JPH05294003A - Image recording device - Google Patents

Abstract

PURPOSE:To reduce change of dot area with respect to speed difference of vertical scanning by providing an image formation means for forming dots with scanning lines, as a start line or end line, to be scanned by light point positioned in the lowest stream in the vertical scanning direction among light point groups. CONSTITUTION:By making a scanning order to be I=n-1 or I=n+l, a cluster is generated at a simple cycle. Dots are formed synchronizing with this cluster so as to reduce fluctuation of a dot area. Concretely, dots 10 are formed with scanning lines, as a start line, to be scanned by a laser beam L4 located at the lowest end, i.e., lowest stream in the vertical scanning direction among a plurality of laser beams L1-L4 to be scanned at a time. The density of a dot image is expressed by the number of picture elements which constitute the dots 10. A single dot image is formed in a single cluster 11. As intervals among the scanning lines in one cluster are the same, intervals among the picture elements will be equalized. Accordingly, the dot area does not change.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image recording apparatus such as a digital copying machine or a laser beam printer, and more particularly to a multi-beam image recording apparatus capable of simultaneously writing with a plurality of laser beams.

[0002]

2. Description of the Related Art For example, in an image recording apparatus for forming an electrostatic latent image with a laser beam such as a digital copying machine or a laser beam printer, a laser beam from a semiconductor laser is applied to a rotary polygon mirror called a polygon scanner, The reflected beam from the rotating polygon mirror is applied to the surface of the charged photoconductor that moves at a constant speed. The laser beam is scanned at right angles to the moving direction of the photoconductor by the rotation of the rotary polygon mirror. Since the laser beam is modulated according to the image to be output, an electrostatic latent image is formed on the photoconductor, and this electrostatic latent image is developed into a visible toner image.

In the image recording apparatus which forms an electrostatic latent image by such a laser beam, it is necessary to narrow the interval between scanning lines in order to increase the definition of the output image. High-definition image recording device using this laser beam,
The biggest problem in increasing the speed is that the rotation speed of the polygon scanner is limited.

In order to solve this problem, a multi-beam scanning system in which the surface to be scanned is scanned with a plurality of laser beams at once is already known. As a matter of course, in this system, a plurality of laser beam spots are in a direction (hereinafter, referred to as sub-scanning direction) perpendicular to the scanning direction (hereinafter, referred to as main scanning direction) by the polygon scanner.
Must be close enough to.

For this reason, efforts have been made to manufacture a plurality of semiconductor lasers in close proximity to each other, and at present, a semiconductor laser array in which a plurality of semiconductor lasers are brought close to each other at an interval of 10 μm has been prototyped (for example, JP-A-2-39583 and R. L.
Thornton et al. , "Properti
es of closely paced inde
pendently addressable las
ers qualified by impurit
y-induced disordering ”, Ap
pl. Phys. Lett. 56 (17), 1623-
1625 (1990)). However, even if a plurality of semiconductor lasers are brought close to each other up to 10 μm by employing the technique disclosed in the above publications, it is still insufficient to perform scanning without gaps in the sub-scanning direction.

As a means for compensating for this, a method of filling a gap in the sub-scanning direction by interlaced scanning has been proposed (for example, see Japanese Patent Laid-Open No. 56-110960). In addition, a multi-beam scanning optical system using a semiconductor laser array with an interval of 10 μm has been proposed and filed by the applicant as Japanese Patent Application No. 2-44435.

FIG. 9 shows an example of interlaced scanning. In this example, interlaced scanning is performed by two laser beams (L1, L2). In FIG. 9, d _X is a laser spot diameter that is electrophotographically defined (hereinafter referred to as an electrophotographic spot diameter). This electrophotographic spot diameter is the diameter of the visible image obtained when the laser beam is imaged on the surface to be scanned to form an electrostatic latent image on the surface to be scanned and this electrostatic latent image is developed. is there. Distance r between the centers of the two spots imaged on the surface to be scanned by the two laser beams
₃ is 3d _X. Since the sub-scan is performed by 2d _{X for} each main scan, the second scan line is scanned by the laser beam L2 in the first scan and the laser beam L1 in the second scan as shown in FIG. Thus, the first scanning line and the fourth scanning line are scanned by the laser beam L2, and so on. That is, although there is a gap in each scan, a scan line scanned in a certain time is skipped in the next scan to perform scanning without gap as a whole.

In the interlaced scanning, the following three conditions must be satisfied in order to prevent overlapping scanning and unscanned scanning lines.

1) First, the main scanning must be sub-scanned by nd _{X with} respect to the number of laser beams n.

2) Next, the distance r ₃ between the two laser beams on the surface to be scanned must be an integral multiple of the electrophotographic spot diameter d _X.

3) When the integer r obtained by dividing the distance r ₃ between the two laser beams by the electrophotographic spot diameter d _x is called the scan order I, this scan order I is the same as the laser beam number n. It must be prime (the greatest common divisor is 1).

The minimum interval between scanning lines is called a scanning pitch, which is represented by p in FIG. Generally, p = d _X
Is.

By the way, according to the above-mentioned interlaced scanning, it seems that no matter how wide the image forming spot interval is made on the surface to be scanned, it is enough to select an appropriate scanning order. However, in reality, when a large imaging order is used and a large scanning order (hereinafter referred to as a high-order scanning order) is adopted, there is a problem that the mechanical accuracy required for the scanning device becomes extremely high. ..

The problem of accuracy will be described below.

As shown in FIG. 10, when the scanning surface is scanned with one spot, the scanning surface pitch p must be within a certain error Δp, and the speed in the sub-scanning direction is allowed. The error rate δ _o will be expressed by the following equation.

Δ _o = Δv / v = Δp / p where v is the sub-scanning speed and Δv is the sub-scanning speed error.

FIG. 11 shows the number of semiconductor laser elements n = 2,
FIG. 10 is a diagram showing the case of the scan order I = 3, and in this case, the allowable error rate δ _2,3 = Δp / 4p = δ _o / 4, and FIG.
The permissible error rate of the speed is reduced to 1/4 as compared with the case of. It should be noted that δ _{n, I} is an allowable error rate of speed when the number of semiconductor laser elements is n and the scanning order is I. Although the allowable error in the speed in the sub-scanning direction has been described above, this causes not only the speed error in the sub-scanning but also the same problem in the dimensional accuracy of the precision semiconductor laser array of the optical system.

It is known that halftone reproduction by halftone dots is most sensitively affected by the sub-scanning speed error with respect to an image. That is, the area of the halftone dots changes due to the temporal change in the sub-scanning speed, which causes density unevenness, resulting in an image in which lines run in the vertical direction of the sub-scanning (main scanning direction).

In order to solve such a problem, it is effective to make the image forming spot intervals on the surface to be scanned as close as possible. Therefore, it is necessary to properly select the scanning order I with respect to the number of laser beams (semiconductor laser elements).
Japanese Patent Application No. 3-15860 filed by the applicant
No. 8 specification. For example, when the number n of semiconductor laser elements is an odd number, the position (speed) error in the sub-scanning direction may be the largest if the scanning order I = 2. FIG. 12 shows the case where n = 5 and I = 3, and in this case, the position (speed) error δ _5,3 = δ _o / 10. FIG. 13 shows n
= 5, I = 2, and in this case δ _5,2 = δ _o / 5
Therefore, the position (velocity) error is twice as large as when I = 3, and is equal to δ _5,1 = δ _o / 5 when I = 1 in FIG.

The position (speed) error δ _{n, I} in the sub-scanning direction is
It can be generally expressed as a function of n and I by the following equation.

Δ _{n, I} = δ _o / mn where m is a maximum integer satisfying m ≦ {I · (n−1) +1} / n. In this specification, m is referred to as a correlation length as follows. Define.

When the spots are closely arranged in the sub-scanning direction as in the case of FIGS. 10 and 14, the spot drawn by a certain scan and the spot drawn by the next scan are only adjacent to each other. However, in an interlaced scan, a spot drawn in one scan can be adjacent to a spot drawn in a scan up to m times later. This m is defined as the correlation length. This relationship is schematically shown in FIG. When the number n of semiconductor laser elements is odd and the scanning order is I = 2, the correlation length m = 1
This means that the required position (speed) error in the sub-scanning direction is the same as when the spots are densely arranged in the sub-scanning direction. Further, when the number of semiconductor laser elements n is an even number, and the scanning order I is a natural number of 3 or more and a minimum natural number that is coprime to n, the allowable position (speed) error in the sub-scanning direction is the maximum (the accuracy is the minimum). ) Can be

[0023]

However, upon further studying the interlaced scanning method using the above-mentioned multi-beam laser, it was found that the problem of scanning line clustering occurs in this method.
Clustering means that the intervals between scanning lines are not constant and a periodic group of scanning lines (cluster) occurs.

An example of scan line clustering is shown in FIG. FIG. 16 is a diagram showing that when the number of beams n = 5 and the scanning order I = 2, a cluster of scanning lines is formed when the sub-scanning speed is slightly higher than the design value. In this example, the scan line is a cluster 1 of one scan line.
2, a cluster pattern 2 having two scanning lines and a cluster 14 having two scanning lines are repeated to form a cluster pattern having a complicated structure. As is apparent from FIG. 16, such a phenomenon occurs because the scanning line interval is contracted and expanded in some cases with respect to the same change in sub-scanning speed. In the figure,
P indicates the scanning line interval on the wide side, and p indicates the scanning line interval on the narrow side. It has been known that even in the case of scanning with a single beam, if the sub-scanning speed changes with time, the scanning line intervals are not uniform, which greatly affects the reproduction of halftone dots. However, in the case of the interlaced scanning method using a multi-beam laser, this phenomenon does not occur due to the time variation of the sub-scanning speed, but occurs only when the sub-scanning speed deviates from the design value. The pattern of this cluster has the property of changing depending on the number of beams n and the scanning order I.

It is an object of the present invention to prevent the dot reproduction from being greatly affected even when clusters are generated during interlaced scanning with multi-beams.

[0026]

In order to achieve the above-mentioned object, the present invention provides a light source section having n (n is an integer of 2 or more) light sources capable of being driven independently of each other, and a plurality of light source sections from the light source section. Optical system for forming a plurality of light beams on the surface of the photosensitive medium as a plurality of light spots, a main scanning unit for scanning the plurality of light beams from the light source unit in the main scanning direction, and the photosensitive medium at a right angle to the main scanning direction. And a means for moving at a constant speed in the sub-scanning direction,
In an image recording apparatus that performs interlaced scanning by the light spot group, the distance between two adjacent light spots on the photosensitive medium surface,
When a value divided by the diameter of the visible image corresponding to the light spot obtained when the latent image corresponding to the light spot formed on the photosensitive medium is developed is defined as the scan order I, the scan order is I. =
n-1 or I = n + 1, and a halftone dot image forming means for forming a halftone dot with a scanning line scanned by a light spot located at the most downstream position in the sub-scanning direction in the light spot group as a starting line or a finishing line. It is characterized by being provided.

[0027]

Now, let us consider a case where interlaced scanning is performed with n beams. When the scanning order I is I = n−1 or I = n + 1, clusters of n scanning lines are formed for every n scanning speed errors. The scanning order I
Is an integer obtained by dividing the distance r ₃ between the two laser beams by the electrophotographic spot diameter d _X (see FIG. 9).

FIG. 1 is a diagram showing the manner of scanning when the scanning order I is I = n-1. L (i, k) represents a scan line scanned by the k-th main scan of the i-th laser beam of the n laser beams simultaneously exposed. Here, attention is paid to the laser beam positioned at the most downstream side in the sub-scanning direction of the n laser beams that are simultaneously exposed. L (n, 1) indicates the n-th scanning line of the laser beam located at the most downstream in the sub-scanning direction.

The scanning order I = n-1 means that n laser beams which are simultaneously exposed are arranged at intervals of (n-1) scanning lines. That is, since I = r ₃ / d _X , I = r ₃ / d _X = n−1 and r ₃ = (n−1) · d _X.

As described above, since the main scanning is performed once for each n scanning lines, the spatial order is L (n, 1), and then L (n- 1,2), then L (n-2,3), and so on.
Up to n), n scanning lines are sequentially main-scanned.
However, as is apparent from FIG. 1, the temporal order is L (1, n), followed by L (n, 2), which was scanned in the second main scan. If the sub-scanning speed is equal to the design value, the scanning lines should be evenly spaced. However, for example, if the sub-scanning speed deviates from the design value, an error in the scanning line interval proportional to the amount of movement in the sub-scanning direction will occur. Therefore, since L (n, 1) to L (1, n) are sequentially scanned, the error in the scanning line interval becomes an error corresponding to the sub-scan of n · d _X. On the other hand, the error between the scanning line intervals of L (1, n) and L (n, 2) is an error corresponding to the sub-scanning of (n-1) · n · d _X. Letting p be the interval between adjacent scanning lines from L (n, 1) to L (1, n) and P be the interval between L (1, n) and L (n, 2), The errors Δp and ΔP are expressed by the following equations (1) and (2), respectively.

Δp / p = nΔv / v (1) ΔP / P = mnΔv / v = (I-1) nΔv / v (2) Therefore, | ΔP |> | Δp | , L (1, n)
And L (n, 2) scan lines form one group (cluster)
Will be formed. However, m is the correlation length described in the prior art, and in the case of the scanning orders I = n-1 and I = n + 1, m = I-1 is always satisfied. Since I = n-1 here, m = n-2 after all. The same applies to the scan order I
It also holds when = n + 1.

Focusing on the most downstream laser beam in the sub-scanning direction, as shown in FIG. 2, L (n, 1) to L (1, n) are sequentially main-scanned. In the case of FIG. 1, the sub-scanning direction coincides with the time-sequential order, but when the scanning order I = n + 1, the sub-scanning direction and the time-sequential order are opposite. The error of the scanning line interval within the cluster and the error of the reference line interval between the clusters are also given by the above equations (1) and (2). Since I = n + 1, m = n, and the error of the line spacing between the clusters is larger than that when I = n−1, but the error of the scanning line spacing within the cluster is I = n−.
The error amount is the same as in the case of 1.

As mentioned above, for the same amount of sub-scan velocity error, the scan line spacing within a cluster is less affected than the scan line spacing between clusters. Therefore, according to the image recording apparatus of the first invention and the second invention of the present application, the change of the halftone dot area due to the speed error of the sub-scan can be reduced.

[0034]

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

FIG. 3 is a perspective view showing the configuration of a laser printer to which the present invention is applied. A plurality of laser beam lights emitted from the multi-beam semiconductor laser array 21 are collimated by the collimator lens 22 and the cylindrical lens 2.
After passing through 3, the polygon scanner 24 deflects it in the main scanning direction. The deflected laser beam light forms an image on the photoconductor 27 through an imaging lens 25 called an f-θ lens and a cylindrical lens 26. The photoconductor 27 rotates in the direction of arrow 28. The rotation of the photoconductor 27 in the direction of arrow 28 is called sub-scanning. The cylindrical lenses 23 and 26 are for correcting the surface tilt of the polygon scanner 24 and form a so-called anamorphic optical system. The optical image drawn on the photoconductor 27 is imaged by an electrophotographic process (not shown). In addition,
Reference numerals 1 ₁ , 1 ₂ and 1 ₃ in the figure schematically show scanning lines formed simultaneously by main scanning of a plurality of laser beam lights.

In FIG. 4, the number of beams n = 4 and the scanning order I = 3.
FIG. 6 is a diagram showing how halftone dots are formed in the case of. If the sub-scanning speed is equal to the design value, the scanning lines should be evenly spaced. However, for example, when the sub-scanning speed becomes slower than the design value, a group (cluster) 11 is formed every four scanning lines as shown in FIG. When the same sub-scan speed error is given, the interval P between the clusters is as described above.
It is greatly affected as compared with the interval p of the scan lines in the cluster. (1) when the number of beams n = 4 and the scanning order I = 3
Substituting into equations (2) and (2), the error in cluster spacing is ΔP = 8Δv / v, which is twice the error in scanning line spacing in the cluster Δp = 4Δv / v.

Therefore, when the halftone dots are formed across the clusters, the variation of the halftone dot area becomes large. In view of the above facts, the present invention sets I = n−1 or I = n + 1 to generate clusters in a simple cycle,
Halftone dots are formed in synchronism with this cluster to reduce variations in halftone dot area. Specifically, in the first embodiment, as shown in FIG. 4, it is located at the lowest end of the plurality of laser beams L _{1 to} L ₄ that are simultaneously scanned, that is, the most downstream in the sub-scanning direction. The halftone dots 10 are formed with the scanning line scanned by the laser beam L4 as the starting line.

The density of the halftone dot image is expressed by the number of pixels forming the halftone dot 10, but in the present embodiment, one halftone dot image is formed within one cluster 11. Since the scan lines in one cluster have the same spacing, the pixels have the same spacing. Therefore, the halftone dot area does not change.

FIG. 5 shows an example of the principle configuration of an image processing circuit for forming a halftone dot with a scanning line scanned by a laser beam positioned at the most downstream side in the sub-scanning direction as a start line. The multi-beam laser array 21 is connected to drive current modulators 31 to 34 that individually modulate the intensities of the laser beams L _{1 to} L ₄ from the four laser elements included in the multi-beam laser array 21. .. The halftone image signal generation circuit 35 converts the image signal from the image signal source 36 into four modulation signals for modulating the laser beams L _{1 to} L ₄ according to the number of laser beams and the scanning order.

FIG. 6 is an enlarged view of a part of FIG. 4, showing each pixel constituting a halftone image, and FIG. 7 is a multi-beam laser for each scanning for generating each pixel shown in FIG. The modulation signal supplied to the array 21 is shown.

In order to form a halftone image as shown in FIG. 6, in the first scanning, as shown in FIG. 7A, laser beams corresponding to the laser beams L ₁ , L ₂ and L ₄ are used. No modulation signal is supplied to the element, and the laser beam L ₃
Pixels A1, A2, B1, B2 are added to the laser element corresponding to
A modulated signal corresponding to is supplied. In the second scan,
As shown in FIG. 3B, no modulation signal is supplied to the laser elements corresponding to the laser beams L ₁ and L ₄ , and the pixels A3 and B are applied to the laser elements corresponding to the laser beam L _2.
Pixels C1, C2, D are supplied to the laser elements corresponding to the laser beam L ₃ by being supplied with the modulation signals corresponding to 3, 3 and B4.
Modulated signals corresponding to 1 and D2 are supplied. In the third scanning, as shown in FIG. 7C, no modulation signal is supplied to the laser elements corresponding to the laser beams L ₁ and L ₄ , and the pixel C3 is applied to the laser element corresponding to the laser beam L ₂ . The modulation signals corresponding to D3 and D4 are supplied,
The pixel E is attached to the laser element corresponding to the laser beam L _3.
Modulated signals corresponding to 1, E2, F1 and F2 are supplied. Similarly, a predetermined modulation signal is supplied to each laser element of the multi-beam laser array 21 so that a halftone dot is formed with the scanning line scanned by the laser beam L4 located at the most downstream side in the sub-scanning direction as a start line. To be done.

In the example shown in FIG. 4, since one cluster is formed from four scanning lines, the size of the matrix forming the halftone image in the sub-scanning direction must be within 4 pixels. However, the size in the sub-scanning direction is
It depends on the number of beams and the scanning order.

In order to generate the predetermined modulation signal as described above, for example, the image signal from the image signal source 36 is once built in the halftone image signal generation circuit 35, or
A bitmap image of an image to be output is generated by expanding it on an externally attached image memory, and as described above, the scanning line scanned by the laser beam L4 located at the most downstream side in the sub-scanning direction is used as a start line to form halftone dots. So that the halftone dot image signal generating circuit 35 sequentially reads out the image signals of the corresponding lines in the image memory and then supplies them simultaneously to the multi-beam laser array 21.

The example shown in FIG. 4 is an embodiment corresponding to the first invention of the present application, while FIG. 8 is an embodiment corresponding to the second invention of the present application. In the case of FIG. 8 as well, clusters of four scanning lines are formed, but unlike the case of the halftone dot forming order in FIG. 4, the scanning line scanned by the laser beam L4 at the bottom end is Halftone dots are formed so as to be the final line.

As a structure for forming this halftone dot,
A circuit similar to the image processing circuit shown in FIG. 5 can be used. However, the halftone image signal generation circuit 35 forms halftone dots so that the scanning line scanned by the laser beam L4 at the lowermost end is the end line.

In the second embodiment, the scanning order I = n + 1
Therefore, the array spacing of the semiconductor laser array may be wider than that in the first embodiment. Nevertheless, the error of the scanning line spacing within the cluster is the same as in the first embodiment. However, the error of the scanning line interval between clusters is larger than that in the first embodiment.

The case where the number of scanning lines forming a halftone dot matches the number of beams n has been described above. However, even when the number of scanning lines forming a halftone dot is an integer multiple of the number of beams n, This has the effect of reducing the variation in area.

[0048]

According to the image recording apparatus of the present invention, since the halftone dot image is generated within the range where the scanning line intervals are equal, the halftone dot caused by the speed error of the sub-scanning speed is generated even when clusters occur. The fluctuation of the area can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing interlaced scanning when I = n−1.

FIG. 2 is a diagram schematically showing interlaced scanning when I = n + 1.

FIG. 3 is a configuration perspective view showing a laser printer to which the present invention is applied.

FIG. 4 is a schematic diagram showing how halftone dots are formed so that a scanning line scanned by a laser beam at the lowermost end is a starting line.

FIG. 5 is a principle configuration diagram of an image processing circuit for forming halftone dots.

FIG. 6 is an explanatory diagram showing pixels forming a halftone image.

FIG. 7 is a timing chart showing a modulation signal supplied to the multi-beam laser array for each scanning.

FIG. 8 is a schematic diagram showing how halftone dots are formed so that the scanning line scanned by the laser beam at the lowermost end is the final line.

FIG. 9 is a diagram showing the principle of interlaced scanning, and is a diagram showing a relationship between an imaging spot of laser light on a surface to be scanned and a scanning line.

FIG. 10 is a diagram showing spot position errors when scanning with a single beam.

FIG. 11 is a diagram showing a positional error of a spot when scanning is performed with a beam number n = 2 and a scanning order I = 3.

FIG. 12 is a diagram showing a positional error of a spot when scanning is performed with a beam number n = 5 and a scanning order I = 3.

FIG. 13 is a diagram showing a positional error of a spot when scanning is performed with a beam number n = 5 and a scanning order I = 2.

FIG. 14 is a diagram showing a positional error of a spot when scanning is performed with a beam number n = 5 and a scanning order I = 1.

FIG. 15 is a diagram showing the relationship among the number of beams n in interlaced scanning, the scan order I, and the correlation length m.

FIG. 16 is a schematic diagram showing how clusters of scanning lines are formed.

[Explanation of symbols]

10: Halftone dots, 11-14: Scan line clusters, 21:
Multi-beam semiconductor laser array, 22: Collimator lens, 23: Cylindrical lens, 24 polygon scanner, 25: Imaging lens, 26: Cylindrical lens, 27: Photoconductor, 28: Sub scanning direction, 31 to 3
4: modulator, 35 halftone image signal generator, 36 image signal source, L1 to L5: laser beam

Claims (2)

[Claims] 1. A light source section having n (n is an integer of 2 or more) light emitting sources that can be driven independently of each other, and a plurality of light beams from the light source section are imaged as a plurality of light spots on a surface of a photosensitive medium. An optical system, a main scanning unit that scans a plurality of light beams from the light source unit in the main scanning direction, and a unit that moves the photosensitive medium at a constant speed in a sub-scanning direction substantially perpendicular to the main scanning direction. In an image recording apparatus that performs interlaced scanning by the light spot group, when a latent image corresponding to the light spot formed on the photosensitive medium is developed with a space between two adjacent light spots on the photosensitive medium surface. When a value obtained by dividing by the diameter of the visible image corresponding to the light spot obtained in step S is the scan order I, the scan order is I = n−1, and the most downstream position in the sub-scanning direction of the light spot group. Halftone image that forms a halftone dot with the scanning line scanned by the light spot in The image recording apparatus characterized in that a formed unit. 2. A light source unit having n (n is an integer of 2 or more) light emitting sources that can be driven independently of each other, and a plurality of light beams from the light source unit are imaged as a plurality of light spots on a surface of a photosensitive medium. An optical system, a main scanning unit that scans a plurality of light beams from the light source unit in the main scanning direction, and a unit that moves the photosensitive medium at a constant speed in a sub-scanning direction substantially perpendicular to the main scanning direction. In an image recording apparatus that performs interlaced scanning by the light spot group, when a latent image corresponding to the light spot formed on the photosensitive medium is developed with a space between two adjacent light spots on the photosensitive medium surface. When the value obtained by dividing by the diameter of the visible image corresponding to the light spot obtained in step S1 is the scan order I, this scan order is I = n + 1 and is at the most downstream position in the sub-scanning direction of the light spot group. A halftone image that forms halftone dots with the scanning line scanned by the light spot as the end line The image recording apparatus characterized in that a formed unit.