Image reading apparatus and shading correction data...

Image analysis – Image enhancement or restoration – Intensity – brightness – contrast – or shading correction

Reexamination Certificate

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Details

C358S001900, C358S461000

Reexamination Certificate

active

06801670

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an image reading apparatus and shading correction data acquiring method and, more particularly, to an image reading apparatus for reading a shading correction plate (in general, a white reference plate having a reference density) which has a uniform reference density and extends in the main scan direction, by using a line sensor constituted by a plurality of photoelectric conversion elements one-dimensionally aligned in the main scan direction, creating and storing shading correction data on the basis of an output from each photoelectric conversion element, and shading-correcting, by using the shading correction data, data of an original image that is read by each photoelectric conversion element, a shading correction method applied to the image reading apparatus, and a storage medium which stores a program for executing the shading correction method.
BACKGROUND OF THE INVENTION
FIGS. 1 and 2
are a side sectional view and perspective view, respectively, showing the schematic arrangement of a general image reading apparatus.
An original cover holding an original
12
is set on a platen
10
. The original
12
is irradiated from its lower side with light emitted by a light source
15
. The light reflected by the original
12
enters a line sensor
20
via a lens
14
. The line sensor
20
has photoelectric conversion elements (CCDs) one-dimensionally aligned in the main scan direction. Each CCD converts the received light into an electrical signal, thereby obtaining electrical signals corresponding to an image (for one scan line) of the original
12
in the main scan direction.
After the original is read by one scan line, a motor
62
moves a reading unit
13
by one scan line in a direction (subscan direction) indicated by the arrow X in FIG.
2
. Then, an image of the next scan line is similarly read. This operation is repeated to read the entire original
12
.
An image signal read by the line sensor
20
is influenced by variations in the sensitivities of the CCDs, variations in dark current, and irregularities in light quantity from an optical system. As for x CCDs which constitute the line sensor
20
, the light quantity of the light source
15
and an output from each CCD have a relationship as shown in FIG.
3
A. That is, input/output characteristics are different between the CCDs. To eliminate these influences and make the input/output characteristics of all the x CCDs coincide with each other, as shown in
FIG. 3B
, CCD output correction (called “shading correction”) is done.
Shading correction is performed as follows.
The motor
62
moves the reading unit
13
to a position (to be referred to as a “home position” hereinafter) where a shading correction plate (reference white plate)
11
attached to the end of the platen
10
is read. At this position, an output from each CCD of the line sensor
20
when the light source
15
is turned on to irradiate the shading correction plate
11
is stored as correction data W(n). (n) means the nth CCD. Then, an output from each CCD of the line sensor
20
when the light source
15
is turned off (or light is shielded) is stored as correction data B(n).
When an image of the original
12
is read next time, these correction data are read out, and shading correction is performed for each CCD by
SD
(
n
)=
k
(
n
)·[(
S
(
n
)−
B
(
n
)]/[(
W
(
n
)−
B
(
n
)]
where S(n) is the output data from the nth CCD, B(n) is the correction data of the nth CCD when the light source
15
is OFF (to be referred to as OFF correction data of the nth CCD hereinafter), W(n) is the correction data of the nth CCD when the light source
15
is ON (to be referred to as ON correction data of the nth CCD hereinafter), k(n) is the coefficient of the nth CCD, and SD(n) is the shading-corrected data for the output data from the nth CCD.
FIG. 4
is a block diagram showing the peripheral arrangement of a shading correction circuit.
An A/D converter
32
converts an analog signal output from the nth CCD of the line sensor
20
into a digital value S(n) via an amplifier
31
. In a shading correction circuit
33
, a subtracter
34
subtracts OFF correction data B(n) of the nth CCD from the digital output value S(n) of the nth CCD, and a multiplier
35
multiplies the difference by shading correction data ShC(n) to obtain shading-corrected data SD(n). The shading correction data ShC(n) is given by k(n)/[W(n)−B(n)].
OFF correction data B(n), ON correction data W(n), shading correction data ShC(n), and the like change for each pixel of the CCD, so that correction in the shading correction circuit
33
is executed for each pixel of the line sensor.
In this manner, output variations between CCDs are corrected, and the original
12
is more faithfully read.
The conventional shading correction method, however, suffers the following problem.
Dirt or dust attached to the shading correction plate
11
makes ON correction data W(n) erroneous. At a portion where a defect such as dirt or dust exists on the shading correction plate
11
, this defect decreases the output level of the CCD. If shading correction is performed by using ON correction data W(n) obtained from the CCD corresponding to the defect portion of the shading correction plate
11
, an output from that CCD is excessively corrected, and a stripe is formed on the read image.
For example, assume that dirt
21
is attached to the shading correction plate
11
, as shown in FIG.
5
A. ON correction data W(n) obtained by reading the shading correction plate
11
decreases in level at a position &bgr; under the influence of the dirt
21
, as represented by a curve
501
. Shading correction is done by using this ON correction data W(n) so as to make corrected data SD(n) flat (ideal value) at the positions of all the CCDs, as shown in FIG.
5
B. Hence, shading correction data ShC(n) has an irregular portion &ggr;, as represented by a curve
502
in FIG.
5
A.
If a uniform-density original is read, each CCD outputs output data S(n) as represented by a curve
601
in FIG.
6
. The output data S(n) is shading-corrected by multiplying it by shading correction data ShC(n) represented by the curve
502
(the same data as the curve
502
in FIG.
5
A). The obtained shading-corrected data SD(n) has a projection &dgr;, as represented by a line
603
.
In other words, the irregular portion &dgr; is generated on the shading-corrected data SD(n) under the influence of the excessively corrected portion (projection &ggr; of the curve
502
of
FIG. 6
) of the shading correction data ShC(n) due to the dirt
21
. The irregular portion
6
appears as a stripe of the read image.
To solve this problem, there is provided a method of reading a shading correction plate before image reading to create shading correction data, reading the shading correction plate at another position after displacement in the subscan direction, performing shading correction by using the shading correction data, and detecting from this result the presence/absence of defects of the shading correction plate at the portion where the shading correction plate is first read. If a defect exists, the reading portion is displaced in the subscan direction to search for a nondefective portion and shading correction data is created there. If no nondefective portion is found, shading correction data is created at a portion having fewest defects.
However, the following problems occur in the conventional shading correction using the method of reading the shading correction plate at another position displaced in the subscan direction.
(1) The operation is performed every time before image reading, so image reading takes a long time.
(2) Strict quality management of reducing defects on the shading correction plate is required on the assumption that the shading correction plate has a position free from any dirt in the subscan direction.
(3) When no nondefective position is found in the subscan direction on the shading correction plate, shading correction data is created at a port

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