Image forming apparatus

Abstract

An input dither pattern determiner divides an input dither image to plural blocks, and determines an input dither pattern for each block. An output dither image portion converts the input dither pattern for each block to an output dither pattern. A reference emission time determiner portion determines a reference emission time for each block based on dark electric potential distribution and intermediate sensitivity distribution of a photoconductive drum, a light intensity distribution of a laser beam in a main scanning direction, and the output dither pattern. An area emission time calculator determines area emission time for each area based on a reference emission time for each block. An actual emission time calculator adjusts emission time for each area so that differences between reference emission times of blocks adjacent one another in the main scanning direction become a predetermined value or lower, and calculates actual emission time for each area.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side schematic diagram mainly showing a mechanical construction of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a block diagram showing an electric construction of the image forming apparatus shown in FIG. 1.

FIG. 3 is a graph showing one example of a dark electric potential distribution stored in a dark electric potential distribution storing portion, an intermediate sensitivity distribution stored in an intermediate sensitivity distribution storing portion, and a light intensity distribution in a main scanning direction of a laser beam stored in a light intensity distribution storing portion.

FIGS. 4A to 4C are diagrams respectively showing an output dither pattern exposed by the image forming apparatus having features shown in FIG. 3. FIG. 4A shows an emission time in the area A in FIG. 3. FIG. 4B shows an emission time in the area B in FIG. 3. FIG. 4C shows an emission time in the area C in FIG. 4.

FIG. 5 is a flowchart showing one example of an operation of the image forming apparatus according to the embodiment.

FIGS. 6A, 6B are diagrams showing a relation between each of blocks constructing the output dither image and respective exposing positions. FIG. 6A is a diagram showing blocks set for the output dither image, and FIG. 6B is a diagram showing a photoconductive drum.

FIG. 7 shows tables showing an example of a reference emission time determined by a reference emission time determining portion.

FIG. 8 is a diagram for describing a processing of calculating an area emission time performed by an area emission time calculating portion.

FIG. 9 is a flowchart showing detailed processing of Step S12 shown in FIG. 5.

FIG. 10 is a diagram for describing processings performed in steps S23 through S25 shown in FIG. 9.

FIGS. 11A to 11C are diagrams for describing a merit in the case whereof calculating actual emission times consecutively from the largest area emission time area toward an area positioned at an end. FIG. 11A is a diagram showing a case where actual emission times are not calculated consecutively from the largest area emission time area. FIG. 11B is a diagram showing the case where actual emission times are calculated consecutively from the largest area emission time area at a left end. FIG. 11C is a diagram showing the case where actual emission times are calculated consecutively from the largest emission time area in a vicinity of a central portion.

FIG. 12 is a diagram for describing a technique disclosed in the U.S. Pat. No. 5,134,495.

FIG. 13 is a diagram for describing a condition where a pixel is interpolated as a result of emission of a laser beam in accordance with the output dither pattern.

Claims

1. An image forming apparatus for forming on a recording sheet an original image having a higher resolution than a reproducible resolution by pseudo high resolution technique, the image forming apparatus comprising: an input dither image producing portion for producing an input dither image by converting an original image to a dither image;an input dither pattern determining portion for dividing the input dither image to a plurality of blocks, and determining which one of advancedly-prepared input dither patterns corresponds to a dither pattern of each block;an output dither image producing portion for producing an output dither image by converting the input dither pattern determined for each block in accordance with an advancedly-prepared output dither pattern for each input dither pattern to simulatedly form on a recording sheet the original image at a resolution of the original image;a reference emission time determining portion for determining a reference emission time indicating a laser beam emission time for each of pixels forming each block to correct, based on a dark electric potential distribution and an intermediate sensitivity distribution of an image bearing member, a light intensity distribution of a laser beam in a main scanning direction, and the output dither patterns, scatterings in the dark electric potential, the intermediate sensitivity, and the light intensity, and simulatedly form the output dither image at the resolution of the original image; andan actual emission time calculating portion for calculating an actual emission time indicating the emission time for each block that is adjusted such that a difference between the respective reference emission times of the blocks adjacent to each other in the main scanning direction becomes a predetermined value or lower.

2. An image forming apparatus according to claim 1, further comprising: a dark electric potential distribution storing portion for storing in advance a dark potential distribution of the image bearing member;an intermediate sensitivity distribution storing portion for storing in advance an intermediate sensitivity distribution of the image bearing member;a light intensity distribution storing portion for storing in advance a light intensity distribution of a laser beam to the image bearing member; anda reference emission time table storing portion for storing in advance a table showing a relation between the dark electric potential, the intermediate sensitivity, the light intensity and the output dither pattern, and the reference emission time, whereinthe reference emission time determining portion determines a dark electric potential of the image bearing member for each block by referring to the dark potential distribution storing portion, an intermediate sensitivity of the image bearing member for each block by referring to the intermediate sensitivity distribution storing portion, and a light intensity of the laser beam for each block by referring to the light intensity distribution storing portion, and reads out from the reference emission time table storing portion the reference emission time for each block corresponding to the determined dark electric potential, intermediate sensitivity, light intensity and output dither pattern.

3. An image forming apparatus according to claim 2, wherein: the dark potential distribution storing portion stores a dark electric potential measured in advance at a plurality of sample points set on the image bearing member;the intermediate sensitivity distribution storing portion stores an intermediate sensitivity measured in advance at the plurality of sample points;the light intensity distribution storing portion stores a light intensity of the laser beam measured in advance at the plurality of sample points; andthe reference emission time determining portion determines the exposing position on the image bearing member that corresponds to a coordinate of an exposing pixel of the output dither image, and determines a sample point positioned minimally spaced apart from a center of each block of the output dither image among the plurality of sample points on the image bearing member, and determines a dark electric potential, an intermediate sensitivity and a light intensity of the laser beam to the specified sample point by referring to the dark electric potential distribution storing portion, the intermediate sensitivity distribution storing portion and the light intensity distribution storing portion.

4. An image forming apparatus according to claim 1, further comprising an area emission time calculating portion for dividing the output dither image to a plurality of areas in the main scanning direction, and calculating an area emission time for each area based on a reference emission time determined for blocks falling in each area, wherein the actual emission time calculating portion calculates the actual emission time for each area that is adjusted such that a difference between the respective emission times of the areas adjacent to each other in the main scanning direction becomes a predetermined value or lower.

5. An image forming apparatus according to claim 4, wherein the area emission time calculating portion calculates an average value of the respective reference emission times determined for the blocks of an area falling in a specified one of the plurality of areas as an area emission time of the specific one area.

6. An image forming apparatus according to claim 4, wherein the actual emission time calculating portion calculates the actual emission time for each area such that an actual emission time of the particular area subjected to a calculation of the actual emission time has an actual emission time falling within a range which is one-tenth of the actual emission time of adjacent area higher or lower than the actual emission time of adjacent area.

7. An image forming apparatus according to claim 4, wherein the actual emission time calculating portion calculates, in the case where an area emission time of a particular area subjected to calculation of an actual emission time does not fall within a predetermined range with respect to an actual emission time of an adjacent area, the actual emission time of the particular area so as to fall in the range.

8. An image forming apparatus according to claim 7, wherein the actual emission time calculating portion determines an area having the largest area emission time of each block line consisting of blocks aligned in the main scanning direction, calculates an actual emission time consecutively from the determined area toward an area positioned at an end of the block line, and calculates a lower limit of a predetermined range as an actual emission time of the particular area in the case where the area emission time of the particular area does not fall within the predetermined range with respect to an actual emission time of an adjacent area.