This application is a continuation of application Ser. No. 07/417,067, filed Oct. 4, 1989, which is a continuation-in-part of application Ser. No. 07/205,750, filed Jun. 13, 1988, which is a continuation-in-part of application Ser. No. 07/070,589, filed Jul. 7, 1987. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thin, readily portable book, and, in particular, to a thin portable book which is mainly comprised of two parts of a recording medium freely detachable and a read-out apparatus which retrieves and displays the data stored in the recording medium. 
     Coded data is recorded on the recording medium as the content of the book and can be read out by a read-out means in the main body of the read-out apparatus, decoded or converted, and displayed in the form of alphanumeric characters, symbols, and images on a liquid crystal display which is part of the read-out apparatus. 
     2. Description of the Prior Art 
     Conventional books are mainly in a form whereby the alphanumeric characters, symbols, and images and so on used to display the contents of a book are printed on the surface of the paper of that book. The reader views the alphanumeric characters, symbols, and images, and the like to comprehend the contents of the book. 
     In many conventional books made from paper, one volume will weigh in excess of 1 kg. For example, literary novels and specialized books frequently exceed one kilogram in weight. For this reason, it frequently happens that when the books are shipped to a retail store, several tons of books are loaded onto a truck. The transportation costs and the like mount up so that the cost of one volume becomes rather high. In addition, the size of the paper on which the alphanumeric characters, symbols, and images are printed is not uniform, even with the similar content of the book, the similar type, the similar cost, and the like, so there is the problem that when a large volume is stored, a large paper area is required. In addition, primary and junior high school students whose bodies are still in a state of growth must carry many heavy texts books to and from school every day for study. This is believed to have a major adverse effect on the bodies of these students. 
     The applicant of the present invention has proposed a “Personal Computer System with a Built-in Solar Cell” (U.S. patent application Ser. No. 07/292,098 filed on Dec. 30, 1988) relating to all types of data processing devices to provide energy saving, in which the terminology “personal computer system” is used to indicate all types of data processing devices. This personal computer system is equipped with a built-in battery, a secondary battery, a solar cell, a liquid crystal display, a keyboard, and a cover. The liquid crystal display is provided inside the cover which opens and closes by means of rotating shaft with reference to the main body including the keyboard. The liquid crystal display is adapted to face toward the user when the cover is open. The solar cell is provided on the side of the cover where there is no liquid crystal display. Therefore, when the cover is closed, the solar cell can be seen but the liquid crystal display cannot. 
     This personal computer system may also be provided with a mechanism so that when the cover is closed, the secondary battery is electrically connected to the solar cell, so that the secondary battery becomes charged, while when the cover is opened, the secondary battery is electrically connected to a terminal to operate the personal computer system. In this mechanism, when the cover is closed, the secondary battery is charged by the electric power generated by the solar cell, and when the cover is opened, the electric energy with which the secondary battery had been charged can be used to operate the personal computer system. At this time, the liquid crystal display would face toward the user because the cover is opened. 
     The above is one example of this personal computer system which demonstrates energy saving in the operation. 
     This personal computer system can also be utilized as the thin portable book described previously. 
     In the case where the above-mentioned personal computer system in the conventional technology is applied to the thin portable book, it has some superior points and some inferior points when compared with the usual type of book prints. For example, in the personal computer system used as a thin portable book, a strong point is the fact that when only the freely detachable recording mediums are carried with the read-out apparatus left behind, this book is considerably lighter than a printed book. On the other hand, there are the drawbacks that when the personal computer system is used as the thin portable book and the read-out apparatus is also taken outdoors to read the book, the life of the charge of a dry cell and a chargeable cell is short, and when the weather is poor so that very little power can be generated by the solar cell, the personal computer system is less reliable than the normal book. 
     For this reason, the appearance of a new type book superior to a standard book on all points has been awaited. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide, with due consideration to the drawbacks of such conventional methods, a thin portable book which has improved electricity generating volume, reduced power consumption and increased electrical charge amount to realize high reliability for use outdoors over a long period with its portability and convenience maintained. 
     This object is achieved in the present invention by the use of a liquid crystal display with memory capabilities, in order to reduce power consumption. 
     When a ferroelectric liquid crystal display is used, the display drive signal may be applied to the ferroelectric liquid crystal display only when the display image is converted to another display. After the image display is changed, even if the display drive signal is not applied, the image continues to be displayed. In this type, the power consumption becomes very small in comparison with a conventional TN-type or STN-type of liquid crystal display in which a liquid crystal display drive signal must be applied 30 to 60 times per second for an image to be continued. 
     In addition, in the present invention, a magnetic floppy disk, an optical disk, or a non-volatile semiconductor memory is used as the freely detachable recording medium. Among these, the non-volatile semiconductor memory has the lowest power consumption. This is because the magnetic floppy disk and the optical disk is provided with a read-out machine, so that when the data stored on the magnetic floppy disk or on the optical disk is read out, electric power is necessary to activate the read-out machine, but when the data stored in the non-volatile semiconductor memory is read out, power is only used for the electrical read-out and is not required to activate such a read-out machine. 
     Also, the solar cells used in the present invention have been improved to upgrade the amount of electricity to be generated and to expand the replenishment of the electrical charge. Specifically, in the configuration of the solar cells, a non-single crystal semiconductor layer of PIN, NIP, or a combination of these is formed on a transparent conductive film, e.g. of SnO 2 , ITO, ZnO, or laminations thereof on a translucent or transparent substrate by commonly known vapor reaction technology and another transparent conductive film is formed on the above-mentioned semiconductor layer as a reverse side electrode. The “non-single crystal” mentioned above is used to mean an amorphous body and polycrystalline body other than the single crystal. 
     With this sort of configuration, the solar cell generates electricity both when light strikes the front and the rear of the cell. When this solar cell is mounted on the cover of the main body of the read-out apparatus, the solar cell generates electricity whether the cover is open or closed. Further, in addition to the solar cell provided on the cover, a solar cell is also provided at an optional position on the main body of the read-out apparatus, which effectively leads to the generation of a considerable amount of electricity. The electrical power generated from solar cells mounted in this manner is used to power the thin portable book of the present invention or is charged into the charging device, so that when the thin portable book of the present invention is used in a dark location, or when it is used in cloudy days, the electric power is replenished by the charging device. 
     In this way, the thin portable book of the present invention can be used out of doors for a prolonged period with improved reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other objects, features, and advantages of the present invention will become more apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view showing the general outer appearance of the thin portable book in one embodiment of the present invention. 
         FIG. 2  is a side elevational view of the thin portable book of  FIG. 1 . 
         FIG. 3  is a block diagram of the thin portable book of  FIG. 1 . 
         FIG. 4  is a graph comparing the power consumption of a conventional liquid crystal display system with the power consumption of the ferroelectric liquid crystal display in one embodiment of the present invention. 
         FIG. 5  is a schematic view of the solar cell in one embodiment of the present invention. 
         FIG. 6  is an electric circuit provided in the thin portable book of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment No. 1 
     Now referring to  FIG. 1  and  FIG. 2 , a read-out apparatus  1  of a thin portable book comprises a display section  2 , an external input keyboard  3 , a rotatable shaft  4 , a cover  5 , a solar cell module  6  for a power source, attached to the cover  5 , a data read-out device  7 , an external power input terminal  8  and a pair of external input terminals  9 ,  10 . 
     In this embodiment of the present invention, a ferroelectric liquid crystal display is used for the display device  2 , and a cassette-type EPROM  22  is used for the freely detachable recording medium. This EPROM is a non-volatile type recording medium in which the contents of a book are recorded in the form of coded signal. The solar cell module is comprised of an amorphous silicon solar cell having a size of a 200 mm×300 mm. 
     With a conventional TN-type or STN-type of liquid crystal display, it is necessary to continuously apply a liquid crystal display drive signal at the rate of 30 to 60 times per second in order to read out and display the alphanumeric characters, symbols, images, and the like which make up the contents of the book. Reducing the number of signals per second causes a flicker to appear in the display, making it difficult to read the book. 
     Furthermore, in the case that the some drive signals are applied to the liquid crystals only when the display image is being changed and not applied after that, no display appears making no sense. Thus, frequent display drive signals must be applied to the liquid crystals in the TN and STN types of liquid crystal display to continue the image display, resulting in the drawback that the power consumption is high. 
     As opposed to this, with a ferroelectric liquid crystal display, each time a different drive signal is applied to the liquid crystals, the image display changes. When a drive signal is not applied to the liquid crystals, the image display does not change but instead continues, so that it is not necessary to further apply the drive signal, and therefore power consumption is low. 
       FIG. 4  is a graph comparing the ferroelectric liquid crystal display system of this embodiment with the conventional liquid crystal display system in power consumption, wherein the image display is changed every 45 seconds. The dotted line  12  represents this embodiment while the solid line  13  represents the conventional system. This clearly shows that the power consumption of the present invention is lower than that of the conventional system. 
     The display of the book by the ferroelectric liquid crystal display of this embodiment can contain 50 characters by 30 lines in lateral display. The reader presses the “next page”, “previous page”, “scroll key” and the like on the external input keyboard  3  as required, to go from the page or display currently being displayed to another page or display. A new page or display thus displayed continues until the next drive signal is applied to the ferroelectric liquid crystals, so that the image is in a static condition and there is no flickering problem. Used for the ferroelectric liquid crystal display in the present invention is a liquid crystal which has a ferroelectric property in the temperature range of 0° C. to 70° C., and has an A4 size containing 720×480 dots. 
       FIG. 5  is a cross sectional view showing the solar cell used in this embodiment of the present invention. In the drawing, the solar cell comprises a translucent or transparent substrate  14 , a first transparent conductive film  15 , an amorphous silicon semiconductor layer  16 , and a second transparent conductive film in a laminated structure. The translucent or transparent substrate  14  is made of, for example, glass, plastic or the like and the first transparent conductive film  15  is made of, for example, ITO, SnO 2 , ZnO or a lamination thereof and formed on the translucent or transparent substrate  14 . The first transparent conductive film  15  functions as a base electrode, on the surface of which the amorphous silicon semiconductor layer  16  with a PIN junction is formed by a commonly known plasma CVD method. In addition, the second transparent conductive film  17  is formed on the surface of the layer  16  and constructed as a reverse side electrode. At this time, 25 stages of elements are formed in series on a 200 mm×300 mm substrate by a laser process. A slight difference in output from this solar cell module can be observed according to the intensity of the light, but whether indoors or outdoors, it can supply adequate power to drive the thin portable book. Also, because the substrate and the upper and lower electrodes are transparent, light from both the upper and lower surfaces strikes the amorphous silicon semiconductor layer. 
     Again referring to  FIG. 1 , the cover  5  is mounted to a section of the read-out apparatus  1  containing the display section  2  and the external input keyboard  3  through the rotatable shaft A. The cover  5  is opened and closed by rotation around the rotatable shaft  4 . The solar cell module  6  is provided generally over the entire surface of the cover  5 . Because the substrate  14  and both the upper and lower electrodes  15 ,  17  of the solar cell module  6  are transparent, when the cover  5  is closed, the light strikes on the outside of the solar cell to generate electricity in the solar cell, and when the cover  5  is open, the light strikes on the inside of the solar cell to generate electricity in the solar cell. Incidentally, the solar cell substrate  14  and the upper and lower electrodes  15 ,  17  do not have to be completely transparent—semitransparency is satisfactory. However, the higher the degree of transparency the better, as a greater quantity of electrical power will be generated by the solar cell. 
     Further, solar cells are provided in positions on the surface of the read-out apparatus where nothing is used, so that a larger amount of electricity can be generated. 
     A cassette-type EPROM is installed in a thin portable book which has ferroelectric liquid crystals and a solar cell module of the type outlined above. The data stored in the EPROM is read out by the read-out function of the present invention before being decoded or converted. The data is then displayed on the ferroelectric liquid crystal display in the form of alphanumeric characters, symbols, images and the like. At this time, the read-out function and the display function are activated from the external input keyboard  3 . The ferroelectric liquid crystal display is characterized in that it is thin and light, and that, in addition to providing low power consumption, it is less tiring on the eyes than a CRT display, even if used for a long period. 
     Now referring to  FIG. 6 , a switch  19  provided in the thin portable book of this embodiment is turned OFF by means of a cam mechanism linked to the rotatable shaft  4  (see  FIG. 1 ) when the cover  5  is closed, and the electricity generated by the solar cell module  6  is charged into a charging device  18 . When the cover  5  is opened, the switch  19  is turned ON. 
     The electrical power stored in the charging device  18  is fed to the read-out apparatus  1 , so that the read-out apparatus  1  is activated. Specifically, in the present invention, when the cover is closed, the power supply to the read-out apparatus is automatically turned off. This mechanism acts to prevent the power stored in the charging device  18  from being wastefully used until the cover  5  is next opened. Also, a mechanism is provided in the read-out apparatus  1  to turn off the power when the cover  5  is opened. A rectifier  20  is provided to prevent reverse power flow. 
     The thin portable book of the above-outlined configuration is small and light, and has a low power consumption. It is possible to take both of the read-out apparatus and the freely detachable recording medium outdoors to read the book out of doors. 
     In addition, the reader can possess one read-out apparatus and a plurality of EPROM cassettes which can be simply exchanged in the read-out apparatus. This is equivalent to owing many volumes of books. Generally one EPROM cassette weighs about 10 gm to 30 gm, so that it is not necessary to provide for a larger storage and shipment for a large volume of books. 
     Further, if the read-out system for the read-out apparatus is made uniform, whereby a uniform standard is set, so that it is made possible to read such a book on any read-out apparatus, the provision of one desk integrated with a read-out apparatus for each pupil at the educational site, such as a school or the like, would mean that the student would no longer have to carry many volumes of heavy books back and forth to school, but instead could carry several EPROM cassettes, each weighing only about 10 gm to 30 gm. 
     Embodiment No. 2 
     In this embodiment of the present invention, a card-type EPROM is used as the freely detachable recording medium. 
     In this embodiment, a temporary recording region is provided in the read-out apparatus, and several pages or displays of data stored on an EPROM card are read out at one time by the read-out means and recorded in the temporary recording region. By activating the external input key, the data on the single page or display in the temporary recording region which the reader would like to see, is displayed on a ferroelectric liquid crystal display in the form of alphanumeric characters, symbols, images, and the like. 
     Different pages or displays can be read out in the same way as reading a normal book. 
     Also, by using a liquid crystal display device in which sensors  21  are positioned in matrix form on the display section  2 , the part of the displayed image which is important to the reader can be indicated by a touch pen or light pen  22  or the like, and this important part can be recorded in the temporary recording region. Then, using a microcomputer  11  shown in  FIG. 3 , when this important part is displayed, it can be underlined or displayed in characters formed with the white dots in a black background. By this means, the same effect is obtained as when underlining a section in a normal paper book or when emphasizing that section with a fluorescent pen. 
     In all other respects, this embodiment is substantially identical to Embodiment No. 1. 
     Since other modification and changes (varied to fit particular operating requirements and environments) will be apparent to those skilled in the art, the invention is not considered limited to the examples chosen for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention.
A thin, readily portable book has a memory-type liquid crystal display in the display section of the thin portable book so as to obtain low power consumption along with compact size and reduced weight, a solar cell and a charging device in the energy section of the thin portable book, so that low power consumption is further promoted, a freely detachable cassette-type or card-type non-volatile semiconductor memory in the recording medium section of the thin portable book so as to provide further savings in power consumption.