US3107314A

US3107314A - Electroluminescent-ferroelectric-photoconductive display device

Info

Publication number: US3107314A
Application number: US133366A
Authority: US
Inventors: Juris A Asars
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1961-08-23
Filing date: 1961-08-23
Publication date: 1963-10-15
Anticipated expiration: 1980-10-15

Description

Oct. 15, 1963 J. A. AsARs 3,107,314

ELECTROLUMINESCENT-FERROELECTRIC-PHOTOCONDUCTIVE DISPLAY DEVICE Filed Aug. 23, 1961 be Db Fig. I

euoFF ecoN edled2- ecoFF ebON eboFF WITNESSES 2 INVENTOR Q- Juris A. Asors 3,197,314 ELECTROLUMINESQENT-FERROELEQTRF- PHOTOCONDUiZ'EEVE BZSELAY BEVECE Enris A. Asars, Monroeviile, Pa, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of iennsyivania Filed Aug. 23, 196i, Ser. No. 133,366 (lit-rims. (Cl. 315-151) The present invention relates to electrical-to-light transducer apparatus, and more particularly to electroluminescent-ferroelectric-photoconductive display apparatus having two stable states.

In most electrical-to-light information display transducer apparatus, which are compatible with high-information-capacity solid state display devices, the screen of the display apparatus receives electrical information from a digital data processing computer. As a result, the screen must be able to accept electrical information at a rate which is compatible with the computer, and in most cases only a bistable (ON-OFF) mode of operation is required. Because the computer must perform other functions in addition to providing electrical information to tie display screen, an infinite display storage time and both selective and total erasure of old information is very desirable. Also, it is desirable to have a screen display which can be used as a memory source with read-out capabilities. The electroluminescent-ferroelectric (ELF) display screen, as known in the prior art, possesses the fast writing or access speed and selective erasure, as well as read-out capabilities; however, it lacks the infinite display storage device and the whole display cannot be non-selectively erased in a short period of time. On the other hand, the electroluminescent-photoconductor display screen, known in the prior art, has infinite display storage time and can be erased on a total display basis; however, it is not compatible with computer writing speeds and selective erasure of old information is not readily obtainable. By combining the two display methods, apparatus, as disclosed herein, may be provided with fast writing speed and infinite storage time.

It is, therefore, an object of the present invention to provide a new and improved electroluminescent-ferroelectric-photoconductor display apparatus having fast writing speed and infinite storage time.

Broadly, the present invention provides bistable electroluminescent-ferroelectric-photoconductor display apparatus in which a light output is supplied from an electroluminescent cell in response to electrical information applied as a control potential to an electroluminescentferroelectric circuit. A photo-conductive element is provided to be responsive to the light output as a feedback control to maintain the light output at the then present stable state.

These and other objects will become more apparent when considered in view of the following specification and drawings, in which:

FIGURE 1 is a schematic diagram of the circuitry embodied in the present invention; and

FIG. 2 is a waveform diagram used to aid in the explanation of the operation of the present invention.

Referring to FIG. 1, an electroluminescent-ferroelectrio bridge circuit and excitation sources are shown including the nonlinear fcrroelectrio capacitors P and F the electroluminescent cell EL, the alternating excitation sources e and e and the D.C. bias source E The waveforms for the excitation sources are also shown in FIG. 1. Letting 11;, be the instantaneous voltage across the electroluminescent capacitor EL, the A.C. component of this voltage V is the light-producing excitation. The average or 11C. component of V is V the control potential. Due to the nonlinearity of the ferroelectric caadd pacitors and the dissymmetry of the excitation and bias sources, V is a function of V Therefore, the amount of light emitted by the electroluminescent cell EL is dependent upon the control potential V Referring also to the Waveform diagram shown in FIG. 2, for bistable (ON-OFF) operation direct control potentials must be supplied to the point f in FIG. 1 in order to establish the ON-OFF conditions. The potential e is defined as the ON control potential, and the potential ecoFF is defined as the OFF control potential, as shown in FIG. 2. The potentials e and ecoFF are established at the point 0 shown in FIG. 1. These potentials are stored on the storage capacitor C and transferred through the isolation resistor R to the junction point 1 between the ferroelectric capacitors F and F It is assumed in the present example that the ON potential e is greater than the OFF potential e Point g is supplied with a DC. potential which is equal to or greater than the ON control potential e and point It is supplied with a DC. potential which is less than or equal to the OFF potential e A photoconductive resistor R is connected to the resistor R, and the resistor R at the common point c. The other end of the resistor R is connected to the point h. While the electroluminescent cell BL is in the OFF state, the resistance of the photoconductive resistor R is greater (several times) than the resistance of the resistor R and so the D.C. level at points 0 and f drifts in the direction of the OFF state to maintain stability in that state. However, when the electroluminescent cell EL is in the ON state, the light feedback from the electroluminescent cell EL acting upon the photoconductive resistor R reduces the resistance of R below that of the resistor R and the DC. level at points 0 and then drifts in the direction of the ON level. i

In order to provide bistable operation incoming information is applied at points a and b. As shown in 1G. 2, potentials e and e are supplied at the points a and b, respectively, to provide the ON state, while the potentials eaoFF and c are supplied in order to provide the OFF state. nonconducting to maintain the stored charge on the capacitor C within the potential limits of e to eaOFF for point a, e to eboFF for point 1;, and e to ecOFF for point 0.

Assuming that the electroluminescent cell is in its ON state, the control potential c is established through the potentials e and e When new information is received for the particular element, it is stored on the indirect storage capacitor C, by applying a switching burst e see FIG. 2, to the point 61 common to the capacitor C, in coincidence with the proper potentials eaOFF and e at the points a and b, respectively. The clamping action of the diodes D and D then may readjust the potential on the storage capacitor C to the new control potential e and thus turning the electroluminescent cell EL to its OFF position. With the cell EL in its OFF position, no light feedback is provided to the photoconductive resistor R so the resistor R goes to its high impedance state being several times larger than the resistor R The effect then is to maintain the control points c and f at the OFF condition by having the control potential drift in the direction of the potential 2 or that at point h. Now with the electroluminescent cell EL in its OFF state and new information e and e being applied to the points a and b, respectively, the switching burst e causes the storage capacitor C to be charged to the ON potential e as the diodes D and D allow the capacitor to so readjust in the presence of the switching burst e With the cell EL in the ON condition, light feedback is then supplied, as shown in the feedback path P in FIG. 1,

The diodes D, and D are 7 thus effecting the low impedance state of the photoconductive resistor R in its low impedance state, the photoconductive resistor R is lower than the resistance of the resistor R The control potential at points and thus tends to drift in the direction of the ON potential or that at point g, thus maintaining stable operation in the ON state with the feedback path being provided to activate the photoconductive resistor R While in the embodiment shown in H6. 1 the photoconductive resistor R and the resistor R are shown as separate circuit components with special input terminals, in practical circuits they may be combined with the other circuit elements. Because semiconductor diodes and electroluminescent cells are photoconductors, the photoconductor resistor R in this circuit can be combined with either of the diode D or the electroluminescent capacitor EL. Similarly, the resistor R may be combined with the diode D the storage capacitor C or the ferroelectric capacitor F FIG. 1 is only illustrative of one of the many possible circuit configurations that could be utilized with the same basic concepts. Direct storage without the need for the storage capacitor C could also be used. See Patent No. 3,018,412 issued .lanuary 23, 1962 to I. A. Asars and assigned to the same assignee as the instant application.

Only one element of a display screen has been specifically described herein. In order to provide a display screen a plurality of elements may be arranged in a suitable matrix to visually display incoming electrical information.

Although the present invention has been described with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to Without departing from the scope and the spirit of the present invention.

1 claim as my invention:

1. In image display apparatus operative with information signals having two levels the combination of, storage means operative to store said information signals at either of said levels, electroluminescent-ferroelectric circuit means operative to provide a light output having either of two stable states in response to said stored information signals, storage adjust means operative with said storage means to selectively allow said storage means to adjust in response to the then present level of said information signals, and photoconductor stabilizing means responsive to said light output to maintain the then 1 resent stable state.

2. A bistable electrical-to-light transducer information signal having two levels the combination of, storage means operative to store electrical information at either of two levels, electrolurninescent-ferroelectric circuit means operative to provide a light output having either of two stable states in response to said stored information level, storage adjust means operative with said storage means to selectively allow said storage means to adjust in response to the then present level of said information, and photoconductor stabilizing means responsive to said light output to maintain the then present stable state.

3. in ima e display apparatus operative with information signals having two levels the combination of, storage means operative to store said information signals at either of said levels, electroluminescent-ferroelectric circuit means operative to provide a light output having either of two stable states in response to said stored information signals, storage adjust means operative with said storage means to selectively allow said storage means to adjust in response to the then present level of said information signals, stabilizing means responsive to said light output to maintain the then present stable state, said stabilizing means including a photoconductive device having an impedance proportional to the light applied thereto.

4. in image display apparatus operative with information signals having two levels the combination of, storage means operatively connected to receive said information signals and being operative to store said information signals at either of said levels, electroluminescent-ferroelectric cncuit means operativery connected to said storage means and being operative to provide a light output having either of two stable states in response to said stored information signals, storage adjust means operatively connected to said storage means and being operative to selectively allow said storage means to adjust in response to the then present level of said information signals, and stabilizing means operatively connected to said circuit means and being responsive to said light output to maintain the then present stable state, said stabilizing means including a photoconductive element having an impedance proportional to the amount of light appli d thereto.

5. In image display apparatus operative with information signals having two levels the combination of, storage means operatively connected to receive said information signals and being operative to store said information signals at either of said levels, electroluminescent-ferroelectric circuit means including a ferroelectric capacitor connected to an electroluminescent capacitor with a common control junction therebetween to which said stored information signals are applied, said circuit means operatively connected to said storage means and being operative to provide alight output from said electroluminescent capacitor having either of two stable states in response to said stored information signals, storage adjust means operatively connected to said storage means to selectively allow said storage means to adjust in response to the then present level of said information signals, and stabilizing means responsive to said light output, said stabilizing means including a photoconductive device having an impedance proportional to the amount of light applied thereto, with said device being connected to said control junction and being responsive to maintain the then present stable state.

No references cited.

Claims

1. IN IMAGE DISPLAY APPARATUS OPERATIVE WITH INFORMATION SIGNALS HAVING TWO LEVELS THE COMVINATION OF, STORAGE MEANS OPERATIVE TO STORE SAID INFORMATION SIGNALS AT EITHER OF SAID LEVELS, ELECTROLUMINESCENT-FERROELECTRIC CIRCUIT MEANS OPERATIVE TO PROVIDE A LIGHT OUTPUT HAVING EITHER OF TWO STABLE STATES IN RESPONSE TO SAID STORED INFORMATION SIGNALS, STORAGE ADJUST MEANS OPERTIVE WITH SAID STORAGE MEANS TO SELECTIVELY ALLOW SAID STORAGE MEANS TO ADJUST IN RESPONSE TO THE THEN PRESENT LEVEL OF SAID INFORMATION SIGNALS, AND PHOTOCONDUCTOR STABLIZING MEANS RESPONSIVE TO SAID LIGHT OUTPUT TO MAINTAIN THE THEN PRESENT STABLE STATE.