Laser Scan Correction in Image Forming Apparatuses for Uneven Density
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Solution Overview
Problem
Image forming apparatuses without a scanning lens having an fθ characteristic face issues with uneven density due to changes in image clock frequency, leading to responsiveness limitations and potential image defects.
Innovation Solution
An image forming apparatus that includes a photosensitive member, an exposing unit with a laser light that changes scanning speed, a generating unit for clock signal control, first and second correcting units for frequency and image data correction, and a modulating unit to correct image data with a correction coefficient, incorporating inflection points near the end portion in the main scanning direction to maintain consistent pixel width and light amount.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a scanning lens with fθ characteristic is used, then the laser spot moves at constant speed and appropriate exposure is achieved, but the apparatus size increases
Solution Approach 1:
The patent removes the scanning lens with fθ characteristic from the optical system, extracting the problematic component that causes large apparatus size while maintaining the essential function of laser scanning through alternative means (polygon mirror alone with corrected drive signals)
Solution Approach 2:
The patent changes the drive parameters of the polygon mirror (rotation speed, acceleration, deceleration) to compensate for the absence of the fθ lens, adjusting the timing and speed of laser spot movement to maintain constant effective exposure across the scanning surface
2Manufacturing precision
If image clock frequency is changed in main scanning direction to correct pixel width, then pixel width becomes constant, but laser turn-off time becomes too short causing responsiveness issues and uneven density
Solution Approach 1:
The patent applies different correction strategies to different regions of the scan: in the central region where constant speed is maintained, normal clock frequency is used, while in the end regions where speed variation occurs, adjusted clock frequency and duty cycle corrections are applied locally to prevent both pixel width distortion and laser responsiveness issues
3Productivity
If higher clock frequency is used in higher speed apparatus, then scanning speed increases, but corrected pixel width becomes even smaller leading to image defects
Solution Approach 1:
The patent implements dynamic adjustment of the polygon mirror rotation speed and clock frequency throughout the scanning cycle, accelerating in the central region for high productivity while decelerating at the ends with appropriate corrections, creating a dynamic balance between speed and precision
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration suppresses uneven density in images by adjusting clock frequency and light exposure, ensuring consistent image quality without the need for an fθ scanning lens, thereby maintaining image integrity.
Implementation Method 1
The optical scanning unit emits a laser light based on image data
Implementation Method 2
by reflecting the laser light with a rotating polygon mirror
Implementation Method 3
The scanning lens is a lens having a so-called fθ characteristic. The fθ characteristic is an optical characteristic which causes the laser light to form an image on the surface of the photosensitive member so that the spot of the laser light on the surface of the photosensitive member is moved at a constant speed
Implementation Method 4
a latent image is formed on the photosensitive member
Data Source
AI summary
An image forming apparatus includes a photosensitive member, an exposing unit scanning the photosensitive member with a laser, a generating unit generating a clock signal, a first correcting unit correcting a frequency of the clock signal for each position in a main scanning direction, a developing unit, a driving unit, a second correcting unit correcting an image date by a correction coefficient depending on the position in the main scanning direction, and a modulating unit modulating the image date corrected by the second correcting unit into the driving signal. The correction coefficient increases as it goes toward an end from a center in the main scanning direction and includes an inflection point where an inclination, which is an increase amount of the correction coefficient to a unit increase amount of the position, is changed. The inflection point is positioned near the end in the main scanning direction.