1. Field of the Invention
The present invention relates to a technique for improving lighting-up control for a laser.
2. Related Background Art
In a laser beam printer apparatus or the like for recording an image by an electrophotographic process, it is necessary to scan a photosensitive member with a laser beam to form an image. In particular, the stable rotating speed control for a rotary polygon mirror for deflecting a laser beam is important.
In general, in order to perform constant speed rotation control for a rotary polygon mirror driving motor, there is a method (PLL control method) for controlling a voltage applied to an IC for driving a rotary polygon mirror driving motor so that a phase difference between an oscillation frequency to become a reference, and an actually measured rotational frequency of the rotary polygon mirror driving motor between the magnetic poles, and the like); the fluctuation of the magnetic force of the magnet resulting from a change in temperature; and the like.
On the other hand, the other technique utilizes a sensor (BD sensor) provided inside an optical scanner for detecting a scanning timing for a light beam in order to control the writing position of an image. Since this BD sensor is not influenced by the fluctuation in magnetic force due to a magnet, the constant speed rotation control for the rotary polygon mirror driving motor can be carried out with high accuracy.
Such an apparatus for stably controlling a rotating speed is disclosed in Japanese Patent Application Laid-Open No. H09-183251.
However, in order to make a light beam incident to the BD sensor, it is necessary to forcibly light up a laser for emitting the light beam. Now, as for a timing at which the laser is forcibly lighted up in order to make the light beam incident to the BD sensor, the continuous light emission has to be started after a light beam used in scanning passes through an effective image area and before the light beam reaches a light receiving portion of the BD sensor. However, there is a problem that light reflected by a part of a writing optical system, refracted at a corner of an optical part, or reflected at a corner of a polygon mirror during forcible lighting-up, i.e., so-called flare light, reaches a photosensitive drum to write an unnecessary image. In addition, it is not preferable that a photosensitive member for image formation is needlessly exposed with a laser beam, and it is also not preferable that a laser is needlessly lighted up to shorten a laser lighting-up life. From a viewpoint of preventing these situations, it is desirable that a time period required to forcibly light up a laser to emit a light beam is as short as possible, and it is also desirable that a timing of starting to forcibly light up the laser to emit the light beam is as late as possible. Accordingly, various means for lighting up a laser in consideration of a timing of passing through a detection portion have been taken.
However, the laser is forcibly lighted up to emit the light beam every period during rotation of the polygon mirror, and this becomes a large factor for shortening a laser lighting-up life because a time zone for forcible lighting-up, if accumulated, always occupies about 5% of a time period during the rotation control as compared with the case of data of an image area to be drawn.
Thus, if the rotation of the polygon mirror is stopped during stand-by, then the lighting-up life can be saved to some degree. However, a reactivation time of the polygon mirror exerts an influence on delay of the next first print out time to delay the first print out time. In addition, if the beam detection timing is lost once, then it becomes necessary to expose an image area of a photosensitive member in order to find out the detection timing again. In particular, it is not preferable that such exposure is always carried out right before the start of the image formation. For these reasons, the means for stopping the rotation of the polygon mirror may not always be taken in some cases.