JP4360002B2 - Light quantity correction method for optical writing head - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light amount correction method for an optical writing head, and more particularly to a light amount correction method for an optical writing head that evaluates both the light output and the shape of a light spot.
[0002]
[Prior art]
A write head (optical write head) of an optical printer is a light source for exposing a photosensitive drum to light, and has a light emitting element array. FIG. 1 shows a principle diagram of an optical printer having an optical writing head. A photoconductive material (photosensitive member) such as amorphous Si is made on the surface of the cylindrical photosensitive drum 2. This drum rotates at the speed of printing. The surface of the photosensitive drum of the rotating drum is uniformly charged by the charger 4. Then, the optical writing head 6 irradiates the photosensitive member with the light of the dot image to be printed, neutralizes the charging where the light hits, and forms a latent image. Subsequently, the developing unit 8 applies toner to the photoconductor according to the charged state on the photoconductor. Then, the toner is transferred onto the paper 14 sent from the cassette 12 by the transfer device 10. The sheet is heated and fixed by the fixing device 16 and sent to the stacker 18. On the other hand, the drum that has been transferred is neutralized by the erasing lamp 20 over the entire surface, and the remaining toner is removed by the cleaner 22.
[0003]
The structure of the optical writing head 6 is shown in FIG. The optical writing head is composed of a light emitting element array 24 and a rod lens array 26, and the focal point of the lens is formed on the photosensitive drum 2.
[0004]
The light-emitting element array used in such an optical writing head is connected to a driver circuit having a sequential lighting function or includes a driver having a sequential lighting function in order to light up sequentially.
[0005]
[Problems to be solved by the invention]
Even if the same light power is incident on the photosensitive drum from each light emitting point of the light emitting element array as described above, the density of the formed latent image changes depending on the spot shape. For this reason, even if the light output of the light emitting point is corrected by correction, the obtained image density is not always uniform.
[0006]
An object of the present invention is to provide a light amount correction method for an optical writing head in which not only the light output but also the shape of a light emitting spot is evaluated and incorporated in the correction when measuring the light output after passing through the rod lens.
[0007]
[Means for Solving the Problems]
The present invention relates to a light amount correction method for an optical writing head having a light emitting element array that is sequentially turned on.
Obtaining a first correction coefficient sequence when the light output is corrected so that the diameter of the light spot on the photosensitive drum by the light emitting point of the light emitting element array is uniform;
Obtaining a second correction coefficient sequence when the light output of the light emitting point of the light emitting element array is corrected to be uniform;
Obtaining a third correction coefficient sequence that is intermediate between the first correction coefficient sequence and the second correction coefficient sequence;
Correcting the light output of the light emitting point of the light emitting element array by the third correction coefficient sequence;
Thus, the image density is made uniform.
[0008]
According to the present invention, more specifically, the spot diameter corresponding to the threshold of the photosensitive drum determined, a correction coefficient sequence C _A in the case of correcting the light output so that the spot diameter is uniform, simple obtain a correction coefficient sequence C _B corrected so that the optical output is uniform, decide _{rC a × (1-r)} C B becomes the correction coefficient sequence. Here, r is determined by the development conditions, and is a constant selected from 0.01 to 0.99, where the diameter of the light spot is the diameter of an equal incident energy density line that gives a 50% density in the output image. is there.
[0009]
Such a constant r is preferably corrected for temperature fluctuations of the printer.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described based on examples.
[0011]
[Example 1]
FIG. 3 shows an inspection apparatus for evaluating the light output distribution of the optical writing head and the light emission spot shape of each light emission point. This apparatus includes a light receiving head 200. In this light receiving head, an imaging optical system 202 provided on the side facing the light emitting element array 80, a half mirror 203, and a light receiving element array provided in a portion where light transmitted through the half mirror 203 forms an image. 204 and a light receiving element 201 for light output monitoring provided in a portion where the light reflected by the half mirror 203 forms an image. In the figure, 205 is a signal line of the light receiving element 201, and 206 is a signal line of the light receiving element array 204.
[0012]
Light emitted from a light emitting point of the light emitting element array 10 passes through the coupling optical system 202 and is divided into two paths by the half mirror 203. One light is directed to the light receiving element 201 and the other light is directed to the light receiving element array. An image is formed on 204. These light emission point images move on the light receiving element 201 and the light receiving element array 204 as the position of the light emitting point of the light emitting element array moves. Therefore, the light receiving range of the light receiving element 201 and the light receiving element array 204 needs an area in which the light emitting point image of the moving light emitting element array can be accommodated.
[0013]
Specifically, the light receiving element array 204 is a one-dimensional CCD sensor having a pixel pitch of 6 microns and a number of pixels of 1024 bits. The refresh rate of the CCD was 50 Hz. On the other hand, as the light receiving element 201, a Si photodiode having a light receiving portion of 10 mm × 10 mm was used, and the short-circuit current was subjected to current-voltage conversion of 10 ⁸ V / A and extracted as a voltage signal. In addition, the image forming optical system 202 is 1 ×. The distribution ratio of the half mirror 203 is 50:50. For this reason, the pitch of the 600 dpi light emitting points of the light emitting element array corresponds to about 7 pixels on the CCD.
[0014]
Assume that a 600 dpi, 128 light emitting element built-in driver type light emitting element array is measured. First, when the light emitting element array 80 is lit at the upper limit speed at which it is actually used, it is confirmed whether there is any abnormality in the light emitting point transfer operation. Here, all the points were lit at a transfer rate of 2 Mdot / s. Since 128 pixels are drawn at 2 Mdot / s, the time required for the light emitting element array to draw one line is 64 μs. Since the frame frequency of the CCD is 50 Hz, an average light amount distribution of light emission of about 300 times is obtained in the light receiving element array 204. A certain threshold is set for this light quantity distribution, the number of peaks exceeding this threshold is counted, and when the number of light emitting points of the light emitting element reaches 128, a normal transfer operation is performed. Judge that Since there is a spatial sensitivity distribution unique to the optical system, the output signal is corrected by numerically multiplying the inverse function of the spatial sensitivity distribution function. An example of the evaluation result signal is shown in FIG. In the figure, the 30th to 90th portions of the CCD pixel numbers (1 to 1024) for the first 7 light emitting points are shown. The vertical axis is the output of the light receiving element array, and 255 is the output of the AD converter that becomes full scale.
[0015]
Next, only the odd-numbered and even-numbered light emitting points were turned on, and the same light quantity distribution was measured. The results are shown in FIG. The method of emitting light by dividing the odd-numbered and even-numbered light emitting points in this way is particularly effective when the light emitting spot spreads due to the influence of a rod lens or the like and it becomes difficult to distinguish between two adjacent light emitting points. .
[0016]
Next, the transfer rate was set to 1 kdot / s, the output signal of the light receiving element 201 was measured, and the light output distribution of each light emitting point was obtained. The light output was measured 100 μs after the target light emitting point started to emit light. Here, the reason why the transfer speed of the light emitting point is slowed down to 1 kdot / s is that the current-voltage conversion speed is not so fast and that the measurement is repeated in order to increase the measurement accuracy. Since there is a spatial sensitivity distribution unique to the optical system, the output signal is corrected by numerically multiplying the inverse function of the spatial sensitivity distribution function. An example of the evaluation result signal is shown in FIG. The vertical axis is converted to output power by multiplying by a certain correction coefficient.
[0017]
When the incident energy density at which 50% density was obtained in the output image was examined, it was found that the photosensitive drum used this time corresponds to level 50 in the output value of FIG. Therefore, from FIG. 5, the width that cuts the level 50 of each light emitting point was calculated as the spot diameter. The calculation was performed by linearly interpolating from the intensity of two pixels before and after the level 50 was cut. The calculation results are shown in FIG. The vertical axis is a spot (μm).
[0018]
If the correction coefficient sequence C _A in the case of correcting the light output so that the spot diameter is uniform, showing the relationship between the spot diameter and the correction coefficient sequence C _A in FIG. The straight line in FIG. 8 is determined as this linear regression line by actually emitting one point of a certain light emitting element on the head and taking the relationship between the light output and the spot diameter. When y is a spot diameter (μm) and x is a light output (μW), this linear regression line is represented by y = 2.4x + 21.
[0019]
From the results of FIGS. 7 and 8, to obtain a correction coefficient sequence C _A to adjust the light output so that the spot diameter 30 microns. The results are shown in FIG. In FIG. 9, the vertical axis represents the correction value. FIG. 10 shows a correction result in which the spot diameter is made constant by such a correction value. The spot diameter could be corrected to 30 ± 0.5 μm.
[0020]
However, when the corrected head was incorporated into a printer and displayed, a uniform image could not be obtained. This is because, even with the same spot diameter, the way in which the latent image spreads changes depending on the amount of light exceeding the slice level. Therefore, it can be seen that the optimum correction coefficient is intermediate between correction with a constant spot diameter and correction with a constant light output.
[0021]
The correction coefficient sequence corrected so that the optical output becomes uniform when the C _B, was determined correction coefficient sequence C _B. FIG. 9 also shows a correction result for making the light output constant.
[0022]
A correction coefficient sequence C _A as the spot diameter becomes constant as shown in FIG. 9, and a correction coefficient sequence C _B simply corrected so that the optical output is uniform, becomes _{rC A × (1-r)} C B Determine the correction coefficient sequence. This constant r is determined by the development conditions and the positional relationship between the head and the drum. In this experiment, when r = 0.75, the density distribution of the obtained image was minimized. FIG. 11 shows the correction result. The vertical axis represents the image density (relative value).
[0023]
Next, the optimum r range was examined by changing development conditions and exposure conditions. For example, although the slice level varies depending on the development conditions, if the spot level is lowered, the correction of the constant spot diameter increases the difference in the amount of light exceeding the slice level, so the constant r must be selected to be small. Further, if the photosensitive drum surface is deviated from the focal plane of the lens, the light spot is lost, and it is necessary to select a larger constant r as the effect is equivalent to the fact that the slice level is deviated higher. .
[0024]
Image under a combination of three development conditions (that is, large, medium, and small slice levels) and three conditions of lens focus shift on the drum surface (that is, defocus in the + direction, just focus, and defocus in the − direction) The density unevenness was examined. The results are shown in FIG. The vertical axis represents image density unevenness (%), and the horizontal axis represents coefficient r. Assuming that the allowable value of image unevenness is 2%, the constant r can be adjusted within the allowable value in the range of 0.1 <r <0.92. Note that defocus + 0.2 mm means that the focal plane of the lens is 0.2 mm outside the drum surface. FIG. 12 shows that the r value varies considerably depending on the slice level, but does not change much with respect to the defocus.
[0025]
In this example, the spot diameter was obtained for all points. However, since the spot diameter variation often occurs in a longer period than the period of the light emitting point, the measurement is thinned out as necessary. Also good.
[0026]
[Example 2]
In Example 1, a constant r = 0.75 was obtained. This is a value determined when the temperature inside the printer is 30 ° C. While the optical writing head was driven and the continuous printing operation was being performed, when the temperature inside the apparatus rose, unevenness in image density began to appear again. This is due to the expansion of the lens due to the temperature rise. Therefore, the following method has been devised in which the value of the constant r is changed with respect to the fluctuation of the temperature in the apparatus so that the function of the constant r can be maintained even if there is a fluctuation in temperature.
[0027]
Advance to determine the value r ₀ of the constant r at a constant temperature T _0. Monitoring the device temperature T constantly in proportion to the temperature difference between T ₀ (T-T _0), to correct the r. That is, r always varies according to the following equation.
[0028]
[Equation 3]
r = r ₀ + (T−T ₀ ) × θ _r
Here, θ _r is a temperature correction coefficient of r. As described above, in the apparatus in which r ₀ = 0.75 is obtained at T ₀ = 30 ° C., when θ _r = 0.001 is selected, the density unevenness due to the influence of the temperature rise of the printer apparatus can hardly be detected. Note that the value of θ _r depends on process conditions, the structure of the head, and the like.
[0029]
【The invention's effect】
As described above, according to the present invention, when measuring the light output after passing through the rod lens, not only the light output but also the shape of the light emission spot is evaluated and incorporated in the correction, thereby realizing a more uniform image density. can do.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a principle diagram of an optical printer including an optical writing head.
FIG. 2 is a diagram showing a structure of an optical writing head.
FIG. 3 is a view showing an optical writing head inspection apparatus;
FIG. 4 is a diagram illustrating an evaluation result signal.
FIG. 5 is a diagram showing a measured light amount distribution by turning on only odd-numbered and even-numbered light emitting points.
FIG. 6 is a diagram showing an evaluation result signal.
FIG. 7 is a diagram illustrating a result of calculating a width that cuts a level 50 of each light emitting point as a spot diameter.
8 is a diagram showing the relationship between the spot diameter and the correction coefficient sequence C _A.
FIG. 9 is a diagram showing correction coefficient sequences C _A and C _B.
FIG. 10 is a diagram showing a correction result with a constant spot diameter.
FIG. 11 is a diagram showing a correction result.
FIG. 12 is a diagram showing the results of examining image density unevenness under a combination of three development conditions and three drum surface lens focus shift conditions.
[Explanation of symbols]
80 Light emitting element array 200 Light receiving head 201 Light receiving element 202 Imaging optical system 203 Half mirror 204 Light receiving element array 205 Signal line 206 of light receiving element 201 Signal line of light receiving element array 204

Claims (4)

In the light quantity correction method of the optical writing head having the light emitting element array that is sequentially turned on,
A first correction coefficient sequence is obtained when the light output is corrected so that the diameter of the light spot on the photosensitive drum by the light emitting point of the light emitting element array is uniform, and the light output of the light emitting point of the light emitting element array is Obtain the second correction coefficient sequence when corrected to be uniform,
Based on the first correction coefficient sequence and the second correction coefficient sequence, a third correction coefficient sequence is obtained so that the image density is uniform ,
It said third correction coefficient amount correction method for an optical write head, characterized in that that correct auxiliary light output of the light emitting point of the light emitting element array by columns. In the light quantity correction method of the optical writing head having the light emitting element array that is sequentially turned on,
Obtaining a correction coefficient sequence C _A when the light output is corrected so that the diameter of the light spot on the photosensitive drum by the light emitting point of the light emitting element array is uniform;
Obtain a correction coefficient sequence C _B when the light output of the light emitting point of the light emitting element array is corrected to be uniform,
Wherein the _{C A} and _{C B,} so that the image density becomes uniform,

However, r is set a correction coefficient sequence composed constants, the light quantity correcting method of an optical writing head, characterized in that that correct auxiliary light output by the correction coefficient sequence. The constant r determined at a predetermined constant temperature, always in response to the temperature difference in the internal temperature constantly monitors and said predetermined temperature, according to claim 1 or 2, wherein the temperature compensation Light quantity correction method for optical writing head. When the predetermined temperature is T ₀ , the constant determined by the temperature T ₀ is r ₀ , the temperature in the apparatus to be constantly monitored is T, and the temperature correction coefficient of the constant r is θ _r , the temperature correction of the constant r is ,

4. The method of correcting a light amount of an optical writing head according to claim 3, wherein

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