JPS6219726B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an optical system for a laser beam printer, and more particularly, a rotating polygon mirror capable of scanning light at different speeds.

Conventionally, when trying to change the scanning speed in a laser beam printer using a rotating polygon mirror, the only option was to change the speed of the rotating polygon mirror. Speed control of a rotating polygon mirror has had problems in terms of cost, such as the need to use a variable speed motor and the need to use a drive circuit with speed control.

An object of the present invention is to eliminate this problem, and more specifically, to obtain different scanning speeds at the same speed by having multiple types of polygon mirrors.

Prior to explaining the present invention, a conventional plane polygon mirror will be briefly explained.

Figure 1 shows a plane polygon mirror. Modulated laser light is incident on the polygon mirror from a fixed position, and the direction of the light is changed by a reflecting surface whose angle changes with rotation, and the light scans the photosensitive drum. From the state shown as damage in Figure 1 to the state shown by the solid line, θ
The polygon mirror is rotating to the right. If it is a polygon mirror with n sides, θ=π/n. Reflective surface when damaged
The normal to A′C′ is indicated by OD perpendicular to the reflecting surface. Similarly, the normal to the solid line is indicated by OE, which is perpendicular to the plane AC. It is understood that the angle θ formed by the straight line OE and the straight line OB is the normal rotation angle, and is the same as the rotation angle of the polygon mirror. In the case of an n polygon mirror, θ in Figure 1 is the surface
In ABC, B is the midpoint of AC, so 2π/
It becomes 2n.

Figure 2 is a diagram of the normal rotation angle, but in relation to Figure 1, the triangle in Figure 1
Comparing OBA with the triangular OBA in Figure 2, the first
The normal OD in the figure corresponds to the side OA in Figure 2. Therefore, the line OP connecting OA in Fig. 2 and arbitrary points P and O on side AB
As can be seen from the explanation of FIG. 1, it can be understood that the angle formed by this is the rotation angle of the normal line and the rotation angle of the polygon mirror.

Next, embodiments of the present invention will be described using the drawings.
FIG. 3 shows a double rotating polygon mirror 5 having a flat surface and a concave surface. In FIG. 3, is an n-plane plane polygon mirror, and is an m-plane concave polygon mirror. In the example of FIG. 3, n=6 and m=12.

FIG. 4 shows a part of the double-rotating polygon mirror shown in FIG. 3, where the curve adc is the concave reflective part and the straight line ab is the flat reflective part. A straight line OQ ₁ having an angle of θ' from the straight line Oa indicates the normal line of the plane polygonal mirror portion A when the double rotating polygon mirror is rotated by θ'.
Therefore, θ' is also the normal rotation angle of the plane polygon mirror portion A at that time. Similarly, a straight line OQ ₂ having an angle of m/nθ' from the straight line Oa indicates the normal line of the plane polygonal mirror portion A when the double rotating polygon mirror is rotated by m/nθ'. Therefore, m/nθ' is also the normal rotation angle of the plane polygon mirror section A at that time.

If the normal rotation angle of the concave polygon mirror section when the double rotating polygon mirror 5 is rotated by θ' is equal to the normal rotation angle of the plane polygon mirror section when it is rotated by m/n θ', then the concave polygon mirror It can be said that part B has a scanning speed m/n times faster than that of plane polygon mirror part A.

This is because of the optical principle that the reflected light of light incident on a reflective surface from a certain direction is determined by the normal to that surface. In the case of a rotating polygon mirror, the reflected light is 2 times the angular velocity of the rotating polygon mirror.
It rotates at twice the angular velocity.

The normal rotation angle of the concave surface when the double rotating polygon mirror 5 rotates by θ' can be determined as follows.
In Fig. 4, the straight line OQ ₁ intersects at point d in the concave surface ac. Tangent line gd and straight line ab at point d of concave surface adc
When the angle formed by the concave surface is rotated by θ', the normal rotation angle of the concave portion becomes θ'+γ. This is because, first, when θ' is 0, that is, at the start of rotation, if the normal to the concave surface is at point a, then the tangent at point a is straight line ab, so this normal line coincides with the normal line to plane ab. ing. Next, when the double rotating polygon mirror 5 is rotated by θ', the normal line of the concave portion moves to point d. However, the normal rotation angle of plane ab is θ′,
This is because the tangent line gd is further inclined by γ with respect to θ', so the normal rotation angle of the concave surface portion is the sum of both, θ'+γ.

Next, if the concave part is formed so that the angle γ is θ'+γ=m/nθ', the normal rotation angle of the plane mirror part at the rotation angle m/nθ' becomes m/nθ', and the rotation of the angle θ' described above At this time, the normal rotation angle of the concave surface portion becomes θ'+γ, and the same normal rotation angle is obtained.

As mentioned above, in Fig. 4, let d be the intersection of the straight line OQ, which forms an angle between the straight lines Oa and θ', and the concave part ac, and let γ be the angle between the tangent to the concave part d and the straight line ab, then θ'+γ= The concave surface adc such that m/n θ' becomes a double rotating polymirror having a scanning speed m/n times that of the plane mirror section.

Next, an embodiment of a laser beam printer using the above-mentioned double rotating polygon mirror will be described with reference to FIG.
In FIG. 5, 5 is a double rotating polygon mirror, 8 and 9 are reflecting mirrors, 10 is a light emitter such as a semiconductor laser, and 11 is a photosensitive drum. The optical path when the reflector 9 is inserted is 10, 9, 8, 5-, 11, and the optical path when the reflector 9 is removed is 10, 5-, 1.
1, and images with different scanning densities can be formed on the photosensitive drum 11 depending on the presence or absence of the reflecting mirror 9. That is, when the reflecting mirror 9 is not included, one scanning line is formed on the photosensitive drum at m/n times the speed when the reflecting mirror 9 is included. Therefore, if the speed at which the semiconductor laser 10 is modulated is constant, the photosensitive drum 1 is
The ratio of the amount of information represented on one is m to n.
Therefore, when a polygon mirror is used, high-density recording is possible, and when a polygon mirror is used, high-speed recording is possible.

As described above, according to the present invention, a laser beam printer that can change the scanning speed of light without changing the rotational speed of the polygon mirror can be realized.

[Brief explanation of the drawing]

Fig. 1 is a diagram explaining the normal rotation angle of a planar rotating polygon mirror, Fig. 2 is a model diagram of the rotation angle, Fig. 3 is a diagram of a double rotating polygon mirror according to the present invention, and Fig. 4 is a diagram of a double rotating polygon mirror. FIG. 5 is a diagram showing the relationship between the polygonal mirror rotation angle and the normal rotation angle in a rotating polygon mirror, and FIG. 5 is a diagram showing an example of application to a laser beam printer. 5... Double rotating polygon mirror, 8... Reflecting mirror, 9... Reflecting mirror, 10... Semiconductor laser, 11... Photosensitive drum.

Claims (1)

[Claims] 1. A multi-rotation polygon mirror having a concentrically integrated planar polygon mirror having an n-plane flat reflecting surface and a concave polygon mirror having an m-plane (m>n) concave surface, wherein the multi-rotation polygon mirror is separated by θ. The curvature of the concave polygon mirror is set so that the normal rotation angle of the concave polygon mirror when rotated is θ, while the normal rotation angle of the concave polygon mirror is mθ/n. Features a multi-rotating polygon mirror.