Electrophotography – Control of electrophotography process – Of plural processes
Reexamination Certificate
1999-10-25
2001-05-08
Beatty, Robert (Department: 2852)
Electrophotography
Control of electrophotography process
Of plural processes
Reexamination Certificate
active
06229969
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image forming apparatus and image forming method of generating a desired image formation pattern by converting an input image signal.
2. Description of the Related Art
Generally, an electrophotographic system, particularly a color copying machine using color toners of a plurality of colors has a look-up table for converting an image signal into a signal value meeting the characteristics of the engine used, in order to obtain desired density gradation characteristics. A color copying machine has such look-up table for each of yellow, magenta, cyan, and black and can output a desired full-color image by individually optimizing these colors.
An electrophotographic image forming apparatus, however, readily changes its characteristics in accordance with, e.g., the surrounding environment and the use state, so it is difficult to constantly output images stable in the tone of color by using fixed look-up tables. Accordingly, conventional apparatuses of this sort include a development density detecting means for detecting the density of a developed image formed on a photosensitive drum or the like. On the basis of information of the density detected by this development density detecting means, look-up tables are newly formed or corrected so that desired gradation characteristics can be obtained.
A conventional image forming apparatus having this development density detecting means will be described below.
FIG. 9
shows the overall arrangement of a conventional electrophotographic digital copying machine.
In this conventional digital copying machine shown in
FIG. 9
, a CCD
1
reads an image of an original
21
, and an amplifier
2
amplifies the obtained analog image signal to a predetermined level. After that, an analog/digital converter (A/D converter)
3
converts the amplified signal into a digital image signal of, e.g., 8 bits (0 to 255 gray levels).
This digital image signal is supplied to a &ggr; converter (e.g., a converter which is constructed of 256-byte data and converts density in a look-up table manner)
5
where the signal is &ggr;-corrected. This digital image signal is again converted into an analog image signal and supplied to one input terminal of a comparator
11
.
A triangular-wave signal of predetermined period generated by a triangular-wave generator
10
is supplied to the other input terminal of the comparator
11
. The analog image signal supplied to one of the input terminals of the comparator
11
is compared with the triangular-wave signal of predetermined period generated by the triangular-wave generator
10
, and is modulated in pulse width. This pulse-wave-modulated binary image signal is input to a laser driver
12
and used as an emission ON/OFF control signal of a laser diode
13
.
A laser beam emitted from the laser diode
13
is scanned in a main scan direction by a well-known polygonal mirror
14
and irradiates a photosensitive drum
17
, which is an image carrier rotating in an arrow direction, via an f&thgr; lens
15
and a reflecting mirror
16
, forming an electrostatic latent image.
This photosensitive drum
17
is evenly charged-removed by an exposing unit
18
and evenly charged, e.g., negatively, by a primary charger
19
. After that, the photosensitive drum
17
is irradiated with the aforementioned laser beam to form an electrostatic latent image corresponding to the image signal. A developing unit
20
develops this electrostatic latent image into a visual image (toner image).
During the development, an AC bias component is superposed on the developing unit to improve the DC bias component and development efficiency meeting the electrostatic latent image formation conditions. This toner image is transferred, by the action of a transfer charger
22
, on to a transfer medium
23
held on a transfer medium carrier belt
27
that stretches between two rollers
25
and
26
and is endlessly driven in an arrow direction.
The residual toner on the photosensitive drum
17
is scraped off and collected by a cleaner
24
later. For the sake of simplicity,
FIG. 9
shows only a single image forming station (including the photosensitive drum
17
, the exposing unit
18
, the primary charger
19
, the developing unit
20
, and the like). In the case of a color image forming apparatus, however, image forming stations corresponding to yellow, magenta, cyan, and black are arranged above the transfer medium carrier belt
27
along its moving direction. Alternatively, developing units of yellow, magenta, cyan, and black are placed in a rotatable housing, and a desired developing unit is opposed to the photosensitive drum
17
to develop a desired color.
When the development by the developing unit
20
progresses, the toner amount in the developing unit
20
reduces, so no desired density can be secured any longer. Therefore, a video counter
4
is used to extract by analogy the use amount of toner from the image formation pattern.
A CPU
6
stores the toner use amount extracted by the video counter
4
into a RAM
6
a
. In the case of a color image forming apparatus, the CPU
6
adds up the use amounts of toner of each color. When a predetermined use amount is reached, the CPU
6
activates a toner supply driver
7
and rotates a toner supply motor
28
to drive a toner supply mechanism
30
, thereby supplying toner
29
in a toner cartridge
8
into a predetermined developing unit
20
. Reference numeral
21
denotes toner supplied into the developing unit.
Not only for copying machines, several halftoning methods (image processing patterns) of gradation reproduction methods can be used. So-called multi-valued reproduction reproduces gradation in units of dots by using a triangular wave described previously. So-called binary reproduction reproduces gradation by forming a plurality of dot matrices at two values: a dot is formed and a dot is not formed. In another method, a matrix is formed by multiple values. An optimum one is chosen from these methods in accordance with the type (e.g., a printed original, a character original, or an image signal from a controller) of input image.
In any method, desired gradation characteristics can be secured only when the toner supply amount in the developing unit
20
is controlled to a desired amount. To control this toner supply amount, the conventional approach is to detect the density of a patch-like toner image (to be referred to as a “gradation control patch” hereinafter) obtained by developing an electrostatic latent image formed by a gradation control image signal. That is, this gradation control patch is irradiated with light from a light source such as an LED, the reflected light is received by a built-in photoelectric element, and the output value is converted into density. In accordance with information of the detected density signal, look-up tables are newly formed or corrected. In this manner, the toner supply amount in the developing unit
20
is controlled to maintain desired gradation characteristics.
Some conventional apparatuses, however, obtain gradation characteristics by using various image processing patterns for one printer engine. If this is the case, it is necessary to form gradation control patches for the individual image processing patterns, measure the respective density characteristics, and form look-up tables and the like on the basis of the measurement results. This is very time-consuming and inconvenient.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above situation, and has as its object to provide an apparatus capable of predicting density information of one image processing pattern from density information of a gradation control patch of another image processing pattern, thereby reducing the control time by one-half compared with gradation control using two different image processing patterns.
It is another object of the present invention to provide an apparatus capable of predicting density information of one image processing pattern from density info
Beatty Robert
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
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