JPS607780B2 - Device with halftone image forming optical system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image scanning device having a halftone image forming optical system for recording or displaying a halftone image on a display medium.

In general, scanning optical systems using rotating polygon mirrors or vibrating mirrors are widely used in laser-based facsimile machines, various display devices, printing devices, etc. because they can provide a large scanning angle and have little chromatic dispersion.

In particular, when a rotating polygon mirror is used, it is widely used as a high-speed scanning device. A conventional method for recording or displaying a halftone image in such a scanning optical system is to apply intensity modulation to a scanning beam using an optical modulator and record or display using this intensity-modulated beam. One embodiment is shown in FIG. In Figure 1, laser 1
The scanning beam is converged by an input lens 2 and enters an acousto-optic light modulation element 3 (hereinafter referred to as an AO element).
The AO element generates an ultrasonic wave based on an AM signal whose intensity is modulated by the drive system 5, and the scanning beam causes black diffraction due to the wavefront of this ultrasonic wave. Since the intensity of this diffracted light is proportional to the intensity of the signal from the drive system 5 applied to the AQ element, the intensity of the diffracted light is changed by operating this signal intensity according to the density of the image. The scanning beam whose intensity has been modulated in this way is collimated again by the output lens 4, and only the diffracted light passes by the IJ cut 6. The beam diameter is expanded by the beam expander 7, and then the rotating polygon mirror 8 The image is scanned by the image forming lens 9 and formed onto the recording surface 10 via the imaging lens 9. The spot on the recording surface 10 that has undergone such intensity modulation is
The driving system 5 is controlled to continuously or discontinuously change between a spot with a strong intensity as shown in FIG. 2A and a spot with a weak intensity as shown in FIG. 2B. When a recording surface or display surface is irradiated with an intensity-modulated scanning spot as described above, the density distribution when recorded or displayed differs depending on the shape of the characteristic curve showing the relationship between the exposure amount and density of the recording surface or display surface. .

FIGS. 3 and 4 show two different idealized characteristic curves of the relationship between exposure and density. Figure 3 shows that when the characteristic curve (solid line) is completely a step function,
When the light spots (C, , C2) shown in FIG. 2 are recorded or displayed, so-called silk dots with clear outlines and different areas are obtained. The characteristic curve (solid line) shown in Figure 4 is a linear shape, and the density distribution obtained when irradiated with the light spot shown in Figure 2 does not have a clear outline as shown in Figure 3. Different shading can be obtained depending on the distribution of the light amount of the light spot. The characteristic curves shown in FIGS. 3 and 4 represent ideal models, and actual characteristic curves deviate from this shape due to various factors. For example, in FIG. 3, the actual characteristic curve is sloping as shown by the dotted line, resulting in halftone dots with blurred edges. Moreover, the strength of the light intensity is not directly proportional to the size of the dots, and as the light intensity increases, the change in the diameter of the halftone dots becomes slower, so a correction system is required and there is a limit to the size of the halftone dots. . Furthermore, even in the method shown in FIG. 4, the actual characteristic curve deviates from a straight line as shown by the dotted line. Therefore, if a portion other than the linear portion of the characteristic curve is used, a concentration saturated environment will occur, so it is necessary to use only the linear portion. Moreover, if this straight line portion is narrow, many gradations cannot be achieved. In this method of forming a halftone image by utilizing the photosensitive characteristics of the photosensitive section, the method of modulating the intensity of the scanning beam must be changed each time depending on the characteristics of the photosensitive section. Furthermore, the characteristics of the photosensitive area are easily affected by changes in the surrounding environment, such as changes in temperature and humidity, and this effect is particularly noticeable when electrophotography is used for the photosensitive area because static electricity is involved. In addition, with this method, it was impossible to eliminate the influence of the drift phenomenon in which the output of a light source such as a laser changes over time, making it extremely difficult to obtain a stable halftone image over a long period of time. . The present invention aims to improve the above-mentioned drawbacks, and provides an optical system capable of obtaining stable halftone images without being affected by fluctuations in photosensitive characteristics on the recording display surface. .

In the present invention, an optical system that changes the cross-sectional area of the scanning beam is used in place of the conventional method of utilizing changes in the intensity of the scanning beam and the photosensitive characteristics of the photosensitive member to obtain a halftone image. .

That is, a light source such as a laser, a halftone image forming optical system that receives a beam from the light source and changes the cross-sectional area of the light beam, and focuses the light beam from the halftone image forming optical system onto a light receiving surface. It consists of an imaging optical system for imaging. This halftone image forming optical system has a first aperture formed from the light source side.
, a first imaging optical system, an optical deflector made of crystal, a second imaging optical system, and a second aperture are arranged in this order, and the first aperture is located at one focal position of the first imaging optical system. Similarly, an optical deflector is disposed at the other focal position, and an optical deflector is disposed at one focal position of the second imaging optical system, and a second aperture is similarly disposed at the other focal position. The light beam from the light source passes through the first aperture, becomes a parallel beam by the first imaging optical system, enters the optical deflector made of crystal, receives a deflection action in the deflector, and then becomes a parallel beam. An image is formed on the second aperture by the imaging optical system.
The position of the image of the first aperture formed on the second aperture is displaced by the action of the optical deflector made of crystal. The light east passing through the second aperture is
The area of the overlapping portion where the image of the first aperture and the second aperture overlap is adjusted by the deflector, and a desired halftone image is obtained by changing this area. The light from the second aperture is transmitted by the imaging optical system.
An image is formed on a light-receiving surface for recording, display, etc. The deflector made of the crystal mentioned above is a prism using an acousto-optic crystal, an electro-optic crystal, or an electro-optic crystal and a wollast prism.
It is a combination of optical elements that separates natural light such as Lotschenbrism into two linearly polarized light beams with orthogonal polarization planes, and the two light beams have an intersecting angle, that is, the two light beams are not parallel. . Further, when the first aperture is illuminated with a parallel beam, the position where the first aperture is imaged by the imaging optical system and the spectral plane position of the diffracted light by the first aperture by the imaging optical system are different. Therefore, a deflector is placed at this spectral position.

Further, in the present invention, a portion of the photosensitive member whose photosensitive characteristics are saturated is used.

Therefore, in the present invention, since a halftone image is formed by changing the cross-sectional area of the scanning beam, it is possible to use almost only the saturated portion of the characteristic curve of the photosensitive member, and it is possible to use only the saturated portion of the characteristic curve of the photosensitive member. This makes it possible to eliminate the influence on images caused by

Furthermore, since an acousto-optic crystal or an electro-optic crystal is used as a deflector, the deflection speed and reliability are improved. Furthermore, by providing the deflector on the spectral image plane formed by the imaging optical system of the first aperture, the deflector can be miniaturized, and in the case of an acousto-optic crystal, the response when the modulation frequency of the crystal is changed. is fast. The present invention will be explained in detail below. 5A and 5B are diagrams showing an embodiment of the halftone image forming optical system according to the present invention, in which FIG. 5A is a perspective view and FIG. 5B is a front view.

5A and 5B show a case where an acousto-optic light modulation element (AO element) is used as the deflector, and both apertures are brought into an optically conjugate relationship with one imaging lens. In the following description of the embodiments of the present invention, members given the same numbers refer to the same members. After the light emitted from the laser 11, which is a light source, has its diameter expanded by a beam expander 12, it illuminates an incident aperture 13a provided in an incident slit plate 13 while maintaining parallel light. This entrance aperture is also the exit slit plate 14.
The exit aperture 14a and the imaging lens 1 provided above
7 at an optically conjugate position. An AO element 15 is provided on the optical path between the imaging lens 17 and the exit aperture 14a. When the signal from the control circuit 16 vibrates a piezo element (not shown) fixed to the AO element to generate an ultrasonic wave, the ultrasonic wave propagates within the Aq element. When a light beam is incident on the AO element in this state, Bragg diffraction occurs and the direction of the incident beam is changed. Since the Bragg diffraction angle can be freely changed by frequency modulation, so-called FM modulation, of the ultrasonic waves propagating within the AO element, this Bragg diffraction angle can be changed arbitrarily by a signal from the control circuit 16. Therefore, by changing the plug diffraction angle, the cross-sectional area of the scanning beam passing through the exit aperture 14a can be changed. The dotted line d shown in FIG. 5 indicates the 0th order diffracted light passing through the incident aperture 13a,
The spectral image is focused on the focal plane P of 7. Also solid line d2
1 shows light beams propagating through the imaging lens 17 using a point 13c within the entrance aperture 13a as a secondary light source. The parallel light from the beam expander 12 is diffracted by the incident aperture 13a, and the diffracted light focuses its spectral image on the focal plane P of the imaging lens 17. If an AO element, which is a deflector, is provided near this position P, the AO element can be made smaller, and the influence of non-uniformity within the element can be alleviated. This means that the spectral plane P of the input aperture 13a formed by the imaging lens 17 and the rear surface 14a of the input aperture 13a can be spatially separated because the light beam illuminating the input aperture 13a is not focused on the input aperture 13a. It is from. Figure 6A
6B is another embodiment of the halftone image forming optical system shown in FIG. 5A, and FIG. 6A uses an electro-optic light modulation element (hereinafter abbreviated as EO element) in place of the AO element. In the case of a prism, Fig. 6B shows an EO element as a deflector, a Wollaston prism, a Roschen prism, etc. When natural light enters, it separates into two light rays with mutually orthogonal polarization planes. This is a combination with an optical element (hereinafter referred to as a polarizing optical element) that can extract the light beams from the book at an angle of polarization, that is, the two light beams are not parallel to each other.

The deflector 18 shown in FIG. 6A uses the fact that when an electric field is applied to a crystal, the refractive index of the crystal changes, and by creating a prism 18 using this EO element and changing the electric field, This changes the angle of the light emitting light. Similar effects can be obtained by using this element interchangeably with the AO element shown in FIG. The deflector shown in FIG. 6B is a combination of an EO element 19 and the polarizing optical element 20, and can divide the emitting direction of the light beam into two different directions depending on when an electric field is applied to the EO element 19 and when no electric field is applied. can. At this time, it is desirable that the light beam from the laser 11 be polarized; if it is non-polarized, it is necessary to insert a polarizing filter. That is, the output deflected beam emitted from the laser 11 enters an EO element 19 and a polarizing optical element 20 such as a Wollaston prism via a beam expander 12, an entrance aperture 13a, and an imaging lens 17. At this time EO
The deflection direction of the polarized beam from the laser is determined so that the beam from the polarizing optical element 20 exits upward when no electric field is applied to the element 19. Next, when an electric field is applied to the EO element 19, the polarization direction of the beam exiting the EO element 19 has been rotated by 90 degrees, so that the beam exiting the polarizing optical element is bent downward. Therefore, two discontinuous output angles can be selected using this pair of EO elements and polarizing optical elements, and by continuing N sets of this silk, 2N discontinuous output angle changes can be obtained. Even if these deflectors 19 and 20 are used interchangeably with the AO element 15 shown in FIG. 5, similar effects can be obtained. 7A, B, and C show other embodiments of the halftone image optical system, and each embodiment has both apertures 13,
A deflector is disposed at a position where the beam becomes a focal point due to the imaging optical systems 21 and 22 provided between the two.

FIG. 7A shows a case where the AO element 15 is used, and the beam 4 propagating from a point 13c in the input aperture 13a as a secondary light source is transmitted through the input lens 22, which has one focal plane at the aperture 13a. This results in a parallel beam. After passing through the AO element 15, the parallel beam is focused onto the exit aperture 14a by an exit lens 22 having one focal plane at the exit aperture 14a. If the AO element 15 is provided on the spectral image plane P formed by the incident lens 21 of the incident aperture 13a, the AO
The element can be made smaller. FIG. 7B shows a halftone image forming optical system using a prism 18 using an EO element as a deflector, and FIG. 7C shows an EO element 19 as a deflector and a polarizing optical element 20.
In this figure, each deflector 18, 19, 20 is provided between the parallel beams between the input lens 21 and the output lens 22 and in the vicinity of the spectral image plane P. ing. In the halftone image forming optical system shown in FIGS. 5, 6, and 7, a single deflector is used, but as shown in FIGS.
, 18, 19, 20 in series, a large deflection angle can be obtained.

Even when a plurality of deflection elements are arranged in this way, the deflector can be made smaller by providing it at the position of the spectral image plane P. FIG. 9 is a perspective view schematically showing a scanning optical system using an embodiment of the halftone image forming optical system according to the present invention.

Koto comes out of Laser 11 and is Beam Expander 12.
to illuminate the entrance aperture 13a. The beam q, which uses the illuminated input aperture 13a as a secondary light source, becomes a parallel beam by the input lens 21, is deflected by the AO element 15 controlled by a signal from the control circuit 16, and then is directed to the output aperture 1 by the output lens 22.
The image of the input aperture is focused on 4a. At this time, the scanning beam passes through only the portion where the image of the input aperture and the output aperture overlap. The scanning beam using the exit aperture 14a as a new light source is converted into a parallel beam by a collimator lens 23 and scanned by a rotating polygon mirror 24, and is then passed onto a photosensitive drum 26, which is a scanning surface, by a scanning imaging lens 25. image is formed. As the rotating polygon mirror 24 rotates, the scanning beam linearly scans the drum 26, and as the drum 26 rotates, two-dimensional scanning is performed. 10A, B, and C show that in the optical system shown in FIG. 9, the angle of the scanning beam after exiting the AO element is changed by the AO element 15 which has been frequency modulated by the control circuit 16, and the incident aperture This figure shows how the conjugate image 13'a formed by the 13 lenses 21 and 22 overlaps with the exit aperture 14a, that is, how the cross-sectional area of the scanning beam changes. FIG. 10A shows an overlapping portion S between the conjugate image 13'a of the input aperture 13a formed by the lenses 21 and 22 and the output aperture 14a when the ultrasonic frequency propagating in the AO element 15 is o. Figure 10B shows the overlapped portion S2, the first
0 diagram C is the same o ten △ 2 (1 △ 21 > l △, l
) shows an overlapping portion S3. At this time, the shape of the photosensitive characteristic curve of the drum 26 is represented by the solid line in FIG. 11, and the light intensity distribution on the drum 26 of the scanning beam having the shape to be recorded as shown in FIG. The maximum intensity is located at a fully saturated portion of the curve. There is no aberration in the optical system between the output aperture 14a and the photosensitive drum 26, and there is no spread due to diffraction on the photosensitive drum 26.
6, the light intensity distribution of the scanning spot on the photosensitive drum is determined as a geometric optical size. Therefore, the photosensitive drum 26
The scanning spot above can be regarded as a constant light intensity distribution in the shape of the cross section (S, , S2, S3) of the light beam passing through the output aperture 14a, and this situation is shown in Fig. 12. has been done. In FIG. 12, the surface of the photosensitive drum is also expressed in xy coordinates, and the light intensity 1 is shown on the vertical axis. Figure 12A is attached to Figure 10A, Figure 12B is attached to Figure 10B,
FIG. 12C shows the light intensity of the spots corresponding to FIG. 10C, and when these are recorded on the photosensitive drum 26, as shown in FIG.
'3 and are recorded as halftone dots with different areas. The characteristic curve shown by the broken line in Figure 11 shows the state in which the shape of the characteristic curve fluctuates due to changes in the environment, and the maximum value Q (bright area) and minimum value 6 (dark area) of the light distribution to be recorded. If there is a sufficient difference in intensity, there will be little variation in the characteristic curve with respect to that intensity, and the shape of the total points recorded will be the same as the shape shown in FIG.

Further, a recording medium with a steep slope as shown in FIG. 3 is not required. Furthermore, since a sufficiently saturated portion of the characteristic curve is used for fluctuations in the output of the laser light source, it is possible to always record a shape that corresponds to changes in the cross-sectional area of the scanning beam obtained by the halftone image forming optical system. It will be done. Next, the relationship between the incident aperture and the spatial spectrum of the aperture will be explained.

FIG. 14 shows an embodiment of the halftone image forming optical system, in which the focal length of the input lens 21 is fb, and the focal length of the output lens is fc. The AO element 15 is placed at a position fb from the input lens and fc from the output lens,
The entrance aperture 13a is located fb away from the entrance lens 21, and the exit aperture 14a is located fc away from the exit lens 22. Coherent light emitted from the laser 11 is converted into a plane wave expanded by the beam expander 12 and illuminates the input aperture 13a. The diffracted light by the input aperture 13a is caused by the input lens 21 to produce a Fraun-Hofer spectrum at a position fb behind the input lens. The spread of this spectrum g(
xi, yi) is the transmission distribution of the incident aperture f(xo
, yo), and the wavelength of light is the target f(
X. ,y. ) e-2 bamboo ig (xi, yi) = Nono (Xx
o ten Yyo) d cutting dy.

It is given by

However, xo, yo, xi, and yi are perpendicular to the optical axis and correspond to x = magazine and Y = vessel. For example, the entrance aperture has a diameter of 1
As shown in Fig. 15, the intensity distribution of the spectrum is a so-called Bayley pattern given by y = Bessel function of the first kind and J, and its energy distribution is Approximately 84% of the values are concentrated in the 0th order, which is the center.

The first zero of that energy distribution is y. =・-22 pieces-
It is given by potato, and it varies slightly depending on the value of input, fb, and 1, but it is 1
》If it is, yo becomes extremely small and becomes a thing. This also applies to apertures having shapes other than circular apertures. As for the characteristics of the AO element, it is better to make the scanning beam enter the AO element as a narrow beam, or in other words, it is better to place the AO element at the spectral image plane position where the beam spread by the input lens of the input aperture is the smallest. If you change the frequency of the ultrasound propagating within the AO
The response of the diffraction angle of the scanning beam emitted from the element to the frequency of the ultrasonic wave is fast.

FIG. 16 is a diagram explaining this state.
When a vibration is given, the wavefront of the ultrasonic wave propagates in the direction of the arrow due to the vibration of the dth frequency. When the frequency changes from , to another frequency, the time required for the deflection angle to change to the angle corresponding to Becomes a water vine.

FIG. 17 shows this change with time on the horizontal axis and deflection angle on the vertical axis. Therefore, in order to make the response time long, it is necessary to reduce the beam spread E within the element. For this purpose, as described above, if the AO element is provided with the incident aperture at the position of the Fraunhofer diffraction image formed by the imaging lens, that is, at the focal point of the imaging lens, the fastest response characteristics can be obtained. Moreover, if the deflector is provided at the position of this spectral image, the size of the deflector will be the smallest. Note that the spectral image of the input aperture by the input lens 21 is inversely transformed by the output lens 22, but if the focal length of the input lens 21 and the focal length of the output lens 22 are not equal, the imaging magnification is not 1. It is necessary to determine the size of the entrance aperture and exit aperture according to the imaging magnification, and the same can be said for the halftone image forming optical system shown in Figures 5 and 6. It is.

Next, the shape of the incident aperture and the method of providing the AO element will be described in relation to the response time 7.

When the entrance aperture 13a is not circular but has a shape elongated in one direction, such as a rectangle, it is provided so that its long side direction coincides with the traveling direction of the ultrasonic wavefront propagating within the AO element. This is related to the spread of the image diffracted by the incident aperture, and the spread of the image diffracted in the direction of the longer opening □ width of the input aperture is the spread of the image diffracted in the direction of the shorter width of the opening □. This is because it is narrower than . Therefore A
The response speed can be increased by matching the direction in which the spread of the diffraction image on the O element is narrow with the traveling direction of the ultrasonic wave propagating through the AO element. Furthermore, since the Caesar beam is a Gaussian beam, it is desirable that the shape of the aperture be in accordance with the Gaussian beam. As described above, in the apparatus having a halftone image forming optical system according to the present invention, an intermediate halftone image forming optical system is provided between the light source section and the light receiving surface, and the cross-sectional area of the light beam is changed by the optical system. This is to obtain a halftone image, and a stable halftone image can be obtained at high speed.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an embodiment of a scanning optical system having a conventional halftone image forming optical system, and FIGS. Diagrams for explaining how images are formed, Fig. 5 A, B, Fig. 6 A, B
7A, B, and C each show an embodiment of the halftone image forming optical system according to the present invention, and FIGS. 8A, B, and C each show an embodiment of the optical deflector according to the present invention. Figure shown, No. 9
The figure is a perspective view showing an embodiment of a scanning optical system having a halftone image forming optical system according to the present invention. 14 is a diagram for explaining the relationship between the incident aperture and the spatial spectrum of the aperture, and FIG. 15 is a diagram for explaining the spectral intensity of the incident aperture. FIGS. 16 and 17 are diagrams showing the distribution, and are diagrams for explaining the response speed of the AO element. 13a...Incidence aperture, 14a...
... Output aperture, 15 ... Acousto-optic light deflection element, 16 ... Control circuit, 17 ... Imaging lens, 18 ... Electro-optic light Prism using polarization element, 19... Electro-optic light polarization element, 2
0... Polarization optical element, 21... Input lens, 22... Output lens. Arithmetic/Figure 2 Figure 3 Chrysanthemum Figure 3 Tail a Hair; Figure 8 Hair Tazu younger brother Zuken/Z Figure Hair/3 Figure Qin Buddha Figure Qin Body Figure Tanosan Diagram

Claims (1)

[Scope of Claims] 1. A light source section, a halftone image forming optical system that changes the cross-sectional shape of the light beam from the light source section, and an imaging optical system that images the light beam emitted from the optical system on a light receiving surface. The halftone image forming optical system includes, in order from the light source side, a first aperture, a first imaging optical system, an optical deflector made of crystal, a second imaging optical system, A second aperture is provided, the first aperture is disposed at one focal position of the first imaging optical system, an optical deflector is disposed at the other focal position, and the second aperture is disposed at one focal position of the second imaging optical system. 1. An apparatus having a halftone image forming optical system, characterized in that a light deflector is disposed at one of the focal positions, and a second aperture is disposed at the other focal position. 2. An apparatus having a halftone image forming optical system according to claim 1, which illuminates the first aperture with a parallel light beam. 3. An apparatus having a halftone image forming optical system according to claim 2, wherein the optical deflector is an acousto-optic light modulator. 4. An apparatus having a halftone image forming optical system according to claim 2, wherein the optical deflector is a prism using an electro-optic light modulation element. 2. Halftone image forming optics according to claim 2, wherein the light deflector is a combination of an electro-optic light modulation element and an optical element that separates natural light into a beam having two non-parallel orthogonal polarization planes. A device with a system. 6. The halftone image forming optical system according to claim 3, wherein the longest direction of the first aperture is the same as the traveling direction of the wavefront of the ultrasonic wave propagating within the optical acousto-light modulator. Device.