected for variations.

[Operation] In the present invention, first, before the optical modulation element array is incorporated into the optical writing device, the emission intensity of each element of the optical modulation element array is measured using an automatic multipoint light amount measuring device or the like. Then, the variation correction signal for each element, which is obtained by arithmetically processing the emission intensity data, is written in the storage element or the like provided in the optical modulator array drive device.

The light modulation element array drive device has one main scanning write time Tr.
Is divided into Ni time intervals, and a lighting signal of various widths selected in advance is supplied to each interval. For example, Ni =
When set to 4, the lighting time for each section is selected to be 8: 4: 2: 1.

The variation correction signal indicates selection of any combination of these lighting times having different widths.
By combining lighting times having different widths, 15 kinds of total lighting times from "1" to "15" can be selected.

The total lighting time Ti may be obtained under the conditions described above. In this way, one main scanning writing time can be divided into a relatively small number to realize various total lighting times.

Also, the maximum value of the lighting time, in this example, the lighting time is "8".
The lighting signal may be supplied twice within one main scanning write time Tr, and a lighting time of 8: 8: 4: 2: 1 may be selected. As a result, the ratio of the total lighting time to the main scanning write time Tr (duty)
And improve both efficiency and accuracy.

Furthermore, in the present invention, the lighting signal except at least the lighting signal of the maximum width is divided into a large number of even pulses, and the pulses are distributed and arranged in each temporal section. By doing so, the total lighting time after the variation correction can be corrected as it is based on the condition of other factors of the device.

[Example] (Automatic multi-point light quantity measuring device) In carrying out the method of the present invention, the emission intensity of each light modulation element forming the light modulation element array is measured in advance.

FIG. 2 shows a block diagram of an automatic multipoint light quantity measuring device suitable for the work.

In this device, an optical sensor 10 is attached so as to be movable in the direction of arrow A, and an LED array unit 15 is installed below it. The optical sensor 10 is supported by two pins 13 and 13 'which are laid over bearings 12 provided upright on both ends of a fixed base 11. One of the two pins 13 and 13 '13
A male screw is cut on the outer periphery of the motor so that it can be rotated by the drive motor 14. This drive motor 14 has
A 5-phase stepping motor was used. A female screw that fits into the male screw of the pin 13 is attached to the upper part of the optical sensor 10, and the optical sensor 10 is configured to move in the direction of arrow A when the pin 13 is rotated by the drive motor 14.

On the other hand, the LED array unit 15 on the fixed base 11 is
16, a light converging rod lens array 15a, and a bracket 15b.

On the other hand, the control device 17 for controlling the operation of this device comprises a microprocessor (not shown), a memory element storing a program for its operation, a recording device for temporarily storing signals, and the like. The control device 17 uses, as the drive device control signal 17a, a command for moving and positioning the optical sensor 10 in the direction of arrow A and a command corresponding to LED address information indicating which light emitting element of the LED array 16 is to be turned on. Drive 18
It is a circuit for sending to. The driving device 18 is a circuit that interprets these commands, creates a lighting control signal 18a for the LED and an optical sensor fine movement signal 18b, and applies them to the LED array 16 and the optical sensor fine movement motor 14, respectively. For the drive device 18, a so-called personal computer digital output board including a decoder and a driver is used.

The optical power meter 19 is a circuit that performs analog-digital (A / D) conversion of the signal output from the optical sensor 10 to obtain a digital light emission intensity signal 19a and outputs it to the control device 17. The optical power meter 19 is composed of an A / D conversion circuit and the like, and GPIB (IEEE488 compliant) is used as an interface with the control device 17.

The above-described automatic multipoint light amount measuring device operates as follows.

First, based on the command 17a output from the control device 17, when the drive device 18 outputs the lighting control signal 18a and the optical sensor fine movement signal 18b, each light emitting element of the LED array 16 measures the light intensity one by one from the left. Lights up for a sufficient amount of time. The optical sensor 10 is an optical element (LED) that is turned on each time.
The light is received and photoelectrically converted.

The output signal of the optical sensor 10 is the optical power meter 19
Then, it becomes a digital light emission intensity signal 19a, which is input to the control device 17. The control device 17 temporarily stores the signal in a built-in storage device. This procedure is executed for all the light emitting elements that form the LED array 16, and the emission intensity signals corresponding to all the light emitting elements are stored in the storage device.

In this embodiment, a variation correction signal 17b corresponding to all the light emitting elements is created based on the light emission intensity signal temporarily stored in the storage device, and the variation correction signal 17b is created through the ROM writer 20.
Write to ROM21 (electrical programmable read only memory). This EPROM 21 is incorporated in an electrophotographic printer, which will be described later, together with the LED array unit 15, and is used to correct the variation in the light emission intensity of each light emitting element of the LED array 16.

(Electrophotographic Printer) FIG. 3 shows a block diagram of an electrophotographic printer suitable for carrying out the present invention.

This device includes an LED array 16 arranged to face the photoconductor 23,
It is composed of a light converging lens array 15a and a circuit for controlling lighting of the LED array 16. This circuit includes a 2-line buffer memory 24, a counter circuit 25, and a memory control circuit 26.
And an AND gate 27 and a lighting signal generation circuit.

The 2-line buffer memory 24 includes two shift registers 24.
a and 24b, one of which stores the image signal 30 for one line and the other of which is provided for outputting the already stored image signal 24d. A changeover switch 24c is added.

The counter circuit 25 receives the input reference clock 31 and the line synchronization signal 32, and causes the two-line buffer 24 to input a reference clock 25a for image signal input, a transfer clock 25b for image signal output, and a switch switching switch. This circuit supplies the switching pulse 25c. This circuit is composed of a counter and a frequency divider (not shown). Further, this circuit supplies the memory control circuit 26 with the address signal 25d for memory reading, and also to the lighting signal generation circuit 28,
The line synchronization signal 25e for creating the control signal is supplied to the transfer clock 25b. Further, the shift clock 25f for blinking data and the latch signal 25g are supplied to the LED array 16.

The memory control circuit 26 is an arithmetic circuit equipped with the EPROM 21 in which the variation correction signal previously created for the LED array 16 is stored. This circuit is provided with a variation correction signal 26a corresponding to the address signal 25d input from the counter circuit 25.
Is output to the AND gate 27.

The lighting signal generation circuit 28 is a circuit that repeatedly outputs a lighting signal 28a having a preset width toward the LED array 16 in a fixed order. This circuit is reset by the line synchronization signal 25e input from the counter circuit 25 and counts the transfer clock 25b.
It comprises a decoder 28c which receives the count value of 28b and outputs a high-level or low-level lighting signal.

(Operation of Electrophotographic Printer) In the electrophotographic printer having the above-mentioned configuration, the image signal 30, the reference clock 31, and the line synchronization signal are supplied from the image output device (not shown).
32 is received and an electrostatic latent image corresponding to the image is formed on the photoconductor 23. Between the image output device and this electrophotographic printer, process control commands for controlling various known electrophotographic processes such as paper feeding, charging, development, transfer, cleaning, and fixing, and paper jam and fixing device temperature are appropriate. Although the printer status is exchanged to notify the ready / not ready indicating whether or not it is not related to the present invention, the description thereof is omitted.

The image signal 30 is input to the 2-line buffer 24, stored in either one of the shift registers 24a and 24b, and then alternately read out. The reference clock 31 given as a pulse synchronized with the image signal 30 is input to the counter circuit 25. The counter circuit 25 is the above two line buffer.
Switching pulse 25c is output to 24 and 2 line buffer 2
The connection of the switch 24c in 4 is switched alternately to allow reading / writing to proceed simultaneously. This method is well known.

At the same time, the counter circuit 25 outputs an address signal 25d for reading the variation correction signal to the memory control circuit 26. 2 line buffers 24 for each variation correction signal
It corresponds to the image signal 24d output from. The memory control circuit 26 outputs a variation correction signal 26a based on the address signal 25d. The AND gate 27 takes the logical product of the image signal 24d and this variation correction signal 26a to create blinking data 27a, and outputs it to the LED array 16.

FIG. 1 shows a time chart showing the transfer of the blinking data 27a to the LED array 16 and the state of the lighting signal.

The LED array 16 incorporates circuits such as the shift register 2 shown in FIG. Here, in FIG. 1, blinking data 27a is input to the shift register 2 of the LED array (FIG. 1a). This blinking data 27a is transferred in synchronization with the shift clock 25f. Blink data 2
When the transfer of 7a is completed, the latch signal from the counter circuit 25
25 g is input, and this blinking data 27 a is held in the latch circuit 3.

For example, in this embodiment, the time Tr for one main scan is divided into four time intervals N1 to N4. Also, the same image signal
30 is stored in the 2-line buffer 24 (Fig. 3) 1
During the main scanning writing time Tr, the image signal is repeatedly read four times. Therefore, both the transfer clock 25b and the shift clock 25f have four times the frequency of the reference clock 31.

Then, when the transfer of the blinking data 27a of the first section N1 is completed, the lighting signal 28a of the lighting time T1 is supplied from the lighting signal generation circuit 28 (FIG. 3) to the LED array 16. Then 2
The blinking data 27a of N2 for the second section is transferred. The lighting signal 28a for the lighting time T2 is supplied to the second section N2. Similarly, the lighting time until the fourth section N4
The lighting signals of T3 and T4 are supplied to complete the exposure for one main scanning. After that, in the 2-line buffer 24, the switch 24c is switched to output the next image signal.

In this way, focusing on one light emitting element of the LED array 16, a certain light emitting element is
It lights up for T1 + T2 + T3 + T4 hours, and a certain light emitting element lights up for T1 + T4 hours. That is, the lighting time of each light emitting element is selected by combining the lighting signals of these four types. In the example of FIG. 1, T1: T2: T3: T4 was set to 8: 4: 2: 1. In this way, the light emitting element having a low light emission intensity is lit for a long time, and the light emitting element having a high light emission intensity is lit for a short time, and the total light energy applied to the photoconductor 23 within one main scanning writing time is adjusted to be uniform.

(Structure of Lighting Signal) Here, in the present invention, as shown in FIG. 1, the lighting signal of the lighting times T2, T3, and T4 is divided into a large number of pulses having a uniform width.

The partially enlarged view is shown in FIG.

Although a detailed description will be given later, in FIG. 8, the lighting signal of the lighting time T2 is, for example, T2 / K as shown in FIG.
Pulses of width T2 '(where K is a constant) are arranged at equal intervals. Similarly, the lighting signal for the lighting time T3 is configured as shown in FIG. 7B, and the lighting signal for the lighting time T4 is configured as shown in FIG. The ratio of the widths of the respective pulses is T2: T3: T4 = T2 ': T3': T4 '. In this example, K = 512.

Hereinafter, the content of the variation correction signal and its specific processing will be described assuming that the lighting signal is a continuous signal that has not been divided, and then the operation when the lighting signal is divided will be described.

(Example 1 of variation correction signal LED) In which section each light emitting element is turned on is determined by the content of the variation correction signal 26a (FIG. 3).

Table 1 shows the variation correction signal 26 for the embodiment of FIG.
It shows an example of the contents of a.

Table 1 shows the case where there are 16 types of variation correction signals (including the case where no lighting occurs at all). One of these variation correction signals is prepared for each light emitting element and stored in the EPROM 21 in the memory control circuit 26. In this table, d _j is called duty, which is 1
The total lighting time of each light emitting element with respect to the main scanning writing time Tu is shown. For example, in FIG. 1, when a certain light emitting element is lit for T1 + T3 hours, its duty d _j is (T1
+ T3) / Tr.

Since T1: T2: T3: T4 is 8: 4: 2: 1, the minimum total lighting time is T4, and the duty at that time is T4 / Tr. Also, since the maximum total lighting time is T1 + T2 + T3 + T4, the maximum duty is (T1 +
T2 + T3 + T4) / Tr.

In Table 1, σ _1j , σ _2j ... σ _4j are the sections N 1 and N, respectively.
It is data having the content indicating whether or not the light emitting element is turned on at 2, N3, and N4. Here, j indicates the number of the light emitting element. If σ _1j is “1”, it indicates that the lighting is performed for the lighting time T1 in the section N1, and if it is “0”, it indicates that the lighting is not performed in the section N1. When σ _1j (i = 1 to 4) are all “0”,
Since it means that the light emitting element does not light at all, if this is excluded, 15 kinds of total lighting time can be prepared from steps s _{1 to} s ₁₅ .

In addition, if the duty is expressed by a general formula, it becomes the following formula.

i: Section number (the number of sections is Ni in the above formula) j: Light emitting element number Tr: 1 Main scanning writing time σ _ij = 1: Light emitting element j is turned on in section Ni σ _ij = 0: Do not turn Ti: The width of the lighting signal in the section i, that is, the exposure time for one main scanning is divided into a plurality of time sections Ni, and the lighting time in each section Ni is Ti (i =
1, 2, 3, ... Ni), the unit lighting time is T, and an arbitrary integer from 1 to N is ni (i = 1, 2, 3, ... Ni).
Let ni (min) be the minimum value of ni and max (ni) be the maximum value of ni.
A = {ni | i = 1, 2, 3, ... Ni} and a set of integers B = {x | min (ni) ≦ x ≦ max (ni)}, A = B Or there is a relationship of A⊃B
ni is selected so that the lighting time Ti satisfies Ti = 2 ⁿⁱ T.

Looking at Fig. 1, Ti is as follows.

T1 = 2 ³ T4 T2 = 2 ² T4 T3 = 2 ¹ T4 In this example, Tr / 40 was selected as T4.

Thus, when d _j is obtained, the above-mentioned Table 1 is obtained.

(Variation correction signal example 2) FIG. 4 shows an example in which the time for one main scan is divided into five sections N1 to N5. In this example, T1: T2: T3: T4: T5 was chosen to be 8: 8: 4: 2: 1. The maximum total lighting time in this case is T
1 + T2 + T3 + T4 + T5. The variation correction signal in this embodiment is shown in Table 2.

Table 2 shows the case where there are 16 types of variation correction signals (not including the case where no lighting occurs at all).

In this example, since T1: T2: T3: T4: T5 is 8: 8: 4: 2: 1, the minimum total lighting time is T5, and the duty at that time is T5 / Tr.

Since the maximum total lighting time is T1 + T2 + T3 + T4 + T5, the maximum duty is (T1 + T2 + T3 + T4 + T5) / Tr.

The lighting time Ti of each section is as follows.

T1 = T2 = 2 ³ T5 T3 = 2 ² T5 T4 = 2 ¹ T5 In this example, Tr / 50 is selected as T5.

In Table 1, σ _1j is all set to “1” because the variation of the light emission intensity of the light emitting element is generally about ± 5% with respect to the average value in a small device and with respect to the average value in a large number of devices. It is about ± 40%. The duty range required for correction may be about 2.4 times the ratio of the maximum value to the minimum value. Therefore, in this case, σ _1j does not need to be controlled and may be always 1. Further, by providing two or more sections having the maximum lighting time, the maximum value of the duty can be increased, the corrected light amount can be increased, and high-speed optical writing can be performed.

By controlling σ _1j (i = 2, 3, 4, 5) for each element as described above, 2 ₄ = 16 in this embodiment as well.
The same duty can be obtained.

(Correction of Variation) As described above, the exposure amount P _wj (corresponding to the total lighting time) by the j-th light emitting element of the LED array 16 is represented by the following formula.

P _wj = d _j Trp _j p _j : Light intensity before correction By selecting d _j (combination of σ _ij ) in Table 1 or 2 according to this p _j , variation in light amount (variation in p _j ) You can make corrections. Furthermore, when the image output device outputs a multi-valued image, that is, when the image signal is multi-valued instead of binary, the exposure amount can be positively multi-valued. In this case as well, the variation correction can be similarly performed.

In this embodiment, σ _ij is determined for the uncorrected light amount p _{j as} follows, and its variation is corrected.

First, find the minimum value pmin of p _j . If the maximum duty d _j is applied to the element having the minimum emission intensity, the light amount after the variation correction of the entire LED array can be maximized. Therefore, the maximum step number shown in Table 1 for pmin can be obtained. s
Apply ₁₅ .

In FIG. 5, the horizontal axis indicates the emission intensity of each light emitting element before correction,
FIG. 7 is a correction diagram in which the vertical axis represents the total emission energy after correction, that is, the exposure amount. The light intensity of each element before correction varies from pmin to pmax due to manufacturing variations and the like. Since the maximum duty is applied to the element of pmin as described above, the corrected exposure amount Pmin is obtained by the following equation.

Pmin = d ₁₅ Tr pmin This d ₁₅ is the step number s ₁₅ k duty. First
Since d ₁₅ = 15/40 = 3/8 from the table, substituting this gives the following formula.

If the exposure amounts of the other elements are matched with the reference exposure amount, the exposure amounts of all the elements can be made uniform.

When this reference exposure amount is set to the lower limit threshold value and the graph of FIG. 5 is viewed from left to right, first, the duty of step s _j = 15 in Table 1 is used is If the previous emission intensity is between pxin and px and exceeds px, the step
When the duty of s _j = 14 is used, the corrected value becomes closer to the reference exposure amount Pmin. Thus, more appropriate duty selection is repeated, and it becomes possible to correct pmax, which is the maximum emission intensity before correction, with the duty of step s _j = 8. As a result, the corrected exposure amount can be suppressed within the range from Pmin to Pmax.

When there is a variation of about pmin / pa ≈ 0.7 pmax / pa ≈ 1.3 when pa is the average value of the emission intensity of each element before correction, the corrected exposure dose is Pmin by the method of this embodiment. /Pa≒0.946 Pmax / Pa ≒ 1.054 That is, it is possible to reduce the light amount variation of ± 30% before correction to ± 5.4% after correction.

Actually, the measurement error of p _j measured by the automatic multi-point light quantity measuring device or the like makes the variation of P _{w j} after correction somewhat large. In the case of the above embodiment, when the corrected P _wj was measured for each element, the variation was ± 5.9%.

The correction diagram for the embodiment shown in Table 2 is shown in FIG.

Also in this case, the corrected exposure amount Pmin is calculated by the following equation.

Pmin = d ₁₅ Tr pmin From Table 2, since d ₁₅ = 0.46, substituting this gives the following formula.

Pmin = 0.46 Tr pmin If the exposure amounts of other elements are matched with this reference exposure amount, the exposure amounts of all the elements can be made uniform.

When this reference exposure amount is set to the lower limit threshold value and viewed from the graph of FIG. 6 from left to right, first, the duty of step s _j = 15 in Table 1 is used before correction. If the light emission intensity of P is from pxin to px and exceeds px, the corrected value becomes closer to the reference exposure amount Pmin when the duty of step s _j = 14 is used. In this way, more appropriate duty selection is repeated, and in step s _j = 3, it becomes possible to correct the emission intensity before correction up to the maximum pmax. As a result, the corrected exposure amount is suppressed within the range from Pmin to Pmax.

When there is a variation of about pmin / pa ≈ 0.6 pmax / pa ≈ 1.4 when pa is the average value of the light intensity of each element before correction, the corrected exposure amount is Pmin / pa by the method of this embodiment. It is improved such that Pa ≈ 0.945 Pmax / Pa ≈ 1.055. That is, it is possible to reduce the light amount variation of ± 40% before correction to ± 5.5% after correction.

Actually, the measurement error of p _wj measured by the automatic multi-point measuring device and the like slightly increase the variation of P _wj after correction. In the case of the above embodiment, when the corrected P _wj was measured for each element, the variation was ± 5.9%.

(Specific Signal Processing) Returning to FIG. 3 again, a more specific circuit operation using the above variation correction signal will be described with reference to the time charts of FIGS. 3 and 7.

First, the image signal 30 is input to the 2-line buffer memory 24 in synchronization with the reference clock 31 (FIGS. 7A and 7C). In this case, the counter circuit 25 supplies the reference clock signal 25a to the 2-line buffer memory 24 so as to adjust the input timing.
Further, in order to synchronize the start end of the image signal 25a for one line, the line synchronization signal 32 is also input to the counter circuit (FIG. 7b).

The switching signal 25c is input from the counter circuit 25 to the two-line buffer memory 24, and the switch 24c switches between the input shift register 24a and the output shift register 24b (FIG. 7d).

Meanwhile, the image signal transfer clock 25b is output to the output shift register 24b (FIG. 7e).

Assuming that the apparatus of this embodiment operates so as to divide one main scanning writing time into four time intervals as shown in FIG. 1, the image signal transfer clock 25b has a frequency four times as high as the reference clock. Then, while the same image signal for one line is stored in the output shift register 24b, 4
The reading is repeated repeatedly.

On the other hand, the counter circuit 25, with respect to the memory control circuit 26,
The address signal 25d is supplied. This address signal 25d is
The image signal 24d is output from the two-line buffer 24 at the same timing as the output of the image signal 24d.
A variation correction signal 26a is output from (FIG. 7i).
The address signal 25d is reset in the counter circuit 25 every time a line synchronization signal is input (FIG. 7h).

Image signal 24d output from the 2-line buffer memory 24
And the variation correction signal output from the memory control circuit 26
When 26a and 26a are simultaneously input to the AND gate 27, the logical sum is taken to obtain the blinking data 27a, which is supplied to the LED array 16.

The latch signal 25g is output once each time one line of blinking data 27a is input to the LED array (FIG. 7f).

The content of the image signal 24d is always "1" for the light emitting element to be turned on.

On the other hand, the variation correction signal 27a is selected and output with a value of "0" or "1" in a manner as shown in Table 1 for each section. That is, the section shown in FIG.
Since N1 outputs σ _1j , N2 outputs σ _2j , and so on,
By taking the logical product of both, blinking can be performed according to the light emission intensity of the light emitting element. The subsequent operation is as described with reference to FIG.

When printing was performed using the above electrophotographic printer, high-quality printed images with little unevenness were obtained not only for letters and figures but also for halftone images. A slight one-dimensional noise remained on the screen in the halftone image, but it was confirmed that this was not due to the variation in the light amount, but to the poor alignment accuracy of the light emitting elements of the LED array. The electrophotographic printer uses an As-Se type photosensitive member, the process speed (photosensitive member moving speed) is 140 mm / sec, the LED array uses a GaAsP type light emitting element, the emission wavelength is about 680 nm at the peak wavelength, and the dot density (LED Array density) 240SPi (spot per inch (2.54cm for 1 inch)), LED forward current is constant current drive of 5mA / dot, and aperture angle is 20 ° as selfoc lens array.
SLA-20 manufactured by Nippon Sheet Glass Co., Ltd. was used.

In the above embodiment, the lighting signals are assigned and supplied in order from the longest one within one main-scan writing time, but this order may of course be arbitrary.

Further, the emission intensity data of each light emitting element measured by the automatic multi-point light quantity measuring device may be transferred to the electrophotographic printer, and the variation correction signal may be generated on the electrophotographic printer side based on this data. In this case, the electrophotographic printer is provided with a so-called non-volatile random access memory (RAM) backed up by a battery for storing the variation correction signal. For example, lithium battery and CMOS
A design life of 6 years can be achieved by combining low power consumption RAM.

Further, a silver salt film or the like may be used as the photoreceptor.

(Operation when Splitting Lighting Signal) As described above, according to the method of the present invention, the lighting for individually and highly accurately correcting the variation in the emission intensity of each light modulation element forming the light modulation element array is performed. A signal can be supplied.

On the other hand, in the optical writing device, there are various other factors that change the image quality of a recorded image, such as environmental changes, changes over time, and changes in the concentration of developer such as toner that develops the photoconductor.

Various stabilization control devices are provided in order to prevent image quality variation due to these factors and to stabilize image quality.

Here, when the density of the recorded image becomes too dark as a whole, the exposure time by the light modulation element is extended, and when the density of the recorded image becomes too light as a whole, the exposure time by the light modulation element is increased. It should be shortened.

This is, for example, the lighting signal T1 obtained by the previous embodiment,
It can be implemented by expanding and contracting T2, T3, T4, and T5 at the same rate.

However, it is not preferable to incorporate a circuit that responds to another correction factor into the circuit for processing the variation correction signal, because the circuit becomes unnecessarily complicated.

Therefore, in the present invention, in the variation correction circuit, at least the lighting signal excluding the lighting signal of the maximum width is
As shown in the figure, it was divided into a large number of even pulses and distributed in each temporal segment.

Then, for example, in all the time intervals, as shown in FIG. 8, the time T _LIM for supplying the lighting signal is expanded and contracted in proportion to other correction factors.

By doing so, it is possible to expand or contract each lighting signal in the same ratio for each temporal section in which the lighting signal width is substantially different.

The expansion and contraction of the lighting signal supply time T _LIM is performed, for example, by opening and closing a gate inserted in the lighting signal supply circuit with a correction signal. Alternatively, the counter circuit 25 shown in FIG. 3 supplies a lighting signal cutting signal 25k for each time interval. A predetermined correction signal 25h is input to the counter circuit 25 from a correction circuit (not shown) in order to control the output timing of the lighting signal cutting signal 25k.

The correction signal may be supplied from a circuit that controls the process of the entire optical writing device, or may be input by an operator operating a predetermined key on the console panel.

FIG. 9 shows another embodiment of the present invention, in which the lighting signal is
When dividing in the same manner as in the figure, it is characterized in that the pulse widths after division are all constant.

In this case, as shown in the figure, the ratio of the lighting signal width is T2: T3: T4.
When the ratio is 4: 2: 1, the ratio of the divided pulse numbers is set to 4: 2: 1. For example, in FIG. 9a, T2 / T2 '= N
Looking at the range of one unit of time T2 'when (N is an integer), 4 pulses are used when the lighting signal is T2 (a in the figure), 2 pulses are used when the lighting signal is T3 (b) in the figure, and One is composed of one pulse (Fig. 6c).

It goes without saying that T2 ′: T3 ′: T4 ′ = T2: T3: T4. In this case, the pulses are not necessarily arranged at equal intervals, but uniform correction is possible as long as they are divided into a large number of pulses. Of course, it does not matter if they are all evenly spaced.

In any of the above cases, expansion / contraction of the entire lighting signal can be performed while preventing the counter circuit and the lighting signal generating circuit in FIG. 3 from having complicated configurations.

[Advantages of the Invention] According to the driving method of the optical writing device of the present invention described above, the variation in the emission intensity of each light emitting element can be corrected at high speed and with high precision, so that the efficiency of optical writing is increased and high quality recording is performed. It can be performed. Furthermore, the cost is low, maintenance is easy, and there is no problem of high frequency noise.

[Brief description of drawings]

FIG. 1 is a time chart showing an embodiment of a driving method of an optical writing device of the present invention, FIG. 2 is a block diagram of an automatic multipoint light quantity measuring device suitable for carrying out the method of the present invention, and FIG. FIG. 4 is a block diagram of an electrophotographic printer used for carrying out the method, FIG. 4 is a time chart showing another embodiment of the method of the present invention, FIG. 5 is a graph showing the effect of the variation correction, and FIG. 6 is the variation correction. 7 is a graph showing the effect of another embodiment, FIG. 7 is a time chart for explaining the actual operation of correcting variations by the electrophotographic printer, and FIGS. 8 and 9 are explanations of the operation for correcting the total lighting time of the present invention. FIG. 10 and FIG. 10 are explanatory views of a drive circuit of an LED array used in a general optical writing device, and FIGS. 11 and 12 are time charts showing a driving method of a conventional optical writing device. 16: Light modulator array, 23: Photoconductor, 25g: Latch signal, 27a: Blinking signal, N1 to N5: Time interval, Tr
…… One main scan time, T1 to T5 …… Lighting time.

Claims (2)

[Claims] 1. An optical modulation element array, in which a plurality of optical modulation elements are arranged in a row, is opposed to a photoconductor, the light modulation elements are selectively caused to emit light, and exposure for one main scanning is performed. In the optical writing device that exposes the photoconductor in correspondence with the recorded image by repeating the operation of moving the optical modulator and the light modulation element relatively in the sub-scanning direction and performing exposure for one main scanning again. The time for performing exposure for scanning is divided into a plurality of time sections, and predetermined lighting times different from each other in at least two sections are selected to create a lighting signal. As for the lighting signal, the lighting signal is composed of a plurality of pulsed lighting signals that are evenly divided, and the pulsed lighting signal is evenly dispersed in a region of a predetermined width within the temporal section, and each light Modulator In order to obtain an appropriate total lighting time according to the emission intensity, the sections to be lighted are combined and selected, and the light modulation element is lighted only in the selected section to perform exposure for one main scanning. And a method for driving an optical writing device. 2. The width of a region in which the pulsed lighting signal is dispersed is made equal to the width of the lighting signal having the maximum lighting time,
2. The method for driving an optical writing device according to claim 1, wherein the total lighting time is corrected by expanding and contracting the width of the region and the width of the lighting signal having the maximum lighting time at the same ratio. .