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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus for reading out an image stored in a
stimulable phosphor sheet by two-dimensionally scanning the stimulable
phosphor sheet by stimulating rays. This invention particularly relates to
an apparatus for reading out an image by scanning wherein backward scan
lines, i.e. return scan lines, in forward and backward scanning in the
main scanning direction are erased.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays,
.alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a
part of the energy of the radiation. Then, when the phosphor which has
been exposed to the radiation is exposed to stimulating rays such as
visible light, light is emitted from the phosphor in proportion to the
stored energy of the radiation. A phosphor exhibiting such properties is
referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent
Publication No. 56(1981)-11395, it has been proposed to use a stimulable
phosphor in a radiation image recording and reproducing system.
Specifically, a sheet provided with a layer of the stimulable phosphor
(hereinafter referred to as a stimulable phosphor sheet or simply as a
sheet) is first exposed to a radiation passing through an object to have a
radiation image stored therein, and is then scanned with stimulating rays
such as a laser beam which cause it to emit light in the pattern of the
stored image. The light emitted from the stimulable phosphor sheet upon
stimulation thereof is photoelectrically detected and converted to an
electric image signal, which is processed as desired to reproduce a
visible image on a recording medium such as a photographic light-sensitive
material or on a display device such as a cathode ray tube (CRT).
In an apparatus for reading out an image by scanning in the aforesaid
radiation image recording and reproducing system, the stimulable phosphor
sheet carrying a radiation image stored therein is two-dimensionally
scanned by stimulating rays such as a laser beam, and light emitted by the
stimulable phosphor sheet is sequentially detected and converted into
electric image signals by a photodetector such as a photomultiplier. The
two-dimensional scanning is conducted by scanning the stimulable phosphor
sheet by stimulating rays in the main scanning direction and in the
sub-scanning direction. For this purpose, in general, the stimulable
phosphor sheet is moved in the sub-scanning direction and, at the same
time, the stimulating rays are moved forwardly and backwardly in the main
scanning direction normal to the moving direction of the sheet.
As a means for conducting the scanning in the main scanning direction, a
galvanometer mirror moveable forwardly and backwardly at a predetermined
speed along a predetermined path is generally used. In this case, since
the stimulable phosphor sheet is continuously moved at a predetermined
speed in the sub-scanning direction, the scan lines traced on the sheet by
the stimulating rays moved forwardly and backwardly in the main scanning
direction by the main scanning means zigzag at a predetermined angle with
respect to the direction normal to the sub-scanning direction. Since the
beam of stimulating rays has a predetermined diameter, the scan lines
overlap at portions where the scanning direction is changed over from the
forward scanning direction to the backward scanning direction and vice
versa. Particularly when the scan line density is increased to conduct
image read-out at a high accuracy, the forward scan lines and the backward
scan lines become closer to each other and, therefore, the areas of the
overlapping portions of the scan lines increase. When the stimulable
phosphor sheet is once exposed to stimulating rays to emit light in
proportion to the radiation energy stored therein, the level of the
radiation energy stored decreases. Therefore, when overlapping of the scan
lines arises as described above during image read-out, the problem that
the portions of the stimulable phosphor sheet previously exposed to
stimulating rays to release the stored radiation energy as light emission
and now having a decreased level of residual radiation energy stored
therein are again exposed to stimulating rays for image read-out arises in
the overlapping portions of the scan lines. As a result, the electric
image signals obtained by scanning the stimulable phosphor sheet by
stimulating rays in this manner become incorrect.
SUMMARY OF THE INVENTION
The primary object of the present invention is to provide an apparatus for
reading out an image by scanning, which correctly reads out the image by
eliminating the overlapping of forward and backward scan lines in the main
scanning direction.
Another object of the present invention is to provide an apparatus for
reading out an image by scanning, which quickly and correctly reads out
the image by eliminating the overlapping of forward and backward scan
lines in the main scanning direction.
The present invention provides an apparatus for reading out an image by
scanning, which is provided with a stimulating ray source for emitting a
light beam, a main scanning means for scanning a stimulable phosphor sheet
carrying an image stored therein by said light beam forwardly and
backwardly in a main scanning direction, a sub-scanning means for scanning
said stimulable phosphor sheet by said light beam in a sub-scanning
direction approximately normal to said main scanning direction, and a
read-out means for photoelectrically detecting light emitted by said
stimulable phosphor sheet in the pattern of the stored image when said
stimulable phosphor sheet is scanned by said light beam,
wherein the improvement comprises the provisions of a converging means
positioned in the optical path of said light beam for converging said
light beam at a point on said optical path, and a light shielding means
for intercepting the passage of said light beam at the converging position
of said light beam converged by said converging means during the period of
backward scanning in said main scanning direction in synchronization with
said forward and backward scanning in said main scanning direction.
As the converging means of the apparatus for reading out an image by
scanning in accordance with the present invention, it is possible to use,
for example, a pair of convex lenses. As the driving section of the light
shielding means, any device may be used insofar as a light shielding
member of the light shielding means is moved in synchronization with the
forward and backward scanning of the light beam in the main scanning
direction conducted by the main scanning means so as to intercept the
passage of the light beam during the backward scanning period. Thus the
driving section may be of the type rotating the light shielding member
forwardly and backwardly, or rotating it in a single direction, or
linearly moving it forwardly and backwardly.
In the apparatus for reading out an image by scanning in accordance with
the present invention, since the light shielding means for moving in
synchronization with the forward and backward scanning in the main
scanning direction conducted by the main scanning means and intercepting
the passage of the light beam during the backward scanning period is
installed at the converging position of the light beam, the stimulable
phosphor sheet can be prevented from being exposed to the light beam
during the backward scanning period. Therefore, the level of the image
information stored in the stimulable phosphor sheet is not decreased by
the light beam scanning along backward scan lines, and it becomes possible
to correctly read out the image stored in the stimulable phosphor sheet.
Further, since the interception of the light beam is conducted at the
position where the light beam is converged to a small beam diameter,
deflection of the light shielding member of the light shielding means
necessary for achieving the light shielding may be very small, and the
size of the light shielding member can be minimized. Accordingly, it is
possible to increase the speed of rotation or forward-backward movement of
the light shielding member conducted by the driving section of the light
shielding means. As a result, it becomes possible to conduct image
read-out at a high speed.
The present invention also provides an apparatus for reading out an image
by scanning, which is provided with a stimulating ray source for emitting
a light beam, a main scanning means for scanning a stimulable phosphor
sheet carrying an image stored therein by said light beam forwardly and
backwardly in a main scanning direction, a sub-scanning means for scanning
said stimulable phosphor sheet by said light beam in a sub-scanning
direction approximately normal to said main scanning direction, and a
read-out means for photoelectrically detecting light emitted by said
stimulable phosphor sheet in the pattern of the stored image when said
stimulable phosphor sheet is scanned by said light beam,
wherein the improvement comprises the provision of a converging means
positioned in the optical path of said light beam for converging said
light beam at a point on said optical path, an acousto-optic modulator
positioned in said optical path between said converging means and said
stimulating ray source for diffracting said light beam and generating
diffracted light components other than a zero-order diffracted light
component during the period of backward scanning in said main scanning
direction in synchronization with said forward and backward scanning in
said main scanning direction, and a light shielding plate positioned at
the converging position of said light beam converged by said converging
means and having a pinhole for passing therethrough only the zero-order
diffracted light component coming from said acousto-optic modulator.
By "diffracted light components other than a zero-order diffracted light
component" is meant high-order diffracted light components such as
first-order, second-order, minus first-order, and minus second-order
diffracted light components. By "acousto-optic modulator" is meant a light
modulator utilizing acousto-optic effects, which modulates light by
modulating an ultrasonic input by utilizing the Debye-Sears effect or the
effect that the intensity of the first-order diffracted light component in
Bragg diffraction is approximately proportional to the ultrasonic input.
Therefore, the acousto-optic modulator is advantageous in that, though the
modulation band width is approximately several tens of megahertz, the
extinction ratio can be made very small and the operation stability
against a change in temperature is good.
In the apparatus of the present invention mentioned last, the light
shielding plate having a pinhole for passing only the zero-order
diffracted light component therethrough is positioned at the converging
position of the light beam converged by the converging means. Since the
diffracted light components coming from the acousto-optic modulator are
very close to each other, it becomes difficult to intercept only the
diffracted light components other than the zero-order diffracted light
component unless the interception is conducted at the position where the
diameter of the light beam, i.e. the zero-order diffracted light
component, is small.
In the apparatus of the present invention mentioned last, the acousto-optic
modulator is positioned in the optical path between the stimulating ray
source and the light beam converging means, and modulation by the
acousto-optic modulator is carried out in synchronization with the forward
and backward scanning in the main scanning direction conducted by the main
scanning means. Thus the light beam emitted by the stimulating ray source
is diffracted by the acousto-optic modulator to generate diffracted light
components of first order and higher or minus first order and lower only
during the backward scanning period. Further, the light shielding plate
having a pinhole for passing therethrough only the zero-order diffracted
light component used for stimulating the stimulable phosphor sheet is
installed at the converging position of the light beam converged by the
converging means. In this manner, the diffracted light components other
than the zero-order diffracted light component which are generated during
the backward scanning period are intercepted by the light shielding plate.
Therefore, the apparatus exhibits a very high response speed (several tens
of thousands times the speed of a backward scan line erasing apparatus
using a mechanical light shielding means), and can simply realize
high-speed image read-out by scanning. That is, even when image read-out
is conducted at a high speed, it is possible to prevent the stimulable
phosphor sheet from being exposed to stimulating rays during the backward
scanning period, and to correctly read out the image stored in the
stimulable phosphor sheet since the level of the image information stored
in the sheet is not decreased by stimulating rays scanning along backward
scan lines. Further, the acousto-optic modulator can accurately and
quickly control the modulation with respect to fluctuation in light beam
output of the stimulating ray source and, therefore, is advantageous also
as a means for stabilizing the light amount. Also, the apparatus is very
reliable since no moving section is used therein. Accordingly, the
apparatus using the acousto-optic modulator in accordance with the present
invention is very advantageous in practical use.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing an embodiment of the apparatus for
reading out an image by scanning in accordance with the present invention,
FIG. 2 is an enlarged schematic view showing the beam expander and the
light shielding means of the apparatus of FIG. 1,
FIG. 2A is a schematic view showing a modified form of the sector employed
in the light shielding means of FIG. 2,
FIGS. 3A and 3B are respectively a perspective view and a schematic view
showing another embodiment of the light shielding means,
FIG. 4 is a schematic view showing a further embodiment of the light
shielding means,
FIG. 5 is a schematic view showing another embodiment of the apparatus for
reading out an image by scanning in accordance with the present invention,
and
FIG. 6 is an enlarged schematic view showing a part of the apparatus of
FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will hereinbelow be described in further detail with
reference to the accompanying drawings.
Referring to FIG. 1, an endless belt 2 provided with a magnet layer 2a
overlaid on the surface is driveably positioned around a pair of rollers
1a and 1b. A stimulable phosphor sheet 3 is closely contacted with the
magnet layer 2a of the endless belt 2 by magnetic attraction. The rollers
1a and 1b are rotated to move the endless belt 2 in the direction as
indicated by the arrow A. While the stimulable phosphor sheet 3 is moved
together with the endless belt 2 in the direction as indicated by the
arrow A, a laser beam 4 emitted from a laser beam source 5 is made to
impinge upon the stimulable phosphor sheet 3 via a galvanometer mirror 6
so as to scan the sheet 3 in the direction as indicated by the arrow B
normal to the moving direction A of the endless belt 2. As a result, the
stimulable phosphor sheet 3 is two-dimensionally scanned by the laser beam
4. That is, the stimulable phosphor sheet 3 is scanned in the sub-scanning
direction as indicated by the arrow A by the sub-scanning means comprising
a pair of rollers 1a and 1b and the endless belt 2, and in the main
scanning direction as indicated by the arrow B normal to the sub-scanning
direction by the galvanometer mirror 6 acting as the main scanning means.
The endless belt 2 comprises a flexible endless belt substrate 2b and the
magnet layer 2 a overlaid on the substrate 2b.
A beam expander comprising convex lenses 9a and 9b and acting as a beam
converging means is positioned in the optical path between the laser beam
source 5 and the galvanometer mirror 6. At the beam converging position
between the beam expander lenses 9a and 9b is positioned a rotatable light
shielding means 10 for intercepting the laser beam 4 during the backward
scanning period in synchronization with the forward and backward scanning
in the main scanning direction conducted by the galvanometer mirror 6.
The purpose of expanding the laser beam 4 by the beam expander lenses 9a
and 9b is to minimize the size of the light spot ultimately formed on the
stimulable phosphor sheet 3. The laser beam 4 expanded by the beam
expander lenses 9a and 9b is converged by an image forming lens 11 onto
the stimulable phosphor sheet 3.
The stimulable phosphor sheet 3 comprises a magnetic material layer 3a and
a stimulable phosphor layer 3b laid on the magnetic material layer 3a. The
stimulable phosphor layer 3b carries a radiation transmission image of an
object stored therein. When the laser beam 4 impinges upon the stimulable
phosphor sheet 3, the portion of the sheet 3 exposed to the laser beam 4
emits light in proportion to the radiation energy stored. The emitted
light enters a light guide member 7 fabricated, for example, by forming an
acrylic plate, and is guided inside of the light guide member 7 by total
reflection to a photodetector 8 such as a photomultiplier, which detects
and converts the light into electric image signals. Thus the electric
image signals are sequentially obtained as the stimulable phosphor sheet 3
is two-dimensionally scanned by the laser beam 4 and then processed and
used for reproducing the image.
FIG. 2 is an enlarged schematic view showing the rotatable light shielding
means 10 and the beam expander lenses 9a and 9b of FIG. 1. The laser beam
impinging upon the first lens 9a of the beam expander is converged thereby
and then diverges. The beam diameter of the laser beam is then expanded by
the second lens 9b. At the converging position of the laser beam between
the beam expander lenses 9a and 9b is positioned a sector 10a provided
with a rotation shaft approximately parallel to the laser beam and rotated
by a rotating motor 10b in synchronization with the forward and backward
scanning in the main scanning direction conducted by the galvanometer
mirror 6. The sector 10a is constituted by a fan-like light shielding
member, and the rotation timing and the angle at the circumference of the
sector 10a are adjusted to intercept the laser beam only during the
backward scanning period in accordance with a synchronizing signal sent
from the galvanometer mirror 6. In the embodiment of FIG. 2, since the
sector 10a is installed at the beam converging position, it is possible to
minimize the size of the sector 10a. Further, since interception of the
laser beam can be achieved simply by rotating the sector 10a in a single
direction by the rotating motor 10b, the interception of the laser beam
can be conducted easily in conformity with the high speed scanning by the
galvanometer mirror 6. As a modified form of the sector 10a, FIG. 2A shows
a disk 10h which is advantageous in that it does not generate wind during
rotation. The disk 10h is fabricated of a transparent glass plate or a
transparent acrylic plate and provided with two light shielding portions
on which a light shielding member is applied to intercept the laser beam
during the backward scanning period. In this embodiment, the light
shielding portions and portions permeable to light are alternately
positioned at approximately 90.degree. intervals on the disk 10h, and the
rotation of the disk 10h is timed so that the light shielding portions
intercept the passage of the laser beam only during the backward scanning
period on the basis of the synchronizing signal sent from the galvanometer
mirror 6.
However, in FIG. 2A, the angle ratio of the light shielding portions to the
portions permeable to light or the numbers of the respective portions may
be changed when necessary.
FIG. 3A is a perspective view showing another embodiment of the light
shielding means of the apparatus for reading out an image by scanning in
accordance with the present invention, and FIG. 3B is a schematic view
showing the light shielding means of FIG. 3A positioned in the optical
path of the laser beam. In this embodiment, a light shielding member 10c
is secured to a rotation shaft 10d of a galvanometer 10e. The rotation
shaft 10d is positioned approximately normal to the laser beam, and the
light shielding member 10c is rotated forwardly and backwardly by the
galvanometer 10e. The light shielding member 10c consists of a cylindrical
light shielding material having a through groove laterally extending in
the upper surface, and is installed at the beam converging position
between the beam expander lenses 9a and 9b. The galvanometer 10e is
rotated forwardly and backwardly in synchronization with the forward and
backward scanning in the main scanning direction conducted by the
galvanometer mirror 6. That is, the galvanometer 10e rotates the light
shielding member 10c forwardly and backwardly so that the laser beam
passes through the groove of the light shielding member 10c during the
forward scanning period and is intercepted by the wall portions on both
sides of the groove during the backward scanning period. In this
embodiment, since the light shielding member 10c is installed at the beam
converging position, it is possible to minimize the size of the light
shielding member 10c. Further, since the wall portions on both sides of
the groove are used to intercept the laser beam, interception of the laser
beam can be achieved by a smaller rotation angle of the light shielding
member 10c than when only a single side wall portion is used, and can be
conducted in conformity with the high speed scanning by the galvanometer
mirror 6.
FIG. 4 is a schematic view showing a further embodiment of the light
shielding means of the apparatus for reading out an image by scanning in
accordance with the present invention. In this embodiment, a piston type
light shielding member 10f consisting of a metal plate is installed
approximately normal to the laser beam at the beam converging position
between the beam expander lenses 9a and 9b. A solenoid 10g is positioned
below the light shielding member 10f to vertically move the light
shielding member 10f in synchronization with the forward and backward
scanning in the main scanning direction conducted by the galvanometer
mirror 6. Namely, the light shielding member 10f is moved vertically so
that it is positioned below the laser beam to pass the laser beam during
the forward scanning period and raised to intercept the laser beam during
the backward scanning period. In this embodiment, since the light
shielding member 10f is installed at the beam converging position and the
moving distance of the light shielding member 10f may be very small,
interception of the laser beam can be conducted in conformity with the
high speed scanning by the galvanometer mirror 6.
FIG. 5 shows another embodiment of the apparatus for reading out an image
by scanning in accordance with the present invention. In FIG. 5, similar
elements are numbered with the same reference numerals with respect to
FIG. 1. In this embodiment, an acousto-optic modulator 12 is positioned at
the stage next to the laser beam source 5 in the optical path of the laser
beam 4 emitted from the laser beam source 5. Between the acousto-optic
modulator 12 and the galvanometer mirror 6 is positioned a light beam
converging means 9 comprising a pair of convex lenses 9a and 9b. As shown
in FIG. 6, there is installed at the beam converging position of the light
beam converging means 9 a light shielding plate 10' having a pinhole for
passing therethrough only the zero-order diffracted light component coming
from the acousto-optic modulator 12.
The acousto-optic modulator 12 is controlled in synchronization with the
forward and backward scanning in the main scanning direction conducted by
the galvanometer mirror 6 so as to modulate the laser beam 4 emitted by
the laser beam source 5 only during the backward scanning period and not
to modulate the laser beam during the forward scanning period.
That is, when the stimulable phosphor sheet 3 is scanned by the laser beam
4 along the forward scan lines, the acousto-optic modulator 12 does not
modulate the laser beam, and most of the laser beam 4 impinging upon the
modulator 12 (approximately 97% to 98% thereof) is passed through the
modulator 12 as the zero-order diffracted light component.
The zero-order diffracted light component passing through the acousto-optic
modulator 12 is converged by the first convex lens 9a of the beam
converging means 9 and passed through the pinhole of the light shielding
plate 10' installed at the converging position. The zero-order diffracted
light component is then converted by the second convex lens 9b into
parallel rays of light and made to impinge upon the galvanometer mirror 6.
During the backward scanning period, the acousto-optic modulator 12
diffracts the laser beam at the maximum modulation efficiency and converts
most (90% to 95%) of the light amount of the laser beam impinging upon the
modulator 12 into the diffracted light components other than the
zero-order diffracted light component. At this time, a portion within the
range of approximately 5% to 10% of the light amount of the laser beam
impinging upon the modulator 12 remains as the zero-order diffracted light
component. The diffracted light components other than the zero-order
diffracted light component are intercepted by the light shielding plate
10', and the zero-order diffracted light component is passed through the
pinhole of the light shielding plate 10'.
As shown in the enlarged view in FIG. 6 showing the acousto-optic modulator
12, the light beam converging means 9, and the light shielding plate 10',
the diffracted light components other than the zero-order diffracted light
component, e.g. a first-order diffracted light component 14 and a
second-order diffracted light component 15, which are generated by the
diffraction by the acousto-optic modulator 12 during the backward scanning
period are intercepted by the light shielding plate 10' installed at the
light beam converging position. In this case, a zero-order diffracted
light component exists in an amount within the range of approximately 5%
to 10% of the light amount of the laser beam impinging upon the
acousto-optic modulator 12. This is caused by the general characteristics
of the acousto-optic modulator 12. However, practically, no adverse effect
arises when the zero-order diffracted light component in the small amount
as described above impinges upon the stimulable phosphor sheet 3. Further,
when scanning in the main scanning direction is conducted, backward
scanning is generally conducted at a speed higher than the speed of the
forward scanning. Therefore, the energy of the zero-order diffracted light
component impinging upon the stimulable phosphor sheet 3 during the
backward scanning further decreases.
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