Optical printer

Details Optical printer Optical printer

1999-09-15

2001-05-15

Adams, Russell (Department: 2851)

Photocopying

Projection printing and copying cameras

Multicolor picture

C355S035000, C355S041000, C347S232000, C347S238000, C347S239000, C347S240000, C347S255000, C347S251000, C358S474000, C358S487000, C358S497000

Reexamination Certificate

active

06233036

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an optical printer apparatus having a scanning optical head capable of moving on a sensitized sheet to expose it with given timing, thereby forming an image, and more specifically, to a technique for controlling the exposure timing for the optical head to expose the sensitized sheet.
BACKGROUND ART
Disclosed in Japanese Patent Application Laid-Open No. 2-169270 is an optical printer apparatus in which an optical head is moved relative to a sensitized sheet to form an image on the sensitized sheet. This optical printer apparatus will now be described with reference to FIG.
9
.
A sensitized sheet
60
is driven at a constant speed in the direction of arrow A with respect to the optical head
10
by means of feed rollers
70
. The optical head
10
comprises a white light source
20
for radially emitting white light, a cylindrical lens
30
for linearly converging the white light on the sensitized sheet
60
, a three-color separation liquid crystal shutter
40
, and a liquid crystal shutter
50
.
The three-color separation liquid crystal shutter
40
is composed of three shutters
40
r
,
40
g
and
40
b
that linearly extend in the width direction (spreading direction) of the white light from the cylindrical lens
30
. These three shutters
40
r,
40
g
and
40
b
are driven independently of one another, and are provided individually with color filters that transmit red (R), green (G), and blue (B) light beams, respectively.
The liquid crystal shutter
50
includes a plurality of pixels that are arranged in the same direction as the lengthwise direction of the shutters
40
r,
40
g
and
40
b.
The following is a description of a method for forming an image on the sensitized sheet
60
by means of the apparatus shown in FIG.
9
.
The optical printer apparatus receives gradated color image data, controls the shutters
40
r,
40
g
and
40
b
in accordance with the image data, and exposes the surface of the sensitized sheet
60
, thereby forming the image thereon. First, the shutter
40
r
opens for a given time to transmit the white light; next, the shutter
40
g
opens for the given time to transmit the white light; and then, the shutter
40
b
opens for the given time to transmit the white light.
This given time section is just equal to a period of time during which the sensitized sheet
60
moves for a distance X in FIG.
9
.
Thus, the sensitized sheet
60
is exposed to the red light beam (R), which is first transmitted through the shutter
40
r,
for the distance X in its moving direction (direction A). Then, the shutter
40
r
is closed, while the shutter
40
g
opens. Since the sensitized sheet
60
is moved for the distance X by this time, that portion of the sensitized sheet
60
which is already exposed to the light beam R is exposed again to the green light beam (G) that is transmitted through the shutter
40
g.
When the sensitized sheet
60
further moves for the distance X, thereafter, the portion already exposed to the light beams R and G is exposed again in like manner to the blue light beam (B) that is transmitted through the shutter
40
b
. An image of full-color display can be obtained by repeating these processes of operation in the feeding direction of the sensitized sheet
60
.
In a direction perpendicular to the feeding direction of the sensitized sheet
60
, an image is formed by means of the liquid crystal shutter
50
.
Referring now to
FIG. 10
, there will be described further in detail a method for gradation control by means of the printer apparatus shown in FIG.
9
.
FIG. 10
is a diagram showing the relation between exposure time and position within the pixel width. For ease of illustration, in this case, the liquid crystal shutter
50
(and the optical head) is supposed to move in the z-direction with the sensitized sheet
60
kept stationary.
Let it be supposed that the light beam R with a width W, having passed through the liquid crystal shutter
50
, is further moved at uniform speed in the z-direction so that an image A
2
is formed after an image A
1
is formed on the sensitized sheet
60
.
Thereupon, the region that is exposed all the time from the formation of the image A
1
on the sensitized sheet
60
to the formation of the image A
2
is a region E within the pixel width. In this case, an exposure time t1 has a value obtained by dividing the distance for the movement of the image A
1
to the image A
2
(which is equal to the distance given by D and F in
FIG. 10
) by the moving speed of the optical head. In the regions D and F, moreover, the change of the exposure time, compared to the change of the z-direction position, is a linear increase or decrease. Thus, the relation between the position on the sensitized sheet
60
and the exposure time can be represented by a trapezoid B, as indicated by full line in FIG.
10
.
When the light beam R with the width W, having passed through the liquid crystal shutter
50
, is further moved in the z-direction so that an image A
3
is formed after the image A
1
is formed on the sensitized sheet
60
, the relation between the position on the sensitized sheet
60
and the exposure time can be represented by a trapezoid C, as indicated by broken line in
FIG. 10
, if the distance for the movement of the image A
1
to the image A
3
is D/2.
The region that is exposed all the time from the formation of the image A
1
on the sensitized sheet
60
to the formation of the image A
3
corresponds to the top side of the trapezoid C indicated by broken line. In this case, the exposure time is t2 (=t½).
In the case where the gradation control is effected in a manner such that the light beam with the width W having passed through the liquid crystal shutter
50
is further moved in the z-direction to form the image A
2
or A
3
(or any other image) after forming the image A
1
on the sensitized sheet
60
, as described above, the trapezoid that is indicative of the relation between the position on the sensitized sheet
60
and the exposure time is always situated closer to one end (starting end) within the pixel width despite variation in height. Inevitably, therefore, there is a wide monochromatically exposed or unexposed portion in the region near the other end of the trapezoid. Accordingly, the image color mixture property is so poor that the image quality is lowered. Depending on the image pattern, moreover, interference fringes may be produced to make the image unclear in many cases.
The following problem is aroused, however, if the gradation control is effected in a manner such that the sensitized sheet
60
is exposed only for a time (t1, t2, etc.) corresponding to the given distance X by moving the image A
1
on the sensitized sheet at uniform speed for the distance X in the scanning direction, as described above.
In general, the photosensitive properties of the sensitized sheet
60
with respect to time are non-linear. More specifically, the sensitization speed for the sensitized sheet is not always constant without regard to the exposure time. As the exposure time becomes longer, the sensitization speed lowers gradually.
Let it be supposed that the maximum exposure time is divided into N number of equal parts (N: integer), and image data i are made to correspond to [maximum exposure time×(i/N)] (i: an integer not larger than N). If the gradation is controlled by means of the image data i, the image data cannot be reproduced with correct gradation.
In general, moreover, the sensitization speed of the sensitized sheet
60
varies also depending on the waveform of light. Thus, in a color image, the gradation inevitably delicately varies with color without any change in exposure time.
DISCLOSURE OF THE INVENTION
The object of the present invention is to provide an optical printer apparatus, capable of forming a high-quality image with satisfactory color mixture property and of accurately reproducing the gradation of image data on a sensitized sheet to cope with the fact that the photosensitive properties of the sensitized sheet w

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