Image analysis – Color image processing – Color correction
Reexamination Certificate
1995-05-15
2003-04-08
Lee, Cheukfan (Department: 2622)
Image analysis
Color image processing
Color correction
C358S504000, C358S520000, C358S522000, C358S523000
Reexamination Certificate
active
06546131
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an image processing apparatus for reproducing half-tones, for example, a color-image processing apparatus having a color correcting function, and an image processing method implemented by these apparatus.
2. Description of the Prior Art
A copying apparatus equipped with a laser-beam printer is well known as an apparatus of the above-mentioned kind. The operation performed within the laser-beam printer involves pulse-width modulating an image signal by means of a reference signal (e.g., a triangular-wave signal) and reproducing tones by making the obtained pulse width correspond to laser emission time.
The use of full-color images has recently increased not only in the printing and design fields but also in the office, and this has been accompanied by the popularization of color copying apparatus which faithfully read and output color images. Here also it is well-known to employ the aforementioned pulse-width modulated laser-beam printer in order to faithfully reproduce color tones.
FIG. 1
is a simplified view showing the construction of an image reading section. Shown in
FIG. 1
are an original
1
, a platen
2
on which the original
1
is placed, a lamp
3
for illuminating the original
1
, an imaging element array
4
, an infrared cutting filter
5
, a contact-type CCD sensor (hereinafter referred to simply as a “CCD”)
6
, and an optical unit
7
which holds the lamp
3
, imaging element array
4
, filter
5
and CCD
6
.
The above-described arrangement is so adapted that when a copy key (not shown) is pressed, the lamp
3
illuminates the original
1
and light reflected from the original
1
is imaged on the CCD
6
through the imaging element array
4
and infrared cutting filter
5
.
The optical unit
7
is scanned in the direction of the arrow, and electrical signals of main and subordinate scans are produced by the CCD
6
and sent to a signal processing circuit shown in
FIGS. 2A and 2B
.
FIGS. 2A and 2B
are block diagrams illustrating the construction of an image forming apparatus. A signal from the CCD
6
is amplified by an amplifier
8
, and the amplified signal is converted into digital image data by an A/D converting circuit
9
. The resulting data is converted into image (density) data by a LOG converter circuit
29
, the data is then subjected to various types of image processing by an image processing circuit
30
, and the processed digital data is fed into a D/A converter
14
, where the digital signal is converted back into an analog signal. The analog signal is applied to a comparator
16
, where the signal is pulse-width modulated by being compared with a signal of a predetermined period generated by a triangular-wave generating circuit
15
. This pulse-width modulated binarized image signal is fed into a laser driving circuit
17
(
FIG. 2B
) in order to be used as a signal for on/off control of an emission from a laser diode
18
. The laser light emitted by the laser diode
18
is made to scan in the main scanning direction by a well-known polygon mirror
19
an f/&thgr; lens
20
and a reflecting mirror
21
so as to irradiate a photosensitive drum
22
, which serves as an image carrier rotating in the direction of the arrow, An electrostatic latent image is thus formed on the drum
22
.
After the photosensitive drum
22
is uniformly de-electrified by an exposing device
28
, it is uniformly electrified to a minus charge by a corona charging device
23
. Thereafter, the drum
22
is acted upon by the laser light so that an electrostatic latent image conforming to the image signal is formed on its surface. A so-called image scanning method, in which the portions (black pixels) that undergo development are exposed, is one type of a developing method often used by a laser-beam printer. By means of a well-known reverse developing method, a developing device
24
causes a toner having a negative electrification characteristic to adhere to the portion of the photosensitive drum
22
de-electrified by the laser, thereby forming an image.
The developed image (the toner image having a negative charge) formed on the photosensitive drum
22
by the above-described method is transferred to a transfer material (generally paper)
26
by a transfer corona charging device
25
. The visible toner image transferred to the transfer material
26
is then fixed by a fixing device (not shown). Residual toner left on the photosensitive drum is subsequently scraped off by a cleaner
27
, after which the process described above is repeated.
In the image forming apparatus described above, conventionally the LOG converter circuit
29
performs a digital conversion by a look-up table method. For example, two types of look-up tables are provided and a changeover is made between a character mode and a photograph mode. An inconvenience encountered in the above-described prior art is that the changeover of the LOG conversion tables for deciding output density must be performed manually by an operator, who makes the changeover upon looking at the original. In the case of an original that is a photograph, maximum density differs greatly depending upon whether the original is a silver chloride photograph or printed matter.
Even silver chloride photographs are of various types, such as photographs that are bright overall and photographs that are dark and have almost no high density. Selecting the mode in each of these instances is difficult.
Furthermore, in a case where the appropriate LOG conversion table has been selected, the following problem can arise:
When a bright photographic original having many portions of a reflective density of 1.7-2.0 or greater is processed by a table of LOG 1.7 (in which reproduction is possible from a density of 0.05 to 1.7), tone reproduction of the portions having a density greater than 1.7 is impossible. Conversely, when LOG 1.7 is selected in case of a faint original whose maximum density is on the order of 1.0, the maximum copy density falls below 1.0 and an image having an overall faint density results.
In a case where a copied image is used as an original to produce another copy which is then copied in the same manner, the foregoing problem becomes more pronounced with each generation of such copies and picture quality deteriorates markedly.
Though a variety of color-image forming apparatus are known, those in widest use employ a laser printer in which image exposure on a photosensitive body is performed by laser light and the image is developed to obtain a visible image. Such a laser printer exhibits the advantages of high picture quality and speed and is widely employed as the output unit of a color copier or as an ordinary printer.
A color-image forming apparatus will now be described with reference to
FIGS. 1
,
3
and
4
.
First, the process for reading a color original will be described. When a copy key (not shown) is pressed, the lamp
3
(
FIG. 1
) illuminates the original
1
and light reflected from the original
1
is imaged on the CCD
6
through the imaging element array
4
and infrared cutting filter
5
, thereby forming an image of the original on the CCD
6
. The optical unit
7
is made to scan the original
1
successively in the direction of the arrow. As shown in
FIG. 3
, filters for red (R), green (G) and blue (B) are correctly mounted on the CCD
6
for each and every pixel. As the original is scanned, electrical signals from the CCD
6
are processed by a signal processing circuit shown in FIG.
4
.
In
FIG. 4
,
6
B,
6
G,
6
R represent signals from each of the B, G, R elements on thee CCD
6
shown in FIG.
3
. The R, G, B signals are applied to an A/D converter circuit
41
and a LOG converter circuit
42
, whereby these signals are converted into signals Y
1
, M
1
, C
1
of digital form. The signals Y
1
, M
1
, C
1
are introduced to a circuit
43
, which performs black extraction and UCR (under-color rejection) to produce signals Y
2
, M
2
, C
2
, Bk
2
by processing expressed by the following equations:
Y
2
=Y
1
−k
3
Fukushima Satoru
Gu Sono
Mizoguchi Yoshito
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