JPH0592617A - Assembling method for led printer head - Google Patents

Abstract

PURPOSE:To prevent deterioration of printing quality arising from systematic errors in outputs on the right and the left of an LED array by a method wherein control is made so that sudden change of outputs may not occur between the adjoining LED arrays. CONSTITUTION:Selection is made among LED arrays and those in good quality only are picked up. Then mean value of outputs is obtained for the ends on both the right and left of each of the arrays. The difference in the mean values of outputs obtained for both the ends on the right and left is then taken as unevenness between the right and left of the array. Then, the arrays that exceed a specified limit in the difference of outputs on the right and left ends are eliminated, and those having unevenness of output exceeding a specified limit are also eliminated. The arrays are then sorted out in groups graded by every 5% of the output mean values. Then a printer head is assembled by using the LED arrays picked up out of the same group. The difference of outputs among the heads is corrected by changing amperage in power sources supplying electric current for emission of light to the LED's. Thereby deterioration of printing quality arising from systematic errors in outputs on the right and the left of the LED array can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a printer head using a large number of LED arrays, and more particularly to prevention of print quality deterioration due to LED output variations.

[0002]

2. Description of the Related Art A printer head is known in which a large number of LEDs (light emitting diodes) are used to drive in a static or time-division manner to expose a photosensitive drum for printing. LE is one of the problems with LED printer heads.
There is a decrease in print quality due to the output variation of D. LED
The variation in the output of (1) appears as a variation in the beam diameter of the printing dots, which causes unevenness in density and the like, and deteriorates the printing quality.

In response to this problem, the actual fairness is 2-21829.
Japanese Patent Laid-Open Publication No. 2003-242242 discloses control of light emission time in LED array units and L
It is shown that the output is made uniform by combining with the control of the light emission time for each individual LED in the ED array. Similarly, Japanese Utility Model Publication No. 2-1983 discloses that the light emission time is controlled for each individual LED to compensate the output variation. However, controlling the light emission time for each LED requires a hardware configuration that is almost the same as that for realizing gradation printing. To adjust the light emission time for each LED, L
A memory that stores the optimum light emission time for each ED is required, and a control circuit that can change the light emission time is required. From the viewpoint of software, each time the LED is operated, the memory that stores the light emission time must be read and transmitted to the light emission control circuit, which increases the signal processing amount. The reason why the light emission time is adjusted for each LED despite these problems is that the problem of LED output variations is extremely important. What is clear from these known techniques is that
The variation in the average output for each LED array and the variation in the individual output within the array are being focused on as management factors.

However, the inventor has found that there is another systematic error at the left and right ends of the LED array as a factor of the LED output variation. Here, the left and right means a direction parallel to the arrangement direction of the LEDs of the LED array, which coincides with the direction of the printing line in the printer. The systematic error mentioned here means that the output systematically changes between the left side and the right side of the LED array. Such a systematic error is caused by a slight shift of the mask or a slight shift of the etching conditions in the LED array etching process or the like.

If there is a systematic error on the left and right of the LED array,
The output will change significantly between adjacent arrays.
For example, it is assumed that there is a total output difference of 20% at both ends of ± 10%. At the seam of the arrays, a ± 10% output difference results in a total output difference of 20%, even if the output averages of the two arrays are equal. If the difference of the average output for each array is added to this, the output change at the joint of the arrays becomes extremely large.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of assembling an LED printer head which limits the fluctuation of the LED output between adjacent arrays at the boundary between the arrays and enables high quality printing. It is in.

[0007]

According to the present invention, in an LED printer head assembly method in which a large number of LED arrays in which a large number of LEDs are integrated are arrayed, the average value of the light emission output of a plurality of LEDs at the left and right ends of each LED array. Is detected by the light receiving element and stored in memory, and the displacement of the average value of the emission outputs at the left and right ends stored in the memory is compared with the allowable range in the arithmetic circuit, and the displacement of the average value is used within the allowable range by using an LED array, LE
It is characterized in that the D printer head is assembled.

[0008]

In the LED array, due to the difference in the etching conditions in the manufacturing process and the mask alignment accuracy,
There is a systematic error in the output on the left and right of the array. LED
The array is made to emit light before assembly, and the average output at the left and right ends is obtained by the light receiving element. As the light receiving element, for example, a phototransistor array, a photodiode array, a photocell array, a CdS cell, a CCD element, a photomultiplier, or the like may be used. The light emission output to be measured may be obtained at least on the left and right ends, and if possible, only the left and right ends of the array may be made to emit light.

In the LED array, in addition to the systematic error on the left and right,
There are individual variations for each LED. However, since the individual accidental variations are large and the influence on the print quality is small, the outputs at the left and right ends are, for example, about 3 to 8 LEs.
Obtain an average value for D. Even if the output of one LED at the left and right ends is obtained, the systematic error is generally larger than the systematic error, so the systematic error cannot be detected because it is hidden by the random system. Since what is needed is the average output at both left and right ends, it is possible to obtain one output for each LED in the light receiving element, but the output of multiple LEDs is guided to one light receiving element by a lens, etc. The average value may be obtained from

The calculated average values of the outputs at the left and right ends are stored in a memory and compared with an allowable range by an arithmetic circuit, and those having an average value displacement within the allowable range are selected to assemble a printer head. This permissible range is selected from a range in which the print quality is not affected by the output change in the adjacent portion of the array. As the displacement, a difference or a ratio may be used. The average may be a weighted average or the like other than the simple average. For example, if the outputs of a plurality of LEDs are guided to a light receiving element via a lens, the guided output may be a weighted average of the plurality of LED outputs.

It is not practical in practice to keep the displacement of the outputs at both ends of the array within an allowable range, and it is also necessary to manage the average output of the array and control the maximum value of the output variation within the array. Preferably, the light-receiving element is used to obtain the output (light-emission output) for each LED, and from this the average output of the array and the maximum value of the output variation within the array are also obtained. Then, the printer heads are assembled by excluding those in which the output variation in the array exceeds the allowable range and those in which the displacements at the left and right ends exceed the allowable range by grouping the output averages of the array. For example, let the average value of the array output be A, the maximum variation value of the output be ± D, and the difference between the average value at the left and right ends and the average value A of the array be ± P. Assuming that the average value change is S or less in the array selection condition based on the average value, the output change at the adjacent portion of the array is determined by S + 2P. The average value change for each array is S or less, and the output change for the entire printer head is S + 2D or less. These symbols A,
S, D, and P will be used in a unified manner below.

Another example of the management of the average value of the outputs at both ends of the array is not to compare the average value A with the average output of both ends, but to fix the allowable range for the output displacement at both ends with an absolute value from the beginning. It is to be. In this case, for example, if the average output displacement at both ends is within the permissible range and the maximum output difference within the array is within the permissible range, it is considered as a good product, and, for example, the array is sorted by output average A and classified. The heads may be assembled using arrays of the same classification.

When the arrays are classified and grouped by any of the methods, one printer head has all the arrays classified by the average value A. Therefore, it is preferable to adjust the light emission current to the LED for the entire head to compensate for the magnitude of the output due to grouping. Such adjustment may be performed not for each LED but for the entire head, and for example, the output adjustment resistance of the LED constant current power supply may be adjusted.

[0014]

FIG. 1 shows an outline of an LED printer head assembling method in an embodiment, and FIG. 2 shows an outline of an assembling apparatus used. In the printer head to be assembled, 40 LED arrays of 64 dots (one LED is represented by 1 dot) are arranged on a straight line, and the photosensitive drum is exposed by time division drive of 40 divisions. In the printer head assembled in the embodiment, an LED array with uniform output is selected and used.
The light emission time for each D and each array is not adjusted. Not adjusting the light emission time here means not adjusting the light emission time for compensating the output variation of the LED. When the light emission time can be made variable, gradation printing such as halftone printing is performed. Just go for it. Further, in order to improve the yield, the LED array, which has a high average output or a low average output, is used without being regarded as a defective product. LED
The arrays are grouped and selected according to average output, and the light emission current is changed for each group to make the outputs uniform.

As shown in FIG. 1, the LED array is selected and only non-defective products are taken out. First, all the LEDs in the array are made to emit light with a uniform drive current for the same time, the output is measured by the light receiving element for each LED, and the output is stored for each LED. The obtained outputs are averaged to calculate the average output A, and the average output A is compared with the allowable range D for the maximum value of the output variation. Such variations are a mixture of random variations for each LED and systematic errors on the left and right sides of the LED array, and the allowable range D is preferably ± 20%, more preferably ± 10% with respect to the average value A. And The variation of ± 10% is set so that the variation of the beam diameter for each dot is within the allowable range (here, ± 1%) when the photosensitive member is exposed in the saturated region.

Next, the variations at the left and right ends of the array are calculated. First, the average value of the output is obtained for each of the left and right ends of the array. The average value is preferably 3 to 8 L
The average value of the ED output. Next, the difference between the average values of the outputs at the left and right ends is defined as the left-right variation of the array (the left-right systematic error due to etching conditions and the like). A ratio may be used instead of the difference. The reason why the average value is obtained here is that even if the displacement is obtained for each one LED at both ends, only a random error appears and no systematic error appears. For example, the allowable range D for accidental error is, for example, ± 10%, while the allowable range P on the left and right is, for example, 2.
The reason for setting 5% is that the variation for each individual LED is larger than the systematic error at the left and right ends. The permissible range of the average value of the left and right variations is ± P or less, and P is, for example, 6% or less, more preferably 4% in comparison with the average value A.
Hereafter, it is more preferably 2.5% or less. The allowable range of 2.5% is when the selection condition of the average value A is 5%,
(The average value A is grouped in 5% increments), which means that the output difference at the adjacent portion of the array is ± 10% of 5% + 2.5% × 2. This is when driving in the saturation region,
This means that the change in beam diameter that occurs on the photoconductor is 1% or less.

The output difference between the left and right ends of the array is ± P or more (the ratio with the average value A is the same below), and the output variation is ± D or more (the ratio with the average value A is the same below). ) Are excluded.

The average value A of the output of the array is grouped and sorted in steps of 5%, for example. And LE of the same group
The printer head is assembled using the D array, and the output difference between the heads is corrected by changing the current value of the power source for supplying the light emission current to the LED, for example.

In this way, the output variation of the printer head is 5% + 2P, for example, 1 at the adjacent portion of the array.
0%, the variation within the array is 2D, for example 20%, and the maximum variation in the output of the entire head is S (5%) + 2D
25% of The output change in the adjacent portion of the array is set to, for example, 10% at the maximum, and the variation in the array is set to 20% at the maximum. This is because the variation in the array is accidental and difficult to be noticed. This is because the variations are systematic and easily noticeable.

FIG. 2 shows an assembling apparatus of the embodiment. In the figure, 2 is an LED array, 4 is a phototransistor array, and includes a photovoltaic array, a CdS cell array, a CCD element,
Alternatively, it may be a light receiving element such as a photomultiplier. Instead of using the phototransistor array 4, one phototransistor may be used, which is moved along the LED array 2 to scan the output from the 64 LEDs.

Reference numeral 6 designates the light emission output from the LED array 2 as L
A memory for storing each ED, for example, the output of the first LED is stored in the address 1 and the last LED is stored in the address 64.
Memorize the output of. 8 is an average value calculation circuit, which is an LED
The average output of the array 2 is calculated, and the calculated average value A is stored in the memory. Reference numeral 10 is an average output calculation circuit for the left and right ends of the LED.
The average value of about 3 to 8 LED outputs at the left and right ends of the array 2 is calculated and stored. 12 is a sorting circuit,
If even one LED has an output outside the range of A ± D, the LED array 2 is selected as a defective product. Further, the selection circuit 12 outputs the average output at the left and right ends of the array 2.
Check that it is in the range of A ± P, while A ± P
If it is out of the range, the array 2 is determined as a defective product.

The selection circuit 12 outputs all the LEDs.
Within the range of A ± D, the average output at both left and right ends is A
The LED array 2 in the range of ± P is regarded as a non-defective product, and the average output A is classified into groups 1 to n. The standard of grouping is, for example, the average output A in steps of 5%.

FIG. 3 shows an example of changes in the light emission output in the adjacent portion of the LED array 2. The central portion of the figure is the change portion from the array A to the array B, and the light emission output suddenly changes by about 20% in this portion. Also, the output of array A on the left side of the figure tends to increase from left to right. FIG. 4 schematically shows the adjacent portions of the array A and the array B of FIG. In the figure, 20 is a light emitting portion in the array A, 22 is a light emitting portion in the array B, and 24 is a light emitting portion.
Is an electrode of array A, and 26 is an electrode of array B. As is apparent from the figure, the electrode 26 of the array B is thicker than the electrode A of the array A, and the light emitting area of the array B is smaller. Therefore, the output is lower than that of the array A.

As in the array A on the left side of FIG. 3, the cause of the change in output between the left and right sides of the LED array 2 will be examined. LED
It is already known that the average output A of the LED array 2 varies due to subtle differences in etching conditions, mask alignment error, etc. during the manufacture of the array 2, and the output varies among the LEDs in one array. LED array 2
Form a light emitting part and electrodes at the wafer stage. These formations involve etching and a resist exposure process using a mask before etching. The concentration and temperature of the etching solution are not uniform over the entire surface of the wafer, and there are temperature differences and concentration differences depending on the location. These irregularities cause irregularities in the etching rate of the electrodes, and cause differences in the thickness of the electrodes on the left and right sides of the LED array 2. Similarly, if there is a mask alignment error in the exposure process using the mask before etching the electrodes, the positions of the electrodes are systematically shifted from the center of the light emitting portion to the left and right. The same applies to the etching before forming the light emitting portion, and the area of the light emitting portion changes between the left and right sides of the LED array 2 due to a subtle difference in etching conditions and a mask alignment error. Due to these, a difference in output between the left and right sides of the LED array 2 occurs.

FIG. 5 shows the variation of the LED array 2 in the embodiment, and FIG. 6 shows the variation in the conventional example. Since the average output A of the LED array 2 is classified by 5%, the difference between the average output A of the array A and the average output B of the array B is within 5%. If a permissible range P (here, ± 2.5%) of change in average output at the left and right ends of the arrays A and B is added to this,
The output change at the boundary between the array A and the array B is such that the difference between the average outputs A and B is within 5%, and the change from the average output at the left and right ends is within ± 2.5%, for a total of 5% + 2. ×
It becomes 10% or less of 2.5%. On the other hand, in the conventional example of FIG. 6, a maximum of 5% of the difference between the average output A of the array A and the average output B of the array B is set to ± 10% of the variation range of the LED outputs of the arrays A and B. In addition, the maximum output change at the boundary between array A and array B is 5% + 2 × 10%, which is 25%.

7 and 8 show the influence of the output variation of the LED array 2 on the print quality. The surface potential of the photoconductor is saturated with exposure energy of about 0.5 μJ / cm ² , and becomes a constant value of about 30V. On the other hand, the beam width generated on the photoconductor by the exposure from the LED is 1.0 μJ / cm.
Up to about ² energies will increase rapidly with energy,
It saturates at an energy of 1.0 μJ / cm ² or more and becomes a substantially constant value. Therefore, in order to avoid the influence of LED output variations, it is preferable to use it in a saturated region of energy of 1.0 μJ / cm ² or more and avoid fluctuations in surface potential and beam width. 1.0 μJ for the purpose of gradation printing
It is also possible to make use of the unsaturated region of / cm ² or less and increase the change in beam width and surface potential. Even when used in a saturated region of 1.0 μJ / cm ² or more, the exposure energy is 10
When it changes by%, the beam width changes by about 1%.

In a printer head having a resolution of 300 DPI, the width occupied by one dot is about 80 μm.
Even if the exposure energy of each dot changes about ± 10% from the average value A and the beam width changes about ± 1%, the influence on the print quality is small. From this, the allowable range D of the output variation in the LED array 2 with respect to the average output A is preferably ± 10% or less. The output change between the arrays and the turn of the array has a great influence on the print quality because the output change is abrupt and reflects the systematic error on the left and right of the array. Therefore, in the embodiment, the change in the beam width at the transition between the arrays is suppressed to ± 1%,
Therefore, the difference in exposure energy is set to ± 10% or less. 1
From the 0% tolerance, subtract 5% of the maximum difference in average output,
When the rest is divided by 2, the change in average output at the left and right ends of one LED array 2 is ± 2.5% or less. The change from the average output A of the left and right average outputs in the array is ± 2.5
% Is within ± 10% of the change in output at the changing part of the array
The following is a condition for changing the beam width within ± 1%.

Although the time-division drive LED printer head has been described here, the present invention is not limited to this, and the same applies to a static drive LED printer head.

[0029]

According to the present invention, the sudden change of the output between the adjacent LED arrays is limited, and the deterioration of the printing quality due to the systematic error of the output on the left and right of the LED array is prevented.

[Brief description of drawings]

FIG. 1 is a flowchart showing a method of assembling an LED printer head according to an embodiment.

FIG. 2 is a block diagram of the assembling apparatus used in the embodiment.

FIG. 3 is a characteristic diagram showing variations in the output of the LED array.

FIG. 4 is a diagram showing a cause of a change in output in a portion adjacent to an LED array.

FIG. 5 is a characteristic diagram showing an output variation in the printer head of the embodiment.

FIG. 6 is a characteristic diagram showing output variations in a conventional printer head.

FIG. 7 is a characteristic diagram showing the relationship between exposure energy and the surface potential of the photoconductor.

FIG. 8 is a characteristic diagram showing a relationship between exposure energy and a beam diameter of a photoconductor.

[Explanation of symbols]

 2 LED array 4 Phototransistor array 6 Memory of LED output 8 Average value calculation circuit 10 Average output calculation circuit at left and right ends 12 Sorting circuit 20 Light emitting part 22 Light emitting part 24 Electrode 26 Electrode

Claims (1)

[Claims] 1. A method of assembling an LED printer head in which a large number of LED arrays integrated with each other are arranged, wherein a plurality of LEDs are provided at both left and right ends of each LED array.
The average value of the light emission output of is detected and stored by the light receiving element, and the displacement of the average value of the light emission outputs of the left and right ends stored in the memory is compared with the allowable range by the arithmetic circuit, and the displacement of the average value is within the allowable range. A method for assembling an LED printer head, which comprises assembling an LED printer head using only an LED array.