JPH07234371A - Image forming device - Google Patents

Abstract

PURPOSE:To suppress density unevenness in the direction perpendicular to a scanning line of an image formed on a photosensitive material without providing a special correction mechanism by using the photosensitive material whose relative sensitivity is lowered in multiple exposure. CONSTITUTION:This device is constituted of a supply unit 2, a transfer unit 3, an exposure unit 4, a laser optical unit 5, a housing unit 6 and an electric circuit part 7, and is controlled by a control circuit in the electric circuit 7, and an image is formed by scanning and exposing a laser beam (light beam) from a laser optical unit 5 for the photosensi-tive material 9. At this time, the device is provided with a scan control means controlling the scan so that adjacent scanning lines each other by the light beam are superimposed partially each other, and a multiple exposed part occurs on the photosensitive material 9, and the photosensitive material 9 is made the photosensitive material having a multiple exposure effect lowering the sensitivity at the time of multiple exposure. Thus, the mutual scanning lines are superimposed each other, and are multiple exposed, and the concentration of the part multiple exposed on the photosensitive material 9 is lowered.

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 more particularly to an image forming apparatus for scanning and exposing a light beam onto a photosensitive material to form an image.

[0002]

2. Description of the Related Art In an image forming apparatus such as a laser printer or a laser imager used for medical treatment, which scans a light beam on a photosensitive material to form an image, density unevenness may occur in the formed image. Density unevenness in the direction perpendicular to the line is likely to occur, which is a problem in image quality. The causes of the density unevenness in the direction perpendicular to the scanning lines of the image formed on the photosensitive material are roughly classified into the unevenness of the scanning line pitch and the unevenness of the light beam intensity.

In conventional image forming apparatuses, various measures have been taken to suppress uneven density due to such causes as much as possible.

For example, in Japanese Unexamined Patent Publication (Kokai) No. 5-35061, the cause of uneven density in the direction perpendicular to the scanning line of the image formed on the photosensitive material is unevenness in the moving speed of the relative position between the photosensitive material and the light beam ( That is, an image exposure apparatus is disclosed which pays attention to (unevenness of the scanning line pitch), detects the unevenness of the moving speed, and modulates the intensity of the light beam so as to correct the uneven density due to the unevenness.

Further, in Japanese Patent Laid-Open No. 2-237267, density unevenness due to a rotating polygon mirror (hereinafter referred to as polygon) for scanning is taken as density unevenness in a direction perpendicular to a scanning line of an image formed on a photosensitive material. (That is, the unevenness of the light beam intensity due to the difference in the reflectance of each surface of the polygon and the unevenness of the scanning line pitch due to the tilt of the surface of the polygon.) However, there is disclosed a density unevenness correction method for suppressing the density unevenness by selecting a pattern having the smallest density unevenness among these patterns.

[0006]

By the way, in the apparatus and method disclosed in the above publication, a special means for correcting uneven density such as a means for detecting uneven moving speed of the relative position between the photosensitive material and the light beam is used. Therefore, a special method such as preparing various types of correction exposure patterns for correcting density unevenness caused by polygons is necessary.
In this case, there are problems that the cost of the image forming apparatus becomes high and the apparatus becomes large.

When the density unevenness is not corrected as in the above-described apparatus and method, each component of the image forming apparatus, that is, a means for relatively moving the photosensitive material and the light beam, a polygon or the like is high. Precision is required, and as a result, cost is also high.

The present invention has been made in view of the above points, and it is possible to suppress uneven density in the direction perpendicular to the scanning line of an image formed on a photosensitive material without providing a special correction mechanism. The purpose is to provide a device.

[0009]

In order to achieve the above object, the present invention provides an image forming apparatus for forming an image by scanning exposure of a light beam emitted from a light source that can be modulated onto a photosensitive material. By scanning control means for controlling scanning so that adjacent scanning lines partially overlap with each other to cause multiple exposure on the photosensitive material, and the sensitivity of the photosensitive material is lowered during multiple exposure. The photosensitive material has an exposure effect.

[0010]

According to the present invention, the scanning lines are overlapped with each other and are subjected to multiple exposure, and the density of a portion of the photosensitive material subjected to the multiple exposure is reduced, whereby unevenness of the scanning line pitch can be suppressed.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 is a schematic view of an image forming apparatus according to the present invention. FIG. 1A is a side view and FIG. 1B is a schematic view of an optical system of the image forming apparatus.

In FIG. 1A, the image forming apparatus 1 is
Supply unit 2, transport unit 3, and exposure unit 4
A laser optical unit 5, a housing unit 6, and an electric circuit unit 7, which are controlled by a control circuit (not shown) in the electric circuit unit 7 so that the laser light from the laser optical unit 5 is directed to the photosensitive material 9. An image is formed by scanning and exposing. In this embodiment, laser light is used as the light beam.

The photosensitive material 9 in the supply tray 8 is the transport unit 3
And is wound around the exposure drum 10 in the exposure unit 4. The photosensitive material 9 wound around the exposure drum 10 is irradiated with the laser light L from the laser optical unit 5, and scanning exposure is performed as the exposure drum 10 rotates. The photosensitive material 9 for which image formation has been completed by scanning exposure is conveyed by the conveying unit 3 and is accommodated in the accommodation tray 11 in the accommodation unit 6.
Housed in.

FIG. 1B shows an optical system in the laser optical unit 5 shown in FIG.

Reference numeral 20 denotes an optical system in the laser optical unit 5, and the laser light from the laser 21 travels as shown by a chain line. That is, the laser light is reflected by the mirrors 22 and 23, its intensity is modulated by the ultrasonic light modulation element 24, and after being reflected by the mirror 25, its cross-sectional shape is shaped by the beam expander 26 and the cylindrical lens 27. Then, the photosensitive material 9 is irradiated through the F-θ lens 29 and the mirror 30 while moving in the main scanning direction as the polygon 28 rotates. The polygon 28 in this embodiment is composed of eight mirror surfaces as shown in FIG.

Each time the laser light reaches the mirror 31 as the polygon 28 rotates, the laser light reflected by the mirror 31 is applied to the horizontal synchronizing signal generator 32. The horizontal synchronizing signal generator 32 is used for timing such as the start of writing an image in the width direction of the photosensitive material 9.

Photosensitive material 9 of image forming apparatus 1 according to the present invention
As the material, a light-sensitive material having a multiple-exposure effect in which the sensitivity is lowered when the same portion is exposed a plurality of times (called multiple exposure) is used. Here, the multiple exposure effect will be described. Here, the case where the photosensitive material is exposed a plurality of times, ie, the pre-exposure and the main exposure are performed twice, and the main exposure and the post-exposure are performed twice, will be described. It is a figure which shows the relative sensitivity of the photosensitive material (herein called photosensitive material A) which has a multiple exposure effect in which sensitivity decreases, (a) is a figure regarding pre-exposure, (b) is a figure regarding post-exposure.

FIG. 3 is a diagram showing the relative sensitivity of a photosensitive material (herein referred to as photosensitive material B) having a multiple exposure effect in which the sensitivity is increased when multiple exposure is performed, (a) is a diagram relating to pre-exposure, and (b) is a diagram. FIG. 4 is a diagram relating to post-exposure.

2 (a) and 2 (b) and FIG. 3 (a),
In (b), the vertical axis represents relative sensitivity and the horizontal axis represents exposure ratio.

Here, the relative sensitivity on the vertical axis means that the density of the image formed when the photosensitive material is irradiated with laser light of a predetermined intensity is 1, and the density of the image at each exposure ratio with respect to this. Is the ratio.

Explaining with reference to FIG. 2A, the exposure ratio on the horizontal axis is such that the laser light in the pre-exposure when the double exposure in which the main exposure is performed after the pre-exposure of the photosensitive material A is performed. The intensity of the laser light in the pre-exposure and the intensity of the main exposure in the case of forming an image so that the total of the intensity and the intensity of the laser light in the main exposure becomes equal to the predetermined intensity of the laser light in the case of the relative sensitivity 1 described above. It is the ratio (%) to the intensity of laser light.

As can be seen from FIG. 2A, the photosensitive material A
In this case, when the double exposure of the pre-exposure and the main exposure is performed, the relative sensitivity becomes lower than 1.

Next, referring to FIG. 2B, the exposure ratio on the abscissa is determined by the main exposure when the double exposure is performed in which the photosensitive material A is subjected to the main exposure and then the post-exposure. The intensity of the laser beam in the post-exposure when an image is formed so that the sum of the intensity of the laser beam and the intensity of the laser beam in the post-exposure becomes equal to the predetermined intensity of the laser beam in the case of the relative sensitivity 1 described above. It is the ratio (%) to the intensity of the laser beam in the main exposure.

As can be seen from FIG. 2 (b), the photosensitive material A
In this case, when the double exposure of the main exposure and the post exposure is performed, the relative sensitivity becomes lower than 1.

In FIG. 3A, the exposure ratio on the horizontal axis is
When double exposure is performed in which the main exposure is performed after the pre-exposure is performed on the photosensitive material B, the total of the intensity of the laser light in the pre-exposure and the intensity of the laser light in the main exposure is equal to The ratio (%) of the intensity of the laser beam in the pre-exposure and the intensity of the laser beam in the main exposure when forming an image so as to be equal to the predetermined intensity of the laser beam in the case.

As can be seen from FIG. 3A, the photosensitive material B
In the case of 2, the relative sensitivity becomes higher than 1 when the double exposure of the pre-exposure and the main exposure is performed.

In FIG. 3B, the exposure ratio on the horizontal axis is
When double exposure is performed in which the photosensitive material B is subjected to main exposure and then post-exposure, the sum of the intensity of the laser light in the main exposure and the intensity of the laser light in the post-exposure is equal to the above relative sensitivity 1. The ratio (%) of the intensity of the laser beam in the post-exposure and the intensity of the laser beam in the main exposure when an image is formed so as to be equal to the predetermined intensity of the laser beam in the case.

As can be seen from FIG. 3B, the photosensitive material B
In this case, the relative sensitivity becomes higher than 1 when the double exposure of the main exposure and the post exposure is performed.

In the present invention, FIG. 2 (a) and FIG. 2 (b)
The photosensitive material A whose relative sensitivity decreases when multiple exposure is performed as shown in FIG. Generally, a light-sensitive material whose relative sensitivity increases when multiple exposure is performed, but a light-sensitive material whose relative sensitivity decreases when multiple exposure is performed is also known. For example, the document “Journal of Imaging Science and Technology 37: 117-129 (199)
3) ”, the photosensitive material indicated as A8 in Table 1 is
It has an octahedral grain structure and has a composition of AgBr + 1% I ⁻ (mo
1%) unsensitized silver salt emulsion is coated, and this film is an example of a light-sensitive material whose relative sensitivity decreases when multiple exposure is performed.

This film has a total exposure time of about 3 × 1 as shown by curve 3 in FIG. 10 of the above-referenced document.
^Within the range of 0 ^-7 seconds or less, when forming an image of the same density, it is necessary to expose a long time for a larger exposure amount (exposure energy). That is, if the total exposure time is within the range of about 3 × 10 ⁻⁷ seconds or less, the sensitivity decreases when the exposure is performed for a long time. Exposure time of the image forming apparatus is generally about 1 × 10 ^-7 seconds, the total exposure time in the case of double exposure is about 2 × 10 ^-7 seconds, about 3 × less 10 ^-7 seconds above It is within the range, and the sensitivity decreases when double exposure is performed. Further, when the relative sensitivity increases due to the exposure time interval (intermittent exposure effect) in the double exposure, it is necessary to set the exposure time interval to the extent that the sensitivity is not improved beyond the above-mentioned sensitivity decrease amount. That is, the time interval of the sub-scanning by the light beam may be set within this exposure time interval range.

FIG. 4 is a diagram showing the intensity of laser light used in forming an image on the photosensitive material 9.

In FIG. 4, the vertical axis represents the intensity of the laser light and the horizontal axis represents the distance from the center point of the cross section of the laser light.
The intensity distribution of the laser light is strongest at the center point of the cross section and is represented by a Gaussian distribution curve as shown in FIG. Although the radius of the cross section of this laser light is 60 μm (beam diameter 120 μm), it can be seen from FIG. 4 that the laser light has intensity even at a point 60 μm or more away from the center point of the cross section.

A darker image is formed on the photosensitive material 9 as the intensity of the laser light is higher.
By rotating the polygon 28, the laser light shown in FIG. 4 is laterally scanned (main scanning) on the photosensitive material 9 to form an image with one scanning line, and then the photosensitive drum 9 is rotated by rotating the exposure drum 10. The image is formed by moving (sub-scanning) in the vertical direction by the next one scanning line.

FIG. 5 is a diagram showing the intensity of the laser light received by the photosensitive material 9 when the laser light is scanned five times.

In FIG. 5, the vertical axis represents the intensity of laser light, and the horizontal axis represents the position of the photosensitive material 9 in the sub-scanning direction (vertical direction). In the figure, the curve indicated by the solid line shows the distribution of the intensity of each laser beam when the same laser beam as that shown in FIG. 4 is irradiated five times at a scanning line pitch of 80 μm,
The curve indicated by the alternate long and short dash line is an intensity distribution curve of the laser light received by the photosensitive material 9, which is obtained by superposing the intensities of the laser light indicated by the solid line.

Reference numerals a, b, and c are curves representing the intensity distributions of the laser light corresponding to the scanning lines, and they overlap each other at their tail portions, and the photosensitive material 9 at these portions.
Will be exposed multiple times.

FIG. 5 is a diagram when there is no unevenness in the scanning line pitch. Here, the case where there is unevenness in the scanning line pitch will be described.

FIG. 6 is a diagram when the curve b shown in FIG. 5 is shifted to the curve a as a result of uneven scanning line pitch. In the figure, the curve indicated by the alternate long and short dash line is the intensity distribution curve of the laser light received by the photosensitive material 9, in which the intensity of the laser light of the curve indicated by the solid line is superimposed, as in FIG.

By the way, when the image forming apparatus 1 always uses a photosensitive material whose relative sensitivity does not change in multiple exposure, the density of the image formed on the photosensitive material 9 is determined by the laser received by the photosensitive material 9. It is proportional to the logarithm of the light intensity. That is, as shown in FIG. 6, the curve b is displaced toward the curve a by the distance d, so that the interval between the scanning line of the laser light of the curve a and the scanning line of the laser light of the curve b is narrowed (80-d). .mu.m, and the distance between the scanning line of the laser light of curve b and the scanning line of the laser light of curve c widens to (80 + d) .mu.m. That is, as a result of unevenness in the scanning line pitch, unevenness in density occurs in the formed image.

However, if the photosensitive material 9 whose relative sensitivity is lowered in multiple exposure is used for the image forming apparatus 1 as in the present invention, it is possible to suppress the uneven density due to the uneven scanning line pitch. This point will be described below.

FIG. 7 is a diagram showing the concentration distribution of the photosensitive material 9 when the laser beam having the intensity distribution shown by the one-dot chain line in FIG. 6 is received.

In the figure, the curve shown by the solid line is the same as the curve shown by the solid line in FIG. 6, and the vertical axis thereof is the intensity of the laser beam as in FIG. On the other hand, the curve indicated by the alternate long and short dash line in FIG. 7 is the concentration distribution of the photosensitive material 9 when the laser beam having the intensity distribution indicated by the alternate long and short dash line in FIG. 6 is received, and the vertical axis thereof is the concentration.

As can be seen from FIG. 7, when the photosensitive material 9 whose relative sensitivity is lowered in multiple exposure is used, the curve b
The point g having the highest density exposed by the scanning line by the laser beam of (1) approaches ((df)) the center position of the scanning line by the laser beam of the curve b when there is no scanning line pitch unevenness. Thus, according to the present invention, it is possible to reduce uneven density due to uneven scanning line pitch.

By the way, the curve shown by the one-dot chain line in FIG. 7 has a larger amplitude than the curve shown by the one-dot chain line in FIG. Therefore, let us consider whether or not this adversely affects the uneven density.

FIG. 8 is a diagram showing the distribution of the level of the visual recognition limit unevenness (“Applied Optics, Vol. 25, No. 21”).
Issue (published November 1, 1986), page 3880, by reference to the graph in the paper entitled "Periodic Image Artifacts in Continuous Tone Laser Scanners" by Paul See Schubert. ).

Conventionally, the level of visible limit unevenness (visual contrast sensitivity to a sine wave pattern), which indicates the limit at which density unevenness can be visually identified, is known, and if it is above the curve in FIG. It is possible to identify uneven density on the light-sensitive material.

The amplitude of the curve indicated by the alternate long and short dash line in FIG. 7 affects the density unevenness of the scanning line period. Since the scanning line pitch is 80 μm in this embodiment, the spatial frequency of the scanning line period is 12.5 (lines / mm). According to the level of uneven visibility as shown in FIG. 8,
When the spatial frequency is 12.5 (lines / mm), density unevenness cannot be visually identified unless there is a considerably large density difference. That is, since the density unevenness is not within the visible range due to the density difference of about the amplitude of the curve shown by the one-dot chain line in FIG. 7, there is almost no adverse effect on the density unevenness of the formed image.

As can be seen from FIG. 8, even a small density difference can be visually recognized as density unevenness when the spatial frequency is 1.
This is the case before and after (lines / mm). In other words, how to reduce the density difference of the spatial frequency around this is an important point in suppressing the density unevenness.

For example, the polygon 2 shown in FIG.
Since 8 has an eight-sided structure, the spatial frequency of the polygon period is about 1.6 (lines) when the scanning line pitch is 80 μm.
/ Mm). Therefore, reducing the density difference of the polygon cycle leads to suppressing the density unevenness of the image.

FIG. 9 is a scanning line pitch uneven distribution of the polygon period when an image is formed using the photosensitive material A having the characteristics shown in FIGS. 2A and 2B.

In FIG. 9, the vertical axis represents the scanning line pitch, and the horizontal axis represents the distance between each of the eight polygons. In the figure, the vertical lines connect the maximum and minimum values of the scanning line pitch for each face of the classified polygons, and connect the average value of the scanning line pitch for each face of the classified polygons with a polygonal line. It is. When the photosensitive material A was used, the average value of the fluctuation width of the scanning line pitch was 1.253 μm.

FIG. 10 shows a frequency distribution obtained by Fourier transforming the scanning line pitch unevenness distribution of the polygon period shown in FIG.

In FIG. 10, the vertical axis represents the scanning line pitch in decibels, and the scanning line pitch of 1 μm is 0d.
Corresponds to B. The horizontal axis represents the spatial frequency, which represents the frequency of fluctuations in the scanning line pitch on the photosensitive material A. As shown in FIG. 10, the spatial frequency of the polygon period is about 1.6 (lin).
The scanning line pitch unevenness component h exists at the position (es / mm). The scanning line pitch unevenness of this scanning line pitch unevenness component h is -15.75 dB, which means that the scanning line pitch fluctuates by a maximum of 0.3 [mu] m when converted into a length.

On the other hand, FIG. 11 differs from FIG. 9 in that only the photosensitive material is changed, and scanning lines having a polygonal cycle when an image is formed using the photosensitive material B having the characteristics shown in FIGS. 3A and 3B. It is a pitch uneven distribution.

In FIG. 11, the vertical axis represents the scanning line pitch, and the horizontal axis represents the distance between each of the eight polygons. In the figure, the vertical lines connect the maximum and minimum values of the scanning line pitch for each face of the classified polygons, and connect the average value of the scanning line pitch for each face of the classified polygons with a polygonal line. It is. When the photosensitive material B was used, the average value of the fluctuation width of the scanning line pitch was 2.763 μm.

FIG. 12 is a frequency distribution obtained by Fourier transforming the scanning line pitch unevenness distribution of the polygon period shown in FIG.

In FIG. 12, the vertical axis represents the scanning line pitch in decibels, and the scanning line pitch of 1 μm is 0d.
Corresponds to B. The horizontal axis represents the spatial frequency, which represents the frequency of fluctuation of the scanning line pitch on the photosensitive material B. As shown in FIG. 12, the spatial frequency of the polygon period is about 1.6 (lin).
The scanning line pitch unevenness component i exists at the position (es / mm). The scanning line pitch unevenness of the scanning line pitch unevenness component i is -7.62 dB, which means that the scanning line pitch fluctuates by a maximum of 0.8 μm when converted into a length.

As can be seen by comparing FIGS. 9 and 10 with FIGS. 11 and 12, scan line pitch unevenness is suppressed more when the photosensitive material A, that is, the photosensitive material whose relative sensitivity is lowered in multiple exposure is used. Has been.

Further, FIG. 13 shows the structure shown in FIGS. 2 (a) and 2 (b).
3 is a density unevenness frequency distribution obtained by Fourier transforming the density unevenness distribution when an image is formed using the photosensitive material A having the characteristics shown in FIG.

In FIG. 13, the vertical axis represents the density in decibels, and the density 1 corresponds to 0 dB. The horizontal axis represents the spatial frequency, which represents the frequency of density fluctuations on the photosensitive material A. As shown in FIG. 13, the uneven density component j exists at a position of about 1.6 (lines / mm) which is the spatial frequency of the polygon period. The density unevenness of the density unevenness component j is −5.
It is 1.83 dB, and when converted into a concentration, the concentration difference has a maximum variation of 0.005.

On the other hand, FIG. 14 shows a density unevenness distribution in the case where an image is formed using a photosensitive material B having the characteristics shown in FIGS. 3A and 3B, only the photosensitive material is changed from that of FIG. It is the converted density unevenness frequency distribution.

In FIG. 14, the vertical axis represents the density in decibels, and the density 1 corresponds to 0 dB. The horizontal axis represents the spatial frequency, which represents the frequency of density fluctuations on the photosensitive material B. As shown in FIG. 14, the uneven density component k exists at a position of about 1.6 (lines / mm) which is the spatial frequency of the polygon period. The density unevenness of the density unevenness component k is −4.
It is 9.34 dB, which means that when converted to a concentration, the concentration difference fluctuates by a maximum of 0.007.

As described above, the density difference in the case of FIG. 13 using the photosensitive material A is 0.005, and the density difference in FIG. 14 using the photosensitive material B is 0.007. This difference of 0.002 is subtle but very important. That is, as can be seen from FIG. 8, when the spatial frequency is about 1.6 (lines / mm), the density difference of 0.
In the case of 007, it is in the visible range, and in the case of a density difference of 0.005, it is out of the visible range.

Next, assuming the photosensitive materials having various multiple exposure effects, the calculation result of the density unevenness of the polygon period when each photosensitive material is used will be described.

FIG. 15 is a diagram for explaining the density unevenness of the polygonal cycle when each of the photosensitive materials having various multiple exposure effects is used for image formation. FIG. 15A shows the relative sensitivity of the photosensitive material. b) is a list of density irregularities in the polygon cycle.

As shown in FIG. 15A, the photosensitive material F is a photosensitive material whose relative sensitivity is always 1 in multiple exposure,
The photosensitive material G is a photosensitive material in which the degree of decrease in relative sensitivity in multiple exposure is larger than that of the photosensitive material A described above.

Next, the calculation conditions will be described.

The intensity of the laser light used for exposure is the same for all scanning lines, and the intensity distribution is the Gaussian distribution shown in FIG.
The beam diameter was 120 μm. Also, the scanning line pitch is
80, 80.1, 80.2, 80.1, 80, 79.
Eight repetitions of 9, 79.8 and 79.9 μm were made. Further, the sensitivity of each light-sensitive material in one exposure has a γ characteristic as shown in FIG. 16, and the exposure amount has an average density of 1.
It was set to 06.

The calculation result is as shown in FIG. 15B, and it can be seen that the density irregularity of the polygon period is small in the order of the photosensitive material G, the photosensitive material A, and the photosensitive material F. That is,
From the calculation, it is clear from the calculation that the light-sensitive material having a lower relative sensitivity in multiple exposure has less density unevenness.

By the way, according to the present invention, since the uneven density of the formed image is suppressed by using the photosensitive material whose relative sensitivity is lowered in the multiple exposure, the uneven density is suppressed if the amount of overlapping scanning lines at the time of image formation is small. Less effective.

FIG. 17 is a diagram showing the density unevenness of the polygon period when the beam diameter of the laser light is changed.

In the figure, the vertical axis represents the density difference, that is, the density unevenness, in decibels, and the horizontal axis represents the beam diameter of the laser light used for exposure. White circles are measurement results when the photosensitive material A is used, and black circles are measurement results when the photosensitive material B is used. The scanning line pitch was 80 μm.

As described above, the spatial frequency of the polygon period is about 1.6 (lines / mm), and as can be seen from FIG. 8, the density unevenness is outside the visible range because the density difference is about 0.005 or less. Is the case. This density difference is −52 dB when converted to decibel display, so that the effect of the present invention is remarkable when the beam diameter of the laser light is 100 μm or more, that is, when the scanning line overlap amount is 25% or more of the scanning line pitch. .

By the way, in FIG. 17, the beam diameter is 80 μm.
The density unevenness in this case is smaller than that when the beam diameter is 100 μm, which is considered to be due to other factors than the effect of the present invention.

Next, the examination will be made on the density unevenness of the scanning line period.

In the above description of FIG. 7, even if the amplitude of the curve indicated by the alternate long and short dash line in FIG. However, the present inventors have noticed that the formed image looks somewhat grainy due to such a large amplitude.
Therefore, this measure will be described.

FIG. 18 is a diagram showing density unevenness in the scanning line period under the same conditions as in FIG.

In the figure, the vertical axis represents the density difference, that is, the density unevenness, in decibels, and the horizontal axis represents the beam diameter of the laser light used for exposure. White circles are measurement results when the photosensitive material A is used, and black circles are measurement results when the photosensitive material B is used. The scanning line pitch is 80 μm, which is the same as the measurement condition of FIG.

As a result of actually checking the formed image visually, if the density unevenness in the scanning line period, that is, the density difference due to the amplitude of the curve shown by the alternate long and short dash line in FIG. It was found that no image was observed and the image did not appear grainy.

Since the density difference of 0.2 is converted to a decibel display of -20 dB, the beam diameter of the laser beam is 140 μm or more according to FIG. 18, that is, the scanning line pitch is 75% or more of the scanning line pitch. For example, it is possible to suppress the density unevenness of the scanning line cycle as well as the density unevenness of the polygon cycle, so that the image quality of the image formed by the image forming apparatus is greatly improved and a high effect is obtained.

Further, in order to suppress the density unevenness of the scanning line period, it is also effective to change the intensity distribution of the laser light used for exposure.

As can be seen from FIG. 5, when the intensity distribution of the laser light is a Gaussian distribution, the exposure amount received by the photosensitive material becomes a valley between a certain scanning line and an adjacent scanning line and is small. . For this reason, when a photosensitive material whose relative sensitivity is lowered in multiple exposure is used, this valley becomes deeper and becomes a factor of increasing the density unevenness of the scanning line period.

Therefore, the shape of the intensity distribution of the laser light may be changed so that the exposure amount received by the photosensitive material is larger than the scanning line position between a certain scanning line and the adjacent scanning line.

FIG. 19 shows the intensity of the laser beam received by the photosensitive material 9 in the case where the shape of the intensity distribution of the laser beam is different from that of FIG. 9 is a diagram showing the density distribution of an image formed on the photosensitive material 9 at this time, and the horizontal axis represents the position of the photosensitive material 9 in the sub-scanning direction (vertical direction).

In the figure, the curve indicated by the solid line shows the distribution of the intensity of the individual laser light, and the curve indicated by the alternate long and short dash line is a superposition of the intensity of the laser light indicated by the solid line.
4 is an intensity distribution curve of laser light received by the photosensitive material 9.
The vertical axes of the two are the intensity of the laser light.

On the other hand, the curve shown by the broken line in FIG. 19 is the density distribution of the photosensitive material 9 when receiving the laser beam having the intensity distribution shown by the alternate long and short dash line, and the vertical axis thereof is the density.

When the shape of the intensity distribution of the laser light is set to the curve shown by the solid line in FIG. 19, the intensity of the laser light received by the photosensitive material 9 is such that the intensity of the laser light received by the one-dot chain line and the adjacent scanning line And (the portion where multiple exposure is performed) is larger than the scanning line position.

Thus, if the photosensitive material 9 whose relative sensitivity is lowered in multiple exposure is used, the amplitude of the scanning line period of the curve of the density distribution can be reduced as shown by the broken line, that is, the scanning line period It is possible to reduce uneven density.

Regarding the point that the density unevenness of the scanning line period can be reduced, the intensity of the laser beam becomes larger between the scanning line having the exposure amount received by the photosensitive material and the adjacent scanning line than the scanning line position. As another example of the distribution, a trapezoidal intensity distribution will be further described.

FIG. 20 is a diagram when the intensity distribution of the laser light is trapezoidal, and FIG. 20A is a diagram showing the intensity of the laser light received by the photosensitive material when scanning with the trapezoidal laser light. FIG. 3B is a diagram showing the density of an image formed on a photosensitive material when scanning with a trapezoidal laser beam.

In FIG. 20A, the vertical axis represents the intensity of laser light, and the horizontal axis represents the position of the photosensitive material in the sub-scanning direction (vertical direction). In the figure, the distribution of the intensity of each laser beam is shown by a broken line, and the intensity of the laser beam received by the photosensitive material is shown by a solid line. Further, one of the distributions of the intensities of the individual laser beams is indicated by a thick line in order to make the trapezoidal shape easy to understand.

As can be seen from FIG. 20A, when the shape of the intensity distribution of the laser light is trapezoidal, the exposure amount received by the photosensitive material is larger than the scanning line position between two adjacent scanning lines. Become.

FIG. 20B shows the characteristics of the multiple exposure effect when the photosensitive material A shown in FIGS. 2A and 2B is used. As shown in FIG. 20B, the fluctuation of the density of the scanning line period becomes extremely small, and its effect can be seen.

FIG. 21 shows the calculation results of the density unevenness in the polygon period and the density unevenness in the scanning line period when the intensity distribution of the laser beam is trapezoidal.

The calculation conditions in FIG. 21 are shown in FIG.
The trapezoidal shape is the same as the calculation conditions in.
The shape of the trapezoid was 4 μm and the bottom was 124 μm.

As can be seen from FIG. 21, when the laser light intensity distribution is trapezoidal and the photosensitive material A is used, both the density unevenness in the polygon period and the density unevenness in the scanning line period are most suppressed.

In order to obtain a laser beam having a trapezoidal intensity distribution as described above, a filter having a trapezoidal transmittance may be inserted at an image forming position in the optical path of the laser beam. For example, the beam expander 26 At the image formation position inside,
A filter formed by coating glass with chrome or a filter formed by developing a photosensitive material may be inserted.

Further, as the F-θ lens 29, an F-θ lens having large spherical aberration in the sub-scanning line direction may be used,
Even if a laser that is not coherent in the sub-scanning line direction (not a point light source) is used, it is possible to obtain laser light having a trapezoidal intensity distribution.

The light-sensitive material used in the present invention may be any one as long as it has low sensitivity in multiple exposure, and may be any of silver salt film, selenium photoconductor, silicon photoconductor, OPC (organic photoconductor) and the like. I don't care.

[0101]

As described above, according to the present invention,
It is possible to suppress the density unevenness in the direction perpendicular to the scanning line of the image formed on the photosensitive material without providing a special correction mechanism.

Therefore, the correction mechanism is not required, and the cost is reduced, and high precision is required for each component of the image forming apparatus, that is, the means for moving the relative position between the photosensitive material and the light beam, the polygon, and the like. This also leads to cost reductions.

Further, the present invention can be applied to a wide range of applications, even for an image forming apparatus which is already in use, because the photosensitive material can be switched to one having a lower sensitivity in multiple exposure.

[Brief description of drawings]

FIG. 1 is a schematic view of an image forming apparatus according to the present invention,
FIG. 1A is a side view, and FIG. 1B is a schematic view of an optical system of the image forming apparatus.

FIG. 2 is a diagram showing a relative sensitivity in a photosensitive material having a multiple exposure effect in which sensitivity decreases when multiple exposure is performed,
(A) is a figure regarding pre-exposure, (b) is a figure regarding post-exposure.

FIG. 3 is a diagram showing relative sensitivity in a light-sensitive material having a multiple exposure effect of increasing sensitivity when multiple exposure is performed,
(A) is a figure regarding pre-exposure, (b) is a figure regarding post-exposure.

FIG. 4 is a diagram showing the intensity of laser light used in forming an image on a photosensitive material.

FIG. 5 is a diagram showing the intensity of laser light received by a photosensitive material when scanning is performed five times with laser light.

FIG. 6 is a diagram in the case where the curve b shown in FIG. 5 is shifted to the curve a side and the scanning line pitch becomes uneven.

FIG. 7 is a diagram showing a concentration distribution of a photosensitive material when a laser beam having an intensity distribution shown by a dashed line in FIG. 6 is received.

FIG. 8 is a diagram showing a distribution of levels of unevenness in visual recognition limit.

9 is a scanning line pitch unevenness distribution of a polygon period when an image is formed using the photosensitive material A having the characteristics shown in FIGS. 2A and 2B. FIG.

FIG. 10 is a frequency distribution obtained by Fourier transforming the scanning line pitch unevenness distribution of the polygon period shown in FIG.

FIG. 11 is a scanning line pitch unevenness distribution of a polygon period when an image is formed using the photosensitive material B having the characteristics shown in FIGS. 3A and 3B.

FIG. 12 is a frequency distribution obtained by Fourier transforming the scanning line pitch unevenness distribution of the polygon period shown in FIG.

FIG. 13 is a density unevenness frequency distribution obtained by Fourier-transforming the density unevenness distribution when an image is formed using the photosensitive material A having the characteristics shown in FIGS. 2A and 2B.

FIG. 14 is a density unevenness frequency distribution obtained by Fourier-transforming the density unevenness distribution when an image is formed using the photosensitive material B having the characteristics shown in FIGS. 3A and 3B.

FIG. 15 is a diagram for explaining density unevenness of a polygonal period when each of various photosensitive materials having multiple exposure effects is used for image formation, FIG. 15A shows relative sensitivity of the photosensitive material, and FIG. Is a list of density irregularities in the polygon cycle.

16 is a diagram of a γ characteristic which is the calculation condition of FIG.

FIG. 17 is a diagram showing density unevenness in a polygon period when the beam diameter of laser light is changed.

FIG. 18 is a diagram showing density unevenness in a scanning line period under the same conditions as in FIG.

FIG. 19 shows the shape of the laser light intensity distribution different from that in FIG.
It is a figure which shows the intensity | strength of the laser beam received by the photosensitive material in the case of scanning 5 times by the laser beam, and also shows the density distribution of the image formed on the photosensitive material at that time.

FIG. 20 is a diagram when the intensity distribution of laser light is trapezoidal, and FIG. 20A is a diagram showing the intensity of laser light received by a photosensitive material when scanning with trapezoidal laser light, FIG. 6B is a diagram showing the density of an image formed on a photosensitive material when scanning with a trapezoidal laser beam.

FIG. 21 is a result of calculation of density unevenness in a polygonal cycle and density unevenness in a scanning line cycle when a laser light intensity distribution has a trapezoidal shape.

[Explanation of symbols]

 1 image forming apparatus 2 supply unit 3 transport unit 4 exposure unit 5 laser optical unit 6 accommodation unit

Claims (5)

[Claims] 1. An image forming apparatus for scanning and exposing a light beam modulated by an image signal onto a photosensitive material to form an image, wherein adjacent scanning lines of the light beam partially overlap each other on the photosensitive material. An image forming apparatus having scanning control means for controlling scanning so that a portion to be multiple-exposed is generated, and the photosensitive material is a photosensitive material having a multiple-exposure effect of lowering sensitivity in multiple-exposure. . 2. The intensity distribution of the light beam is such that the exposure amount received by the photosensitive material at the time of scanning by the light beam becomes larger than the central position of the scanning line in a portion where two adjacent scanning lines overlap each other. The image forming apparatus according to claim 1, further comprising: 3. The image forming apparatus according to claim 2, wherein the intensity distribution of the light beam has a trapezoidal shape. 4. The length of overlap of two adjacent scanning lines in a portion where the light beam intensity is 1 / e ² or more in the multiple-exposed portion is 25% of the scanning line pitch.
Claim 1 characterized by being defined as above.
The image forming apparatus according to item 1. 5. The length of overlapping of two adjacent scanning lines in the portion where the light beam intensity is 1 / e ² or more in the multiple-exposed portion is 75% of the scanning line pitch.
Claim 1 characterized by being defined as above.
The image forming apparatus according to item 1.