JP2549012B2 - Image recording device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image recording device using a laser as a light source,
In particular, it relates to improvement of unevenness in main scanning pixel recording density due to characteristics of the fθ lens.

2. Description of the Related Art An image recording apparatus using a laser as a light source mainly scans a record carrier such as a film with a laser beam deflected in a one-dimensional direction by a polygon mirror or the like, and also moves the record carrier in a direction orthogonal to the main scanning direction. By moving relatively in the sub-scanning direction) and scanning the record carrier two-dimensionally, image information is recorded.

In such a device, an fθ lens is generally used as an imaging lens. A beam spot is formed on the record carrier by this fθ lens.
An image is recorded on the record carrier by a laser beam directly modulated by a laser diode using a modulator such as.

The laser beam is modulated by a halftone dot signal created based on a reference clock having a constant frequency and an image signal after the laser beam passes through the main scanning start point detection sensor.

In the main scanning direction, when a deflector such as a polygon mirror which has a constant angular velocity movement is used as a beam deflecting device and an fθ lens is used as an imaging lens, the beam spot of the imaging surface on the record carrier is scanned. The speed is constant. That is, when an image is recorded on the record carrier on the image plane with a reference clock having a constant frequency, the pixel recording density is constant at any position in the main scanning direction, and an image can be recorded evenly in the main scanning direction.

Next, this will be described using an equation. The relationship between the beam incident angle θ of the fθ lens and the image height x is given by the following equation.

x = f · θ (1) When the above equation is time-differentiated, dx / dt = f · dθ / dt (2) is obtained. In the equation (2), the left side corresponds to the beam scanning speed of the image plane, and dθ / dt on the right side indicates the temporal change of the incident angle θ of the beam entering the fθ lens, that is, the angular velocity.

When the angular velocity of the polygon mirror is constant, dθ / dt is also constant in the main scanning direction effective printing area, and thus the scanning speed dx / dt is also constant. As a result, image recording without unevenness in the main scanning direction is achieved.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention By the way, in recent years, as a demand for the fθ lens, the image forming spot diameter is reduced to increase the image recording density in the entire effective area, and the scanning angle is increased to increase the effective area capable of image recording. There is a demand to take it widely.

When the fθ lens is designed with priority given to such requirements, on the other hand, there arises a problem that the fθ characteristic is somewhat sacrificed. That is, since the relationship between the incident angle θ and the image height x shown in the equation (1) is not sufficiently satisfied, even if the polygon mirror is stably rotated, the scanning speed of the beam spot on the image forming surface. Is not constant, which causes unevenness in the image recording density in the main scanning direction.

In view of such a point, the present invention corrects the fθ characteristic of the lens, which is a sacrifice at the time of lens design, by electrical processing, is inexpensive, and has a uniform pixel density over a wide effective scanning area.
Moreover, it is an object of the present invention to provide an effective image recording apparatus capable of recording an image with high definition.

Means for Solving the Problems In order to achieve the above object, the present invention provides a modulating means for modulating a laser beam emitted from a laser light source based on an image signal and a reference clock, and a modulated laser beam in a main scanning direction. And a first scanning means for scanning the laser beam and an fθ lens for forming the beam spot on the surface of the record carrier by imaging the laser beam (even if the fθ characteristic is not necessarily good, this type of plane scanning lens is It is generally called an fθ lens, which is also followed in this specification.), Second scanning means for relatively moving the record carrier in the direction orthogonal to the main scanning direction, and the characteristics of the fθ lens. Frequency control means for controlling the frequency of the reference clock applied to the modulating means in response to the change in the main scanning speed of the beam spot, and the main scanning of the laser beam for each main scanning. Comprising a synchronizing means for synchronizing the generation start timing of the start position and the reference clock, wherein the frequency control means, one word n
It consists of a bit (n is an integer of 2 or more) line memory, a D / A converter and a voltage controlled oscillator.
One address and one word for each predetermined angle of incidence on the fθ lens, and a value related to the main scanning speed on the image plane of the fθ lens is stored as n-bit data for each word. An n-bit data read from the line memory is converted into an analog amount by the D / A converter by an addressing signal proportional to the incident angle of the fθ lens after the start of scanning, and a frequency proportional to the analog amount is generated by the voltage controlled oscillator. It is characterized by creating a reference clock of.

Further, the image recording apparatus further comprises an analog voltage generating means for obtaining an analog voltage proportional to the frequency of the reference clock, and a multiplication processing means for multiplying the analog voltage by the image signal at the corresponding position in the main scanning direction. It is characterized in that the intensity of the laser beam is changed by the signal obtained from the multiplication processing means.

Action It is known in advance how the main scanning speed changes in the image plane of the fθ lens. Therefore, one address and one word are stored in advance in the line memory for each predetermined incident angle of the fθ lens, and a value related to the main scanning speed on the image plane of the fθ lens is stored as n-bit data for each word. can do. Then, n-bit data is read word by word from this line memory at regular intervals, converted into an analog amount by the D / A converter, and then a reference clock with a frequency proportional to the analog amount is generated by the voltage controlled oscillator. Let In this case, the laser beam 1
By synchronizing the generation start timing of the reference clock with the main scanning start timing of the laser beam for each main scanning, the frequency of the reference clock is high on the surface of the record carrier where the main scanning speed is high. The frequency of the reference clock can be lowered at a slow speed. As a result, the main scanning pitch formed on the surface of the record carrier becomes uniform over the entire main scanning direction.

Embodiment FIG. 1 is a diagram showing the overall configuration of an embodiment of the image recording apparatus according to the present invention. This apparatus is a plate-making scanner provided with a polygon mirror as a first scanning means for main scanning. The X direction and the Y direction shown in the figure correspond to the main scanning direction and the sub scanning direction, respectively. In a plate-making scanner, it is customary to subject the image signal to halftone dot modulation before being output, and this specification will be described accordingly. However, the present invention can also be applied to an image recording device that outputs continuous tone. The laser beam LB generated by the laser light source 1 is incident on an AOM (acousto-optic modulator) 2 as a modulation means. A dot signal for ON / OFF modulating the laser beam LB is input to the AOM 2 from the dot signal generator 3.

The laser beam LB that is ON / OFF modulated by AOM2 is a
The light is incident on the mirror surface of the polygon mirror 4 rotating at a constant speed in the direction. The laser beam LB reflected from the mirror surface of the polygon mirror 4 is periodically deflected in the X direction as the polygon mirror 4 rotates. Laser beam LB after deflection
Forms an image as a beam spot through the fθ lens 5 and on the photosensitive material 6 provided at the image plane position. The photosensitive material 6 is attached to a drum 7 that is driven to rotate in the b direction. By the rotation of the polygon mirror 4 and the rotation of the drum 7,
The laser beam LB records a two-dimensional image on the surface of the photosensitive material 6.

A start sensor 8 for detecting the main scanning start position of the laser beam LB is provided on one side of the drum 7 and on the start point side in the main scanning direction X.

The fθ lens 5 is designed to sacrifice the fθ characteristic to some extent to reduce the image forming spot diameter in the effective scanning region to make the image highly precise and to widen the effective scanning region. Therefore, the relationship between the incident angle θ of the fθ lens 5 and the image height x is not linear as shown by the solid line in FIG. 2 (a) (the broken line shows the characteristic of the ideal fθ lens). .

If the time when the beam spot imaged by the fθ lens 5 is detected by the start sensor 8 is t = 0 and the incident angle at that time is θ = 0, the incident angle θ and the time t are one-to-one. Since it corresponds, the horizontal axis of FIG. 2 (a) is represented by the time axis. Further, since the image height x is the same as the coordinate x in the main scanning direction when the position of the start sensor 8 is the origin, the vertical axis is represented by the coordinate x in the main scanning direction X.

By the way, if the time t and the coordinate x in the main scanning direction are not linear as the fθ characteristic as described above, the main scanning speed V (t of the beam spot imaged by the fθ lens 5 on the photosensitive material 6 is obtained. ) (= Dx / dt) changes every moment as shown by the solid line in FIG. 2 (b).

Now, let fd be the frequency of the reference clock for modulating AOM2.
(T), the main scanning pitch recorded on the photosensitive material 6 by the ON / OFF modulation of the laser beam by AOM2 is ΔP (t),
The main scanning speed of the beam spot on the photosensitive material 6 is V (t)
Then, the relation ΔP (t) = K ₁ · V (t) / fd (t) (3) holds. However, K ₁ is a constant. In this relational expression, the main scanning speed V (t) changes with time t as shown in FIG. 2 (b), so that the main scanning pitch ΔP (t), which is the subject of the present invention, can be kept constant. , Second
The frequency fd (t) of the reference clock must change proportionally to the main scanning speed V (t) as shown in FIG.

The halftone dot signal generator 3 has the frequency fd of the reference clock.
A circuit for changing (t) so as to satisfy such a condition is included, and its details are shown in FIG. As shown in the figure, the halftone dot signal generator 3 comprises a reference clock generating circuit 9 and a halftone dot signal generating circuit 10. The reference clock generation circuit 9 includes an address counter 91, a line memory 92, and a D / A.
It is composed of a converter 93, a low-pass filter 94 and a voltage controlled oscillator (hereinafter abbreviated as VCO) 95.

The address counter 91 receives the beam spot detection signal from the start sensor 8 as a start condition,
Thereafter, clock pulses of a constant frequency output from a clock generator (not shown) are sequentially counted, and the count value at that time is output as an address designation signal of the line memory 92. At this time, since the polygon mirror 4 is moving at a constant angular velocity, the count value, that is, the addressing signal has a value proportional to the incident angle θ. When the count value of the address counter 91 reaches the same number as the number of words in the memory 92, the counting is finished. Then, when the beam spot detection signal from the start sensor 8 is next input, the address counter 91 is reset and the counting operation is repeated again.

The line memory 92 is a 1-word n-bit memory, which has one address for each predetermined angle of incidence on the fθ lens 5.
As one word, a value related to the main scanning speed V (t) on the image plane of the fθ lens 5 is stored as n-bit data for each word. FIG. 4 shows what contents are stored in the memory 92. Figure 4 (a) shows fθ
FIG. 6 is a diagram showing a main scanning speed V (t) on an image plane with respect to an effective incident angle of a lens 5. The effective incident angle is the fθ lens 5
The incident angle of the range required for scanning the effective scanning area determined by This figure is shown in Fig. 2 (b).
The horizontal axis of is only changed from the time to the incident angle θ, and is a diagram substantially equivalent to FIG. 2 (b). In FIG. 4A, the effective incident angle is divided by the total number of words in the line memory 92, and the speed Vi (i = 1 ... M) at the left end of each section is obtained in advance. Then, as shown in FIG. 4 (b), at the address corresponding to the partition number, the main scanning speed in that partition is set.
The value associated with Vi is stored as an n-bit code.

The D / A converter 93 is a circuit for converting n-bit digital data read from the line memory 92 for each word into an analog signal.

The low-pass filter 94 is a filter that removes high-frequency noise from the analog signal output from the D / A converter 93.

The VCO 95 is a circuit that outputs a reference clock DC having a frequency proportional to the output signal of the low pass filter 94. The circuit 95, the D / A converter 93, and the low pass filter 94 are all known circuits.

The halftone dot signal generation circuit 10 includes an image processing unit 101, a halftone dot signal generator 102, and a driver 103. The image processing unit 101 is a known circuit that performs processing such as gradation correction and color correction on the image signal obtained by the input device. The halftone dot signal generator 102 is an image processing unit 101.
This is a circuit that binarizes the image data obtained in step 1 by the reference clock DC obtained from the reference clock generation circuit 9. The driver 103 is a circuit that amplitude-modulates a halftone dot signal obtained from the halftone dot signal generator 102 with a high frequency signal (for example, a carrier wave of 80 MHz). The output from this driver 103 is AOM2
The laser beam LB is ON / OFF modulated. In this case, the frequency of ON / OFF modulation is equal to the frequency fd (t) of the reference clock DC obtained from the reference clock generation circuit 9.

Next, the operation of the apparatus having the above configuration will be described. The laser beam LB emitted from the laser light source 1 is the reference clock in AOM2.
After being ON / OFF-modulated at the frequency of DC, it is periodically deflected in the X direction by the polygon mirror 4 and imaged on the photosensitive material 6 through the fθ lens 5. Here, the fθ lens 5 is a lens with high definition and a wide effective area, but sacrificing the fθ characteristic to some extent. Further, the data relating to the main scanning speed Vi in which the reference clock generation circuit 9 is stored in the line memory 92 is
This is that of the θ lens 5. Therefore, the velocity of the beam sport in the main scanning direction on the photosensitive material 6 is a function of time as shown in FIG. 2B, and is not constant.

It is now assumed that the beam spot is detected by the start sensor 8 at the main scanning start position and one line is scanned from the left end to the right end of the photosensitive material 6. When the beam spit is detected by the start sensor 8, the address counter 91 of the reference clock generation circuit 9 resets the count value and starts counting sequentially from 0 in synchronization with the clock of a constant frequency. The output of the address counter 91 is applied to the line memory 92 as an address designation signal, and the memory contents of the line memory 92 are read word by word. Since the line memory 92 stores an n-bit code in one word, the bit signal of each word is input in parallel to the D / A converter 93 and converted into an analog signal here. FIG. 4 (c) shows a change in the analog signal obtained during the main scan of the laser beam LB once.

This analog signal is converted into a reference clock DC by the VCO 95 and applied to the halftone dot generator 102. The time change of the frequency of the reference clock DC obtained from the VCO 95 is shown in FIG. 2 (c).
This frequency is represented by fd (t) as a function of time t. This frequency fd (t) is a curve similar to the analog signal of FIG. 4 (c), and if the number of words in the line memory 92 is sufficiently increased, the main scanning speed V on the image plane of the fθ lens 5
It becomes similar to (t). This can be confirmed from the fact that the curves in FIGS. 2 (b) and 2 (c) are similar. Therefore, by sufficiently increasing the number of words in the line memory 92, the frequency fd (t) of the reference clock DC has the following relationship with the main scanning speed V (t).

fd (t) ∝V (t) (4) fd (t) = K ₂ V (t) (4) 'However, K ₂ is a constant.

The reference clock DC created as described above is supplied to the halftone dot signal generator 102, and every time the reference clock DC is received, the image signal received from the image processing unit 101 is converted into a halftone dot signal.

Next, this halftone dot signal is applied to AOM2 through the driver 103. AOM2 turns on laser beam LB by halftone dot signal
/ OFF Modulate. In this case, since the frequency of the halftone dot signal matches that of the reference clock DC, the laser beam LB is
ON / O in AOM2 depending on the reference clock frequency fd (t)
FF modulated. Thus, the frequency fd of the reference clock
Since the laser beam modulated by (t) is imaged on the photosensitive material 6 through the polygon mirror 4 and the fθ lens 5 and is subjected to main scanning, the main scanning pitch ΔP (t) formed on the photosensitive material 6 is It is obtained by substituting the equation (4) 'into the equation (3). That is Becomes

Since K ₁ and K ₂ on the right side of the above equation are constants, ΔP on the left side
(T) is a constant that does not depend on time. That is, according to the above apparatus, the main scanning pitch on the photosensitive material 6 can always be kept constant even if the fθ lens that sacrifices the fθ characteristic is used, and recording with uniform pixel density is possible. Means that is possible. Incidentally, FIG. 2 (d) shows the above (5).
The relationship between expressions is shown in a graph.

Next, FIG. 5 shows another embodiment of the present invention.
In the above-described embodiment, the main scanning pitch can be kept constant even if the main scanning speed on the photosensitive material 6 changes, but in this case, if the change in the main scanning speed is too large, the output of the laser light source 1 is increased. Even if the power is constant, it is not possible to record on the photosensitive material 6 at a constant density, which may cause shading.
That is, when the change in the main scanning speed on the photosensitive material 6 is large, even if the output power of the laser light source 1 is constant, the irradiation energy per unit area is low at a high main scanning speed, so that the recording density is high. On the contrary, when the main scanning speed is low, the irradiation energy per unit area is high, so that the recording density is high.

The embodiment shown in FIG. 5 is devised to prevent density unevenness due to a large change in the main scanning speed. Therefore, first, the multiplication processing unit 104 is provided between the halftone dot signal generator 102 of the halftone dot signal generating circuit 10 and the driver 103, and the frequency / voltage converter 99 is connected to the output of the VCO 95 of the reference clock generating circuit 9. This frequency-voltage converter
The signal obtained from 99 is added to the multiplication processing unit 104.

The operation of the above configuration will be described with reference to the waveform chart of FIG.
Since the reference clock DC, which is the output of the VCO 95, is proportional to the main scanning speed of the beam spot on the photosensitive material 6 as described above, the output of the frequency / voltage converter 99 is shown in FIG. 6 (a). As shown in FIG. 2 (b), which is the main scanning intensity waveform, it has a similar shape. On the other hand, the output of the halftone dot signal generator 102 is a halftone dot modulated image signal received from the image processing unit 101 each time the reference clock DC is received. Normally, several clocks to several + clocks are turned on and off. The states are alternately maintained and, for example, a binary signal as shown in FIG. 6 (b) is obtained.
However, in this figure, the image signal is set to a constant level for convenience,
The reference clock frequency is also drawn as a constant frequency. The multiplication processing unit 104 multiplies the above two signals, and therefore, the waveform shown in FIG. 6C appears in the output of the analog processing circuit 104. This waveform is high-frequency modulated by the driver 103 and added to AOM2. AOM
2 intensity-modulates the laser beam LB based on this input. The intensity-modulated laser beam is shown in FIG. 6 (d). As can be seen from this figure, the output light of AOM2 is strong where the main scanning speed of the beam spot on the photosensitive material 6 is fast (areas A and C), and conversely where the main scanning speed is slow (area B). Then the output light of AOM2 is weak. As described above, if the power of the laser beam is originally constant, the irradiation energy is low where the main scanning speed is fast and is high when the main scanning speed is slow. When changed, the irradiation energy can be made uniform over the entire surface of the photosensitive material 6. As a result, recording without unevenness in density is realized.

EFFECTS OF THE INVENTION As described above, according to the present invention, fθ characteristics are sacrificed in order to increase the definition of an image by reducing the diameter of the imaging spot and enlarge the effective area by increasing the scanning angle. Even if a lens is used, the main scanning pitch on the record carrier can always be kept constant, and as a result, it is possible to obtain an image recording apparatus having no pixel density unevenness, high definition and a wide effective scanning width. .

Moreover, since the pixel density unevenness is eliminated by electrical means such as a line memory, a D / A converter, and a VCO, it can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing an embodiment of the image recording apparatus of the present invention, and FIGS. 2 (a) to 2 (e) are photosensitized as a record carrier by using an f.theta. Lens with some sacrificed f.theta. Characteristics. FIG. 3 is a diagram for explaining the reason why the main scanning pitch on the material can be made constant, and FIG. 3 is a detailed block diagram of the halftone dot signal generator.
4 (a) to (c) are diagrams showing a storage method and memory contents in a line memory, FIG. 5 is a block diagram showing another embodiment of the present invention, and FIGS. 6 (a) to (d). FIG. 4 is a waveform chart of each part of FIG. 1 ... Laser light source, 2 ... AOM (modulation means), 3 ... Halftone dot signal generator, 4 ... Polygon mirror (first scanning means), 5 ... fθ lens, 6 ... Photosensitive material (recording) Carrier),
7 ... Drum (second scanning means), 8 ... Start sensor (synchronization means), 9 ... Reference clock generation circuit, 10 ...
Halftone dot signal generation circuit, 92 …… Line memory, 93 …… D / A converter, 95 …… VCO.

Claims (2)

(57) [Claims] 1. A modulation means for modulating a laser beam emitted from a laser light source based on an image signal and a reference clock, a first scanning means for scanning the modulated laser beam in a main scanning direction, and the laser beam. Fθ lens for forming a beam spot on the surface of the record carrier by imaging the image, second scanning means for relatively moving the record carrier in the direction orthogonal to the main scanning direction, and due to the characteristics of the fθ lens. Frequency control means for controlling the frequency of the reference clock applied to the modulating means in response to the change in the main scanning speed of the beam spot, and the main scanning start position of the laser beam and the generation of the reference clock for each main scanning. The frequency control means includes a line memory of 1 word n bits (n is an integer of 2 or more), a D / A converter, and a voltage controlled oscillation. The line memory has one address and one word for each predetermined angle of incidence on the fθ lens, and for each word, a value related to the main scanning speed on the image plane of the fθ lens is n bits. The n-bit data, which is stored as data and is read from the line memory, is converted into an analog amount by a D / A converter by an addressing signal proportional to the incident angle of the fθ lens after the start of each main scan, An image recording apparatus characterized in that a reference clock having a frequency proportional to an analog amount is created by a controlled oscillator. 2. The image recording apparatus according to claim 1, further comprising an analog voltage generating means for obtaining an analog voltage proportional to the frequency of the reference clock, and the analog voltage for multiplying the image signal at a corresponding position in the main scanning direction. An image recording apparatus, comprising: a multiplication processing unit for changing the intensity of a laser beam with a signal obtained from the multiplication processing unit.