JPS5932771B2 - Scanning optics with halftone image forming optics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a scanning optical system in an image scanning device, which has 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 allow 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. As a method of recording or displaying a halftone image in such a scanning optical system, there is a conventional method of imparting intensity modulation to a scanning beam using an optical modulator and recording or displaying with this intensity-modulated beam. An example is shown in FIG. In FIG. 1, a scanning beam from a laser 1 is converged by an entrance lens 2 and enters an acousto-optic light modulation element 3 (hereinafter referred to as AO element). The AO element generates an ultrasonic wave using 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 AO 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 through the slit 6.The beam diameter is expanded by the beam expander 7, and then scanned by the rotating polygon mirror 8 to form an image. An image is formed on the recording surface 10 via the lens 9. The spots on the recording surface 10 that have undergone the new strain intensity modulation are shown in Figure 2A.
The driving system 5 is controlled to continuously or discontinuously change between a strong spot as shown in FIG. 2B and a weak spot 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 (C1, C2) shown in FIG. 2 are recorded or displayed, so-called halftone dots with clearly different areas of limbus are obtained. The characteristic curve (solid line) shown in Fig. 4 is in the case of a linear shape, and the density distribution obtained when irradiated with the light spot shown in Fig. 2 has a clear ring as shown in Fig. 3. Instead, different shades of light and shade can be obtained depending on the distribution of the light intensity 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 halftone dot,
As the light intensity increases, the change in the diameter of the halftone dot becomes slower, so a correction system is required, and there is a limit to the size of the halftone dot. Also the fourth
Even in the method shown in the figure, 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. Furthermore, 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 in the photosensitive area because static electricity is involved. In addition, this method cannot eliminate the influence of a drift phenomenon in which the output of a light source such as a laser changes over time, making it extremely difficult to obtain a halftone image that is stable 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 order to obtain a halftone image, the present invention uses an optical system that changes the area of the cross section of the scanning beam, instead of the conventional method that uses changes in the intensity of the scanning beam and the photosensitive characteristics of the photosensitive member. .

That is, in the present invention, a light source section that can freely change the wavelength of the scanning beam emitted according to a signal is provided, a signal giving information on the shading of an image is inputted to the light source section, and the image information is converted to the wavelength from the light source section. The spot diameter of the scanning beam on the scanning surface is changed using the chromatic aberration characteristics given to the scanning optical system in advance. The recording member or photosensitive member that is the scanning surface may have wavelength sensitivity characteristics.

That is, the photosensitivity of the recording or photosensitive member is different for each wavelength. If the wavelength region of the scanning beam is in a region where the photosensitivity of the recording or photosensitive member changes, changes in the cross-sectional area of the scanning beam due to chromatic aberration of the scanning optical system will not be accurately recorded. In such cases, a filter is installed in the scanning optical system so that the photosensitive characteristics of the recording or photosensitive member are approximately constant in the wavelength range of the scanning beam used, and the changes in the cross-sectional area of the scanning beam are accurately controlled. to be recorded or appear on a photosensitive member. Also, depending on the characteristics of the light source, the intensity of the oscillated scanning beam may change if the wavelength changes, and even in such cases, a filter is installed in the optical system to keep the intensity of the scanning beam constant from the light source. Provided for. The present invention will be explained in detail below. FIG. 5 is a perspective view showing an embodiment of the scanning optical system of the present invention.

In FIG. 5, reference numeral 11 denotes a variable wavelength light source section, which emits a scanning beam having a wavelength corresponding to a signal from the drive system 12. The scanning beam from the light source section 11 passes through a filter 13 that corrects the photosensitive characteristics of the scanning surface, is expanded in beam diameter by a beam expander 14, and then enters a rotating polygon mirror 15. A scanning beam plane-scanned by a rotating polygon mirror 15 is focused through an imaging lens 16 onto a scanning surface 17 which is a recording or photosensitive member. The beam expander 14 or the imaging lens 16 or both the beam expander and the imaging lens are designed to have chromatic aberration. That is, a scanning beam having a certain reference wavelength is imaged on the scanning surface 17 by the scanning optical system, but a scanning beam having a different wavelength from the reference wavelength is not imaged on the scanning surface 17, resulting in a blurred image. Therefore, the scanning beam of the reference wavelength becomes a scanning spot with the smallest cross-sectional area on the scanning plane, and as the beam deviates from the reference wavelength, the amount of blurring of the scanning spot increases and the cross-sectional area of the scanning spot increases. Therefore, by knowing in advance the change in the cross-sectional area of the scanning spot where light is focused on the scanning surface due to the change in wavelength from the light source section 11, a signal corresponding to the image information is transmitted from the drive system 12 and the light source section transmits a signal corresponding to the image information. Control the wavelength of the scanning beam. As mentioned above, the lens system that provides chromatic aberration is the beam expander 14 or the imaging lens 16.
Although both (14, 16) may be used, it is preferable that the beam expander 14 has chromatic aberration.

Further, the optical system that causes chromatic aberration is made of glass that has a large chromatic dispersion effect so that the scanning beam is greatly DefOcused on the scanning surface due to a slight wavelength change.
Furthermore, when the imaging lens 16 has chromatic aberration, the imaging lens is designed so that off-axis imaging characteristics do not deteriorate due to wavelength changes. When the scanning spot diameter on the scanning plane changes due to a change in wavelength, the central illuminance of the spot changes, but
The range of this change is set so that it occurs within a sufficiently saturated region of the recording or photosensitive characteristic curve of the photoreceptor.

Next, a tunable laser will be described as the wavelength-tunable light source section.

For example, by using a dye laser as a tunable laser, the wavelength range of the scanning beam can be changed considerably. FIG. 6 is a schematic diagram showing a tunable dye laser.

The dye cell 20 excited from the outside undergoes a transition depending on the energy level of the dye, and laser oscillation is performed by a resonator made of a diffraction grating 21 and a half mirror 22, which is selected by the inclination angle of the diffraction grating 21. The wavelength can be changed. If the displacement of this diffraction grating 21 is changed according to the optical information to be obtained, signal modulation (
FM modulation). By using a reflection type acousto-optic modulation element instead of this diffraction grating 21 and changing the wavelength pitch of the ultrasonic waves applied to the element, the pitch of the acousto-optic modulation element as a diffraction grating can be changed, Wavelength selection becomes possible. It is desirable to use a dye that has as many energy levels as possible near the wavelength used. Further, as a method for changing the wavelength, methods using optical parametric oscillation, a spin-flip type or polariton type using a Raman laser, a well-known tunable laser such as a semiconductor laser, a high-pressure gas laser, etc. are also possible.

As described above, the scanning optical system having an optical system for forming a halftone image according to the present invention uses a light source section that can freely change the wavelength of the oscillated scanning beam and a scanning optical system that has chromatic aberration to focus light onto a scanning surface. This method changes the cross-sectional area of the spot of the scanning beam, and it is possible to obtain stable halftone images at high speed.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an embodiment of a scanning optical system having a conventional halftone image forming optical system, and FIGS. 5 is a perspective view showing an embodiment of a scanning optical system having a halftone image forming optical system according to the present invention, and FIG. 6 is a tunable laser using a dye. Diagram for explaining. 11... Light source section, 12... Drive system, 1
3...Filter 1, 14...Beam expander 16...Imaging lens, 17...
...Scanning surface.

Claims (1)

[Claims] 1. In a scanning optical system comprising a light source section, a scanning means for scanning a scanning beam from the light source section, and a scanning surface scanned by the scanning beam from the scanning means, the light source section scans a scanning beam according to a signal input to the light source section. and a halftone image forming optical system that emits a scanning beam having a wavelength of 100 nm and changes the spot diameter of the scanning beam on the scanning surface based on the chromatic aberration characteristics of an optical system provided between the light source section and the scanning surface. A scanning optical system featuring: