JPS6017716A - Laser beam printer optical system - Google Patents

Abstract

PURPOSE:To change suitably an imaging magnification in a state that a constant picture quality is left constant, by changing a scanning luminous flux diameter by a beam expander, basing on a relation in which the product of a magnification of an imaging picture and the scanning luminous flux diameter becomes contact, following the magnification of the imaging picture. CONSTITUTION:An optical system consisting of a laser light source, a beam expander, a scanning mirror, and an (f).theta lens is provided with a means for changing a scanning luminous flux diameter by said beam expander, basing on a relation in which the product of a magnification of an imaging picture and the scanning luminous flux diameter becomes constant, following the magnification of the imaging picture. In order to hold a piclture quality in a constant state, a spot diameter rho and a magnification (m) are in a relation of a direct proportion, and when changing the magnification (m), a beam diameter phi is changed so as to be in inverse proportion to the magnification (m). When changing the imaging magnification, the beam expander 2 is rotated by 90 deg., and the beam is expanded, holding said relation by lenses 2c, 2d.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical system for a laser beam printer that can change printing magnification.

In recent years, laser beam printers (hereinafter referred to as LBPs) have been put into practical use, but so far there has been no model that allows the printing magnification to be changed. One problem with LBPs with variable printing magnification is the spot diameter of the laser beam on the recording medium. This spot diameter is determined according to the size of the pixel, but if you want to reduce the size of the image into a small image, it is necessary to reduce the spot diameter according to the pixel.

An object of the present invention is to provide a laser beam printer optical system that can obtain an optimal spot diameter according to the pixel size as the printing magnification is reduced or enlarged.
The gist of this is that in an optical system consisting of a laser light source, beam expander, scanning mirror, and f/theta lens, the product of the magnification of the printed image and the diameter of the scanning beam becomes constant as the magnification of the printed image increases. Based on the above, the present invention is characterized in that it has means for changing the diameter of the scanning light beam using the beam expander.

The present invention will be explained in detail based on illustrated embodiments.

In Figure 1, 1 is a laser light source, and this laser light! The laser beam emitted from the laser beam enters a scanning mirror 3 via a beam expander 2, and further passes through an f/θ lens 4 to be focused on a recording medium 5. The beam expander 2 has two pairs of lens systems, and can be used selectively by rotating, for example. That is, the lens system of the beam expander 2 is a combination of a concave lens and a convex lens (2a, 2
b), (2c, 2cl), or 2 different focal pressures N[
The laser beam is changed to an appropriate beam diameter by a combination of convex lenses.

Therefore, the laser beam emitted from the laser beam 1 has its beam diameter expanded by the beam expander 2, travels to the scanning mirror 3, is deflected and scanned by the scanning mirror 3, and is further directed onto the recording medium 5 by the f/θ lens 4. Form an image.

The scanning mirror 3 is actually a rotating polygo mirror or a vibrating galvano mirror as shown in the figure, and performs main scanning in a direction parallel to the paper surface, and for sub-scanning, the recording medium 5 is mechanically moved in a direction perpendicular to the paper surface. It will be.

Now, let's consider changing the enlargement ratio or sizing ratio of a printed image. To change the magnification while keeping the image quality constant,
The number and size of pixels must be kept constant. If the f/θ lens 4 has no aberration, the spot diameter is set as f-0.
If φ is the focal length of the lens 4, and φ is the beam diameter of the laser beam incident on the f/θ lens 4, and input is the wavelength, then the spot diameter ρ on the recording medium 5 is given by the following equation by diffraction.

p=4f (in/π)φ...(1) Spot diameter p and beam diameter φ are inversely proportional in equation (1),
If C is a constant, it is expressed as follows.

pφ=C...(2) On the other hand, as mentioned above, in order to keep the image quality constant, the spot diameter ρ and the magnification m are directly proportional, and if C' is a constant, then ρ/m=c' ...(3) becomes. From formulas (2) and (3), φm is constant as shown in (4). From formula (4), to change the magnification m, the beam diameter φ must be inversely proportional to the magnification m. You can see what you need to do to change it.

When changing the printing magnification, set the beam expander 2 to 9
The beam can be expanded by rotating by 0° and using the lenses 2c and 2d while maintaining the relationship of equation (4). It goes without saying that it is necessary to appropriately change the main scanning and sub-scanning speeds in conjunction with this conversion of printing magnification. In addition to changing the beam diameter by changing the beam diameter as described in Embodiment 7), it is of course also possible to convert the beam diameter by using sumlenth.

As explained above, the laser beam printer optical system according to the present invention can print images using extremely simple means. The printing magnification can be changed appropriately while keeping the printing magnification 11 constant.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an embodiment of a laser beam printer optical system according to the present invention. 1 is a laser light source, 2 is a beam expander, 3 is a scanning mirror, 4 is an fφθ lens, and 5 is a printed image. Patent applicant Canon Co., Ltd.

Claims (1)

[Claims] 1. Regarding the optical system consisting of a laser light source, a beam expander, a scanning mirror, and an f-(l lens, the magnification of the printed image and the diameter of the scanning light beam change depending on the magnification of the printed image. A laser beam printer optical system characterized by having means for changing the scanning beam diameter by the beam expander based on a relationship in which the product is constant. 2. The changing of the scanning beam diameter is based on a beam expander. The laser beam printer optical system according to claim 1, wherein the magnification changing lens is changed.