JPS6156655B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses a rotating polygonal mirror as a light deflector to irradiate a light beam such as a laser beam onto a photoreceptor while scanning and deflecting it, thereby allowing characters, numbers,
The present invention relates to an electrophotographic image recording device that records images such as symbols.

In laser printers, rotating polygon mirrors are widely used as optical deflectors. The inclination angle precision (inclination angle precision) of each surface of the rotating polygon mirror with respect to the optical scanning surface greatly affects the recorded image. FIG. 1a shows the arrangement of light beam spots when there is no error in the angle of inclination of the rotating polygon mirror, and FIG. 1b shows the arrangement of the light beam spots when the error in the angle of inclination of the rotating polygon mirror is large. In case b, an unirradiated area shown by hatching occurs between the scanning beams. In positive recording in which the light-irradiated areas are developed white, black streaks occur in the non-irradiated areas, resulting in an image with a lot of background stains. In negative recording in which the light-exposed areas are developed black, white streaks occur in the unexposed areas, resulting in white spots in the image.
If the tilt angle error of the rotating polygon mirror is large as described above, image deterioration occurs. When the scanning pitch error is Δp and the focal length of the imaging lens is f, the tilt angle error β of the polygon mirror is expressed by the following equation.

β≦±Δp/f ₂ (1) In a normal laser printer, this inclination angle error is within ±5 seconds to ±10 seconds, which requires very strict processing accuracy. Therefore, the cost of the rotating polygon mirror has become extremely high.

The present invention corrects the inclination angle error of the polygon mirror to prevent image deterioration, thereby making it possible to use a low-cost rotating polygon mirror. Hereinafter, the present invention will be explained based on illustrated embodiments. FIG. 2 is a schematic diagram of the main parts of an embodiment of the present invention. In the figure, a photoreceptor 1 is charged by a corona charger 2, and then a light beam A
An electrostatic latent image is created by illuminating the image.
This electrostatic latent image is visualized by a developing device 9 and transferred onto a recording paper 10 by a transfer charger 11.

The image signal is converted into a light beam by a laser light source 3, and the irradiated light beam is deflected and scanned on the surface of the photoreceptor 1 by a rotating polygon mirror 4. Reference numeral 5 denotes an imaging lens for forming an image of a light beam on the surface of the photoreceptor 1. 6 is a motor that drives the rotating polygon mirror 4. A rotational position detector 7 is connected to the shaft of this motor 6, and a rotational position detector 7 is configured to detect the polygonal mirror surface currently being scanned.
Note that a Hall element or a photodetector is used as the rotation sensor 8. 14 is a mirror surface detection circuit that detects each mirror surface number of the rotating polygon mirror 4. The scanning start position on each mirror surface can be easily determined by a synchronization detector (not shown) provided at the laser scanning start position. Reference numeral 15 denotes a light beam diameter switching means corresponding to each mirror surface number, which selects the light beam diameter in accordance with the angle of inclination of the rotating polygon mirror 4. Here, the angle of inclination of each mirror surface of the polygon mirror is measured in advance. 16 is a laser drive circuit, and when a semiconductor laser is used as 3, a method of changing the drive current is used as a method of switching the light beam diameter. Note that 12 in FIG. 2 is a cleaning device for the photoreceptor 1, and 13 is an optical static elimination device.

FIG. 3a is a diagram showing the relationship between the driving current of the semiconductor laser and the optical power. Standard drive current A2
When the current is decreased A1 or increased A3, the optical power becomes B1 or B3. Since laser light has a Gaussian energy density distribution, the optical power density distribution when the driving current is decreased or increased with respect to the standard output optical power density distribution G2 is as shown in Figure 3b. G1, respectively
It becomes G3. From FIG. 3b, if the exposure amount required for recording on the photoreceptor is _ET , then the diameter of the recording spot on the photoreceptor will change depending on the drive current. Here, the recording spot diameter is d, and the laser light output is P _O
Then, _d =a√( _OT )...(2). Here, a and b are constants determined by recording conditions.

FIG. 4 shows a correction state when the light beam diameter is changed in response to the tilt angle error of the rotating polygon mirror 4 (in the figure, the beam spot diameter is increased). Fourth
It can be seen from the figure that the background stains in positive recording or the white streaks in negative recording have disappeared.

FIG. 5 shows the case of positive recording in which the area irradiated with the light beam is developed white. In the figure, the areas not irradiated by the light beam are recorded as black lines. When the diameter of the light beam is increased to accommodate the tilt angle error of the rotating polygon mirror, the horizontal lines parallel to the main scanning direction become slightly smaller at adjacent points of the enlarged light beam, and the vertical lines perpendicular to the main scanning direction become smaller. The line becomes thinner at the points where the light beam becomes larger. When horizontal lines become thinner, they are hardly noticeable in the image, but when vertical lines become larger or smaller, they may be noticeable. As a countermeasure against this problem, an effective method is to use a vertically elongated light beam spot. In the case of semiconductor lasers, the divergence angle of the light beam is different in the direction perpendicular to the bonding surface and in the horizontal direction, so this property can be used to easily create a long light beam spot in the direction perpendicular to the main scanning direction. .

Another measure is shown in Figure 6. As shown in the figure, in positive recording, when adjacent spots in the main scanning direction are black and the point to be recorded is white, the light beam diameter should be returned to the standard state. In this case, a small black beard of one spot is generated in order to return the light beam diameter to the standard state, but this is
With a spot diameter of about 100 μm, it is almost invisible. Determining whether or not adjacent spots in the main scanning direction are black can be easily realized using a 3-bit shift register and a "1" or "0" determination circuit.

As described above, according to the present invention, the tilt angle error of the rotating polygon mirror can be easily corrected by changing the light beam diameter, and deterioration of recorded images can be effectively prevented. Furthermore, the present invention has a number of excellent features in that it is possible to use a low-cost rotating polygon mirror because the inclination angle accuracy of the rotating polygon mirror can be made loose.

[Brief explanation of the drawing]

FIGS. 1a and 1b are arrangement diagrams of light beam spots for explaining image deterioration caused by tilt angle errors of a rotating polygon mirror, and FIG. 2 is a schematic diagram of the main parts of an image recording device according to an embodiment of the present invention. The configuration diagram, FIG. 3a, is a diagram showing the relationship between the drive current and optical power of the semiconductor laser,
Figure 3b is an optical power density distribution diagram, Figure 4 is an array diagram of optical beam spots showing the state of image deterioration correction when changing the optical beam diameter, and Figures 5 and 6 are for positive recording. FIG. 2 is an array diagram of light beam spots showing the state of image correction based on the light beam diameter of FIG. 1... Photoreceptor, 2... Corona charger, 3... Laser light source, 4... Rotating polygon mirror, 6... Motor,
7... Rotation position detector, 8... Rotation sensor, 9
...Developer, 10...Recording paper, 11...Transfer charger, 14...Specular surface detection circuit, 15...Light beam diameter switching means, 16...Laser drive circuit.

Claims (1)

[Scope of Claims] 1. An electrophotographic image recording device that records an image by scanning and deflecting a light beam onto a photoreceptor using a rotating polygon mirror, wherein each A mirror surface detection means for detecting a mirror surface and a light beam diameter variable means for changing a light beam diameter are provided, and the light beam diameter is changed from the normal size according to the angle of inclination of each mirror surface of the polygon mirror. An image recording device characterized by: 2 In claim 1, when performing positive recording in which the area irradiated with light is developed white,
Image recording characterized in that when the spot to be recorded is white and at least one of the left and right spots in the main scanning direction of the spot is black, the diameter of the light beam is not changed and is set to the normal size. Device. 3. An image recording device according to claim 1, characterized in that a semiconductor laser is used as a light source of the light beam, and the diameter of the light beam is changed by changing the current that drives the semiconductor laser.