The present invention relates to optical scanner for an image recording equipment.
An optical scanner, incorporated in a laser printer, digital copier or similar image recording apparatus, has a light source device, a deflecting device, and image forming optics. The deflecting device has a deflecting surface for reflecting and deflecting a light beam issued from the light source device. The light beam reflected by the deflecting device is focused by the optics onto a target surface to form a beam spot thereon. Assume that the optical arrangement extending from the light source device to the target surface is linearly developed along the optical axis, and let the resulted optical path be referred to as a developed optical path. Then, the start point and the end point of the developed optical path are the light source device and the target surface, respectively. The developed optical path is, therefore, perpendicular to the main scanning and subscanning directions at the end point thereof. The directions parallel to the main scanning and subscanning directions at any point on the developed optical path will hereinafter be referred to as a direction corresponding to main scanning and a direction corresponding to subscanning, respectively.
The deflecting device is implemented with a rotary polygonal mirror, galvano mirror, pyramidal mirror or similar mirror having a reflective surface. It is not practicable to hold the axis of rotation of such a deflecting mirror ideally in one direction, i.e., the axis of rotation slightly changes in direction. The change in the direction of the axis of rotation directly translates into a change in the deflected light beam in the direction corresponding to subscanning and, therefore, a change in the position of the beam spot on the target surface in the same direction. Regarding the polygonal mirror, for example, each of a plurality of reflective surfaces is not accurately parallel to the axis of rotation, also resulting in the change in the position of the beam spot in the above-mentioned direction. To eliminate this problem ascribable to the configuration of the deflecting device, image forming optics interposed between the deflecting device and the target surface may include an elongate anamorphic lens, as proposed in the past. The elongate anamorphic lens is situated in the vicinity of the target surface. Alternatively, the optics itself may be constituted by an anamorphic f-theta lens, as also proposed in the past.
Today, there is an increasing demand for high-quality image recording using optical scanning. Also, attempts have been made to render tones of an image in a dot pattern by changing the size of a dot which constitutes a pixel. To implement high-quality image recording or tones, it is preferable that the beam spot on the target surface be provided with a smooth optical intensity distribution analogous to a Gaussian distribution.
It is a common practice with an optical scanner to locate an aperture member between the light source device and the deflecting device for the purpose of regulating the beam spot to a predetermined diameter. The aperture member intercepts part of the parallel light beam issuing from the light source device and has an aperture which is usually as small as 0.3-0.6 millimeter in width. Such a small aperture brings about a problem that the aperture member intercepts, in the direction corresponding to subscanning, even the part of the light beam surrounding the center and having substantial optical intensity and thereby diffracts the light at the edges thereof. The diffracted light noticeably complicates the optical intensity distribution of the beam spot on the target surface. Especially, when use is made of an elongate anamorphic lens, the influence of the diffraction is prominent and, therefore, the intensity distribution of beam spot is extremely complicated, causing the beam spot to change in diameter and configuration with the image height. This obstructs high-density image recording and tone rendering. When the elongate anamorphic lens is replaced with anamorphic optics, or an anamorphic image forming lens system, the influence of diffraction is not noticeable so long as the curvature of the image surface of the lens system is small. However, when the curvature of the image surface is substantial, an image height area in which the beam spot changes in diameter and configuration appears due to the influence of diffraction.