The present invention relates to a light beam scanning apparatus adapted for use in an image forming apparatus, such as a laser beam printer or a digital copying machine, wherein a beam emitted from a semiconductor laser is scanned across an image bearer to form a latent image on the image on the image bearer and wherein the latent image on the image bearer is printed on an image formation medium.
In an image forming apparatus that uses a laser beam for image formation, a laser beam (light output) emitted from a semiconductor laser is scanned across a photosensitive drum to form an electrostatic latent image thereon by employing a condenser lens, a rotatable polygonal mirror, a mirror, a correcting lens, etc. A synchronization signal, based on which a laser beam is scanned, is detected by a synchronization detecting circuit, which includes a light-receiving element such as a photodiode. A beam detected as the synchronization signal is reflected by a return mirror, which is not located in the optical path used when the scanning operation is performed with respect to the photosensitive drum. By that reflection, the beam is guided to the synchronization detecting circuit. Owing to this, the optical path of the beam used for the detection of a synchronization signal tends to be very complicated optically.
There is a demand for a small-sized image forming apparatus since the space it occupies should be as little as possible in offices. To meet this demand, the optical structure using a semiconductor laser tends to be further complicated. Under the circumstances, an image forming apparatus designed to execute image formation by use of a laser beam must employ a large number of optical elements arranged within a narrow space. In the optical system of the image forming apparatus, therefore, stray light, which is unnecessary reflected light, is inevitably produced.
If the stray light is incident on the synchronization detecting circuit, the synchronization for each scanning operation may be shifted. If the synchronization shift occurs, vertical lines may not be reproduced in an output image. To avoid this problem, factors that give rise to the stray light are removed by arranging optical components properly, determining a desirable position and angle of a reflecting surface, working the reflecting surface in such a manner that the reflected light becomes diffused light, or attaching a felt member or the like in such a manner as to decrease the reflectance. These measures, however, inevitably increase the number of manufacturing steps required, and the cost needed for the structural components.
Since the sensitivity of the photosensitive drum deteriorates with time, it may be necessary to gradually intensify the optical output of a semiconductor laser. In such a case, the intensity of stray light will increase in accordance with an increase in the intensity of an optical output of the semiconductor laser. If the stray light increases in intensity, it may give rise to a problem which does not occur in the initial state.
Furthermore, if the lens, mirror, glass or other members are stained, the optical efficiency may lower. If, in this case, the optical output intensity is constant, a reduced amount of optical energy will reach the synchronization detecting circuit. This may result in an undesirable operation of the synchronization detecting circuit and, in the worst case, failure to detect any synchronization signal.
The present invention has been conceived in an effort to solve the problem that the number of manufacturing steps or the cost needed for structural components is increased when measures are taken to cope with stray light in the optical system of an image forming apparatus. An object of the present invention is therefore to provide a light beam scanning apparatus which does not result in an increase in the number of manufacturing steps or the cost needed for structural components, and yet prevents adverse effects stray light may have on the optical system of an image forming apparatus.
Another object of the present invention is to provide a light beam scanning apparatus capable of preventing adverse effects stray light may have on the optical system of an image forming apparatus, even when the optical output of a semiconductor laser is gradually intensified to compensate for deterioration the sensitivity of the photosensitive drum may undergo with time.
A further object of the present invention is to provide a light beam scanning apparatus capable of preventing adverse effects stray light may have on the optical system of an image forming apparatus, even if the lens, mirror, glass or other members are stained and the optical efficiency becomes lower.
A light beam scanning apparatus according to the present invention comprises: a light emitter for emitting a light beam to an image bearer on which a latent image is to be formed, the light beam being emitted in accordance with the latent image to be formed on the image bearer; an optical system for guiding the light beam emitted by the light emitter to the image bearer and for scanning the light beam in the main scanning direction of the image bearer one line at a time on the basis of a synchronization signal; and a synchronization signal detector for detecting a synchronization signal, the synchronization signal detector including a photoelectric conversion element that detects the light beam scanned by the optical system in units of one cycle and converts the detected light beam into an analog signal, the synchronization signal detector digitizing the analog signal obtained by the photoelectric conversion element on the basis of a reference signal supplied from an external circuit, so as to obtain a digital signal that serves as the synchronization signal.
An image forming apparatus according to the present invention comprises: an image bearer on which a latent image is formed by irradiation of a light beam; a light beam scanning apparatus for radiating a light beam to the image bearer on the basis of image information; a developing unit for developing the latent image the light beam scanning apparatus forms on the image bearer; a transfer section for transferring the image developed by the developing unit onto an image formation medium; and a controller for controlling the light beam scanning apparatus to form the latent image, controlling the developing unit to develop the latent image, and controlling the transfer section to transfer the developed image onto the image formation medium, the light beam scanning apparatus including: a light emitter for emitting a light beam to the image bearer on which a latent image is to be formed, the light beam being emitted in accordance with the latent image to be formed on the image bearer; an optical system for guiding the light beam emitted by the light emitter to the image bearer and for scanning the light beam in the main scanning direction of the image bearer one line at a time on the basis of a synchronization signal; and a synchronization signal detector for detecting a synchronization signal, the synchronization signal detector including a photoelectric conversion element that detects the light beam scanned by the optical system in units of one cycle and converts the detected light beam into an analog signal, the synchronization signal detector digitizing the analog signal obtained by the photoelectric conversion element on the basis of a reference signal supplied from an external circuit, so as to obtain a digital signal that serves as the synchronization signal.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
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