Field of the Invention
The present invention relates to an electrophotographic image forming apparatus.
Description of the Related Art
Conventionally, image forming apparatuses are known which form electrostatic latent images on a photosensitive member by deflecting a light beam emitted from a light source using a rotating polygonal mirror and scanning the photosensitive member using the deflected light beam. This kind of image forming apparatus includes an optical sensor for detecting the light beam deflected by the rotating polygonal mirror (beam detection (BD) sensor), and the optical sensor generates a synchronization signal upon detecting the light beam. By causing the light beam to be emitted from the light source at a time that is determined using the synchronization signal generated by the optical sensor as a reference, the image forming apparatus keeps constant the writing start position for the electrostatic latent image (image) in the direction in which the light beam scans the photosensitive member (main scanning direction).
Also, image forming apparatuses are known which include multiple light emitting elements as light sources for emitting light beams that each scan different lines on the photosensitive member in parallel in order to realize a higher image formation speed and higher resolution images. With this kind of multi-beam image forming apparatus, a higher image formation speed is realized by scanning multiple lines in parallel using multiple light beams, and higher resolution images are realized by adjusting the interval between the lines in the sub-scanning direction.
Japanese Patent Laid-Open No. 2008-89695 discloses an image forming apparatus that includes multiple light emitting elements as a light source and is capable of adjusting the resolution in the sub-scanning direction by performing rotational adjustment of the light source in the plane in which the light emitting elements are arranged. This kind of resolution adjustment is performed in the step of assembling the image forming apparatus. The patent literature above discloses a technique for suppressing shifts in the writing start positions in the main scanning direction for the electrostatic latent image that occur due to light source attachment errors in the assembly step. Specifically, the image forming apparatus uses a BD sensor to detect light beams emitted from a first light emitting element and a second light emitting element and generates multiple BD signals. Furthermore, the image forming apparatus sets a light beam emission time for the second light emitting element relative to the light beam emission time for the first light emitting element based on the difference in the generation times of the generated BD signals. This compensates for the light source attachment error in the assembly step and suppresses shifts in the writing start positions for the electrostatic latent images between the light emitting elements.
However, with the optical scanning apparatus (image forming apparatus) including multiple light emitting elements as a light source, the following problem is present in the method for measuring the difference between the generation times of the BD signals generated by the BD sensor as described above.
During the execution of image formation, the overall temperature of the optical scanning apparatus rises due to heat generated from the polygon motor rotating the rotating polygonal mirror (polygon mirror) that deflects the beams, and the optical characteristics of the optical system, such as the refractive index of the lens, change. According to this, a shift occurs in the imaging positions on the photosensitive member of the light beams deflected by the polygon mirror, and therefore the result of measuring the difference in the generation times of the BD signals also changes. As a result, it may become impossible to align the writing start positions, in the main scanning direction, of the electrostatic latent images formed by the light beams emitted from the light emitting elements. Accordingly, in order to align the writing start positions, in the main scanning direction, of the electrostatic latent images formed by the light beams, it is necessary to control the times at which the light beams are emitted from the light emitting elements so as to follow the change in the temperature in the optical scanning apparatus while image formation is being executed.