Image display apparatus

The present invention provides an image display apparatus including an image bearing web in the form of an endless belt on which an erasable image is formed. The erasable image is then carried to a display portion by the image bearing web. The length of the image bearing web is selected to be equal to or more than twice the length of the displayable region in the display portion. The image bearing web being movable independently of image forming process, whereby the image on the web which has already passed by the display portion can be returned to the display portion, permitting repetitive display function and improving the display apparatus in operation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image display apparatus for displaying 
data in computers, facsimiles and the like, and more particularly to an 
apparatus for forming and displaying an image on a repetitively usable 
image bearing web in the form of a belt. 
2. Description of the Prior Art 
In the prior art, a cathode ray tube (CRT) is currently used as an image 
display apparatus. Where a CRT is utilized to display a still image such 
as a letter, a figure or the like, a random access memory (RAM) as an IC 
memory is generally used together with the CRT in connection with picture 
elements of a picture to store the information of image which will be read 
out for display. 
In general, it may often be required that the image display apparatus 
displays the previously displayed image repetitively. In order to fulfill 
such a requirement, the image display apparatus including the CRT is 
provided with a RAM capable of storing data for a plurality of pictures to 
allow a new picture to be displayed without the necessity of clearing the 
RAM of the data for the picture currently on display. Thus, the respective 
pictures can be re-displayed at any time by reading out the information 
which has been stored in the RAM. The RAM required to store the 
information of all the pictures must have an extremely great capacity and 
is then substantially expensive. Also, the image display apparatus using 
the CRT has another problem of unsatisfactory resolving power. 
An image display apparatus of thermal recording type is known as having a 
great capacity with respect to information of pictures. This apparatus 
comprises a reversible heat-sensitive recording web, for example, of 
Ag.sub.2 HgI.sub.4 which is a compound of silver, mercury and iodine, and 
a thermal head for recording an image on the heat-sensitive recording web. 
Such a compound, Ag.sub.2 HgI.sub.4, changes in color in accordance with 
the change of temperature and has a hysteresis with respect to 
temperature. Therefore, if an image is recorded on a belt-shaped film 
containing Ag.sub.2 HgI.sub.4 by the use of the thermal head, the image 
can be displayed by maintaining the temperature of the film by the use of 
a planar heater and erased by cooling the film at room temperature or by 
the use of a cooler. Thus, the film can repetitively be utilized. 
There is also known an image display apparatus of electrophotographic type 
in which an electrophotographic type photosensitive member is scanned by a 
light beam which has been modulated in accordance with image signals, to 
form a toner image on the photosensitive member. This toner image will be 
displayed. Thereafter, the toner image can be removed by the use of 
subsequent light beam application so that the photosensitive member can 
repetitively be used. 
The image display apparatus of either heat-sensitive recording type or 
electrophotographic type has an advantage in that it has an extremely 
great capacity of display and a good resolving power. For example, if 
there are eight dots per one millimeter and when there is displayed an 
image having dimensions of 200 mm.times.300 mm, picture elements as many 
as three million, eighty hundred and fourty thousand can be displayed. If 
there are 16 dots per one millimeter, the number of picture elements 
reaches fifteen million, three hundred and sixty thousand. In order to 
redisplay the previously displayed image in such an image display 
apparatus, a random access memory (RAM) is similarly used to store all the 
display information, that is, all picture elements of one entire picture 
entirely. Thus, the RAM must have a great capacity, resulting in an 
extremely expensive structure increased in size. For example, if LSI 
memories of 64 kilo-bit are used to store the information of fifteen 
million, three hundred and sixty thousand of picture elements, it is 
required that the number of LSI memories is 235. Considering memory drive 
circuits and associated power supply and others, the image display 
apparatus will be considerably bulky and costly. 
The prior art image display apparatus of thermal recording type or the like 
involves another problem that a speed for which an image is being 
displayed is not very high. This is also one of various causes of 
disadvantages produced when the aforementioned memories are used. For 
example, the thermal recording type image display apparatus is adapted to 
write the respective picture elements on the image bearing web 
successively and exactly while the latter is mechanically moving. 
Accordingly, the complete display of a picture requires a period of time 
in the range of a few seconds to several tens of seconds. Of course, this 
period cannot be reduced even when the previously displayed image is 
re-displayed after it has been once stored in the memory. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the aforementioned 
disadvantages in the prior art image display apparatus of the above type 
and to provide an image display apparatus which can re-display the 
previously displayed image without need of memory means having great 
capacity. 
Another object of the present invention is to provide an image display 
apparatus which has a reduced period of time required to effect the 
re-display. 
To accomplish the above objects, the present invention provides an image 
display apparatus which comprises a display section for allowing 
observation of an image formed on an image bearing web in the form of an 
endless belt which is movable within a casing, the image bearing web 
having such a length that two or more images to be observed in the display 
section can be formed thereon; a plurality of support members movably 
supporting the image bearing web; drive means for moving the image bearing 
web along the support members; means for forming a visible image on the 
image bearing web; and means for making the image forming means 
inoperative when the image formed on the image bearing web is again to be 
introduced into the image display section after the image bearing web has 
passed by the image display section. 
When the image held on the image bearing web is returned to the display 
section, the image bearing web in the form of an endless belt can be moved 
in either the forward or rearward direction. To obtain a proper image, it 
is preferred that means for erasing the formed image and other components 
which would disturb the image are retracted from the passage of the belt, 
if necessary.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in connection with the 
drawings. 
FIG. 1 shows an embodiment of an image display apparatus of 
electrophotographic type to which the present invention is applied. The 
image display apparatus comprises a casing 1 in which a light beam 
outputted from a semiconductor laser and modulated by imagewise electric 
signals impinges on the inner face of an image bearing member 11, and 
scans the same by a scanner 8 in an optical system 7 in one direction, 
which member 11 is a photosensitive web in the form of an endless belt. 
The optical system 7 includes an f.multidot..theta. lens 9 and mirror 10. 
The image bearing member 11 will be referred to simply as "photosensitive 
web" for the purpose of simplicity. The photosensitive web 11 is moved in 
the direction shown by an arrow and formed, for example, of a polyethylene 
terephthalate film which includes an electrically conductive film of 
oxidic indium tin formed on the surface thereof and a photoconductive 
layer of CdS formed on the electrically conductive film with a binder of 
resin. The CdS used is doped with copper and indium to provide a 
sensitivity for the infrared rays in the light beams ejected from the 
semiconductor laser. In the exposure position, a development device 12 is 
located opposite to the photosensitive web 11. The development device 12 
includes a sleeve 4 within which a magnet 3 is disposed to be rotatable in 
the direction shown by an arrow. 
Toner 5 having electrically conductive and magnetic properties is supplied 
to the surface of the sleeve 4 and uniformly regulated by a blade 6 before 
the toner contacts the surface of the photosensitive web 11. DC voltage is 
applied between the sleeve 4 of the development device and the substratum 
of the photosensitive member from a source of DC voltage (not shown). 
Adjacent to the exposure and development positions there are disposed 
rollers 13 and 14 which function to maintain the moving photosensitive web 
11 flat therebetween. Thus, the distance between the surface of the 
photosensitive member and the sleeve 4 of the development device can 
properly be held constant. The toner image formed on the surface of the 
photosensitive web 11 at the position of the development device is moved 
to a display section 15 at which the photosensitive web 11 is once 
stopped. In this display section, the toner image on the photosensitive 
web 11 surface can be observed externally through a transparent glass 
sheet 16. 
In such an arrangement, photo-carriers are produced in the bright area on 
the surface of the photosensitive member which is illuminated by the light 
of image. On the contrary, the toner is charged with a polarity opposite 
to that of the photo-carriers. Therefore, the toner will be deposited on 
the surface of the photosensitive web 11 under the action of coulombic 
force to form a desired toner image thereon as the photosensitive web 11 
is moving. In other words, a writing is carried out by the illumination of 
image light. The toner so deposited will not influence the re-formation of 
image as the photosensitive web 11 is again moved to the development 
position. Therefore, the illustrated embodiment of the image display 
apparatus according to the embodiment does not require any particular 
cleaning means for cleaning the remaining toner on the photosensitive web 
11. 
FIGS. 2 and 3 illustrate the principle of image formation which is used for 
the photosensitive web 11 in the image display apparatus shown in FIG. 1. 
FIG. 2 represents the state of charge in the bright area of the information 
light. When the toner 5 contacts the photosensitive web 11 while a voltage 
is being applied to the toner through the sleeve 4, an electric field is 
applied to the photoconductive layer 11c of the photosensitive web 11. At 
this time, if the information light is projected onto the photoconductive 
layer 11c, photo-carriers e are produced therein and then moved to near 
the surface of the photoconductive layer 11c under the action of the 
electric field. As a result, a strong electrostatic attraction force acts 
between the toner 5 and the photoconductive layer 11c so that the toner 5 
will be deposited on the photoconductive layer 11c, that is, the surface 
of the photosensitive web 11. 
In the illustrated embodiment, the photoconductive layer 11c is of an 
N-type semiconductor, while a positive voltage is applied to the toner 5. 
Accordingly, photo-carriers e, which have been produced within the 
photoconductive layer 11c adjacent to the substratum on the projection of 
the information light L, can satisfactorily be moved toward the surface of 
the photoconductive layer 11c. As a result, a strong electrostatic 
attraction force will act between the toner 5 and the photosensitive web 
11 to deposit the toner 5 on the photosensitive member 11. 
FIG. 3 shows the state of charge in the dark area. When an electric field 
is applied across the toner 5 and the transparent photoconductive layer 
11b of the substratum, an electrostatic attraction force acts 
therebetween. Since, however, the photoconductive layer 11c is located 
between the toner 5 and the photoconductive layer 11b to provide a 
distance therebetween, the above electrostatic attraction force is 
smaller. The toner 5 is therefore forced to separate from the 
photoconductive layer 11c, that is, the surface of the photosensitive web 
11 due to various causes such as the magnetic force of the rotating magnet 
3 disposed within the fixed sleeve 4, and the force by which the particles 
of the toner 5 are attracted to one another and so on. 
When it is desired to form a new image on the photosensitive web 11, it can 
be formed simply by passing the photosensitive web 11 through the exposure 
and development position. In other words, if the toner holding portion of 
the photosensitive web 11 is to be changed to the non-toner-holding 
portion thereof, the toner 18 reduced in electrostatic attraction force is 
removed from the photosensitive web 11 under the influence of the magnetic 
field of the magnet 16 to provide a bright area having no toner. On the 
other hand, if the toner holding portion is not changed to the non-toner 
holding portion, photo-carriers e are again injected thereinto under the 
action of information light so that the toner 18 will still be held on the 
photosensitive web 11 against the action of the magnetic field. Therefore, 
the toner image on the surface of the photosensitive web 11 will not 
affect the subsequent formation of image. This means that an additional 
cleaning means is not required in the image display apparatus. 
In FIGS. 2 and 3, reference numeral 11a denotes a polyethylene 
terephthalate film and reference letter E designates a source of voltage 
for the sleeve 4. 
The first toner image formed on the photosensitive web 11 in accordance 
with the above principle of image formation is moved away from the display 
section 15 by moving the photosensitive web 11 again in the direction 
shown by the arrow after the first image has been observed. At the same 
time, the second toner image may be moved into the display section 15. 
When the second toner image is positioned within the display section 15, 
the photosensitive web 11 is stopped so that the second toner image can be 
observed through the glass 16. 
The illustrated embodiment is characterized by the feature that the 
circumference of the endless belt-shaped photosensitive web 11 is such 
that at least one toner image in addition to the first toner image which 
is being displayed in the image display section 15 can simultaneously be 
kept formed on the photosensitive web 11. 
Namely, when the first toner image is moved away from the display section 
15 and at the same time the second toner image is introduced into the 
display section 15 by moving the photosensitive web 11 counterclockwise as 
viewed in FIG. 1 after the first toner image has been observed, the first 
toner image travels along the run between a drive roller 17 and guide 
rollers 18, 19 without being erased. If the photosensitive web 11 is 
subsequently moved in the reverse direction, that is, clockwise as viewed 
in FIG. 1, the first toner image can be re-displayed in the display 
section 15. This eliminates the need of re-formation of a new image 
corresponding to the first toner image. In this case, the movement of the 
photosensitive member 11 can be set at any speed, for example, a speed 
twice the normal speed through a transmission, since no formation of image 
is required. 
However, the presently displayed toner image will be erased when the 
photosensitive web 11 passes by the development device 5 in the reverse 
movement. It is, therefore, desirable on the reverse movement of the belt 
that the development device 5 can be moved downwardly by a predetermined 
distance into a position where the development device has no influence on 
the already formed image by any suitable means, such as a combination of 
drive means such as a plunger with a linkage, or that an image 
corresponding to the formed image can be re-recorded starting at a 
position to which the belt is returned. 
As another method for re-introducing the previously displayed image into 
the display section, it may be considered that the development device 5 is 
moved downwardly to the position having no influence to all the toner 
images on the photosensitive web 11 and then the belt-shaped 
photosensitive web 11 is moved in the forward direction (that is, 
counterclockwise as viewed in FIG. 1) by a predetermined distance. This 
provides such an advantage that the photosensitive web 11 can be made to 
move only in one direction. 
An example of the drive means for the apparatus shown in FIG. 1 will be 
described with reference to FIG. 4. 
A drive mechanism for the photosensitive web 11, which is shown in FIG. 4, 
includes a DC motor 20 driving the drive roller 17 through a gearing. This 
DC motor 20 is reversible such that it can be rotated either clockwise or 
counterclockwise in accordance with the direction of voltage which is 
applied to the motor. The DC motor also drives an encoder 21 through 
reduction gears with a complete revolution thereof corresponding to the 
movement of the photosensitive web 11 for one complete picture. The 
encoder 21 includes a notch 21a formed therein at the periphery thereof, 
which notch can be detected by a photo-interrupter 22 to sense the 
movement of the photosensitive web 11 relative to the image display 
section. 
FIG. 5 is a circuit diagram of a control for the above DC motor and the 
whole image display apparatus. 
In FIG. 5, if a positive (+) pulse is supplied from a host computer to the 
control circuit, the DC motor 20 is energized to rotate the photosensitive 
web 11 counterclockwise as viewed in FIG. 1. When the notch 21a in the 
encoder 21 is detected by the photo-interrupter 22, the photosensitive web 
11 is temporarily stopped. If the positive pulse is supplied from the host 
computer to the control circuit, a motor 36 for rotating a magnet roller 
within the development device 12 is energized while maintaining a solenoid 
for moving the development device 12 away from the photosensitive web 11, 
which will be described hereinafter, at OFF state. At the same time, an 
instruction signal from the host computer is applied to a laser modulator 
(not shown) to initiate a formation of image in accordance with the 
above-described principle of image formation. A bias is applied to the 
development device to energize the sleeve 4 thereof at the same time as 
the development motor 36 begins to rotate. The above solenoid is normally 
in ON state except on writing an image, so that, the sleeve 4 is held 
spaced away from the photosensitive web 11. If an image corresponding to 
one complete picture is formed on the photosensitive web 11, the latter is 
then stopped by the detection signal from the photo-interrupter. 
If a switch SW1 on an operation board is turned on, the DC motor 20 is 
reversed to rotate through the amount corresponding to one complete 
picture and then stops. If the switch SW1 is kept at its ON state, the DC 
motor 20 continues to rotate in the reverse direction until the same 
switch is released to its OFF state at which the photosensitive web 11 
will be stopped. If a switch SW2 is depressed to its ON state, the DC 
motor 20 is energized to rotate in the forward direction so that the 
photosensitive web 11 will be moved counterclockwise as viewed in FIG. 1, 
through an angular distance corresponding to one complete picture. The DC 
motor 20 is connected to transistors TR1, TR2, TR3 and TR4. Two of these 
transistors TR1 and TR4 are turned on as the DC motor is rotated in the 
forward direction while the other two transistors TR2 and TR3 are turned 
on as the DC motor is rotated in the reverse direction. 
When no image is to be formed on the photosensitive web 11, that is, if the 
solenoid SN is energized, it is preferred to move the photosensitive web 
11 at higher speed. In this case, a voltage for driving the DC motor 20 is 
selected by relay contacts RS in a relay which can be actuated in 
accordance with the ON or OFF condition of the solenoid SN. In other 
words,the revolution of the motor can be controlled in accordance with the 
voltage applied thereto. 
FIG. 6 shows a lifting mechanism for the development device 12, which 
mechanism comprises a pivot shaft 24 pivotably supporting the vessel 23 of 
the development device, a spring 25 urging the vessel 23 counterclockwise 
about the pivot shaft 24, and a plunger 26 adapted to rotate the vessel 23 
clockwise against the action of the spring 25 to move the sleeve 4 away 
from the photosensitive web 11. This plunger 26 is associated with the 
solenoid SN shown in FIG. 5. The counterclockwise movement of the 
development device 12 is limited by a stopper 27. 
FIG. 7 shows an embodiment of a thermal type image display apparatus to 
which the present invention is applied. The thermal recording type image 
display apparatus comprises a heat-sensitive recording web 28 in the form 
of an endless belt which is moved in the direction shown by an arrow and 
on which an image is recorded by a thermal recording head 27. 
Within the image display region, an image formed on the belt-shaped 
heat-sensitive recording web 28 by the recording head 27 is maintained, at 
its state as it is formed, by the use of a planar heater 29 which is 
located inside and opposite to the image display section 15. The 
temperature of the recording web 28 is kept at about 40.degree. C. The 
image on the heat-sensitive recording web 28 can be erased by a cooling 
unit 30. 
The embodiment illustrated in FIG. 7 is also characterized in that the 
circumference of this endless belt-shaped heat-sensitive recording web 28 
is so selected that at least one image in addition to the first image 
which is being displayed in the display section 15 can be formed on the 
heat-sensitive recording web 28, and that even if that portion of the 
recording web 28 on which the image is formed is moved out of the display 
section, this image is held on the recording web 28 by another planar 
heater 31 disposed as shown in FIG. 7. In such a manner, the image can be 
held on the recording web 28, after the image has been observed, and 
re-displayed in the display section by moving the heat-sensitive web 28 in 
the reverse direction. 
Operations of the re-display of image is similar to those of the previous 
embodiment. More particularly, where it is desired to maintain the first 
image or the other one image when the belt-shaped recording web 28 is 
moved in the forward or rearward direction, the thermal recording head 27 
and/or the cooling unit 30 are moved away from the surface of the 
heat-sensitive recording web 28 by drive means such as a plunger or 
solenoid or a linkage including levers and others, if necessary. The image 
display apparatus further comprises a drive roller 32 for driving the 
recording web 28 and guide rollers 33 and 34 for guiding the same. The 
movement of the recording web 28 in the opposite directions can be carried 
out by the same arrangement as in the first embodiment. 
There will now be described a process in which an image is formed on the 
photosensitive web 11 after the first image has been re-displayed and has 
been observed in the image display apparatus shown in FIG. 1. If the 
second image is in the process of being formed on the web 11 upstream 
relative to the first image, it is preferably re-formed from the beginning 
because it is not expected to provide an image of high quality due to the 
slippage and resiliency in the belt if the remaining portion of the image 
is again formed after the image has been halfway formed. The condition of 
image can be detected by comparing the position of the encoder with an 
image formation signal. For example, if the encoder does not reach a 
predetermined position although an image formation signal has been 
supplied, it can be judged that the second image is under the formation. 
The present invention can use any other system which can form and erase an 
image on the image bearing web, such as a magne-stylus system as described 
in U.S. Pat. No. 3,914,771. 
It is understood from the foregoing that the present invention provides 
various advantages in that the display can be effected without need of any 
great capacity memory, in that the previously displayed image can more 
easily be re-displayed, and in that the image bearing web can be moved at 
higher speed on the redisplay to improve the operation of display.